Rockwell Automation Low-Voltage User Manual

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Low-Voltage Switchgear and Controlgear

Technical Document

LVSAM-WP001A-EN-P - April 2009

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Disclaimer

The present document is designed to provide general technical information about the selection and application of low-voltage switching and control devices and does not claim to provide a comprehensive or conclusive presentation of the considered material. Errors or changes – for example as a consequence of changed standards or technical progress – cannot be excluded.

This documentation has been worked out with utmost diligence. Nevertheless the authors and Rockwell Automation do not warrant the correctne cannot exclude typing errors. Claims on the authors or Rockwell Automation based on this documentation cannot be accepted. Rockwell Automation reserves the right to make changes at any time and at its own discretion. Correspondingly, qualified professional advice should be obtained before making decisions and initiating activities that could have an effect on technical equipment.

The authors thank the International Electrotechnical Commission (IEC) for permission to reproduce information from its International Stan IEC 60947-1 ed.5.0 (2007) / IEC 60947-4-1 Am2 (2005) / IEC 60947-2 ed.4.0 (2006) / IEC 60269-1 ed.4.0 (2006) / IEC 60947-8 ed.1.1 (2006) / IEC 60947-5-1 ed.3.0 (2003 ) / IEC 60038 ed.6.2 (2002) / IEC 60079-14 ed.4.0 (2007). All such extr

extracts and contents are reproduced by the authors, nor is IEC in any

way responsible for the other content or accuracy therein.

Rockwell Automation would like to thank the authors of the present document and their assistants for their valuable contributions.

Authors: Dr. Werner Breer, Paul Hug, Urs Hunziker, Rey Kaltenrieder, Heinz Unterweger, Dr. Hans Weichert

With the inclusion of oth

er specialists

ss of the contents and recommendations and

dard:

sibility for the placement

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General preliminary comments

The present technical manual is intended as an aid in project design and the application of lowvoltage switchgear and controlgear in switchgear assemblies and machine control. The focus of the document is on electromechanical switchgear, however electronic devices used in lowvoltage engineering have also been included. They are in many cases an effective alternative to mechanical devices.

The discussions relate – insofar relevant – to the IEC standards, which correspond to the European CENELEC standards. W are listed. The numbering of the CENELEC standards (EN) largely corresponds to that of the IEC standards. National standards (e.g. DIN/VDE or BS) in some cases have differing numbering for historical reasons, but in terms of content are largely identical to the IEC and EN standards, apart from rare national deviations. In relation to the requirements of other standard zones, especially in North America, reference is made to specific publications. The physical characteristics are generally applicable.

For switchgear combinations the standard IEC 60439-1 is referred to that is in effect at issuance of this docum

ent. It is expected that IEC 61439-1 will shortly replace IEC 60439-1. The state-

ments in the present documentation for switchgear assemblies also apply for IEC 61439-1. Statements made in this document concentrate on the underlying principles and fact

– insofar as this is possible – stating technical data relating to specific products in order to avoid premature obsolescence of the information contained. The applicable technical data about the products should be obtained from the latest valid product documentation as published in printed and “electronic” catalogs and electronic documentation like RALVET.

here standards are quoted, the respective IEC designations

s and avoid

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0 Table of contents

0 Table of contents.................................................................................................. 0-5

1 Load characteristics and utilization categories................................................. 1-1

1.1 Utilization categories simplify the selection of devices ................................... 1-1

1.2 Electrical heating devices....................................................................................1-4

1.3 Lamps and illumination equipment .................................................................... 1-4

1.3.1 Incandescent lamps................................................................................................ 1-4

1.3.1.1 Halogen lamps........................................................................................................ 1-4

1.3.2 Discharge lamps..................................................................................................... 1-4

1.4 Transformers......................................................................................................... 1-5

1.5 Reactive power compensation and switching of capacitors............................ 1-6

1.5.1 Reactive power compensation................................................................................ 1-6

1.5.1.1 Individual compensation......................................................................................... 1-6

1.5.1.2 Group compensation .............................................................................................. 1-7

1.5.1.3 Central compensation............................................................................................. 1-7

1.5.2 Switching of capacitors........................................................................................... 1-7

1.5.2.1 Switching-on single capacitors ............................................................................... 1-8

1.5.2.2 Switching of long, screened lines ........................................................................... 1-8

1.5.2.3 Switching capacitors of central compensation units............................................... 1-8

1.6 Control circuits, semiconductor load and electromagnetic load..................... 1-9

1.7 Three-phase asynchronous motors.................................................................... 1-9

1.7.1 Principle of operation.............................................................................................. 1-9

1.7.1.1 Slip-ring motors..................................................................................................... 1-11

1.7.1.2 Squirrel-cage induction motors............................................................................. 1-12

1.7.1.2.1 High efficiency motors .......................................................................................... 1-14

1.7.1.3 Influence of the voltage across the windings........................................................ 1-15

1.7.1.4 Performance of squirrel-cage induction motors with changing frequency............1-16

2 Switching tasks and selecting the appropriate switchgear.............................. 2-1

2.1 Electrical equipment complying with standards and matching

the application requirements............................................................................... 2-1

2.2 Basic switching tasks and criteria for device selection ................................... 2-1

2.2.1 Device types........................................................................................................... 2-2

2.2.1.1 Disconnectors (isolating switches).......................................................................... 2-2

2.2.1.2 Load switches......................................................................................................... 2-3

2.2.1.3 Switch disconnectors.............................................................................................. 2-3

2.2.1.4 Circuit breakers....................................................................................................... 2-3

2.2.1.5 Supply disconnecting devices................................................................................. 2-3

2.2.1.6 Supply disconnecting EMERGENCY STOP devices.............................................. 2-4

2.2.1.7 Summary supply disconnect and EMERGENCY STOP devices............................ 2-5

2.2.1.8 Fuses...................................................................................................................... 2-5

2.2.1.9 Devices for thermal protection................................................................................ 2-5

2.2.1.10 Contactors .............................................................................................................. 2-5

2.3 Parameters for the correct selection and sizing................................................ 2-5

2.3.1 Rated isolation voltage Ui....................................................................................... 2-7 U

2.3.2 Rated operational voltage Ue, rated operational current Ie and utilization category 2-7 U

2.3.3 Rated impulse withstand voltage U

imp

..................................................................... 2-7 U

2.3.4 Short-circuit withstand capacity and short-circuit protection................................... 2-9

2.3.4.1 Joule integral I2t.................................................................................................... 2-10

2.3.4.2 Cut-off current ID................................................................................................... 2-10

2.3.4.3 Rated short-time withstand current lCW................................................................. 2-10

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2.3.4.4 Current limiting protective equipment................................................................... 2-11

2.3.4.5 Coordination of electrical equipment .................................................................... 2-12

2.3.4.5.1 Coordination in respect of the switching capacity of the contactor

(overcurrent selectivity)......................................................................................... 2-12

2.3.4.5.2 Coordination with respect to the operability after a short-circuit........................... 2-13

2.3.4.6 Short-circuit switching capacity............................................................................. 2-14

2.3.4.6.1 Rated short-circuit making capacity Icm................................................................. 2-14

2.3.4.6.2 Rated short-circuit breaking capacity Icu and Ics.................................................... 2-14

2.3.5 Thermal protection................................................................................................ 2-15

2.3.5.1 Ambient temperature............................................................................................2-15

2.3.5.2 Operational overcurrents, heavy-duty starting...................................................... 2-15

2.3.6 Life span............................................................................................................... 2-16

2.3.6.1 Prospective service life......................................................................................... 2-17

2.3.6.2 Mechanical life span............................................................................................. 2-17

2.3.6.3 Electrical life span................................................................................................. 2-17

2.3.7 Intermittent and short-time duty, permissible frequency of operation................... 2-20

2.3.7.1 Intermittent duty and relative ON-time.................................................................. 2-22

2.3.8 Rated frequency and harmonics........................................................................... 2-24

2.3.9 Safety clearances................................................................................................. 2-24

2.3.10 Mounting position.................................................................................................. 2-25

2.3.11 Protective separation............................................................................................ 2-25

2.3.12 Site altitude........................................................................................................... 2-26

2.3.13 Shock and vibration.............................................................................................. 2-26

2.4 Specific application conditions and switching tasks...................................... 2-27

2.4.1 Parallel and series connection of poles................................................................ 2-27

2.4.1.1 Parallelling............................................................................................................ 2-27

2.4.1.2 Series connection................................................................................................. 2-27

2.4.2 AC switchgear in DC applications......................................................................... 2-28

2.4.3 Applications at supply frequencies < 50 Hz and > 60 Hz. Effect of harmonics.... 2-29

2.4.3.1 Effect of the supply frequency on the thermal load............................................... 2-29

2.4.3.2 Effect of the supply frequency on the switching capacity ..................................... 2-31

2.4.3.3 Performance of release units at supply frequencies < 50 Hz and > 60 Hz........... 2-32

2.4.3.4 Switchgear used with soft starters........................................................................ 2-32

2.4.3.5 Switchgear for use with frequency converters (inverters)..................................... 2-33

2.4.4 Application of four-pole switchgear devices.......................................................... 2-35

2.4.4.1 Applications of switchgear with 4 NO contacts..................................................... 2-35

2.4.4.2 Applications of switchgear with 2 NO and 2 NC contacts..................................... 2-36

2.4.4.3 Applications of switchgear with 3 NO and 1 NC contact....................................... 2-37

2.4.5 Application of circuit breakers in IT networks ....................................................... 2-37

2.4.6 Switchgear for safety applications........................................................................ 2-38

2.4.6.1 Mechanically linked contacts................................................................................ 2-38

2.4.6.2 Mirror Contacts..................................................................................................... 2-39

2.4.7 Installations in hazardous atmospheres ............................................................... 2-40

2.4.7.1 History, guidelines and regulations....................................................................... 2-40

2.4.7.2 Classification of hazardous areas......................................................................... 2-41

2.4.7.3 Motors for hazardous areas.................................................................................. 2-43

2.4.7.4 Protection of motors of ignition protection type Increased Safety “e”...................2-45

2.4.7.5 ATEX 100a (Directive 94/9/EC)............................................................................ 2-46

2.4.7.6 IECEx and other approval schemes for hazardous areas .................................... 2-47

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3 Starting and switching motors............................................................................ 3-1

3.1

Selection criteria................................................................................................... 3-1

3.2 Direct starting of squirrel-cage induction motors............................................. 3-3

3.2.1 Starting time............................................................................................................ 3-3

3.2.2 Reversing starters................................................................................................... 3-4

3.3 Star-delta (Y-Δ, wye-delta) starting ..................................................................... 3-4

3.3.1 Normal star-delta starting ....................................................................................... 3-5

3.3.2 Motor connection for clockwise and counterclockwise direction of rotation............ 3-8

3.3.3 Influence of the third harmonic on motor protection relays................................... 3-10

3.3.4 Uninterrupted star-delta starting (closed transition).............................................. 3-11

3.3.5 Amplified star-delta starting.................................................................................. 3-12

3.3.6 Part-winding star-delta starting............................................................................. 3-13

3.4 Auto-transformer starting.................................................................................. 3-14

3.4.1 Circuit and function............................................................................................... 3-14

3.4.2 Rating of the starter.............................................................................................. 3-15

3.5 Starting via chokes or resistors........................................................................ 3-15

3.5.1 Starting via chokes ............................................................................................... 3-15

3.5.2 Starting via resistors............................................................................................. 3-16

3.6 Stator resistance soft starting........................................................................... 3-16

3.6.1 Circuit and function............................................................................................... 3-16

3.7 Pole-changing motors........................................................................................ 3-17

3.7.1 Speed change by pole changing .......................................................................... 3-17

3.7.2 Ratings of starters for pole changing.................................................................... 3-18

3.7.3 Rating of the starter for steps with star-delta starting........................................... 3-19

3.8 Starting wound-rotor motors............................................................................. 3-20

3.9 Electronic soft starters....................................................................................... 3-22

3.9.1 Voltage ramp versus current limitation ................................................................. 3-23

3.9.2 Voltage ramp ........................................................................................................ 3-24

3.9.3 Kickstart................................................................................................................ 3-24

3.9.4 Current limitation................................................................................................... 3-25

3.9.5 Soft stop................................................................................................................ 3-25

3.9.6 Soft starters for pump controls.............................................................................. 3-26

3.9.7 Motor braking........................................................................................................ 3-27

3.9.8 Positioning speed and controlled braking............................................................. 3-27

3.9.9 Linear acceleration and deceleration by speed feedback..................................... 3-28

3.9.10 Direct start with full voltage................................................................................... 3-28

3.10 Frequency converters ........................................................................................ 3-29

3.10.1 Principle of operation............................................................................................ 3-29

3.10.1.1 Rectifier................................................................................................................. 3-29

3.10.1.2 Intermediate circuit ............................................................................................... 3-30

3.10.1.3 Inverter.................................................................................................................. 3-30

3.10.2 Operational performance...................................................................................... 3-30

3.10.3 Change of sense of rotation and braking.............................................................. 3-31

3.10.4 Motor protection.................................................................................................... 3-31

4 Protection.............................................................................................................. 4-1

4.1 Protection requirements ...................................................................................... 4-1

4.1.1 Protection against electric shock............................................................................ 4-1

4.1.1.1 Protection against direct contact............................................................................. 4-1

4.1.1.2 Protection against indirect contact.......................................................................... 4-2

4.1.1.3 Complementary protection...................................................................................... 4-3

4.1.2 Protection against overload and excess temperature............................................. 4-3

4.1.2.1 Different loading curves of various kinds of electrical equipment........................... 4-3

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4.1.2.2 Protection in continuous duty and at transient loads.............................................. 4-4

4.1.2.3 Overload and overtemperature protection by measurement of current and

measurement of temperature ................................................................................. 4-7

4.1.2.4 Protective functions ................................................................................................ 4-8

4.1.2.4.1 Protection during starting, monitoring of starting time, start interlocking.............. 4-10

4.1.2.4.2 Asymmetry protection........................................................................................... 4-10

4.1.2.4.3 Phase failure protection........................................................................................ 4-11

4.1.2.4.4 Stalling protection................................................................................................. 4-13

4.1.2.4.5 Underload protection ............................................................................................ 4-14

4.1.2.4.6 Automatic switching-over during start-up.............................................................. 4-14

4.1.2.4.7 Ground fault protection......................................................................................... 4-14

4.1.2.5 Display, warning and control functions................................................................. 4-15

4.1.3 Protection against high overcurrents, short-circuit protection............................... 4-16

4.1.3.1 Definition and characteristic of a short-circuit....................................................... 4-16

4.1.3.2 Effects of and dangers in case of short-circuits.................................................... 4-17

4.1.3.3 Protection requirements........................................................................................ 4-18

4.1.3.3.1 Switching capacity................................................................................................ 4-18

4.1.3.3.2 Current limitation................................................................................................... 4-18

4.1.3.3.3 Selectivity.............................................................................................................. 4-19

4.1.3.3.4 Short-circuit coordination...................................................................................... 4-22

4.2 Protective devices .............................................................................................. 4-22

4.2.1 Fuses.................................................................................................................... 4-22

4.2.1.1 Principle of operation............................................................................................ 4-22

4.2.1.1.1 Current limitation................................................................................................... 4-23

4.2.1.1.2 Breaking capacity ................................................................................................. 4-23

4.2.1.2 Standards and utilization categories..................................................................... 4-23

4.2.1.2.1 Classification and time/current zones................................................................... 4-24

4.2.1.3 Designs................................................................................................................. 4-25

4.2.2 Circuit breakers..................................................................................................... 4-26

4.2.2.1 Principle of operation and design ......................................................................... 4-26

4.2.2.2 Standards, functions and utilization categories .................................................... 4-26

4.2.2.2.1 Standards ............................................................................................................. 4-26

4.2.2.2.2 Functions and utilization categories...................................................................... 4-26

4.2.2.3 Design of a circuit breaker.................................................................................... 4-28

4.2.2.3.1 Thermal overcurrent releases............................................................................... 4-28

4.2.2.3.2 Electromagnetic overcurrent releases..................................................................4-29

4.2.2.3.3 Main contact system and switching capacity........................................................ 4-29

4.2.2.4 Application of circuit breakers............................................................................... 4-32

4.2.2.4.1 Application as circuit breaker................................................................................ 4-32

4.2.2.5 Installation of circuit breakers, safety clearances................................................. 4-34

4.2.3 Miniature Circuit Breakers MCB ........................................................................... 4-35

4.2.3.1 Principle of operation and design ......................................................................... 4-35

4.2.3.2 Standards, tripping characteristics and rated switching capacity.......................... 4-35

4.2.3.3 Installation of Miniature Circuit Breakers, safety clearances................................ 4-36

4.2.4 Motor protection relays (overload relays) ............................................................. 4-36

4.2.4.1 Thermal motor protection relays........................................................................... 4-36

4.2.4.2 Electronic motor protection relays ........................................................................ 4-40

4.2.4.2.1 Principle of operation............................................................................................ 4-41

4.2.4.3 Thermistor protection relays.................................................................................4-42

4.2.4.3.1 Relays for PTC sensors........................................................................................ 4-42

4.2.4.3.2 Relays for NTC sensors........................................................................................ 4-43

4.2.4.3.3 Metal resistance sensors...................................................................................... 4-43

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5 Control circuits ..................................................................................................... 5-1

5.1

Utilization categories............................................................................................ 5-1

5.2 Control voltages ................................................................................................... 5-1

5.2.1 Alternating voltage.................................................................................................. 5-1

5.2.1.1 Control transformers for contactor controls ............................................................ 5-2

5.2.1.2 Frequencies < 50 Hz and > 60 Hz.......................................................................... 5-2

5.2.2 Direct voltage.......................................................................................................... 5-2

5.3 Switching contactors ........................................................................................... 5-3

5.3.1 Alternating current magnets.................................................................................... 5-3

5.3.1.1 Conventional alternating current magnets.............................................................. 5-3

5.3.1.2 Electronic coil control.............................................................................................. 5-3

5.3.2 Direct current drives................................................................................................ 5-4

5.3.2.1 “Conventional” ........................................................................................................ 5-4

5.3.2.2 Double winding coils............................................................................................... 5-5

5.3.2.3 Electronic coil control.............................................................................................. 5-5

5.3.3 Electromagnetic compatibility and protective circuits.............................................. 5-5

5.3.3.1 Protective circuits in coil circuits............................................................................. 5-5

5.3.4 Effect of long control lines....................................................................................... 5-7

5.3.4.1 Voltage drop ........................................................................................................... 5-7

5.3.4.2 Effect of the cable capacitance............................................................................... 5-8

5.3.5 Contact reliability .................................................................................................... 5-9

6 Considerations when building control systems and switchgear assemblies6-1

6.1 Temperature rise................................................................................................... 6-1

6.1.1 Temperature rise limit values.................................................................................. 6-1

6.1.2 Laboratory test conditions and real practical environment...................................... 6-2

6.1.3 Verification of temperature-rise............................................................................... 6-3

6.1.4 Important aspects regarding device temperature rise; Recommendations............ 6-3

6.1.4.1 Rated current.......................................................................................................... 6-3

6.1.4.2 Thermal protective devices..................................................................................... 6-4

6.1.4.3 Conductor cross sections ....................................................................................... 6-4

6.1.4.4 Conductor length .................................................................................................... 6-5

6.1.4.5 Tightening torques.................................................................................................. 6-5

6.1.4.6 Line ducting ............................................................................................................ 6-5

6.1.4.7 Operating frequency and harmonics....................................................................... 6-6

6.1.4.8 Mounting devices side-by-side ............................................................................... 6-6

6.1.4.9 Mounting position.................................................................................................... 6-6

6.1.5 Thermal imaging cameras......................................................................................6-6

6.2 Short-circuit withstand capacity ......................................................................... 6-7

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1 Load characteristics and utilization categories

The characteristics of the load to be switched or controlled determine the loading of the switchgear and correct selection of the latter for the respective application. In particular the loading of contacts by current and voltage when circuits are made and broken is of high significance. Thus the making and breaking current under resistance load corresponds to the continuous operational current while for example squirrel-cage induction motors draw a multiple of the rated operational current when they are switched on and accelerate.

1.1 Utilization categories simplify the selection of devices

In order to make the choice of devices easier, utilization categories are defined in the standards for low-voltage switchgear (IEC 60947-1, -2, -3, -4, -5, -6) that take into account the intended application and hence the associated loading of the various low-voltage switchgear types, such as contactors, disconnectors, circuit breakers and load switches (Tab. 1.1-1). The rated operational currents or r usually for various rated operational voltages. For the sake of universal applicability the data is usually stated for several utilization categories for one and the same piece of switchgear. For project engineers, the selection of devices is basically reduced to the comparison of performance data of the switchgear for the respective utilization category with the ratings of the load and the choice of a device which meets or exceeds the ratings of the load.

When the rated operational voltage U certain utilization category, the required making and breaking capacity for the item of switchgear is defined. Thus in general no further agreements between users and manufacturers are required. The selection of a suitable device and comparison of products is thus facilitated.

The test regulations in the IEC standards define the test parameters for the individual utilization categories. Manufacturers are oblige ensures the suitability of the tested devices for the respective application and frees the user from getting “bogged down” in technical details.

The conditions for application in practice may differ considerably – in a favorable as well as adverse sense – from these standardized cond frequency of operation, especially long equipment life span. In such cases, the users and manufacturers must agree the permitted loads. In the catalogs as well as in the RALVET electronic documentation, the corresponding performance data are stated for the most common special applications.

Because of the very high and cost-intensive expenditures for testing, data for the most important and common utilization consultation is required.

ated operational powers are listed in the technical data for the devices –

and the rated operational current Ie are stated for a

d to carry out tests according to these standards. This

itions. Examples are heavy-duty starting, high

categories are usually provided. In cases going over and beyond this

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Nature of current

AC-20A, AC-20B

a.c. AC-1

AC-52a

AC-51

AC-12

AC-31A, AC-31B

Category Typical applications

Connecting and disconnecting under no-load conditions

AC-21A, AC-21-B AC-22A, AC-22B AC-23A, AC-23B

AC-2 AC-3

AC-4 AC-5a AC-5b AC-6a AC-6b AC-7a

AC-7b AC-8a

AC-8b

AC-52b AC-53a

AC-53b AC-58a

AC-58b

AC-55a AC-55b AC-56a AC-56b

AC-13 AC-14 AC-15

AC-140

AC-33A, AC-33B AC-35A, AC-35B

AC-36A, AC-36B

Switching of resistive loads, including moderate overloads S

witching of mixed resistive and inductive loads, including moderate overloads Switching of motor loads or other highly inductive loads

Non-inductive or slightly inductive loads, resistance furnaces Slip-ring motors: starting, switching off Squirrel-cage motors: starting, switching off motors during run Squirrel-cage motors: starting, plugging1), inching2) Switching of electric discharge lamp controls Switching of incandescent lamps Switching of transformers Switching of capacitor banks Slightly inductive loads in household appliances and similar ap Motor-loads for household applications Hermetic refrigerant compressor motor control with manual resetti Hermetic refrigerant compressor motor control with automatic resetting of overload releases Control of slip ring motor stators: 8 h duty with on-load currents for start, acceleration, run Control of slip ring motor stato Control of squirrel-cage motors: 8 h duty with on-load currents for start, acceleration, run Control of squirrel-cage motors: intermittent duty Control of hermetic refrigerant automatic resetting of overload releases: 8 h duty with on-load currents for start, acceleration, run Control of hermetic refrigerant compressor motors w automatic resetting of overload releases: intermittent duty Non-inductive or slightly inductive loads, resistance furnaces Switching of electric discharge lamp controls Switching of incandescent lamps Switching of transformers Switching of capacitor banks Control of resistive loads and solid-state loads with isolation by optocouplers Control of solid-state loads with transformer isolation Control of small electromagnetic loads Control of a.c. electromagnetic loads Control of resistive loads and solid state loads with optical isolation Control of small electromagnetic loads with holding (closed) current ≤ 0,2 A, e.g. contactor relays

Non inductive or slightly inductive loads Motor loads or mixed loads including motors, resistive load

s and up to 30 % incandescent lamp loads Electric discharge lamp loads Incandescent lamp loads

ning

plications

ng of overload releases

s: intermittent duty

compressor motors w

Relevant IEC product standard

60947-3

60947-4-1

60947-4-2

ith

60947-4-3

60947-5-1

60947-5-2

60947-6-1

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Nature of current

a.c. AC-40

AC-7a

a.c. and d.c. A

DC-20A, DC-20B

d.c. DC-1

DC-12

DC-31

DC-40

1) By plugging is understood stopping or reversing the motor rapidly by reversing motor primary connections while the motor is running.

2) By inching (jogging) is understood energizing a motor once or repeatedly for short periods to obtain small movements of the driven mechanism.

3) The utilization categories with annex A apply for frequent operations, those with annex B for infrequent/occasional operations

Category Typical applications

Distribution circuits comprising mixed resistive and

AC-41 AC-42

AC-43 AC-44

AC-45a AC-45b

AC-7b

DC-21A, DC21B DC-22A, DC22B DC-23A, DC23B

DC-3 DC-5 DC-6

DC-12 DC-13

DC-14

DC-13 DC-33

DC-36

DC-41 DC43 DC-45 DC-46

reactive loads having a resultant inductive reactance Non-inductive or slightly inductive loads, resistance furnaces Slip-ring motors; starting, switching off Squirrel-cage motors: starting, switching off motors during run Squirrel-cage motors: starting, plugging1), inching2) Switching of electric discharge lamp controls Switching of incandescent lamps Slightly inductive loads for household appliances and similar applications Motor-loads for household applications Protection of circuits, with no rated short-time withstand current Protection of circuits, with a rated short-time withstand current Connecting and disconnecting under no-load conditions Switching of resistive loads, including moderate overloads

witching of mixed resistive and inductive loads,

S including moderate overloads (e.g. shunt motors) Switching of highly inductive loads (e.g. series motors)

Non-inductive or slightly inductive loads, resistance furnaces Shunt-motors, starting, plugging1), inching2). Dynamic breaking of motors Series-motors, starting, plugging1), inching2). Dynamic breaking of motors Switching of incandescent lamps Control of resistive loads and solid-state loads with isolati Control of electromagnets Control of electromagnetic loads having economy resistors in circuit Control of resistive loads and solid state loads with optical isolation Control of electromagnets Resistive loads Motor loads or mixed loads including motors Incandescent lamp loads Distribution circuits comprising mixed resistive and reactive l Non-inductive or slightly inductive loads, resistance furnaces Shunt-motors: starting, plugging1), inching2). Dynamic breaking of d.c Series-motors: starting, plugging1), inching2). Dynamic breaking of d.c. motors Switching of incandescent lamps

ning

y optocouplers

on b

oads

having a resultant inductive reactance

. motors

Relevant IEC product standard

60947-6-2

61095

60947-2

60947-3

60947-4-1

60947-5-1

60947-5-2

60947-6-1

60947-6-2

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1.2 Electrical heating devices

Electrical heating devices are for example used for heating rooms, industrial resistance furnaces and air-conditioning plants.

In the case of wound resistance elements, the making current can be 1.4 times the rated current. In the select

ion of switchgear devices it should be noted with respect to the rated operational current that (in contrast to the motor) the current consumption increases when the mains voltage increases. When contactors are used, utilization category AC-1 should be used as a basis for alternating current and DC-1 for direct current. For manual switching, a loadswitch with corresponding load-switching capacity (AC-21) is sufficient.

Furthermore, if the ambient temperature is very high this must be taken into account. Heating circuits are often single pole circuits. Usually multi-pole switchgear devices with poles

connected in parallel are used, which enables to load-carrying capacity of switchgear units with poles connected in parallel, see section

increase the permissible load current. For the

2.4.1.1.

1.3 Lamps and illumination equipment

The illumination devices are subject to constant change due to developments in energy efficiency and electronics. For the choice of associated switching (e.g. contactors) and protective equipment (e.g. miniature circuit breakers and circuit breakers) not only the type of lighting equipment itself should be taken into account but also the kind of control circuit. Particular attention should be paid to inrush currents caused by compensation capacitors and charging of electronic control devices. This loading may be reduced by the attenuating effect of long lines.

The startup and operational current loads should be obtained from the respective manufacturers. The below descriptions relate to the basic

In general it is recommended to utilize a max. of 90 % of the current capacity of the switchgear as the current consumption of light

ing equipment typically increases when the voltage in-

creases.

characteristics. Also see Tab. 1.3-1.

1.3.1 Incandescent lamps

The filaments of incandescent lamps have a very low ohmic resistance when cold. This creates a high current peak when they are switched on (up to 15 · l

). The making capacity of the

switchgear must thus at least correspond to this value (utilization category AC-5b). Upon switching off, only the rated current has to be disconnected due to the high resistance of the hot filaments.

1.3.1.1 Halogen lamps

Halogen lamps are actually a version of incandescent lamps and their behavior is basically the same as the latter. The lamps are often designed for low voltages and powered via a transformer or electronic mains adapter. Their inrush currents should be taken into account for switching on.

1.3.2 Discharge lamps

Discharge lamps such as fluorescent tubes, energy saving lamps, mercury vapor lamps, halogen metal vapor lamps or sodium vapor lamps require both a starting circuit and a current limitation device. These devices may be conventional or electronic. Discharge lamps with electromagnetic series chokes have a low power factor and are therefore usually compensated. The compensation capacitance leads to high inrush currents that must be taken into account when the switchgear is selected.

Most electronic series devices have a high power factor (e.g. cosφ ≈ 0.95), nevertheless during switching on

When selecting the switchgear for high in-rush currents, the permitted rated power for the switching of capacitor to prevent undesired release of miniature circuit breakers with the simultaneous activation of a number of fluorescent tubes, information is provided by the tube manufacturers on the maxi-

there occurs a charging current surge that loads the switchgear accordingly.

s should be taken into account as per utilization category AC-6b. In order

LVSAM-WP001A-EN-P - April 2009

1-4

mum number of luminescent tubes (including series devices) that can be operated via a single protective switch.

Lamp type, (switch)

Incandescent lamps 15 · I Halogen lamps

- Transformer operation

- ECG

operation

Luminescent lamps (choke operation)

- uncompensated

- parallel compensated

- DUO circuit Luminescent lamps

- ECG

) operation, AC

Mercury vapor high pressure lamps

- uncompensated

- parallel-compensated Halogen metal vapor

lamps

- uncompensated

- parallel-compensated Sodium vapor high

pressure lamps

- uncompensated

- parallel-compensated Dual-source lamps ≈ 1.3 · I

Making current

peaks

10 · I

≈ 2 · I

≈ 20 · I

≈ 2 · I

10 · I

≈ 2 · I

≈ 20 · I

≈ 2 · I

≈ 20 · I

≈ 2 · I

≈ 20 · I

Startup

time

[min]

Starting

current

cos φ Calculation

- - 1 ≤ I

3 – 5 3 – 5

5 – 10 5 – 10

≈ 3 ≈ 1.3 · I

≈ 2 · I ≈ 2 · I

e e

0.95

0.5

0.9

0.9 ≤ 0.7 · I

0.4 – 0.6

0.9

0.4 – 0.5

0.9

0.4 – 0.5

0.9 1 ≤ 0.9 · I

basis for I

see Section1.4

≤ 0.7 · I

≤ I ≤ I ≤ I

≤ 0.5 · I ≤ 0.5 · I

eAC-5b

eAC-5a eAC-1 eAC-1

eAC-3

, ≤ I

eAC-3

eAC-1 eAC-1

eAC-1

eAC-6b

, ≤ I

eAC-6b

Tab. 1.3-1 Making currents for lamps and notes on selecting switchgear

) ECG … Electronic control gear

1.4 Transformers

If a low-voltage transformer is switched on, there is a short-term current surge (rush). The peak surge currents evoked by field set-up can be up to 30 times greater than the transformer rated current. The inrush currents vary according to the transformer type. They depend on the position of the coil, the characteristics of the magnetic circuit and especially on the phase angle of the voltage during switching on. The switchgear must have a correspondingly high making capacity in order to avoid contact welding.

IEC 60947-4-1 provides the utilization category AC-6a for switching transformers. The permitted

rated operational current can be determined as per IEC 60947-4-1 (Tab.7b) from the data of the AC-3 switching capacity:

= 0.45 · I

eT30

eAC-3

for n ≤ 30 n = peak value of the making current/peak value of the rated operational current

In the case of larger rush factors the following applies:

eTn

= I

eT30

· 30/n

The factor «n» should be specified by the transformer supplier. If no specifications are available, the following guideline v

(AC-6a) for switching transformers with a making rush factor of ≤ 30

alues apply for «n»:

LVSAM-WP001A-EN-P - April 2009

1-5

Transformers up to approx. 1 kVA at 230 V n ≈ 20 at 400 V n ≈ 15 larger transf

ormers at 400 V n ≈ 15 ... 30

Note

The thermal continuous current

may not be exceeded.

th(e)

Transformers in welding machines are usually designed so

that inrush current peaks and the short-circuit current with electrodes short-circuited are limited (n ≈ 10). The contactor is selected for switching these currents operationally.

If the individual welding current surges are not switched by power semiconductors but by the primary contactor, this means that th

e latter has a high switching frequency and a very high number of operations. It is essential that the contactor selected is checked with respect to the permitted frequency of operation and the electrical life span. For the electrical endurance the selection can be based on approx. 70 % of the AC-1-ratings as long as the inrush currents are limited.

1.5 Reactive power compensation and switching of capacitors

1.5.1 Reactive power compensation

In electrical networks in which inductive consumers (e.g. motors) are switched on and off, the power factor cos φ often changes with each switching operation. The Power Utilities demand from their consumers that the ratio of the consumed effective power P to the drawn apparent power S does not fall below a certain value, as the transmission of apparent power is uneconomic.

The reactive power of motors, luminescent lamps with series chokes and other inductive loads is therefore fr load of transformers and lines by the reactive current.

In deciding whether it is more advantageous to compensate individual consumers with fixed capacitor definitive. Control units for central compensation have a higher price per power unit (kVA). If allowance is made however for the fact that in most operations not all consumers are switched on at the same time, a lower installed capacitor power is often sufficient for central compensation.

equently compensated by connecting capacitors, in order to reduce the additional

s or to provide central compensation units, economic and technical considerations are

1.5.1.1 Individual compensation

For individual compensation (Fig. 1.5-1 a) the capacitors are directly connected to the terminals of the individual consumer (e.g. motor, transformer, induction heater, luminescent lamp) and switched together with these via a common switchgear unit. Single compensation is recommended with large consumers with constant power consumption and long ON-times. They offer the advantage that the lines to the consumers are also relieved of load. The capacitors can frequently be connected directly to the terminals of the individual consumer and be switched on and off with a common switchgear device.

a) Individual compensation b) Group compensation c) Central compensation

Fig. 1.5-1 Compensation types

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In the case of motors, the capacitors can be connected up- or downstream the motor protection unit (Fig. 1.5-2). In most cases the capacitor will be connected parallel to the motor (case 1). In this case th rated current I

e motor protection unit should be set to a smaller setting current I

as the magnitude of the line current falls due to the compensation:

than the motor

Case 1 Case 2

Fig. 1.5-2

Individual compensation of motors I

= (cos φ1/cos φ2) · IN

cos φ cos φ

= power factor of the uncompensated motor

= power factor of the compensated motor

1.5.1.2 Group compensation

For group compensation each compensation device is assigned to one consumer group. This may consist of motors or also for example of luminescent lamps that are connected to the mains via a contactor or a circuit breaker (Fig. 1.5-1 b).

1.5.1.3 Central compensation

Mostly reactive power control units are used for central compensation which are directly assigned to a main- or sub-distribution station ( many consumers with differing power requirements and variable on-times are instal

Fig. 1.5-1 c). This is especially advantageous if

led in the

network. Central compensation also offers the advantage that

 the compensation device is easy to monitor due its central location,  any retrospective installation or extension is relatively simple,  the capacitive power is continuously adapted to the reactive power requirement of the

consumers and

 making allowance for a simultaneity factor a lower capacitance is often required than for

individual co

mpensation.

See IEC 61921; Power capacitors – Capacitor batteries for correcting the low-voltage power factor

1.5.2 Switching of capacitors

Capacitors form oscillator circuits together with the inductances of the lines and the transformers. During closing, very high transient currents with higher frequencies may flow. Typical values are 10 ... 30 times the capacitor rated current at frequencies of 2 ... 6 kHz. For this reason, the switching of capacitors represents a very heavy load on switchgear and can result in increased contact burn-off or under adverse conditions even welding of the contacts. Especially when capacitors are switched by contactors, it should be ensured that they are discharged before switching-on to avoid even higher transient currents and welding of the contacts in case of adverse phase angles.

A harmonic component in the supply voltage leads to increased current consumption by the capacitors a undesired temperature rise, the rated operational current of the contactors, load switches and circuit breakers shall be higher than the capacitor rated current. Generally this should only be 70 … 75 % of the rated current of the circuit breaker.

LVSAM-WP001A-EN-P - April 2009

nd results in additional heating of the current carrying circuits. To prevent any

1-7

Taking into account the aforementioned facts, the switchgear should be dimensioned so that

 it does not weld at the high making currents and  that no unacceptable temperature rise occurs during continuous duty.

1.5.2.1 Switching-on single capacitors

If a capacitor with a specific capacity is connected to the power supply, then the making current is largely determined by the transformer size and by the network impedance to the capacitors, i.e. from the prospective short-circuit current at the installation site of the capacitor.

The loading of the switchgear increases as

 the capacitance of the capacitors increases,  as the rated power of the supplying transformer increases and hence its short-circuit

impedance decreases,  decreasing impedance of the connecting lines. Table 7 in I

the rated operational current I

−

valid for

EC 60947-4-1 states the below derivation of capacitor switching capacity I

in relation to the prospective short-circuit current ik:

eAC-3

⋅=

)6(

kbACe

ACe

3.13−⋅=

)1( −

)3(

eAC-6b

from

205−⋅>

)3(

ACek

1.5.2.2 Switching of long, screened lines

Long screened lines have comparatively large capacitances and therefore create high transient current loads during switching. Typical applications are variable frequency drives. The peak currents to be expected should be taken into account when selecting switchgear to the same extent as for the switching of single capacitors.

1.5.2.3 Switching capacitors of central compensation units

If individual capacitors of capacitor banks are switched – for example in reactive power control units - especially adverse conditions occur at closing of the switchgear contacts as the capacitors already connected to the power supply represent an additional source of energy.

The inrush current is limited by the impedance of the circuit (conductors, capacitor inductance, inductances

The loading of the switchgear is therefore determined by  the power ratio of the switched capacitors to those already connected to the power supply

and  the impedance of the individual circuit branches For avoiding welding of t

e.g. be increased, by additional of the connecting wires).

With special capacitor-contactors or capacitor-c ces to the power supply via pre-charging resistances, a very high switchable capacitance at a minimum of interference with the supplying network can be achieved, as the making currents are specifically limited by the resistances and strongly reduced.

AC-6b is defined in IEC 60947-4-1 as the utilization category for the switching of capacitor banks.

between the individual capacitor branches).

he switching contacts of the contactors the switchable capacitance can

inductances in the capacitor branches (e.g. a few winding turns

ontactor-combinations that connect capacitan-

LVSAM-WP001A-EN-P - April 2009

1-8

1.6 Control circuits, semiconductor load and electromagnetic load

Regarding the specific aspects of the switching of control circuits, also refer to Section 5. The utilization categories AC-12 to AC-15 for alternating current and DC-12 to DC-14 for direct

current (see control circui

Tab. 1.1-1) make allowance for the specific loading of switchgear for switching of

ts with semi-conductors or electromagnetic loads. When electromagnets are switched, for example contactor coils, particular attention is paid to the increased making load because of the pull-in current of the magnets and the increased breaking load due to the high inductance of the closed magnets.

In addition to the switching capacity of the contact in the sense of a maximum permitted load, very often the key criterion in the switching of co

ntrol circuits is contact reliability, i.e. the capability of a contact or a chain of contacts to reliably switch small signals. This is especially the case for contacts in circuits of electronic controllers and in the signal range of ≤ 24 V / ≤ 20 mA (also see section

5.3.5).

1.7 Three-phase asynchronous motors

The three-phase asynchronous motor – also known as the induction motor – is the most frequently used motor type for industrial drives. Especially in the form of a squirrel-cage induction motor, it dominates the field of industrial electrical drive technology.

1.7.1 Principle of operation

The key functional elements of the three-phase asynchronous motor (see Fig. 1.7-1) are the fixed stator with a three-phase coil supplied by the three-phase supply network and the revolving rotor. There is no electrical connection between the stator and rotor. The currents in the rotor are induced by the stator across the air-gap. The stator and rotor are composed of highly magnetizable dynamo plates with low eddy current and hysteresis losses.

Fig. 1.7-1 Sectional view of a squirrel-cage three-phase motor with enclosed design

When the stator coil is connected to the power supply, the current initially magnetizes the laminated metal body. This magnetizing current generates a field that rotates with the synchronous speed n

n n

f = frequency in s

= 60 · f/p

= synchronous speed in min-1

-1

p = pole pair number (pole number/2) For the smallest pole number of 2p = 2, with a 50 Hz power supply, the synchronous speed is

= 3000 min-1. For synchronous speeds with other pole numbers and for 50 and 60-Hz power

LVSAM-WP001A-EN-P - April 2009

1-9

supplies, see Tab. 1.7-1.

Pole

2 4 6 8 10 12 16 24 32 48

number

s 50 Hz

s 60 Hz

3000 1500 1000 750 600 500 375 250 188 125 3600 1800 1200 900 720 600 450 300 225 150

Tab. 1.7-1 Synchronous speeds for 50 and 60 Hz power supplies

The rotating field of the stator induces a voltage in the coil of the rotor, which in turn creates a current flow therein. With the interaction of the rotating field of the stator with the conductors in the rotor through which a current flows, a torque is created in the direction of the rotating field.

The speed of the rotor is always smaller than the synchronous speed by the so-called slip s.

s = (n

-n)/n

s slip n

synchronous speed

n operational speed

It is only because of this speed differential that a voltage can be induced in the rotor and hence the rotor curr

ent that is the prerequisite for the generation of the motor-torque. The slip increases with the load torque. Its rated value at the rated load of the motor depends on the rotor resistance and hence on the energy efficiency of the motor.

The torque curve of the induction motor is characterized by the breakdown-torque. This means that the torque of the motor increase

s with increasing speed to a maximum value and then rapidly falls back to zero at the synchronous speed. If the mechanical load of a motor running at normal service is increased beyond the value of the breakdown torque, it will stall, i.e. it comes to a halt. The magnitude of the breakdown torque is determined by the electrical reactance of the motor and hence by the motor’s design. The slip that occurs at breakdown torque can be influenced by the rotor resistance. This effect is exploited in slip-ring motors by switching on external resistors (

Fig. 1.7-2 and Fig. 1.7-3).

3*R

2*R

Fig. 1.7-2 The torque characteristic of asynchronous motors can quasi be extended by connecting resistors in the rotor circuit.

breakdown torque

s slip

breakdown slip

1-10

rotor resistance

LVSAM-WP001A-EN-P - April 2009

Asynchronous motors behave electrically like transformers. The secondary winding is the rotor and the mechanical power output of the motor acts on the primary side like a – variable – load resistance. If no mechanical power output is produced at rest (on initiation of start-up), this load resistance is zero, i.e. the transformer is in effect secondarily shorted. This leads – depending on the rotor-internal resistance – to a high or very high current consumption of the motor during starting. In the case of slip-ring motors, the current consumption is reduced by connecting external resistors and hence the torque characteristic is adapted to the driven machine. With squirrel-cage induction motors (see section torque characteristic ar

e influenced by the design of the rotor cage.

1.7.1.2) the current consumption and hence the

1.7.1.1 Slip-ring motors

With slip-ring motors, the rotor winding is connected to slip rings and terminated with external resistances. The resistance of the external resistors influences the current flowing through the rotor and the speed-torque characteristic.

M 3~

Fig. 1.7-3 Principal diagram of a slip-ring motor with external rotor resistances

Slip-ring motors represent the conventional method of controlling starting torque (and the current consumption) by selection of the rotor resistances. The highest attainable starting torque corresponds to the breakdown torque of the motor. This is independent of the magnitude of the rotor resistance. The primary current consumption of slip-ring motors is proportional to the rotor current. Thanks to these characteristics, slip-ring motors can achieve a high starting torque with relatively low current consumption.

The external resistors are usually changed in steps during motor startup. The rotor windings are shorted in normal contin

uous duty. By designing the rotor resistances for continuous duty, it is

even possible to continuously influence the speed, albeit at the cost of high heat dissipation.

LVSAM-WP001A-EN-P - April 2009

1-11

2.0

1.8

1.6

T/T

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0.0

0.0 0.2 0.4 0.6 0.8 1.0

av-acc

≈

n/n

Fig. 1.7-4 Torque characteristic of a slip-ring motor with full-load start-up and stepped change of the rotor resistance during start-up

T4 … T1 motor torque with series-connected resistance stages (R4>R3>R2>R1) T

motor torque with shorted rotor windings

average starting torque

av-acc

≈ TL rated torque corresponds to load torque

1.7.1.2 Squirrel-cage induction motors

In the case of asynchronous machines with squirrel-cage induction rotors, the rotor consists of a grooved cylindrical laminated rotor package with rods of highly conductive metal (preferably aluminum), that is joined on the face side by rings to form a closed cage. The cage – at least in the case of small motors – is usually cast into the rotor.

To reduce the starting current and influence the starting torque characteristic, the coil rods are specially designed so th current displacement. They are usually placed crosswise at an angle to the axis of rotation to avoid variations in torque and to ensure smooth running characteristics.

Fig. 1.7-5 shows the typical characteristic of the torque and of the current in a cage induction

motor in the speed range from rest to synchronous speed. Material and de influence the shape of the characteristic curves.

at they create a high rotor resistance at rest and at low speeds by

sign form of the cage

6.00

5.00

4.00

3.00

I/I

2.00

1.00

0.00

0 20406080100

n/ns [%]

2.50

2.00

1.50

1.00

0.50

0.00

T/T

Fig. 1.7-5 Typical current and torque characteristic of a squirrel-cage induction m otor bet ween rest and synchronous speed.

current characteristic with delta-connected windings

torque characteristic with delta-connected windings

load torque (example)

LVSAM-WP001A-EN-P - April 2009

1-12

The operating characteristics (Fig. 1.7-6) show that the asynchronous motor has a so-called

“hard” speed characteristic, i.

e. the speed changes only slightly with a change in loading. At low loading, the current consumption approaches the value of the idle running current, which is basically the same as the magnetization current of the motor.

1.50

1.25

1.00

n/n

0.75

cos

0.50

0.25

0.00

0.00 0.25 0.50 0.75 1.00 1.25

Fig. 1.7-6 Operating characteristics of an asynchronous motor as a function of load

n = speed

= synchronous speed

s = slip

= power intake

= power output

= rated operational power

η = efficiency

s φ = power factor

co I = current consumption

= rated operational current

n The speed n only decreases slightly with increasing load. Normal squirrel-cage induction

motors thus have “hard” speed characteristics. s The slip s in cos φ The power factor cos φ is strongly d

creases roughly proportionally with increasing load.

ependent on the load and only reaches its highest value in a state of overload. The power factor is relatively adverse in the part-load range, as the magnetization is practically constant.

η The efficien

cy η remains relatively constant and in the upper half-load range remains practically unchanged. It generally reaches its highest value below the rated operational power P

I The current I increase

falls less strongly and then goes over to the idle running current I

s proportionally from around the half-load mark. Below this point it

(constant magnetiza-

tion)

Starting from the idle running consumption, the power intake P1 increases roughly

proportionately to the load. In the overload range, its rate of increase is somewhat higher as losses increase more strongly.

LVSAM-WP001A-EN-P - April 2009

1-13

T The torque in the operating range is calculated as follows:

55.9cos3 ⋅⋅⋅⋅⋅

ηϕ

[]

U voltage across the motor [V] I current [A]

cosφ power factor η efficiency of the motor

n speed [min

-1

]

The rated operational currents, starting currents and the torque characteristic of cage induction motors depend, among other things, on their design, especia

lly the material and form of the cage, as well as on the number of poles. The specifications provided by the motor manufacturer apply in each individual case. Typical values for motors can be obtained from the RALVET electronic documentation.

For switching asynchronous motors, under IEC 60947 the utilization categories AC-2 to AC-4 among others are defined to facilitat (Tab. 1.1-1). These utilization categories make allowance for the loading

e the user in the selection of suitable contactors

of the switchgear by the increased making currents when stationary motors are switched on and for the fact that the effective switching voltage of a running motor is only around 17 % of the rated operational voltage because the running motor develops a back-e.m.f. (counter voltage to supply voltage).

1.7.1.2.1 High efficiency motors

In the context of the efforts of saving energy and pollution control, the efficiency of electric motors and drives has become an issue. This on the background of appr. 40% of the global electricity being used for operating electric motors. The IEC standard 60034-30 (2008) defines efficiency classes for general purpose induction motors of the power range of 0.75 …375 kW and with 2, 4 or 6 poles (Tab. 1.7-2). The term MEPS (Minimum Energy Performance Standard) is being us minimum level for new motors in the area of the European Union, IE3 may be required in a further step (minimum requirements, if any, are subject of national legislation).

IEC Class IEC Code EFF Code

Super Premium Efficiency IE4 Premium Efficiency IE3 NEMA Premium High Efficiency IE2 EFF1 EPAct Standard Efficiency IE1 EFF2 Below stand Efficiency ‘--- EFF3

1) CEMEP classification (CEMEP = European sector committee of Manufacturers of Electrical Machines)

ed in this context [25]. It is expected that efficiency class IE2 shall become the

NEMA

Tab. 1.7-2 Efficiency classes for general purpose induction motors according to IEC 60034-30 (2008) in comparison to the EFF-codes of CEMEP and the codes used by NEMA. IE4 is not yet defined and reserved for the future.

High efficiency motors that comply with the MEPS standard may have higher starting currents and cause hi

gher transient current peaks upon switching. The starting torque may be compara-

tively lower for the same starting current, while the breakdown torque may be higher. When selecting switchgear for starting high efficiency motors, attention should be paid to the

selection of

the proper release class of overload relays (start-up time could become longer at

higher starting current levels). In case of using (current-limiting) circuit breakers the choice of

LVSAM-WP001A-EN-P - April 2009

1-14

breakers with high magnetic trip (c.b.’s for transformer protection) may be required for avoiding nuisance tripping due to high switching transients.

Above factors should particularly be considered in retrofit applications when replacing old standard motors with new high efficiency motors.

Also, if soft should be made to ensure the equip

starters are used, the start current may be higher for a given start torque, so a check

ment is rated accordingly. The available start torque may

also need to be evaluated to ensure matching to the load characteristics.

1.7.1.3 Influence of the voltage across the windings

In order to reduce the high current consumption of squirrel-cage induction motors on starting and the associated, often disruptive, power supply loading and to reduce the high starting torque when driving sensitive machines, a wide variety of methods is available that is based on the reduction of the voltage applied across the motor windings. A reduced voltage across a motor winding results in a proportional reduction in the current flowing through the winding and consequently in a reduction of the torque developed, by the square of the reduction in the voltage applied. ½ voltage for example thus means ¼ torque.

A reduction of the voltage across the motor windings can in principle be achieved in one of two ways:

 Reduction of the voltage on the motor while leaving the internal connections of the individual

windings un

 Rearranging the connections of the motor windings so that the voltage on the windings is

reduced. Example: Star-delta cir

The ratio of the available motor torque to the motor current flowing is different in above cases. This is illu electronic soft starter device (Fig. 1.7-7):

changed (normally connected in delta). Example: electronic soft starter devices.

cuit.

strated with the example of the conventional star-delta circuit compared with the

LVSAM-WP001A-EN-P - April 2009

1-15

U/√3

p/3

Starting method Direct (Δ, delta) Y (star, wye) Soft starting Current in the pole

100 % 33 % 33 % 57 %

conductor Torque 100 % 33 % 11 % 33 % Coil voltage 100 % 57 % 33 % 57 %

2.50

T/T

2.00

1.50

1.00

0.50

0.00

I/I

6.00

5.00

4.00

3.00

2.00

1.00

0.00

0 20406080100

n/ns [%]

Fig. 1.7-7

nt consumption and torques with direct starting (delta-connected), star-co nnected starting and

Curre starting with the aid of a soft starting device by reducing the voltage across the motor term inals.

IΔ Current consumption with delta-connected direct starting T

Torque with delta-connected direct starting

Current consumption for star-connected starting

Torque for star-connected starting

Current consumption when starting by means of soft starting device at torque T

TSS Torque when starting by means soft starting device and same current consumption

as for star-connected starting A startup in star circuit develops a higher torque with the same loading on the power supply, as

the voltage across the motor coils is reduced b

y the circuit change. The windings current is at

the same time the pole current, while in the case of a delta-connected motor two windings currents add vectorial (factor

) to the current of the supplying pole.

In relation to the loading of switchgear and its correct selection for starting and operation of squirrel-cag the possible

e induction motors with voltage reduction, see Sections 3.3, 3.4 and 3.9, in which

solutions are described in detail.

When using soft starting devices, it should be remembered that the line-side switchgear operates the input of the

soft starting device, i.e. usually without load. The motor current is subject to harmonics during start-up, which can lead to undesired tripping, especially where electronic protective relays are used.

1.7.1.4 Performance of squirrel-cage induction motors with changing fre-

quency

The basic form of the current and torque characteristic is independent of the frequency. In the sub-synchronous speed range (n = 0 … n

LVSAM-WP001A-EN-P - April 2009

1-16

), the voltage must be reduced in proportion to the

frequency to keep the magnetic flux constant and to avoid saturation of the ferromagnetic circuits. This means that the magnitude of the breakdown torque remains roughly constant. Motors that are operated for long periods at lower speeds must be externally ventilated due to the decreasing efficiency of their internal ventilation.

If the frequency rises above the supply frequency then a constant voltage is usually available from the frequency converter. This results in a weakening

of the magnetic field with increasing frequency and consequently in a reduction of the breakdown torque in proportion to the square of the frequency. Up to the maximum speed, such drives can typically be operated with constant power output.

2.50

T/T

2.00

1.50

1.00

0.50

0.00 0 50 100 150 200

Fig. 1.7-8 Typical torque characteristic with variation of the frequency. Voltage proportional to frequency in the range 0 … f Voltage constant in the range > f

f/fn[%]

When selecting the line-side switching and protective equipment it should be remembered that frequency converters have large controlled or uncontrolled rectifier circuits on the input-side with large storage capacitances. This results in high charging current surges and in a high harmonic content in the current.

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2 Switching tasks and selecting the appropriate switchgear

The selection and use of electrical equipment for switchgear assemblies and machine control units are regulated under the respective national legislation. Within the European Union (EU) the regulations are based on the CENELEC standards (EN standards) which are largely identical with the IEC standards. The IEC standards also form the basis of the applicable regulations in a large number of other countries. In North America, the standards of UL or CSA as well as the directives of NEMA, NEC etc. have to be applied. All these standards and regulations have as their common goal to guarantee the safety of electrical installations.

2.1 Electrical equipment complying with standards and matching the application requirements

The standards IEC 60439-1 (Low-voltage switchgear and controlgear assemblies) and IEC 60204-1 (Electrical equipment of machines) require among other things that the electrical equipment must correspond to the valid applicable standards. This means that low-voltage switchgear must be built and tested in compliance with the requirements of IEC 60947. Furthermore, the external design of the electrical equipment, its rated voltages, rated currents, life span, the making and breaking capacity, the short-circuit withstand capacity etc. must be suitable for the respective application. If necessary current-limiting protective devices must be used for protecting the electrical equipment. The coordination of electrical equipment, for example of motor starters to short-circuit protection equipment must comply with the applicable norms. When selecting electrical equipment the rated impulse withstand voltages and the generated switching overvoltages have to be considered.

According to these standards, all devices that are available on the market must comply with the applicable standards. For the EU and the EEA, this compliance is confir conformity by the manufacturer and the CE-sign. The same requirements apply to Switzerland, with the exception that the CE-sign is not compulsory (but is permitted). Other countries have their own licensing procedures and signs of conformity. So requires China the CCC-mark and Australia and New Zealand have introduced the C-tick-mark for EMC compliance of electronic products.

med by a declaration of

Fig. 2.1-1 CE-sign for the EU market (left), CCC-sign for China (center) and C-tick-mark for Australia and Ne w Zealand

For special applications such as for example shipping, railroads or applications in hazardous environments where a risk of explosion exists, specific regulations apply in many cases that usually contain additional requirements beyond the basic IEC standards.

The standards to which the devices have been built and tested are listed in the catalogs. For low-voltage switchgear (c markets outside North America, these standards are basically the various parts of the IEC 60947.

ontactors, motor starters, circuit breakers, load switches etc.) for the

2.2 Basic switching tasks and criteria for device selection

Load circuits include functional components in accordance with Fig. 2.2-1, whereby several functions can be combined in a single device.

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Isolation

Short-circuit

protection

Thermal

protection

Operational

switching

Load

Fig. 2.2-1 Functional elements of a load circuit

Example Disconnector (Isolator) Switch disconnector Circuit breaker with isolating function

Fuse Circuit breaker

Fuse (line protection) Circuit breaker with thermal release Motor protection relay (thermal, electronic)

Contactor Load switch Motor protection circuit breaker

Motor Heater Lighting Capacitor

The selection of suitable devices to fulfill the required functions is based on the characteristics of the load (e.g. rated power and utilization category), the operational requirements (e.g. switching frequency, required availability after a short-circuit) and the nature of the power supply (e.g. rated voltage, prospective short-circuit current).

2.2.1 Device types

Various types of device are available for carrying out the switching and protection tasks listed under 2.2 that are specially designed to fulfill the respective requirements. The various parts of IEC 60947 (Low-voltage switchgear and controlg

ear) specify the design, performance and test features of the devices. The most important features of the main device types are presented below.

2.2.1.1 Disconnectors (isolating switches)

The disconnector is a mechanical switchgear that fulfills in the open position the requirements specified for the isolation function (IEC 60947-1). The purpose of the isolating function is to cut off the supply from all or a discrete section of the installation by separating the installation or section from every source of electrical energy for reasons of safety. The key factor here is the opening distance. Isolation must be guaranteed from pole to pole and from input to output, whether this is by means of a visible isolation gap or by suitable design features within the device (mechanical interlocking mechanism).

A device fulfills the isolating function stipulated under IEC 60947-1 when in the “Open” position the isolation circuit of the switchgear. It must also be equipped with an indicator device in relation to the position of the movable contacts. This position indicator must be linked in a secure, reliable way to the actuator, whereby the position indicator can also serve as actuator, provided that it can

at a defined withstand voltage is assured between the open contacts of the main

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only display the position “Open” in the “OFF” position, when all moving contacts are in the “Open” position. This is to be verified by testing.

According to IEC 60947-3, an isolator must only be able to make and break a circuit, if either a current of negligible size is switche

d on or off, or if during switching no noticeable voltage difference between the terminals of each pole occurs. Under normal conditions it can conduct operational currents as well as under abnormal conditions larger currents (e.g. short-circuit currents) for a certain period.

Disconnector

Fig. 2.2-2 Switch symbols The horizontal line in the switch symbol of the contacts indicates that they fulfill the isolating function

(Load-) Switch Switch-disconnector Circuit breakers

The isolator function can be realized with a variety of devices such as for example in disconnectors, fuse-disconnectors, switch-disconnectors, fuse-switch disconnectors and circuit breakers with isolating function.

2.2.1.2 Load switches

Load switches (or only “switches”) are mechanical switching devices capable of making, carrying and breaking currents under normal circuit conditions which may include specified operating overload conditions and also carrying for a specified time currents under specified abnormal circuit conditions such as those of short-circuit.

A load switch may have a short-circuit making capacity, however it does not have a short-circuit breaking capacity (IEC 60947-1 an circuit withstand capacity), but not be switched-off.

For load switches the range of designs is similarly wide as for isolator switches, for example “normal” (load) switches, fuse-switch in all countries.

d -3). Short-circuit currents can be conducted (high short-

es, circuit breakers. Fuse-switches are not legally permitted

2.2.1.3 Switch disconnectors

Switch disconnectors combine the properties of (load) switches and disconnectors. In this case, too, there are a variety of designs such as “normal” switch disconnectors, fuse-

switch-disconnectors an

d circuit breakers. Fuse-switch-disconnectors are not legally permitted

in all countries.

2.2.1.4 Circuit breakers

See also Section 4.2.2. Circuit breakers are mechanical switching devices, capable of making, carrying and breaking currents under normal circuit conditions and also making, carrying for a specified time and breaking currents under specified abnormal circuit conditions such as those of short-circuit (IEC 60947-2). They thus also fulfill the requirements of (load) switches. Circuit breakers are often designed so that they can fulfill the requirements for disconnectors.

2.2.1.5 Supply disconnecting devices

IEC 60204-1 (Machine safety – Electrical equipment of machines) requires a supply disconnecting (isolating) device for each incoming source of supply and for each on-board power supply that completely isolates the machine or the device from the external or internal power supply for the machine, so that cleaning, maintenance and repair work can be carried out or the machine can be shut down for longer periods of time.

- A supply disconnecting device must fulfill the requirements of a switch-disconnector as

defined in IEC 60947-3 (load switch ments of utilization categories AC-23B or DC-23B. Disconnectors are permitted if load shed-

with isolating function) and at the least fulfill the require-

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ding is assured by an auxiliary contact before opening of the main contacts of the disconnector. Also circuit breakers with isolating function or other switchgear with isolating function and motor switching capacity can be used as supply disconnecting devices, provided that they fulfill the corresponding IEC standards.

- A supply disconnecting device must be manually actuated and have unambiguous “ON” and

“OFF” positions that are clearly marked with “О” and “I”. A supply disconnecting device must either have a visible contact gap or a position indicator

which cannot indicate O

FF (isolated) until all contacts are actually open and the require-

ments for the isolating function as specified under IEC 60947-3 have been satisfied.

- If the supply disconnecting device does not simultaneously

serve as EMERGENCY STOP

device, it may not have a red handle (preferred colors black or gray).

- It must be possible to lock the handle in the “OFF” position (for example with a padlock).

- The supply-side terminals of supply disconnecting devices must be equipped with a finger

protection anti-tamper shield and a

warning sign. If required, supply disconnecting devices may be equipped with a door interlock device. The power supply of the below circuits does not necessarily have to be controlled via the supply

disconnect switch:

- Lighting circuits that are required for maintenance work

- Sockets that are exclusively used for service equipment such as drills. The requirements for supply disconnect switches can be fulfilled by switch-disconnectors, fuse-

switch-disco

nnectors an

d circuit breakers.

2.2.1.6 Supply disconnecting EMERGENCY STOP devices

If there is a real and present danger for man or machine, the dangerous parts of the machine or the entire machine must be disconnected from the power supply by actuation of an EMERGENCY STOP device as quickly as possible and brought to a standstill. According to IEC 60204-1 supply disconnecting devices are acceptable as local EMERGENCY STOP devices as long as they are easily accessible for operating personnel. The below conditions must also be fulfilled:

- For use as an EMERGENCY STOP device the handle must be red and on a yellow back-

ground.

- The device must be simultaneously able to interrupt the locked-rotor current of the largest

connected motor plus the sum of the rated curr

- It must be able to conduct the total rated operational current of all connected devices.

- The EMERGENCY STOP switch may not break those circuits that could lead to endanger-

ment of the personnel or the machine.

ents of the remaining loads.

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2.2.1.7 Summary supply disconnect and EMERGENCY STOP devices

Requirements on supply disconnect devices (under IEC 6

Operator handle:

- Black or gray handle yes no

- Red handle with yellow background no yes

- Lockable yes yes Manual actuation from outside yes yes Easily accessible yes yes Only one “ON” and “OFF” position yes yes Position indicator only “О” and “I” yes yes Lockable in “О” position from outside yes yes Touch-protected input terminals with warning symbol yes yes

Tab. 2.2-1 Summary of requirements on switches for use as supply disconnect devices and supply disconnect/EMERGENCY STOP devices

0204-1)

Supply

disconnect

devices

Supply

disconnect/

EMERGENCY

STOP devices

2.2.1.8 Fuses

Fuses have a short-circuit breaking capacity and in the form of full-range fuses are also suitable for overload protection of conductors and certain loads. For details see Section 4.2.1.

2.2.1.9 Devices for thermal protection

See Sections 4.1.2, 4.2 and 4.2.4. Devices for thermal protection are divided into two groups:

 Devices that evaluate the thermal risk to the protected object and provide a protective

disconne

ction in one unit (for example full-range fuses, MCB’s, circuit breakers, motorprotection circuit breakers, electronic motor control devices with integrated motor protection) and

 Devices that exclusively evaluate the thermal risk to the protected object but for protective

shutdown control a power switching

device (usually a contactor). These include for example

overload relays and thermistor (PTC) protection devices.

2.2.1.10 Contactors

Contactors are designed for operational switching and - in accordance with the required high mechanical and electrical life span - use relatively low contact forces. Accordingly they have no short-circuit switching capacity and must be protected against the effects of short-circuit currents by series-connected short-circuit protective devices. See Section

2.3.4.5.

2.3 Parameters for the correct selection and sizing

For the specific application of low-voltage devices additional parameters should be taken into account such as for example the application ambient temperature, the expected device life span, any influences from moisture, mechanical impacts and vibrations etc., to name only a few of the most important. Tab. 2.3-1 provides a summary of the most important parameters when selecting devices. Some of the specific features

are looked at in more detail below.

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Parameter

Rated isolation voltage U

Rated operational voltage Ue (<Ui) Rated impulse withstand voltage U Rated operational current I

imp

Utilization category Short-circuit withstand capacity / short-circuit

protection Rated short-circuit making capacity I Rated short-circuit breaking capacity Icu, I

Cut-off current

Let-through energy (Joule integral)

Rated short time current I

Short-circuit coordination (Type 1, Type 2) with fuses or circuit breakers

Thermal load Ambient temperature Operational overcurrents

(for example heavy-duty starting) Life span Frequency of operation Rated frequency / harmonics

Safety clearances Mounting position Pollution degree Overvoltage category Protective separation Site altitude Shock and vibration Humidity / climatic loading Chemical ambient influences Radioactive radiation UV radiation External form / IP degree of protection

Details chapter

2.3.1

2.3.2

2.3.3

2.3.2

2.3.4

2.3.4.6.1

2.3.4.6.2

2.3.4.2

2.3.4.4

2.3.4.1

2.3.4.4

2.3.4.3

2.3.4.5.2

2.3.5

2.3.5.1

2.3.5.2

2.3.6

2.3.7

2.3.8

2.4.3

2.3.9

2.3.10

2.3.3

2.3.11

2.3.12

2.3.13

Fuses

Disconnectors

ers

Starters

Circuit break-

Contactor

Load switches

P P P P P P P P P P P P P P P P P P P P P P P P P P

P P P P C C

C C

C C C

P P P P C C C C C C C C C C

C C C C

C C C C C

P P P

C C A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A

Tab. 2.3-1 Selection criteria for low-voltage switchgear for main circuits P … Primary selection factors C … Complementary selection factors A … Additional criteria

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2.3.1 Rated isolation voltage Ui

Ui is the voltage on which the selection of creepage distances of electrical equipment and the

dielectric tests are based. U

must always be bigger than (or at least the same as) the voltage

that is applied to the electrical equipment and is thus always larger than or the same size as the rated operational voltage U

. With the selection of Ue is thus Ui (in the responsibility of the

device manufacturer) correctly selected, whereby the pollution degree and the overvoltage category should be taken into account.

2.3.2 Rated operational voltage Ue, rated operational current Ie and utilization category

The rated operational voltage Ue is always to be considered in association with the corresponding rated operational current I suitability of an item of electrical equipment for a certain application. For utilization categories see Section

Corresponding to the universal applicability, an item of electrical equipment can be assigned a variety of different datasets (for example I for a certain operational voltage). Common values for rated operational voltages for switchgear can be seen in Tab. 2.3-2. With 3-phase supply systems, the delta (phase to phase) voltage of the power supply applies.

d.c.

V 6 6

12 12

24 24

36 48 48 60 72 96

110 110 220 230 230/400

440 400/690

1.1.

Supply voltages Supply voltages for three-phase

277/480

and the utilization category. These three variables determine the

for various operational voltages or I

eAC-3

4-wire or 3-wire systems

a.c.

s for supply systems in accordance with IEC 60038 ed. 6.2 and industrial practice

50 Hz

120/208

1000

60 Hz

240

347/600

600

eAC-3

and I

eAC-4

2.3.3 Rated impulse withstand voltage U

The rated impulse withstand voltage U

is a measure for the dielectric strength. From the point

imp

of view of the user it is important, as the required dielectric strength among other things depends on the pollution degree of the installation site and the overvoltage category, i.e. the proximity to the feeding supply network. With respect to the pollution degree see Tab. 2.3-3.

Tab. 2.3-4 shows as an excerpt from Table H.1 of IEC 60947-1 Annex H t

overvoltage category on the applicable impulse withstand voltage U

imp

he influence of the

. The rated impulse withstand voltage is for example important in circuit breakers that are often deployed on the distribution level or also on the supply level.

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6.1.3.2 Pollution degree

The pollution degree (see 2.5.58) refers to the environmental conditions for which the

equipment is intended.

NOTE 1 The micro-environment of the creepage distance or clearance and not the environment of the equipment determines the effect on the insulation. The micro-environment might be better or worse than the environment of the equipment. lt includes all factors influencing the insulation, such as climatic and electromagnetic conditions, generation of pollution, etc.

For equipment intended for use within an enclosure or provided with an integral enclosure, the pollution degree of the environment in the enclosure is applicable.

For the purpose of evaluating clearances and creepage distances, the following four degrees of pollution of the micro-environment are establishe

d (clearances and creepage distances

according to the different pollution degrees are given in Tables 13 and 15):

Pollution degree 1:

No pollution or only dry, non-conductive pollution occurs.

Pollution degree 2:

Normally, only non-conductive pollution occurs.

Occasionally, however,

a temporary conductiv-

ity caused by condensation may be expected.

Pollution degree 3:

Conductive pollution occurs, or dry, non-conductive pollution due to cond

ensation.

occurs which becomes conductive

Pollution degree 4:

The pollution generates persistent conductivity caused, for instance, by conductive dust or by rain or snow.

Standard pollution degree of industrial applications:

Unless otherwise stated by the relevant product standard, equipment for industrial applications is generally for use in pollution degree 3 environment. However, other pollution degrees may be considered to apply depending upon particular applications or the micro-environment.

NOTE 2 The pollution degree of the micro-environment for the equipment may be influenced by installation in an enclosure.

Standard pollution degree of household and similar applications:

Unless otherwise stated by the relevant product standard, equipment for household and similar applications

is generally for use in pollution degree 2 environment.

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Maximum value of

erati

rated op voltage to earth

a.c. r.m.s. or d.c.

onal

300 220/380, 230/400,

600 347/600, 380/660,

1000 - 12 8 6 4

Nominal voltage of the

supply system

(≤ rated insulation

voltage of the

equ

ipment)

a.c. r.m.s.

240/415, 2607440

277/480

400/690, 415/720

480/830

Preferred values of rated impulse withstand

voltage (1,2/50 μs) at 2 000 m

Overvoltage category

IV III II I

Origin of

Installation

(service

entrance)

level

6 4 2.5 1.5

8 6 4 2.5

Distributi

circuit level

Tab. 2.3-4 Correspondence between the nominal voltage of the supply syste m and rated impulse withstand voltage of the device with protection by surge-arrestors according to IEC 60099-1 Excerpt from Table H.1 of IEC 60947-1 ed. 5.0 Annex H Copyright © IEC, Geneva, Switzerland. www.iec.ch

(applia equipment)

Load

level

nce,

Specially

protected

level

2.3.4 Short-circuit withstand capacity and short-circuit protection

See also Section 4.1.3. Adequate protection against the consequences of a short-circuit is one of the most fundamental

safety measures for electrical equip of property. For operational reasons it is often desirable that devices survive short-circuits largely unscathed so that they may become operational again as quickly as possible afterward. The specification of coordination types for starters is also related to this requirement.

The short-circuit withstand capacity of electrical equipment is usually defined by stating the largest permitted short-circuit protect circuit breaker). Current limiting fuses and modern current limiting circuit breakers make a major contribution to the economical rating of devices, as they strongly reduce the thermal and dynamic loading of devices and equipment connected downstream.

Loading in the event of a short-circuit is defined by the Joule integral (I current (I

) and the short-time current (Icw).

ment. This affects both the protection of persons as well as

ive device (for example permissible fuse or permissible

t value), the cut-off

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u,i

t0 t

I2t=

dti.

∫

Fig. 2.3-1 Basic characteristic of current and voltage when clearing a short-circuit with a current limiting circuit breaker u System voltage

Electric arc voltage

Prospective peak short-circuit current

Limited short-circuit current

Cut-off current

Inherent system delay

Electric arc hesitation time

Rise time

Total break time

2.3.4.1 Joule integral I2t

The I2t-value is a measure of the thermal loading of the electrical equipment in the shorted circuit. Fuses and current limiting circuit breakers limit the short-circuit current to values significantly below those of the uninfluenced current and thus reduce the thermal loading of the devices in the shorted circuit, for example of the contact system of a contactor connected downstream. The rule of thumb is that the Joule integral of the short-circuit protective device must be smaller than the permissible I to be protected.

t-value of the conductor and of the electrical equipment

2.3.4.2 Cut-off current ID

The cut-off current is the largest instantaneous value of the current that a current limiting shortcircuit protective device allows through. As the action of the force of the electrical current is proportional to the square of the current, the cut-off current is critical in ensuring the required mechanical strength of connected electrical equipment. This is particularly relevant for the design of bus systems (number and strength of the supports). IEC 60439 takes this circumstance into account by dispensing from the requirement of verification of the short-circuit withstand capacity for cut-off currents ≤ 17 kA.

2.3.4.3 Rated short-time withstand current lCW

Like the Joule integral, the short-time withstand current lCW is a measure of the thermal load capacity. It is important for circuit breakers of category B (suitable for selectivity) and is usually stated as the 1s-current (preferred values under IEC 60947-2 are 0.05, 0.1, 0.25, 0.5 and 1 s). A conversion of I I

The I

cw1

-value is of importance when for selectivity reasons the breaking action of circuit

breakers is delayed. The circuit breakers in the shorted circuit must be able to carry the shortcircuit current until the delay time has expired and then shut down the shorted circuit. For circuit

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currents for other durations is permitted according to the equation

· t1 = I

· t2 = const.

cw2

breakers with I

≥ 30 kA is required.

≤ 2500 A, IEC 60947-2 requires lCW ≥ 12 · In, at least 5 kA. For In > 2500 A

2.3.4.4 Current limiting protective equipment

If the short-circuit withstand capacity of electrical equipment is lower than the prospective shortcircuit current at the installation site, its loading must be reduced in the case of a short-circuit by upstream current limiting protective equipment to the permissible magnitude. For this purpose fuses or current limiting circuit breakers may be chosen.

The I

t- and iD-values of this protective equipment are – usually in diagrams – stated as a function of the prospective short-circuit current I that these quantities vary with the operating voltage. For fuses, limit-curves can be found in the diagrams for the cut-off current for the largest and without direct current component (see example current for the largest direct current

Fig. 2.3-3). As the time of occurrence of a short-circuit is coincidental, the cut-off

component is critical for engineering (i.e. most unfavourable

time point of occurrence of the short-circuit).

(see example Fig. 2.3-2). It should be noted

Fig. 2.3-2

-values and I2t-values as a function of the prospective short-circuit current I

When the cut-off current is limited to ≤ 17 kA, in accordance with IEC 60439-1 no verification of the short-circuit withstand capacity for the downstream circuits is required. This relates in particular to the mechanical strength of the conductors. For the protection of electrical equipment (for example of motor starters) smaller cut-off currents may also be required.

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[A]

Cut-off current i

Prospective short-circuit current I

Fig. 2.3-3 Example of an i

-diagram for fuses as a function of the prospective short-circuit curre nt Icp

1) Peak short-circuit current without direct current component

2) Peak short-circuit current with maximum direct current component

2.3.4.5 Coordination of electrical equipment

The coordination of electrical equipment refers to the assignment of short-circuit protective devices to contactors or starters with respect to the effects of a short-circuit on these devices. Distinction is made between two types of coordination:

- The coordination of the trip characteristic of the overload relay (if present) with the protective

characteristic of the short-circuit pro

tective device in respect of the switching capacity of the

contactor

- The coordination between the short-circuit protective device, the contactor and the overload

relay with respect to the destructive

effect of a short-circuit and their operability afterward.

2.3.4.5.1 Coordination in respect of the switching capacity of the contactor

(overcurrent selectivity)

The coordination between the release characteristic of the overload relay and the short-circuit protective device takes account of the switching capacity of the contactor. Contactors are designed for the operational switching of loads and are not able to switch-off currents of shortcircuit level. The coordination of the devices must ensure that for currents above the switching capacity of the contactor the short-circuit protective device shuts down before the overload relay is responding and dropping-out the contactor (

Fig. 2.3-4 and Fig. 2.3-5).

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I >

M3~

Fig. 2.3-4 Types of load feeders (with electromechanical switchgear) a) Fuse, contactor, motor protection relay b) Circuit breaker with magnetic release, contactor, motor protection relay c) Circuit breaker with motor protection characteristic, contactor d) Operational switching and circuit breaker function combined in one contact system

For starters that are protected by circuit breakers with motor protection characteristic, no coordination with respect to the overcurrent selectivity is required, as circuit breakers switch-off in the event of overloads and short-circuits.

10000

1000

100

0.1

4 3

0.01

0.001 110

100

Fig. 2.3-5 Short-circuit coordination of switching and protective devices. Circuit breakers with motor protection characteristic are used as an alternative to fuse/overload relay. 1 Motor starting current 2 Trip characteristic overload relay 3 Destruction limit curve overload relay 4 Trip characteristic circuit breaker with motor protection characteristic 5 Time/current-characteristic fuse (alternative to circuit brea ker) 6 Rated breaking capacity of the contactor 7 Welding area of the contactor

2.3.4.5.2 Coordination with respect to the operability after a short-circuit

The coordination of contactor and overload relay, if any, with a short-circuit protective device with respect to the operability of starters after a short-circuit is determined by the destructive or damaging effect of the short-circuit current on the starter components. Basic requirement – regardless of coordination type – is that neither persons nor equipment may be endangered.

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- Coordination type 1 permits damage to the starter so that further operation may only be possible after repair or replacement.

- With coordination type

2 the contactor or starter must be suitable for further use after the short-circuit. Slight welding of contacts is acceptable. An early replacement of the starter components is usually required (depending on the severity of the short-circuit) due to the erosion of contact material by the short-circuit current, however this can be carried out at an operationally convenient time.

- Coordination ty

pe “CPS” requires in accordance with IEC 60947-6-2 that a load feeder continues to be usable after a short-circuit, in order to maximize operational continuity. The guaranteed residual electrical life span based on a new device is 6000 cycles. In this case too, the replacement of the starter components as in coordination type 2 is required and may be carried out at a time that is convenient from an operational viewpoint. Load feeders under coordination type “CPS” can be realized in any design (see also

Fig. 2.3-4).

Type “1” Type “2” Type “CPS”

Finding and rectifying cause of short-circuit X X X Checking starter X X Replacing devices X 1) 1) Separating welded contacts, if any X Resume operation X X X Planned maintenance (device replacement) X X

Tab. 2.3-5 The selection of the coordination type with respect to duration of the interruption to operation

1) Replacement of fuses, if used

2.3.4.6 Short-circuit switching capacity

The switching capacity is the r.m.s value of a current at a given power factor cos φ as well as a given rated voltage at which a switchgear or a fuse can still shut-off under specified conditions in an operationally safe way. Both the short-circuit making capacity as well as the short-circuit breaking capacity of circuit breakers must be larger than or equal to the prospective short-circuit current at the place of installation. If this is not the case, then a suitable backup protection (for example a fuse) should be provided to ensure the required switching capacity of the devicecombination. Data regarding devices for backup protection are given in the technical documentation.

2.3.4.6.1 Rated short-circuit making capacity Icm

The rated short-circuit making capacity Icm is a quantity that according to regulations must be in a certain ratio to the rated ultimate short-circuit breaking capacity I

and that has to be guaran-

teed by the device manufacturer. This is not a variable that must be considered by the user, however it ensures that a circuit breaker is in the position to connect onto a short-circuit – and to disconnect it subsequently.

2.3.4.6.2 Rated short-circuit breaking capacity Icu and Ics

IEC 60947-2 makes distinction between the rated ultimate short-circuit breaking capacity ICU and the rated service short-circuit breaking capacity I

- Rated ultimate short-circuit breaking capacity I

is the maximum breaking capacity of a circuit breaker at an associated rated operational

voltage and under specified conditions. I

is expressed in kA and must be at least as large

as the prospective short-circuit current at the site of installation. Circuit breakers that have switched-off at the level of the ultimate short-circuit breaking capacity, are reduced serviceable afterwards and should at least be checked regarding functionality. There may be changes in the overload trip characteristic and increased temperature rise due to the erosion of contact material.

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- Rated service short-circuit interrupting capacity I

values are usually lower than the values for ICU. Circuit breakers that have been switch-

ing-off at the level of the service short-circuit breaking capacity continue to be serviceable afterward. In plants in which interruptions to operations must be kept as short as possible, product selection should be carried-out based on I

- Breaking capacity of fuses The same applies to fuses as to circuit breakers

with respect to the I

: at the given rated

operational voltage, the rated breaking capacity must be at least as large as the prospective short-circuit current at the site of installation.

2.3.5 Thermal protection

Compliance with the permissible operational temperatures of electrical equipment is both a vital safety factor as well as critical regarding its effective life span. The rate of ageing of plastics increases exponentially with their operational temperature.

For all electrical equipment, limiting values for the load currents are defined, compliance with which should be ensured by suitable protective devices and

measures (fuses, overload relays,

temperature sensors).

2.3.5.1 Ambient temperature

Electrical equipment is designed for operation in defined temperature ranges. The upper temperature limit is of special importance, because practically all electrical equipment dissipates power and hence produces heat. The selection of the devices must consider the device’s ambient temperature and the permitted load at this temperature.

The normal ambient temperature range under IEC 60947, IEC 60439 and IEC 60204 is –5 °C to +40 °C with a 24-hour average that does not exceed +35 °C. It should be noted that values of the current load capacity unless otherwise stated are related to an ambient temperature of +40 °C. With other (higher) temperatures the loads should be reduced in accordance with manufacturer specifications or larger devices should be chosen. For industrial switchgear, loading specifications are often provided for an ambient temperature of +55 or +60 °C.

The lower limit of the operational temperature may be critical with electronic devices and it should be assured by provision of h

eating that temperature does not fall below. In conjunction with moisture (freezing), low temperatures can also adversely affect the operability of electromechanical devices.

Rated operational values designated as “open” apply for devices used in free air, while values designated “enclosed” a

pply for devices installed in an enclosure of small size specified by the manufacturer. The reference ambient temperature for “open” is the temperature of the ambient air of the device, even if this is installed in a box or cabinet. The reference ambient temperature for “enclosed” is the air-temperature of the housing environment. The ambient temperature of the device in the housing is higher because of the effect of its own heat dissipation. In practice this means for example that for a contactor “open” at 60 °C I

= 20 A will be stated and “en-

closed” at 40 °C the same value, because due to heating in the housing the contactor is subjected to the same immediate ambient temperature of 60 °C. At 40 °C “open” the same contactor can for example conduct 25 A.

In switchgear, in which the temperature in the cabinet (see Software TRCS) is calculated or measured, the data for “open”, that is in t

he immediate device environment (microclimate) should be taken into account when selecting the devices. It should be ensured by temperature monitoring and cooling measures that the actual temperature does not exceed the reference value on which the component selection is based.

the rated

2.3.5.2 Operational overcurrents, heavy-duty starting

Operational overcurrents occur especially when motors are started. Switchgear such as contactors or load switches should be rated so that it can cope with the regularly occurring overcurrents without difficulty, assuming it has been selected in accordance with the corresponding utilization category. Motor starts that cause normal motor protection relays of trip

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class 10 (tripping between 4 and 10 s at 7.2 · I

A A

) to trip are considered as heavy-duty starts. In

these cases overload protective relays with slower trip characteristics should be selected. See also

1.7.1.2.1.

In addition the load capacity of the switchgear should be checked. The load capacity of contactors and circuit breakers without thermal release basically depends

on their size (cross-se

ction/mass of the conducting parts). It therefore varies from device to device. Up to a starting time of around 10 s for direct starting, the load of the devices during starting needs not to be checked. Furthermore the admissible load capacity can be obtained from the technical documentation (catalog, RALVET; see example should be al

lowed between successive heavy-duty starts that provide sufficient time for the

Fig. 2.3-6). Rest times

switchgear (see RALVET) and the motor to cool down before the next loading.

Heavy starting and regular short-time duty

Contactor size

100

30/37

09/12/16

60/72/85

210A250A300A420

180A140A110

e(AC-3)

Max. starting time / load period [s]

10 100 1000 10000

Starting current / short-time current [A]

Fig. 2.3-6 Example of a loading diagram for contactors for heavy-duty starting of squirrel-ca ge induction motors

2.3.6 Life span

The life span of switchgear basically depends on the size of the load and the number of switching cycles. Instead of a time span, with electromechanical switchgear reference is usually made to the number of operations, as the ageing mainly depends on the stress during switching and less on the on- and off-phases between. The maximum number of operations is usually determined by the wear of heavily loaded components – in contactors, load switches and circuit breakers especially of the contact system.

For switchgear, the mechanical and electrical life spans are separately defined. The mechanical life span sta life span states the number of operations for a certain size of electrical loading and a certain utilization category.

In electronic devices, the life span is usually less dependent on the number of operations but rather on the working te power adapters) age more quickly at higher temperatures. This is why it is recommended to install electronic devices in the cooler parts of switch cabinets.

Ageing is also a problem with fuses, especially in the context of switching of motors. Full-range fuses (gL, gG example to repeated short-term melting. When using them with motors with the latters’ high starting currents, it should therefore be ensured that the starting current does not raise the

tes the number of possible operations without electrical loading, while the electrical

mperature. Thus for example electrolytic capacitors (for example used in

) have a soldered joint for tripping in the overcurrent range that may age due for

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temperature of this solder joint beyond a certain limit. Fuse manufacturers provide information on the smallest fuses that can be selected in relation to given motor currents and starting times.

2.3.6.1 Prospective service life

The prospective service life of switchgear is the number of years, months or weeks that it should complete under the foreseen service conditions in 1-, 2- or 3-shift operation without the replacement of spare parts. It depends on the frequency of operation and the total number of individual switching operations. For the latter in addition to the mechanical also the electrical life span of the devices must be selected accordingly (see Section 2.3.6.3). The required parameters can be determined by means of

∗∗∗=

tot

Total number of operations (life span)

tot

Switching operations per hour Operating hours per day Operating days per year Number of years (life span)

ndh

YYD

tot

∗∗

dhn

YDY

the below formulae:

2.3.6.2 Mechanical life span

The mechanical life span of switchgear is the total number of possible switch operations without electrical switch loading. It depends on the design, the masses moved, the forces and accelerations occurring. Large load switches and circuit breakers operate with high contact forces and large masses, and therefore have a comparatively short mechanical life span. On the other hand, contactors operate with relatively small contact forces and thus achieve longer mechanical life spans.

After the mechanical life span has expired, the devices must be replaced. This life span is only rarely achiev

ed during the foreseen service life. In a few cases, where the complete mechanical life span has to be used, it should be ensured that it is not reduced by adverse ambient conditions, installation position and – in the case contactors – by an excessive control voltage.

2.3.6.3 Electrical life span

The electrical life span for switchgear is the number of possible switching operations under operational conditions. After this number has been reached, the parts subject to wear must be – wherever possible – replaced. With small devices, the entire device must be replaced.

Depending on the application, the loading and the resulting erosion of the contacts varies widely. This is influ dominant role:

- Breaking current

- Making curr

- Voltage

- Power factor cos φ with alternating current

- Time constant

- Frequency of operation

- Malfunctions in the plant and on other devices (contact chatter)

- Ambient conditions (climate, temperature, vibrations)

Usually the electrical life span determined under test conditions is presented in diagrams as a function of the rated op in contactor selection. In practical operation, the loads are usually lower, as the running motor usually carries a current that is below the rated operational current. In the case of longer inching

enced by the following conditions, whereby the first mentioned play the

ent

τ with direct current

erational current. These values may generally be used without hesitation

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operation, the starting current has already dropped somewhat by the time the motor is switchedoff. This usually compensates for the effect of any disregarded adverse conditions.

For the most common applications of contactors the electrical life span is presented in the product documentation with various diagrams:

- AC-1 Non-inductive or slightly inductive loa

ds, for example resistance furnaces (small making current and cos φ higher than with AC-3, however full recurring voltage on switching off)

- AC-3 Squirrel-cage induction motors: Starting, switching off motors during running

- AC-2 Slip-ring motors: Startin

- AC-4 Squirrel-cage motors: Starting, plugg

(high making current, breaking of the motor rated current)

g, switching

off

ing, inching (high making and breaking current at

full voltage)

- Mixed service of slip-ring motors, e.g. AC-3

90 %

AC-4 10 %

With the curves Fig. 2.3-7 for AC-3 and Fig. 2.3-8 for AC-4 the expected electrical life span for specific applications

can be determined. These curves also can be used to determine the electrical life span for any application (for example jogging motors with very high or especially low starting current and any mixed service).

Contactor size I

e(AC-3)

[A]

Fig. 2.3-7 Example of a diagram for determining the electrical life span of contactors as a function of the rated operational current I The diagram applies up to 460 V, 50/60 Hz.

for utilization category AC-3.

Example

Background: Squirrel-cage induction motor 7.5 kW, 400 V, 15.5 A, AC-3 (switching off only when running),

operating cycle 2 minutes

ON / 2 minutes OFF, 3-shift operation, expected service life 8 years. Objective: Selection of the contactor Solution:

2 min ON +

2 min OFF

= 15 switching operations/h. This results for 3-shift operation over 8

years in around 1 million switching operations. From diagram Fig. 2.3-7 yields for a rated operational current of 15.5 A and 1 million required

switching operations the

contactor C16 (electrical life span approx. 1.3 million switching

operations).

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Contactor size I

Fig. 2.3-8 Example of a diagram for determining the electrical life span of contactors as a function of the rated operational current I The diagram applies up to U

for utilization category AC-4.

=690 V, 50/60 Hz.

e(AC-3)

[A]

Example

Background: Squirrel-cage induction motor 15 kW, 400 V, 29 A, plugging, switching off rotor at standstill at

= 6·Ie, expected life span = 0.2 million switching operations.

Objective: Rating of starting and braking contactors. Solution: The starting contactor (circuit making only) is selected according to the maximum permitted

rated power at AC-3 (see Fig. 2.3-7): C30. The brake contactor is selected according to the maximum permitted rated operational power at

AC-4 and 0.2 million swi

tching opera

tions according to Diagram Fig. 2.3-8: C72.

For mixed service, i.e. service of the contactor with AC-3 and AC-4 switching operations, the life span results from the sum of the loadings. In the

catalogs, diagrams for certain %-rates of AC-4 operations, for example 10 %, are provided. The RALVET electronic documentation is available for determining the life span for other percentage rates, or direct inquires must be made.

If in practice the electrical life span was considerably shorter than desired, there are several possible caus

es and explanations:

- More switching operations than expected, e.g. operated by extremely sensitive controller.

- More frequent inching than expected, e.g. unskilled operation.

- Permitted frequency of operation exceeded, e.g. chattering contacts

- Short-circuits, e.g. switching pause too short for reversing or star-delta starters.

- Synchronization with the supply voltage. Semiconductors as

controllers

could for example always switch off at the same phase angle and act in the same direction of current-flow (results in one-sided material migration to the contacts like in direct current control).

Assessment of the contacts

In conjunction with the electrical life span, the question often arises of assessment of contacts after a certain service pe

riod for continued serviceability. At least with large contactors, the

contacts can be inspected. Already after the first few switching operations, there are clear signs of burn-off on the contact

surface. After a relatively small number of switching operations, the entire contact surface becomes roughened an

d blackened. Black deposits and traces of arc extinction can be seen on

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the surrounding components. Serrated edges and loss of contact material toward the arcing

chamber are also normal. The end of the contact life span is really reached when larger areas of the contact plating have

broken off or

there is a danger of the contact touching the substrate material. The below figures

are intended as an aid for an assessment of contacts.

Sectional

views

Fig. 2.3-9 Contacts of a power contactor at various stages of the life span with AC-3 loading Fig.s above: Contacts in new state Fig.s in center: Contacts after approx. 75 % of the electrical life span; Contact material partially eroded; contacts still operable Fig.s below: Contacts at the end of their life span; Substrate material visible, contact material eroded down to the substrate; further use would lead to contact welding and excessive tempera ture rise. The pictures on the right show the contact state in long section. The images of the va rious life span phases originate from various contacts, as the contacts can no longer be used once the section has been cut.

2.3.7 Intermittent and short-time duty, permissible frequency of operation

With continuous duty, i.e. with constant loading over hours, days and longer, the switchgear reaches its thermal equilibrium. The individual components reach their steady-state tempera-

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tures. The rated loading values refer from a thermal view point to continuous duty at a certain ambient temperature (see Section 2.3.5). IEC 60034-1 defines the continuous duty of motors at the rated operational cur

rent until the steady-state temperature is reached as the rated service

type S1. In practice in addition to the continuous duty there is a large number of loading situations with

changing loads. In inter

mittent operation, load-phases and de-energized breaks alternate in regular sequences. The load periods and intervals are so short that the components of the switchgear (and of the load) do not reach their thermal equilibrium neither during the warming nor the cooling phases. For motors the three rated service types S3, S4 and S5 are defined for intermittent operation in IEC 60034-1 (S3…constant load; S4…with additional starting load; S5…with additional starting and braking load).

Fig. 2.3-10 Current loading and temperature rise during intermittent duty

Current loading during intermittent duty S4

Heating and cooling during intermittent duty S4

For meaning of other symbols see Fig. 2.3-11

In short-time duty the current flows for a limited time so that steady-state temperature is not reached. The de-energized interval after the load-period is however so long that the devices can nearly cool down to the ambient temperature. In IEC 60034-1 the short-time duty for motors is named rated service type S2.

Fig. 2.3-11 Current loading and temperature rise in short-time duty S2

Load duration

De-energized interval

Duration of a switching cycle

Thermal continuous current

Current loading with short-time duty

Maximum permissible temperature rise

Δδ

max

Temperature rise with thermal continuous current

Δδ

Heating and cooling with short-time duty

Δδ

Comment: In short-time duty a higher temperature than in continuous duty is permitted!

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With intermittent or short-time duty, the loading current can be higher than in continuous duty, without resulting in the permitted temperature being exceeded. Therefore, for example, for switching ohmic loads and rotor contactors for slip-ring motors smaller contactors can be selected than would be required according to the rated current of the load.

When switching squirrel-cage induction motors, transformers, capacitors and incandescent lamps, the required contact rating is

however the main selection criterion. The size of contactors for these applications is therefore determined by the rated operational current and the respective utilization category for all service types.

2.3.7.1 Intermittent duty and relative ON-time

In order to define a specific intermittent duty, in addition to the value of the current either the load and cycle time or the frequency of operation per hour together with the relative ON-time are preferably stated.

Fig. 2.3-12 Intermittent duty

⋅=

Average current loading (r.m.s. value during a switching cycle and hence also during

the whole service time) [A]

Current during period under load [A]

Load duration [s]

Switching cycle = load duration + de-energized interval [s]

ED relative ON-time = t

B/tS

[%]

The relative ON-time – usually expressed as a percentage – is the ratio of the load duration to the cycle-time, whereby the cycle-time is the sum of the load duration and the de-energized interval.

The average current loading I

must always be somewhat lower than the thermal continuous

current, so that the temperature rise peaks at the end of each period under load do not exceed the permitted values. With stator contactors of slip-ring motors, especially with short switching cycles, the higher current during the starting time (see heating effect of the ele

ctric arcs must be taken into account.

Fig. 2.3-10) as well as the additional

At high frequency of operation the heating effect of the starting current and the switching arc is greater than the cooling

effect of the de-energized intervals so that contactors with higher ratings must be chosen than would normally be required according to the rated operational current. The selection is made based on the graphs for the permissible frequency of operation.

Even without electrical loading, the frequency of operation of contactors is limited by the maximum p

ermissible temperature of the coil or the electronic coil control circuit, if any. The inrush currents of the coil (Fig. 2.3-13) make a considerable contribution at higher frequencies of operation to the overall heating of the

coil and of the contactor. This applies both for alternating current and for direct current magnets with series resistance or contactors with double-windingcoils (economy circuit) and also to electronic coil control circuits.

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Fig. 2.3-13 Coil current at closing a contactor a.c. magnet

Rated current of coil

Inrush current of coil (depending on contactor 6... 15 · IS)

T1 ON-command (coil circuit closed) T2 Magnet closed

The permissible frequency of operation of conventional coils can be exceeded short-time without risk, as the time constants for the heating of coils is 5 to 20 minutes depending on contactor size.

True direct current magnets do not exhibit in-rush currents. Therefore with these the ON and OFF delay times, which in this case

are notably longer, determine the maximum frequency of

operation. With electronically controlled coils, the permissible frequency of operation is determined by the

thermal load capacity of the electronic components and may

not be exceeded.

With electrical loading, the temperature rise of the contacts must also be taken into account for determining the permissible frequency of operation. Although, heat is dissipated during

the deenergized intervals, the contacts are additionally heated to a considerable degree by arcing and by the starting currents when switching motors. The permissible frequency of operation is therefore dependent on the relative ON-time, the size and duration of the motor starting current and on the breaking current. Corresponding diagrams are provided in the catalogs for typical applications (

Fig. 2.3-14).

Contactor size I

9/12/16 A 23/30/37/43 A 60/72/85

e(AC-3

[A]

Fig. 2.3-14 Example of a frequency of operation characteristic for contactors. T he frequency of operation for small loads is limited by the temperature rise of the coil.

At higher frequencies of operation, the contacts are predominantly loaded by the starting currents. This also applies to motor windings.

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When switching motors – assuming that the motor is correctly rated for the stated frequency of operation – it should be checked whether the overload protection device is suitable for the high frequency of operation and that it does not release early or late. See also Section

4.1.2.

Note

equ

Inadvertently exceeding of the permissible fr

ency of operation is the most frequent cause of prematurely eroded contactor contacts. If the contactors are made to “chatter” by rapidly recurring interruptions of the coil current and at the same time switch high currents – e.g. starting currents of motors – this results in heavy wear that can lead to welding or destruction of the contacts.

It is often difficult to identify faults in switching cir

cuits as they can have a variety of causes, for

example:

- Loose terminals

- Gradually opening cont

acts of thermostats, pressure sensors, limit switches etc.

- Strongly bouncing control contact

- Slowly activated control switch

- Incorrectly programmed PLC

- Drop of control voltage

2.3.8 Rated frequency and harmonics

See also Section 2.4.3. The normal supply frequencies for all catalog data are 50 and 60 Hz. Also for direct current

applications whether higher (for example 400 Hz in military and aviation applications) or lower (for example 16 2/3 Hz on railroads) the loading of switching and protective devices changes. An examination of the suitability for the respective application and the determination of the performance data for the specified loadings are essential prerequisites for correct device selection.

Also in applications in which harmonic contents of the current occur – for example in variable speed drives – the performance data of switching

corresponding values are provided in the technical data. With other frequencies,

and protective devices may be affected.

2.3.9 Safety clearances

For devices that generate electric arcs, especially for circuit breakers, safety clearances to adjacent devices, conductors or conductive surfaces may be required, as the arc gases (plasma) can be ejected with very high temperature and speed. The safety clearances specified in the manufacturer documentations must be observed to avoid risks to persons and equipment. If the required safety clearances are not observed a secondary short-circuit may be created on the input side of a circuit breaker – for example when conductive gas is emitted during switching off a short-circuit. Such short-circuit would be switched off by the next short-circuit protective device on the supply side, whose rated current usually would be significantly higher. The destructive energy of the electric arc and the danger to persons and material are correspondingly high.

The safety clearances are usually stated in the dimensional dr example Fig. 2.3-15).

awings for the devices (see

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Fig. 2.3-15 Example of a dimensional drawing stating the required safety clearances to condu ctive materials; the safety clearances do not apply to connected, insulated conductors

Clearances between devices may also be necessary from a thermal viewpoint, in order to ensure adequate heatflow and compliance with the operationally permissible temperatures. These specifications are also available in the catalogs or on request. See also Section 6.1.

2.3.10 Mounting position

Certain electrical devices such as contactors and circuit breakers may be subject to restrictions with respect to the permissible mounting position (Fig. 2.3-16) or their operational parameters may change with the mounting posit mounting type is permissible at all, typically exhibit longer dropout times and are sensitive to impacts in vertical direction. Suspended contactors (overhead position) may require higher pullin voltages, which pushes the lower limit of the tolerance range of the control voltage upward.

ion. Thus standing contactors (table-mounted), if this

Fig. 2.3-16 Permissible mounting position of a contactor (exampl e). The gray zone on the side indicates the required clearance from grounded components.

Specifications on the permissible mounting positions can be found in the dimensional drawings of the devices. With respect to the influence on the operational parameters of installation in positions that differ from the standard position (suspended, upright), it is recommended to inquire directly to the manufacturer.

2.3.11 Protective separation

For protection against electric shock in conjunction with the use of electronic control devices, protection by SELV (Safety Extra Low Voltage) and PELV (Protective Extra Low Voltage) is increasingly being chosen. The maximum voltage levels for SELV and PELV are 50 V a.c. and 120 V d.c. (with exception of special applications with lower limit values).

In circuits with SELV and PELV, all devices that are included in the circuit must be isolated from other power circuits double creepage distances and the next higher impulse voltage withstand level U applies for example for auxiliary circuits of switchgear with respect to their power circuits (Fig. 2.3-17).

by insulation corresponding to that of a safety transformer. That means

. This

imp

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Fig. 2.3-17 Protective separation between power and control circuits

This is usually achieved by a reduction in the rated operational voltage. This means that for example a contactor suitable for 690 V can be used at 400 V in SELV and PELV circuits. The approval of SELV and PELV circuits requires design features that guarantee that protective separation is maintained even in the event of faults (for example broken parts). When selecting switchgear for SELV and PELV circuits attention must be expressly paid to the declaration of protective separation at the respective operational voltage.

2.3.12 Site altitude

The site altitude and hence the air density play a role with respect to the cooling conditions, the dielectric withstand voltage and electric arc extinction. A site altitude of up to 2000 m is considered as normal in accordance with IEC 60947. For higher altitudes, some performance data of the devices must be reduced. With power electronics devices in many cases load reductions already apply from 1000 m. In the product catalogs specifications should be found about the site altitudes on which the performance data is based.

For contactors, bimetallic thermal overload relays and circuit breakers with bimetallic tripping mechanisms, approximate values for the reduction of ratings at altitude provided in

Tab. 2.3-6.

Altitude above sea level [m]

[ft]

Contactors

Reduction factor for I Reduction factor

for I

AC-2, IAC-3, IAC-4

AC-1

n·I up to 415 V n·I up to 500 V n·I up to 690 V n·I

Bimetal overload relays 1)

Adjustment factor on the

n·I

rated operational current of the motor 2)

1) Also applies for circuit breakers with bimetal tripping mechanisms. The trip characteristics of electronic protective relays usually do not change with the site altitude

2) Reduction of the rated operational currents of motors in relation to the site altitude in accordance with specifications of the motor manufacturer to be considered additionally

2000 6600

e e e e

1.0 0.95 0.9 0.85

1.0 0.93 0.85 0.78

1.0 0.87 0.77 0.65

1.0 1.06 1.11 1.18

3000

10000

s above 2000 m are

4000

13000

5000

16500

Tab. 2.3-6 Correction factors for applications at altitudes over 2000 m

2.3.13 Shock and vibration

Low-voltage switchgear is designed and tested for loading by shock and vibration for normal industrial usage. This includes the usual stress in operation, for example, as a consequence of vibrations during switching of contactors.

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In applications with increased stress by shock and vibration such as for example in vehicles, in rail transport or on ships a variety of measures is required to protect the devices from the immediate influence of externally generated shock and vibrations. In the simplest case, by optimization of the mounting position. In case of doubt, the manufacturer should be consulted.

2.4 Specific application conditions and switching tasks

2.4.1 Parallel and series connection of poles

2.4.1.1 Parallelling

Parallel connection of poles in switchgear increases its thermal load capacity. It should be remembered that the resistances of the individual poles vary due to contact burn-off, deposits etc. The current does not distribute itself equally among the parallel poles, but corresponding to their particular impedances.

A reduction factor for the total load must be applied to avoid overloading of the individual contacts. In

- with 2 parallel poles I

- with 3 parallel poles I The making and breaking capacity remain in parallel circuits the same as for single contacts , as

frequently one contact is opening or closin switching work. Therefore it is not possible to increase the contact rating of contactors for switching motors and capacitive loads by parallel connection of contacts. Tab. 2.4-1 shows the switching ca

Three pole

Making capacity 12·I

practice the following values for the permissible total current can be calculated with:

= 1.8 x Ie

= 2.5 x Ie

g first and therefore must take the largest part of the

pacity based on the total current with 2 and 3 contacts connected in parallel.

switching

Æ I

2 poles in

parallel

Æ I

= 1.8 · I

(12·Ie2)/1.8 = 6.7·I

3 poles in parallel

Æ I

= 2.5 · I

(12·Ie3)/2.5 = 4.8·I

Breaking capacity 10 · I

Voltage across each contact U = Ue/√3

Tab. 2.4-1 Making and breaking capacity of contactors as a multiple of the rated operational current I switching and for two and three parallel poles

(10·Ie2)/1.8 = 5.6·I

(10·Ie3)/2.5 = 4·I

for three pole

For this reason contactor poles should only be connected in parallel for switching resistive loads (utilization category AC-1). Where possible they should only be connected in parallel by means of copper bars fed in the center in order to ensure symmetrical current distribution and good heat dissipation. For small contactors special connecting bridges are available.

Any short-circuit currents that occur are distributed between the poles depending on the given pole resistances. In the

case of circuit breakers with parallel contacts, it may happen at small short-circuit currents that the operating current of undelayed electromagnetic short-circuit releases is not reached. Consequently such a short-circuit is only switched off by the thermal release after a delay. The pick-up threshold for undelayed short-circuit breaking rises approximately by a factor given by the number of parallel poles.

2.4.1.2 Series connection

When two or three poles of switchgear are connected in series (Fig. 2.4-1), the advantages include the following:

- Increased dielectric withstand voltage

- Improved s

- Higher operating voltage

- Larger contact life span

witching capacity

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These advantages are exploited by using three-pole contactors and circuit breakers for switching single-phase alternating current and above all direct current. The limit for higher operating voltage is determined by the rated insulation voltage that may in no event be exceeded. The permissible current loading of poles connected in series is the same as for individual poles.

1-pole 3-pole

2-pole

Fig. 2.4-1 Examples of diagrams for poles connected in series. Where grou nded power supplies are used (top graph) with loads switched on both sides, it should be noted that ground faults can lead to bridging of contacts and hence to a reduction in the breakable voltage.

The overload trip characteristics of devices with thermally delayed bimetal tripping mechanisms, such as circuit breakers and overload relays, apply when all three bimetal strips are equally loaded. This is guaranteed by connecting the circuits in series. In devices that are sensitive to phase failure, series connection of all circuits is compulsory. With electronic motor protection relays, it may be necessary to deactivate the phase failure protection.

The impact of series connection of circuits when switching direct currents is dealt with in Section 2.4.2.

For the effect of series connection of circuits on switching frequencies < 50 Hz and > 60 Hz see Section 2.4.3.

2.4.2 AC switchgear in DC applications

Switchgear designed for alternating current can carry at least the same rated continuous operational DC current. With direct current the skin effect in the circuits disappears and none of the specific effects associated with alternating currents such as hysteresis or eddy current losses occur.

DC devices that are operated at low voltage can be switched by AC switchgear without difficulty, as their direct current switching cap current.

With voltages in excess of around 60 V, the direct current switching capacity of AC switchgear with double-breaking co or three circuits in series (

ntacts (for example contactors) decreases strongly. By connecting two

Fig. 2.4-1) this limit can be raised to twice or three times the voltage.

The reason for the reduced switching capacity with DC compared with AC is the absence of the current zero crossover t

hat with AC supports the quenching of the electric arc. The electric arc in the contact system can continue to burn under larger direct voltages and thus destroy the switchgear. With direct voltages, the contact erosion and hence also the contact life span differ from those at alternating voltage. The attainable values for direct current are specifically tested and documented.

With direct current, the load affects the switching capacity more strongly than with alternating current. The energy stored in the ind an electric arc. Hence with a strongly inductive load (large time constant L/R) the permissible switching capacity for the same electrical life span is smaller than with an ohmic load due to the much longer breaking times.

acity at low voltages is practically the same as for alternating

uctance of the load must largely be dissipated in the form of

LVSAM-WP001A-EN-P - April 2009

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Overload release units

The reaction of bimetal strips heated by the operating current depends on the heat generated in the bimetal strips and in

their heating coil, if any. This applies equally for alternating current and direct current. The trip characteristic can be somewhat slower with direct current as there are no hysteresis and eddy current losses. With overload releases that are sensitive to phase failure, all three circuits should always be connected in series to prevent premature tripping.

Overload releases heated via current transformers are not suitable for direct current. Also electronic ov

erload relays in most cases cannot be used in direct current applications as the current is measured via current transformers and their functionality is tailored to alternating current.

Short-circuit releases

Electromagnetic overcurrent releases can be used with direct current. However the tripping threshold cu

rrent is som

ewhat higher than with alternating current.

Undervoltage and shunt-trip releases

Undervoltage and shunt-trip releases operate with magnet circuits. Special designs are required for direct voltage.

2.4.3 Applications at supply frequencies < 50 Hz and > 60 Hz.

Effect of harmonics

Low-voltage switchgear is designed for a supply frequency of 50 ... 60 Hz. If it is desired to use them for other rated frequencies the following device characteristics should be checked:

- thermal load capacity of the circuits,

- switching capacity,

- life span of the contact system,

- release characteristics,

- operating characteristics of magnetic and motor drives. The effect of higher frequencies on the performance of low-voltage devices should be consid-

ered both in

networks wit where current-harmonics occur. Such current-harmonics occur if the supply voltage contains harmonics or if non-linear consumers are connected. Such consumers may for example be compensation devices for luminescent lamps that operate in the range of saturation or devices with phase angle control. With consumers with phase angle control and with frequency converters (Inverters; see Section supply. The harmonic content can be current consumption increases with increasing frequency. Special attention should be paid to this factor in individually compensated motors and a correction of the current settings of the protective relay may be required.

In applications in which current-harmonics arise, the effect of the harmonics (for ex additional heating effects) is added to that of the basic frequency. This can be especially critical in devices that contain coils or ferromagnetic materials (bimetal heating coils, magnetic releases etc.).

In the case of loads with connection to the neutral conductor (e.g. single-phase loads such as luminescent lamps, small power ada the formation of a zero-sequence-system that may lead to thermal overloading. This should also be taken into account in the use of 4-pole switchgear.

h higher basic frequencies (for example 400 Hz) and also in cases

3.10) harmonics with frequencies up to several kHz may arise in the

increased by capacitors connected to the supply, whose

ample

pters etc.), a high harmonic content can result because of

2.4.3.1 Effect of the supply frequency on the thermal load

In contrast to direct current, with AC the current does not flow evenly through the cross-section of a conductor. The current density falls from the surface inward. This effect – known as the skin effect – increases with increasing frequency so that at very high frequencies the core of the

LVSAM-WP001A-EN-P - April 2009

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conductor is virtually de-energized and the current only flows in a relatively thin layer at the conductor surface.

This means that with increasing frequency, the resistance of the circuit increases. In addition, due to magnetic inductio

n, higher hysteresis and eddy current losses are created in adjacent metal parts. Especially ferromagnetic materials (arc extinguishing parts, screws, cage terminals, magnets, base plates) can reach unacceptably high temperatures. Special care should be taken at frequencies > 400 Hz.

Because of the variable cross-sections of the conductive parts as well as the different nature and distances to adjacent metal parts, additiona

l heating effects and especially the local overtemperatures vary according to device type. This has the following consequences for the load capacity of the switchgear and the switchgear combinations.

Individual clarification is required for each individual application as a general statement cannot be provided due to the widely differing design fea

tures, especially at frequencies > 400 Hz.

Load capacity of contactors, load switches and circuit breakers

Devices that are designed for a frequency of 50/60 Hz can, from a thermal viewpoint, be used for the same rated current at a lower frequency. Approximate values for permissible operational currents are stated in Fig. 2.4-2. The actual reduction factors vary according to design and the rated current range of th

e devices. It should be noted that cage-type terminals have an adverse

effect on heating at higher frequencies.

1.00

0.90

0.80

0.70

eAC-1(50/60Hz)

0.60

eAC-1(f)

0.50

0.40

0.30

0.20

0.10

0.00

medium size

devices

(≈100 A )

big devices

big devices wi t h

cage-type

terminals

10 100 1000 10000

small dev i ces

f [Hz]

Fig. 2.4-2 Approximate values for permissible operational currents AC-1 of contactors, load switches and circuit breakers at higher frequencies relative to I

at 50…60 Hz

Higher frequencies and installation provisions

For installation at higher frequencies, special attention must be paid to the effects of current distribution (skin effe

ct), hysteresis and eddy current losses: The conductors to be connected should be rated according to the higher frequency (larger cross-section, flat or tube conductors). The load capacity of circuits at higher frequencies can be roughly estimated by help of Fig. 2.4-3. It depends on the geometry of the rails and their arrangement and should be measured separately in each ind

ividual case. The conductors should be positioned as far as possible from conductive (especially ferromagnetic) parts, to minimize inductive effects.

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1.00

0.90

0.80

e(50Hz)

0.70

e(f)

0.60

0.50

0.40

0.30

0.20

0.10

0.00 10 100 1000 10000

f [Hz]

Fig. 2.4-3 Approximate load capacity of busbars at higher frequencies [12]

Load capacity at 50 Hz

e50

Load capacity at frequency f

In order to reduce losses, no cage-type terminals should be used. This is especially important with currents > 100 A! For single phase loads over 400 Hz, the two outer poles of contactors should be used in parallel for the feed line and the middle pole for the return line. This results in a partial mutual compensation of the magnetic induction.

2.4.3.2 Effect of the supply frequency on the switching capacity

The starting currents of motors for higher frequencies are sometimes higher than those at 50/60 Hz. At 200 Hz this can result in 15 times the rated current and at 400 Hz up to 20 times. The power factor may be significantly worse than for motors at 50/60 Hz. Allowance should be made for the increased making currents when selecting devices.

Switching capacity of contactors and load switches

When breaking a.c. circuits, the clearance between open contacts must be sufficiently deionized during the curre half cycle. At higher frequencies, the increase of the voltage after the zero crossover is usually faster. The electric arc duration per half cycle and hence the ionization of the distance between open contacts is however smaller. Therefore contactors and load switches (zero-point interrupters) up to around 400 Hz have virtually the same switching capacity as at 50/60 Hz.

Insofar as the making currents are permissible (see above) – a reduction of the rated current I according to Fig. 2.4-2 for contactors and load switches is only required because of the larger thermal loading at higher frequencies.

The circumstances at lo the switching chamber due to the lon current limiting switchgear. The switching capacity falls at lower frequencies and becomes more heavily dependent on the voltage and the inductance of the load.

The full rated operational current for three-pole operation at 400V and 50/60 Hz can be permitted at 16 2/3 Hz and 400 and 16 2/3 Hz all poles should be connected in series so that the full rated operational current can be operated.

At frequencies below 16 2/3 Hz, the direct current switching capacity of switchgear in accordance with catalog specifications

Switching capacity of circuit breakers

The short-circuit currents in medium frequency supplies are comparatively low. Any reduction of the switchin

g capacit

cause problems in practice.

nt zero crossover to prevent a re-ignition of the electric arc in the next

wer frequencies are less favorable. The effect of the strong ionization of

ger presence of the electric arc is predominant with non

V with two poles in series. For rated operational voltages up to 500 V

must be applied.

ies at frequencies over 400 Hz compared to 50/60 Hz does therefore not

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The effect of the current limitation is reduced with increasing frequency, as at higher frequencies the peak value of the short-circuit current is already reached during the reaction time of the switch. In view of the comparatively low short-circuit currents in medium frequency supplies, this is not relevant in practice. The short breaking times of current limiting circuit breakers are retained.

With two poles connected in series, circuit breakers with current limitation in single-phase supplies typically reach

up to 400 V the rated breaking capacity at 50/60 Hz. At voltages over 400 V to 690 V a.c. on the other hand, series connection of three poles is required. In singlephase supplies it must always be ensured that all three poles of thermally delayed overload releases are in the current loop.

2.4.3.3 Performance of release units at supply frequencies

< 50 Hz and > 60 Hz

Thermal overload releases

Thermally (current-dependent) delayed overload releases and relays operate with bimetal strips. These are us secondary current of a current transformer.

Up to around 400 Hz, the heat losses in the heating coils (ohmic losses) are the main heat source. The additional these frequencies, so that the tripping characteristic will only be slightly faster than at 50 Hz. At frequencies over 400 Hz the proportion of inductive heating increases and the ultimate tripping current falls with increasing frequency.

Overload relays that are connected to main current transformers with a high overcurrent factor (protective current transformer) or that have integrated curre what faster tripping characteristic in comparison to 50 Hz at frequencies over 50 Hz to 400 Hz.

The trip characteristic of relays with a saturation current transformer for heavy-duty starting becomes considerably f move proportionally to the frequency towards higher currents.

Electronic overload devices

Due to the variety of principles of operation, no general statement can be made on the performance of elec current transformers it should be noted that application at low frequencies is limited because of transformer saturation.

Short-circuit releases

For the activation of electromagnetic overcurrent releases, in addition to the size of the current also the tim electromagnetic overcurrent releases are activated within around 5 ms. During the half-cycle, the force is sufficient to pull the armature all the way through to its end position. At higher frequencies the duration of a half cycle is too short.

The pick-up-threshold of the short-circuit-releases increases over 50/60 Hz and at around 400 Hz approaches 1.4 ti

Increased operating frequencies can lead to increased temperature-rise of the releases.

ually heated via a heating coil by the heat losses of the operating current or the

inductive heating in the bimetal strips itself is practically negligible up to

nt transformers, display a some-

aster with frequency increasing up to 400 Hz as the saturation effect will

tronic overload relays at frequencies over and below 50/60 Hz. With relays with

e is relevant

during which the current is applied. At 50/60 Hz the armatures of the

mes of the 50/60 Hz value.

2.4.3.4 Switchgear used with soft starters

Overload protection

Thermal relays and circuit breakers are equipped with thermal overcurrent releases that can be adjusted to the motor rated current a harmonic components of the currents that also contribute to motor heating are measured.

The performance of electronic motor protective devices with respect to the effect of the harmonics (for exam

ple true r.m.s. value measurements) should be obtained from the respective device

documents.

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nd even during soft starting map the motor heating. The

I >

M 3~

Fig. 2.4-4 The basic design of a power circuit with circuit breaker, contactor and soft starter

Installations that allow heavy-duty starting via a soft starter with a starting time of around 1 minute and longer require besides a specifically selected motor also specifically selected switching and protective devices.

It is a good idea to protect motors for heavy-duty starting that are activated by soft starters with electronic motor protective devices. The circuit br

eaker must be selected and adjusted so that it does not trip before the motor protective device and that it is thermally capable for the specific load (harmonics content and starting time). Circuit breakers without thermal releases can be employed to advantage. The selection and adjustment of the circuit breaker is as for heavy-duty starting conditions (see Section short-circui

t protection and/or line protection.

4.1.2.2). In this case, the circuit breaker only has to provide

Short-circuit protection

Short-circuits are critical and endanger the power semiconductors of the soft starters. Circuit breakers are not sufficiently fast to protect power semiconductors of soft starters against shortcircuits. For short-circuit protection, the specifications provided by the soft starter manufacturer should therefore be observed. Short-circuit protection for the power semiconductors of soft starters is often omitted for cost reasons, whereby such coordination only satisfies the requirements of coordination type 1.

2.4.3.5 Switchgear for use with frequency converters (inverters)

Overload protection on supply side

See also Section 3.10.4 Motors that are controlled by an inverter are not directly connected to the power supply (via

rectifier, intermediate cir devices upstr

eam of the inverter do not receive any direct information about the condition of the motor and thus cannot fulfill the motor protection function. Motor protection functions are usually integrated in the inverter.

LVSAM-WP001A-EN-P - April 2009

cuit and inverter, Fig. 2.4-5). Circuit breakers or motor protective

2-33

M 3~

Fig. 2.4-5 The basic design of a circuit with rectifier, intermediate circuit and converter of the inverter. Freq uently filters are provided on the input side (whether internally or externally) to reduce supply interference.

As the reactive current of the motor is provided by the intermediate circuit capacitance, the supply current is smaller than the motor current and its power factor cos φ is nearly 1. Because of the harmonic content of the current, the thermal overcurrent release of the circuit breaker should be set to approx. to 1.2 times the motor rated current, but no more than the permissible current carrying capacity of the connecting cable.

Short-circuit protection

Inverters usually protect themselves against short-circuits

on the output side. Short-circuits on the supply side between pole conductors or pole conductors and ground are switched off by the upstream short-circuit protective device (circuit breaker or fuse).

Circuit breakers, motor protection relays or contactors in the output circuit of inverters

If low-voltage switchgear is installed in the output circuits of inverters, this must be compatible with the high switching f

requency of the output signal of several thousand Hz. The harmonic content of the current can result in overheating of the devices. Especially in circuit breakers with plunger-operated magnetic releases, the sensitivity to current harmonics increases with decreasing rated current. As a guideline can serve: Due to the large number of windings on plunger coils, excessive heating should be expected with versions with < 10 A rated current. The manufacturers’ specifications should be observed.

With long screened lines between the output of the frequency converter and the motor, switch contacts located between the device

s can be stressed by high peak currents caused by the capacitance of the line that may even result in welding of the contacts. The high charging currents can under certain circumstances also result in undesired releasing. Filters can have similar effects. The switching mode of inverters generating steep voltage slopes can lead to additional voltage stress on long lines by traveling wave effects. Suitable filter measures may be required as a remedy.

Generally it should be ensured by the control circuit that loadside contactors are switching without load, i.e. that the

frequency converters are switching on after the contactor and switch-

ing off before it.

Overload protection on the output side

Overload protective devices with bimetal strips (bimetal relays and cir

cuit breakers with bimetal tripping mechanism) are designed for 50/60 Hz. As their mode of operation is based on the heating of the bimetal strips by the motor current, the release values relate to heating by 50/60 Hz currents. Depending on the design of the device, the switching frequencies of frequency converters extend from several kHz to the ultrasonic range and generate harmonic currents in the output that result in additional temperature rise in the bimetal strips. Long shielded lines to the motor can cause additional increases of the harmonic components because of the line capacitance.

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Fig. 2.4-6 The switching of the output voltage (above) results in a harmonic content of the output cu rrent of frequency converters (below) that affects the performance of prote ctive devices on the load side.

The temperature rise is not only dependent on the r.m.s. value of the currents, but also on the induction effects of the higher frequency currents in the metal parts of the devices. The additional heating effect results in a reduction of the ultimate tripping current of the overload relays, whose extent must be individually determined for the respective combination of frequency rectifier / overload relay / connecting lines / motor. As a consequence of these effects, the current settings should be increased for overload relays with bimetals, the choice of a higher current range may even be necessary. Depending on the device combination, the factor can be up to 150 %.

As the determination of the shift of the ultimate tripping current is very time consuming in individual cases and for

all possible device combinations, it is recommended in practice to set the motor protective devices by physical testing. To this end the drive with a rated load is run for around one to two hours and the overload relay initially adjusted so that there is no risk of it being released. At the end of this operating period, the current setting on the device is successively reduced until the relay trips. The final setting is around 10 % above the trip value. If interruption to operation by tripping during the test is undesired, the release contact can be temporarily bypassed. The current setting also provides the basis for checking the required size of the contactor or other switchgear in the circuit.

Also the performance of electronic overload relays may depend on and be affected by the harmonic content of inverter currents. It is not po

ssible to make a general statement due to the differences in the modes of operation. In electronic overload relays with current transformers it should be remembered that use at low frequencies is limited because of converter saturation.

2.4.4 Application of four-pole switchgear devices

The majority of low-voltage switchgear is equipped with three contacts in the main circuit, which switch three-phase loads in all poles. In some applications switchgear with four main poles is required, either for safety reasons or for an optimum solution of the application. This may require various device configurations.

2.4.4.1 Applications of switchgear with 4 NO contacts

Four NO contacts are required or at least very advantageous for the below applications  Applications which require the interruption of the neutral line for switching off or disconnect-

ing loads. T plies, for protective disconnection in IT or impedance-grounded networks. Attention has to be paid that the neutral pole closes before or at the same time as the other poles and opens after them or at the same time. When switching non-linear consumers, specific attention has to be paid regarding the current loading of the neutral line. See also Section

LVSAM-WP001A-EN-P - April 2009

his can be the case in supplies with adverse grounding conditions, in TT sup-

2.4.3.

2-35

 Switching-over of supply systems (for example for standby power supplies), for which

complete separation of the two supply systems is required.

 Switching of several single-phase loads (heaters, lamps) with one switchgear unit.  Switching direct current loads with higher rated voltage that requires the series connection of

four contacts (see also Section 2.4.2).

2.4.4.2 Applications of switchgear with 2 NO and 2 NC contacts

Devices with two NO and two NC contacts are useful in applications in which one of two circuits must always be closed. These are, for example

 Switching a heater between two levels (Fig. 2.4-7)  Switching-over between single-phase supplies – for example, emergency power supply

stems (Fig. 2.4-7)

 Reversing motors for space saving arrangement of devices (Fig. 2.4-8)  Reversing of 2-step motors with separated windings (Fig. 2.

Fig. 2.4-7 Four-pole contactors with 2 NO / 2 NC contacts for switching single-phase loads (left) or switching-over between two supplies (right)

4-9)

I >

M 3~

I / O

Fig. 2.4-8 Slimline reversing starter with a 2 NO / 2 NC contactor for reversing

LVSAM-WP001A-EN-P - April 2009

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Fig. 2.4-9 Reversing of a two step motor with separate windings

2.4.4.3 Applications of switchgear with 3 NO and 1 NC contact

Devices with three NO and one NC contact are used in applications in which, when the main load is switched off – for example the motor –, another single-phase load must be switched on. Such applications could include:

 Safety circuits  Direct current brake systems that are activated when a drive is switched off  Clutches that must be released when the drive is switched off

2.4.5 Application of circuit breakers in IT networks

IT supplies are used to prevent that a ground fault leads to immediate disconnection of the affected circuit like in a grounded system. Although a first ground fault results in a displacement of the potential of the entire supply, continued operation is still temporarily possible. Special ground fault monitoring equipment reports any ground faults and hence makes it possible to quickly rectify the fault – often without disruption to the operation of the plant. The situation is similar in supplies with high impedance grounding.

If however a second ground fault occurs in another phase, there is a short-circuit that must be immediately cleared by the short-circ depending on the locations of the short-circuits (Fig. 2.4-10). This results in different voltage levels to be switched off

by the short-circuit protective device and in the case of circuit breakers

to different required breaking capacities.

uit protective device. The voltage to be switched off varies

Fig. 2.4-10 Double ground faults on the load side of the circuit breakers do not cause increased stress. If, however, there is one ground fault on the supply side and the other on the load-side, a significantly higher breaking capacity is required because of the increased voltage load.

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If both short-circuits occur on the load-side of the circuit breaker, the breaking work is shared between two contacts and the required breaking capacity corresponds to the normal 3-phase values.

If the location of one short-circuit is on the supply-side of the circuit breaker and the second short-circui

t on the load-side, then one contact only of the circuit breaker has to perform the total breaking work and this at phase-to-phase voltage. In this case, the significantly lower single pole breaking capacity of the circuit breaker at the phase-to-phase voltage is critical. If the values cannot be obtained from the device documents, an inquiry should be made. If the shortcircuit current at the installation site exceeds the single pole switching capacity of the circuit breaker, then a back-up fuse is required.

For three-pole short-circuits there is no difference between IT supplies a The ultimate short-circuit breaking capacity I

and the service short-circuit breaking capacity Ics

nd other supply types.

continue to apply. Circuit breakers under IEC 60947-2 are suitable for use in IT supplies, if they are not

with the symbol . Testing is in accordance with Annex H.

marked

2.4.6 Switchgear for safety applications

The safety of machines, systems and processes with respect to the protection of persons and property from damage of any kind is the primary objective of the legislation and of regulations and norms for technical measures and solutions. Also low-voltage switchgear is being applied in safety applications. In Section 4 “Protection” the dangers that result from electrical energy directly are dealt-with in

In safety applications it is very important to receive reliable feedback about the main contacts position so t

hat for example it can be excluded that an auxiliary contact reports open main contacts but while in fact – for example because of contact welding – they are closed. In this context the term “mirror contacts” for power contactors is of importance. In a similar way, the methods of “positive guidance” or “mechanical link” of contacts of control contactors ensure that the position of NO and NC contacts cannot be mutually contradictory.

In modular systems, mirror and mechanically linked contacts must be me base unit, so that they cannot separate.

more detail.

chanically fixed to the

2.4.6.1 Mechanically linked contacts

Appendix L of IEC 60947-5-1 defines the criteria for the mechanical linking of contacts. The standard defines mechanically linked contact elements as a “combination of n Make contact element(s) and m Break contact element(s) designed in such a way that they cannot be in closed position simultaneously …”. The standard also defines the test conditions: With welding of a contact – for example of a make-contact – the break-contacts of the contactor when dropped out must still have an opening clearance of 0.5 mm or withstand an impulse voltage of

2.5 kV. The same applies with welding of a break-contact.

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Fig. 2.4-11 The make contacts remain open when mechanically linked when the control relay is excited and a break contact has welded.

In accordance with standard, mechanically linked contacts should be clearly labeled on the device or in the documents or in both places.

Fig. 2.4-12 shows the symbols to be used.

Fig. 2.4-12 Symbol for mechanically linked contacts together with contacts th at are not mechanically linked (left) and symbol for mechanically linked contacts, when all contacts are mechanically linked (right)

2.4.6.2 Mirror Contacts

Appendix F of IEC 60947-4-1 defines the requirements for mirror contacts, i.e. for the unambiguous feedback of the state of the main contacts in event of fault, for example with welded main contacts. The standard defines a mirror contact as “normally closed auxiliary contact which cannot be in closed position simultaneously with the normally open main contact under conditions defined ….”, i.e. if a main contact has welded. The test conditions are similar to those for mechanically linked contacts: The auxiliary normally closed contacts that are designed as mirror contacts must still have an opening clearance of 0.5 mm or be able to withstand a test impulse voltage of 2.5 kV when the contactor is de-energized.

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Fig. 2.4-13 Principle of mirror contacts: the normally open auxiliary contact remains open when the contactor is deenergized and a main contact has welded.

A power contactor can have several auxiliary mirror contacts. In large contactors it may be necessary to connect two mirror contacts in series, one of which is mounted on the left and the other on the right side. Thus even if the contact armature is in an inclined position – for example if an outside contact has welded – safe feedback is ensured.

Like mechanically linked contacts, mirror contacts must be marked directly on the device, in the documentation or in both location

Fig. 2.4-14 Symbol for marking mirror contacts

2.4.7 Installations in hazardous atmospheres

2.4.7.1 History, guidelines and regulations

The Directive 94/9/EC (ATEX 05) regulates the requirements for explosion protection in the European Union. It deals with the properties of explosion-protected devices, protective systems and components for free trade in the internal market of the EU and stipulates that the use of such devices in member states may not be prohibited, hindered or restricted.

The Directive is structured in accordance with the so-called “New Approach”. A key feature is renouncing fr comprehensively defined directly in the directive (Appendix II: Essential health and safety

requirements relating to the design and construction of equipment and protective systems intended for use in potentially explosive athmospheres). This is done in a general form, so that

reference to suitable standards is normally preferable. While placing on the market of explosion-protected devices (protective systems, components) is

uniformly regulated by Directive 94/9/EC, the safe operation of these devices is ultim regulated under national ordinances. The Directive 1999/92/EC, also known as the Safety at Work Directive, defines minimum requirements below which the requirements of the national regulations may not fall.

om strict normative regulations; instead the requirements on the products are

ately

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The extensive CENELEC standards for electrical equipment for hazardous areas apply in all West European states and practically cover the same subjects as the IEC standards.

2.4.7.2 Classification of hazardous areas

When handling combustible or oxidizing substances that are present in fine dispersion as gases, vapors, mists or dusts, risks of explosion can arise. An effective source of ignition must be present to initiate an explosion. Sources of ignition can arise in electrical plants as electrical sparks and arcs, mechanical sparks and hot surfaces.

Hazardous areas are zones, in which, due to the local and operational conditions, a potentially explosive atmosphere of a dangerous quantity can occur. H zones (IEC/EN 60079-10, EN 50281-3 and IEC/EN 61241-10) according to the nature of the combustible substances (gases, vapors and dusts) and according to the frequency of occurrence and duration of the explosive gas atmosphere in the Ex-zone (permanent, occasional or very seldom and short-time) (

Tab. 2.4-2).

In accordance with Appendix I of the Directive 94/9 EC distinction is made between 2 groups of devices:

- “Equipment-group I” (Methane or combustible dusts) in the mining industry and

- “Equipment-group II” (gases or dust/air mixtures) for the remaining areas with explosive

atmospheres.

“Equipment-group II” is subdivided among explosion groups IIA (for exa example ethylene) and IIC (for example hydrogen). The hazard of the gases increases from explosion group IIA to IIC. The requirements on the electrical equipment increase accordingly.

Combustible gases and vapors are classified into temperature classes according to their ignition temperatures, the electr

ical equipment according to its surface temperature (T1 ... T6,

Tab. 2.4-4).

As explosion-protected electrical equipment does not always have to satisfy the highest requirements, it is cla

ssified according to zone, temperature classes and the explosion group of the combustible substances. This enables the adaptation of explosion-protected electrical equipment for various ignition protection types (

Tab. 2.4-3).

azardous areas are classified into

mple propane), IIB (for

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Classification of equipment acc. to IEC/EN Categories of zones exposed to the risk of ignition

Equipment group (application area)

Mines

Other areas with

combustible

atmospheres

Combusti-

ble

sub-

stances

Methane,

Dusts

Gases,

vapours

Dusts

Equipment

2.4.7.3 Motors for hazardous areas

Electrical drives that are operated in hazardous areas must be built and engineered so that they cannot become an ignition source. This applies not only to normal operating and starting, but also in case of faults, for example at stalled rotor.

The specified temperature limits for hot surfaces as a potential source of ignition have for ignition protection types excluded) and Pressurized enclosures “p” (Ex-atmosphere is kept away from the source of ignition) to be complied with only on the outside of the enclosure. Due to the lag in temperature changes of the motor housing, short-term temperature rise of the windings over the limit temperature of the temperature class are with these ignition protection types regarded as noncritical from an explosion protection viewpoint. In contrast, with a motor of ignition protection type Increased Safety “e” (suppression of sparks and high temperatures), exceeding the limit temperature of the corresponding temperature class, for which the motor is foreseen, inside the motor even short-term is not permissible.

Temperature class T1 T2 T3 T4 T5 T6 Ignition class IEC/EN 60079-14, Tab.1 Maximum surface temperature EN 50014. Tab.1; IEC/EN 60079-14 Tab.1 Windings class F continuously EEx e, EN 50019, Tab. 3 Winding class F at end of tE EN 50019, Tab. 3

= determined by the temperature class of the gas = determined by the temperature class (isolation class) of the windings

Flameproof enclosures “d” (transfer of an explosion to the outside

Limit temperatures (°C)

> 450 300 200 135 100 85

450 300 200 135 100 85

130 130 130 130 95 80

< <

210 210 195 130 95 80

Based on the requirement that premature damage and ageing of the motor windings must be reliably prevented, there is a further limitation with respect to the heating characteristic of the windings: The permissible ultimate temperature rise corresponding to the insulation material class (temperature class) of the windings is reduced compared to the normal values by 10 to 15 K in motors of ignition protection type “e”. In theory this signifies a doubling of the windings life span and serves to increase safety, also resulting however in a reduction of the power output compared with the standard values for a motor of the same size.

The permissible limit temperature of a winding in an electrical machine of ignition protection type

Increased S

afety “e” depends, on the one hand, on the temperature class from the explosion

protection viewpoint and, on the other, on the insulation material class of the winding. Tab. 2.4-4 shows the relevant limit values for motors of iso

lation class F.

If another insulation material is used, these values change according to the temperature class of the insulati

LVSAM-WP001A-EN-P - April 2009

on material (Tab. 2.4-5).

2-43

Insulation class E B F H „d“, Continuous service 115 120 145 165 „e“, Continuous service 105 110 130 155 „e“, at the end of the tE-time 175 185 210 235

Tab. 2.4-5 Limit temperatures of motors of ignition protection type “e” and “d” in relation to the insulation material class of the windings

Limit temperatures (°C)

With respect to the temperature rise characteristics of an electrical machine, two operating statuses should be taken into account: continuous duty and stalled rotor motor.

At continuous duty under full load the machine slowly heats up and after several hours, depending on its size, re

aches its steady-state temperature. At the highest permissible ambient temperature, this steady-state temperature may not exceed the limit temperature of the insulation material class nor of the temperature class.

In the schematically presented example of the heating characteristics of a machine of insulation ma

terial class F in Fig. 2.4-15, neither the permitted limit of temperature class T4 nor that of

insulation material class F are exceeded once the steady-state temperature has bee

n reached.

The second operating case should be considered as more critical. It occurs if the rotor of the 3phase asynchronous motor becomes stalled aft

er running at service temperature. The current that then flows is several times higher than the rated current and causes the temperature of the rotor and stator windings to rise rapidly. A monitoring device must disconnect the machine from the supply within the heating time t reached. The heating time t

is the time after which the permissible temperature is reached with

, i.e. the time for the limit temperature of the windings to be

a stalled rotor condition starting from service temperature. It is a characteristic quantity of the motor.

As the selected example of a motor with stalled rotor in Fig. 2.4-15 shows, the limit temperatures of the temperature class f

or applications T4 and T3 determine the t

-time.

300

°C

250

200

Iso.-Kl. F =

150

100

0 140

(Limit temparature at T3)

Continuous operation (t / h) Locked rotor (t / s)

195°C

T2=300°C

(T2)

(T3)

(T4)

T3=200°C

Iso.-Cl.

=210°C

(short-time)

T4=135°C

Iso.-Cl.

=130°C

(permanently)

Fig. 2.4-15 Schematic presentation of the heating characteristic of a motor. When locked at service temperature, the motor must be disconnected from the supply within the t

time.

If however the machine is intended for hazardous areas of temperature class T2 (or T 1), the thermal limit is determined by the short-term permissible limit temperature of isolation material class F of 210 °C.

Ex-motors are not inherently explosion protected. They achieve the required explosion protection by means of complementary in

stallation measures, including appropriate selection of

LVSAM-WP001A-EN-P - April 2009

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equipment and service conditions. With explosion protection type Increased Safety “e” this particularly requires connection to a correctly selected and adjusted overload protective device.

2.4.7.4 Protection of motors of ignition protection type Increased Safety “e”

For the overload protection of motors of ignition protection type Increased Safety “e”, the following regulations and standards apply.

- IEC/EN 60947-1, IEC/EN 60947-2 and IEC/EN 60947-4-1 and IEC/EN 60947-8

- IEC/EN 60079-14, Electrical installation in hazardous areas (other than mines), Sections 7 a

and 11.2.1

Among others, the following protective equipment can be considered for use:

- Current-sensing overload protective equipment with delayed release

- Equipment for direct temperature monitoring with the aid of temperature sensors

Protection by current sensing motor protective devices with delayed overload release

Overload protection for motors of ignition protection type “e” must be selected so that it monitors the rated current I time t

. The heating time tE is the time after which the permissible temperature of the motor is

reached when the rotor is locked at service temperature. The overload protection must release according to its cold curve as in most cases the protective relays cool down already after a short break, while motors take much longer to cool down. The starting (locked-rotor) current I the heating time t

The trip characteristics of the overload protection must always be available at the operating site or via Internet. The release charact starting from the cold state as a function the starting current ratio I The actual trip times must lie within a tolerance band of ±20 % of the stated values.

The trip times t the motor to be protected for the I

When selecting the overload relay, it should be noted that although a short tripping time t reliably ensures shutdown within the heating time t without any disruption. IEC 60079-7 and EN 50 019 stipulate minimum values for the t

100

tE, t

[s]

and can switch off a motor with a locked rotor at all poles within the heating

and

should be obtained from the rating plate of the motor.

eristics should indicate the trip times t

A/IN

of the overload protective devices must be smaller than the heating time tE of

ratio given.

A/IN

, the start up of the motor must be possible

time of motors (Fig. 2.4-16).

with 3-pole loading,

– at least 3 to 8 times.

1 10

IA / I

7.7

Fig. 2.4-16 Heating time t characteristic as a function of the starting current ratio I 1 Minimum values for the time t 2 Minimum values for the times t

(minimum value) of motors and tripping time tA of a circuit breaker with motor protection

of motors in accordance with IEC 60079-7 and EN 50019

of motors in accordance with recommendation of the

A/IN

Vereinigung Industrieller Kraftwirtschaft (German “Association of Industrial Power Utilities”) 3 A motor to be protected with t 4 Trip characteristic t

= f (IA/IN) of a typical circuit breaker with motor protection characteristic of

=15 s at IA/IN = 7.7

trip class 10 5 Starting current of the motor

LVSAM-WP001A-EN-P - April 2009

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Current-measuring overload relays for protection of Ex e – motors must be equipped with a phase failure protection.

Protection by temperature sensors

As an alternative to monitoring the current, the windings temperature can be measured directly. If overload protection is exclusively provided by the installation of temperature sensors, then the motor must be especially examined and certified. It should thereby be documented that the temperature sensors installed in the stator windings trip reliably, even when the rotor is locked, before the rotor reaches the critical temperature according to the ignition class.

Direct temperature monitoring is usually realized with PTC thermistors.

2.4.7.5 ATEX 100a (Directive 94/9/EC)

For EEx- applications within the EU and the EEA (EU plus Iceland, Liechtenstein and Norway) only devices and protection systems may be circulated that are certified accordingly under EU Directive 94/9/EC (ATEX 100a or ATEX 95). This also applies to Switzerland on the basis of the bilateral treaties with the EU.

Motor protective devices for the overload protection of motors of ignition protection type “e” are subject to th relays that themselves are installed outside hazardous areas but that are required for the safe operation of the motors with respect to explosion risks, which are installed inside hazardous areas. Strict regulations are in force with respect to the inspection and certification of the devices, their labeling and measures relating to production control and quality assurance at the manufacturer.

In accordance with ATEX 100a, inspection and certification of devices performed by a recognized standards inst stalt Braunschweig PTB”) is required. In addition the “Notified Body” audits the production monitoring and quality assurance systems of the manufacturer. The certification of the manufacturer is subject to periodic renewal.

Device labeling according to ATEX

The protective devices are in accordance with ATEX 100a to be labeled as follows:

• Name and address of the manufacturer

• CE-mark supplemented with the number of the notified testing institution

(for example

• Type designation

• Serial number, where applicable

ese regulations. These include for example circuit breakers and motor protection

itute (“Notified Body”; for example the “Physikalisch Technische Bundesan-

0102 for PTB)

LVSAM-WP001A-EN-P - April 2009

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• Year of construction (or code for the year of manufacture)

• Mark

- of the equipment group (for example II

supplemented with specifications

for miscellaneous areas with explosive atmos-

pheres, not in mines)

- of the equipment category (for example 2 for devices that may be used in zones 1 and 2, supplemented with the letters G and/or D; G for explosive gas mixtures or D for dusts). For devices that are used for protection of devices in the Ex-area but which themselves are not installed in the Ex-area, the 2 is placed in brackets

Æ for example

II 2 G or II (2) G, Code number under ATEX.

• Devices that are directly used in an explosive atmosphere receive an additional code number

to reduce th Æ for example EEx e IIC T3, Code under CENELEC

e risk of misunderstandings:

, “e” for Increased Safety, IIC for the explosions subgroup “Hydrogen”, T3 stands for a maximum surface temperature of 200 °C.

• Number of the ATEX certificate (e.g. PTB 04 ATEX

3039; a “U” at the end of the ATEX number indicates that the device cannot be deployed alone as complete electrical equipment for the Ex-Area – i.e. for example only in conjunction with a motor).

• Relevant standard (for example EN 60079-14 for electrical equipment in hazardous areas with gas)

• The application instructions associated with the device (trip characteristics etc.) must be available (fo

r example via Internet). The place of availability of information must be stated on

the device.

2.4.7.6 IECEx and other approval schemes for hazardous areas

Much of the information in this section refers to ATEX standards. ATEX is a European-based approval scheme, but may not be accepted in some other parts of the world. Other common hazardous area standards include IECEx (mandatory in some countries, such as Australia) and NEC (USA).

Readers should refer to their local national standards for clarification. For IECEx see also

www.iecex.com

LVSAM-WP001A-EN-P - April 2009

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3 Starting and switching motors

3.1 Selection criteria

Electrical motors must be accelerated from rest up to the operating speed with a starting device. In the case of variable speed drives, the motor controller must also manage the motor speed during operation. The motor and method of starting selected depend on the load torque, the desired starting characteristic (starting current, acceleration) and on the characteristic of the supply. See also Section 1.7 with respect to the characteristic properties of induction motors as the most frequently used motors.

Main criteria for the selection of the starting method

When making the decision whether to use a

 direct-on-line starter  electromech  electronic motor control devices (soft starters, inverters)

the following items should be taken into account view of application and productivity:

 How high is  Can transmission components such as belts, gearboxes or chains be damaged by the high

starting torque with dire

 Does the plant require gentle and continuous acceleration or are torque peaks permissible?  Are there any restrictions with respect to supply loading?  Do technologically more complex products offer additional functions for optimization of the

application ( safety controllers, communication links etc.)?

 In addition to starting, are features of controlled coasting to a stop or braking to be taken into

account?

 In addition to starting, are aspects of speed control to be taken into account once the motor

has started (for example

The selection of suitable starting methods is a critical factor in achieving optimum economic efficiency in every motor various methods for starting squirrel-

anical starter for the starting with reduced current or

to find a suitable solution from the points of

the torque required to start the load?

ct starting?

for example pre-warning functions of motor protection relays, mirror contacts for

from process engineering or energy saving perspectives)?

control application. Tab. 3.1-1 provides guidance with respect to the

cage induction motors.

LVSAM-WP001A-EN-P - April 2009

3-1

Kind of motor Starting procedures for squirrel- cag e standard motors compared (typical values)

Method of starting

Direct on

Line

Υ–Δ-

normal

Υ–Δ-closed

transition

Auto-

transformer

Start via

chokes

Start via

resistors

Soft

starters

Frequency

inverters

(DOL)

Mains strong weak weak weak-medium medium medium Load during

start Relative

starting current I

/ I

Relative starting torque T

A/Te

Run-up time (normal)

full low low low-medium

4 ... 8

(= I

1.5 ... 3 (= T

)

1.3 ... 2.7 (= 1/3 IAD)

0.5 ... 1

(= 1/3 TAD)

1.3 ... 2.7 (= 1/3 IAD)

0.5 ... 1

(= 1/3 TAD)

1 ... 5

(=0.25…0.65

) selectable

0.4 ... 2

(=0.25…0.65

)

0.2 ... 5 s 2 ... 15 s 2 ... 15 s 2 ... 20 s 2 ... 20 s 2 ... 20 s 0.5 ... 10 s 0.5 ... 10 s 2 ...10 s 0.2 ... 10 s

low-

medium

low low-medium low-medium medium medium-full

2 ... 4 2 ... 4 2 ... 6 1 ... 2 2 ...4 2 ... 8

0.4 ... 0.8 0.4 ... 0.8

weak-

medium

0.15 … 2 = k2 · TAD

weak medium

adjustable 4)

Run-up time (heavy duty

5 ... 30 s 15 ... 60 s 15 ... 60 s 20 ... 60 s unusual for loaded start 10 ... 60 s 5 ... 60 s 10 ... 40 s 5 ... 30 s

start) Characteristic

features

High

acceleration

with high

starting

current

Start with

reduced

torque an

current;

current

and

torque

peaks at

switchover

Like normal

Y-Δ,

considerably

reduced

peaks at

switchover

Similar to Υ–

Δ, but without

switchover-

interruption;

selectable

steps

Voltage

and motor

current

(and thus

relative torque)

increases

with

speed

Voltage and

torque

increase

less with

speed compared to chokes

Adjustable

starting

characteris-

tics. Also

controlled

Stop

possible.

High available

torque at low

current.

Adjustable

starting

characteristics.

Application area

Drives in

areas with

strong power

supply

which

permit the

high

starting

Drives

which are

only

loaded

after run-

Like normal Υ–Δ, but for

loads with

little inertia

mass and

high

resistance

torque.

Predominantly

English

speaking

countries

Like Υ–Δ-

starters.

Drives

with

resistance

torque

increasing

with

speed.

Cost-

effective for

unloaded

starts. With

resistance

steps more

expensive

and more

flexible.

torque

IA = Motor starting current, Ie = Rated operational current of Motor 2) TA= Motor-Starting torque Te =Rated operational torque of Motor

k = Voltage reduction factor TAD =Motor starting torque at full voltage 4) Start frequency controlled, torque wide range adjustable

Starts which

require a gentle or

adjustable

torque

characteris-

tic (or

reduced

starting

currents).

Usually for

operational

speed

adjustment.

Energy saving

possible.

Special squirrel-cage motors

Υ–Δ-

enhanced

Multi-stage

start

starting

torque

medium-

strong

0.7 ... 1.5 0.7 ... 3

Like normal Υ–Δ, but at

higher

levels of

current and

torque.

Drives with

high available torque at

Starting

currents

and torques

depending

on motor

design.

Usually for operational

speed

adjustment.

start-up..

Tab. 3.1-1 Characteristic features of the commonly used starting methods for squirrel-cage indu ction motors

LVSAM-WP001A-EN-P - April 2009

3-2

3.2 Direct starting of squirrel-cage induction motors

The direct starting (Direct On Line, DOL) is the simplest and most cost-efficient method of starting a motor. This is assuming that the power supply can easily deliver the high starting current and that the power transmission components and the working machine are suitable for the high starting torques.

I> I> I>

Fig. 3.2-1 Example of a two-component starter for direct starting consisting of a motor protection circuit breaker and a contactor

With direct starting, the poles of contactor and motor protective device are connected to the pole conductors (Fig. 3.2-1) and the operating current of the motor flows through them. The motor protective device must therefore be adjusted to t

The contactor is selected according to the rated operational current I

he rated operational current of the motor.

and the respective

utilization category:

- AC-3 Squirrel-cage induction motors: Starting, switching off during running

- AC-4

Squirrel-cage induction motors: Starting, plugging, inching Definition of utilization categories see Section 1.1. For AC-3 operation, allowance must always be made in practice for sporadic inching operations,

for example during commissioning,

in case of faults or in service work. Contactors from Rockwell Automation comply with these requirements and may be rated without risk according to AC-3 values; for the large majority of devices, the rated operational currents for the utilization categories AC-3 and AC-4 are the same.

A considerable proportion of AC-4 operations or exclusive AC-4 operation is in pra

ctice relatively rare. In such cases, a high frequency of operation is often required at the same time and a high electrical life span is expected. Thus the contactor must be selected according to these two criteria. In most cases a larger contactor must be used than would correspond to the maximum permissible AC-4 rated operational current. See also Sections 2.3.6.3 and 2.3.7.

3.2.1 Starting time

The starting time is an important parameter in starter engineering, as the starting current can be many times higher than the rated currents of motor and switchgear and correspondingly places the latter under thermal loading. It depends on the torque of the motor and hence on the selected starting method, as well as on the torque characteristic of the load. The difference between the motor torque and load torque is the acceleration torque. In addition to the resistive torque of the drive, the inertial mass to be accelerated has a key influence on the time taken for motor starting.

The duration of so called no-load starting, i.e. starts without loading of the drive, typically lies, depending on motor size without large flywheel masses) up to around 5 s. For centrifuges, ball mills, calenders, transport

, in the time range of under 0.1 to around 1 s, starting under load (but

LVSAM-WP001A-EN-P - April 2009

3-3

conveyors and large fans, the start times can extend to minutes. In the case of pumps and fans it should be noted that the pumped material (liquid, air) contributes to the effective inertial mass. The above given approximate values apply for direct starting. The times are correspondingly extended with starter methods with reduced starting current and torque.

With respect to the permissible starting time of the respective motor, the manufacturer’s documentation is definitive.

For the sele

ction of contactors for heavy-duty starting, see Section 2.3.5.2.

3.2.2 Reversing starters

In a reversing starter the motor is switched via two contactors, one for each direction. If the motor is started from rest, the contactor is selected according to utilization category AC-3. Often however the motor direction is changed while it is running (plugging), which means a correspondingly higher loading of the contactors and hence requires selection according to utilization category AC-4. Direct reversing requires a reversing delay between the contactors – for example by means of a short-term delay – of around 40 ms, to prevent short-circuits between phases. In addition to electrical interlocking of contactors of reversing starters, mechanical interlocking is recommended.

Corresponding precautions as for reversing starters are required for plugging with stopping at standstill. In this ca controlled by a speed sensor) is switched off and the motor is hence disconnected from the supply.

se when the motor comes to rest, the braking contactor (for example

I> I> I>

Fig. 3.2-2 Reversing starter with motor protection-circuit breakers and m echanical interlock: Diagram and layout

3.3 Star-delta (Y-Δ, wye-delta) starting

Star-delta (in North America the designation “wye-delta” is commonly used instead) starting is the simplest method for reducing the starting current of a motor. The technique can be used with all squirrel-cage induction motors that are delta-connected for normal operation and whose windings ends are individually connected to terminals. The reduction of the motor current causes a reduction of the starting torque. Star-delta starting is therefore especially suitable for drives that are not loaded until after starting. The starting time is longer than with direct starting, which is especially apparent when driving larger inertial masses.

A distinction should be made between

- normal star-delta starting

- star-delta starting with closed transition

- amplified star-delta starting.

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3-4

3.3.1 Normal star-delta starting

Circuit connections and switching-over procedure

On initiation of starting, the supply voltage is applied to the star-connected motor windings. The starting torque and the starting curre connection. Because of the reduced torque in star connection, the motor does not quite reach the rated speed. After star-connected start-up, the windings are switched-over to delta connection.

Fig. 3.3-1 When starting in star connection, the phase voltage is applied to the motor windings and a windings current of I

= IWΔ/√3 flows.

Because of vectorial addition of the windings currents in delta connection I

On switching from star to delta operation, there is a current surge, whose magnitude depends on various factors. In the figures below, typical cases are illustrated.

Fig. 3.3-2 shows the ideal case for such a switchover. The motor nearly reaches its rated speed

in the first st

age, as the load torque during starting is relatively low. The switching-over current

surge is around the same size as the starting current.

nt in this circuit are approx. 30 % of the values for delta

= IeΔ/3.

6.00

I/I

5.00

4.00

3.00

2.00

1.00

0.00

0 20406080100

2.50

2.00

1.50

1.00

0.50

0.00

T/T

n/ns [%]

Fig. 3.3-2 Typical characteristic of current and torque for star-delta starting I = motor current

= rated operational current of the motor

= current in star connection

= current in delta connection

= current characteristic with star-delta starting

T = torque

= rated operational torque of the motor

= torque in star connection

= torque in delta connection

= load torque

n = speed n

= synchronous speed

LVSAM-WP001A-EN-P - April 2009

3-5

Switching-over itself is usually automatic (rarely manual) and performed by a timing relay set to the required operating period of the star contactor. Between switching off of the star contactor and the making of the delta contactor there must be a sufficient time interval to be certain that the breaking arc in the star contactor is quenched before the delta contactor is switched on. If switching-over is too rapid, the breaking arc causes a short-circuit and the short-circuit protection disconnects the circuit (see

Fig. 3.3-3).

On the other hand, when the switching interval is too long, the motor speed falls during the deenergized interval, depending on ine the delta connection is very high, defeating the purpose of the star-delta start up (

rtial mass and load, so strongly that the in-rush current in

Fig. 3.3-4).

A sufficiently long switching interval between breaking of the star contactor and making of the delta contac

tor is achieved in small contactors with short pull-in and dropout times by electronic timing relays with a switching-over delay of approx. 50 ms. Larger contactors have an inherent switching delay of > 25 ms. In this case, timing relays without additional switching delay may be used. The switching interval then is of the optimum length. To avoid phase short-circuits, the star and delta contactors are additionally mechanically interlocked.

If the delta contactor is switched via an auxiliary contactor (e.g. at low control voltages), no switching-over delay is r

equired on the time relay. A switching interval of adequate length

results from the sum of the making delay times of the auxiliary and delta contactor.

6.00

I/I

5.00

4.00

3.00

2.00

1.00

0.00

0 20406080100

2.50

2.00

1.50

1.00

0.50

0.00

T/T

n/ns [%]

Fig. 3.3-3 A switching interval that is too short results in a short-circuit via the switching arc – the short-circuit protection is activated and breaks the circuit

6.00

I/I

5.00

4.00

3.00

2.00

1.00

2.50

2.00

1.50

1.00

0.50

T/T

0.00

0 20406080100

0.00

n/ns [%]

Fig. 3.3-4

With switching intervals that are too long, the speed falls again behind tion

LVSAM-WP001A-EN-P - April 2009

3-6

direct starting in delta connec-

Faults like shown in Fig. 3.3-3 and Fig. 3.3-4 can also be avoided with the interruption-free (closed transition) st

ar-delta circuit (Section 3.3.4).

When the load torque is too high the star-connected motor only accelerates to a fraction of the speed and “s purpose of the star-delta

ticks” at this speed. The switching process would proceed as in Fig. 3.3-5 and the

start up would not be achieved.

Moreover this condition means that the contactors have to switch off a multiple of the motor rated current. In the example in Fig. 3.3-5 the breaking curre contactor is selected according to I

e(Y contactor)

= 0.34 · I

emotor

nt is around 1.3 · I

emotor

. The star

(see below) and must accordingly

switch off

1.3/0.34 ≈ 4 · I

e(Y contactor)

In practice this means AC-4 operation with a correspondingly reduced electrical life span. In this case a motor for amplified star-delta starting (Section 3.3.5) should be used.

6.00

I/I

5.00

4.00

3.00

2.00

1.00

2.50

2.00

1.50

1.00

0.50

T/T

0.00

0 20406080100

0.00

n/ns [%]

Fig. 3.3-5 Switching-over at speed- that is too low

Selecting the starter components

With star-delta circuits in accordance with Fig. 3.3-6 in delta mode the circuits of main contactor, delta contactor and (

Fig. 3.3-7). The devices are therefore loaded with the phase current I

= Ie/√3 = 0.58 · Ie

A F1

motor protection relays are connected in series to the motor windings

I >

I > I >

Fig. 3.3-6 Starter for normal star-delta starting

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Fig. 3.3-7 Contactor contacts and motor protection relays are connected in series to the motor windings in delta connection K1M Main contactor K2M Delta contactor F1 Thermal relay

Rated operational current of the motor

Phase current

For normal star-delta starting, the switchgear should be rated for the following rated operational currents:

Main contactor K1M = 0.58 · I Delta contactor K2M = 0.58 · I Star contactor K3M = 0.34 · I

e e e

Thermal relay F1 = 0.58 · Ie Æ Motor protection in Υ- and Δ- operation, with F1 in Pos. A (Fig. 3.3-6), t Circuit breakers Q1 = 1.00 · I

Æ Restricted motor protection in Υ- operation,

with Q1 in Pos. B (Fig. 3.3-6), t

≤ 15 s (normal starting)

> 15 … 40 s

The selection of contactors according to these values applies for starting times of maximum 15 seconds and 12 start

s per hour. With heavy-duty starting or higher frequencies of operation,

a larger contactor K3M, possibly also K1M, should be selected (see Sections 2.3.5.2 and 2.3.6). Equally the electrical life span of contactors, especially of the star contactor, should be reviewed

(see Section 2.3.6.3 break many times its rat

). If e.g. switching-over occurs at too low a speed, the star contactor has to

ed current (Fig. 3.3-5). This would strongly reduce its electrical life

span.

3.3.2 Motor connection for clockwise and counterclockwise direction of rotation

When the delta contactor connects at adverse vectorial positions of the supply voltage and the rotor field, transient processes could occur in the motor that could lead to larger current peaks than at switching-on the delta-connected motor . This can result in the making capacity of the contactors being exceeded with as a consequence welding of contacts.

The transient currents can be reduced by appropriate wiring of the main circuit (Fig. 3.3-8). Besides the load on the in the motor.

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contactors, this also reduces the dynamic stress on the windings-heads

Lower transient currents peaks with correct wiring (clockwise rotation)

Fig. 3.3-8 Correct connection of motor phases for clockwise rotation

During the de-energized switching interval, the rotor falls back against the rotating field of the power supply. Its magnetic field induces a decaying residual voltage in the stator – in the voltage phasor diagram Fig. 3.3-9 for the pole conductor L1 entered as U

When connecting to delta (Fig. 3.3-8 und Fig. 3.3-9) the mains voltage U

L1’-N

is applied to the

L1-L3

stator winding, across which this residual voltage is still present. The differential voltage ∆U is relatively small, thanks to the favorable vectorial position of the residual voltage U supply voltage U

that are approximately oriented in the same direction. Thus the current

L1-L3

and the

L1’-N

surge generated by this resultant voltage will also remain small.

Fig. 3.3-9 Phasor diagram for star-delta with correctly connected motor phases for clockwise rotation

High transient current surge with incorrect wiring

The motor also turns clockwise when the terminals are connected according to Fig. 3.3-10.

Fig. 3.3-10 Incorrect connection of the motor phases also produces clockwise turning

A decaying residual voltage acts again with lagging phase position in the stator during the switching interval. On switching to delta, the phase winding with the phasor U to the supply phase U

. These two voltages however have totally different vectorial direc-

L1-L2

is connected

L1’-N

tions, the differential voltage ∆U is high and results in a correspondingly high transient current surge.

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Switching from star to delta produces the phasor diagram Fig. 3.3-11

Fig. 3.3-11 Phasor diagram for connections of the motor phases according to Fig. 3.3-10. This produces a high transient current su

rge because of the adverse vector position.

Counterclockwise sense of rotation

To run the motor in the counterclockwise direction, it is not enough to swap around two phases at any point. This would produce the

same relationships as described above. In order to keep the transient current surge from star to delta connection as small as possible the wiring must be arranged as in

Fig. 3.3-12.

Fig. 3.3-12 Correct connection of the motor phases for counterclockwise rotation of motor

3.3.3 Influence of the third harmonic on motor protection relays

In motors, in which there is relatively little core iron (e.g. refrigeration motors, submersible pump motors etc.), with delta connection the third harmonic and its harmonics are excited in the windings as a consequence of iron saturation. Because of the triple frequency the currents of the third harmonic have the same vector position in all three windings. This harmonic current basically flows in a circle through the windings and is not noticeable in the feeding lines. With star connection, no third harmonics can form as the motor star point is not connected to the mains.

Experience shows that the harmonic current values can be up to 30 % and more of the basic current. With measuring instruments that show the true r.m.s. value, the entire windings current can be measured; on the other hand, the harmonic component cannot be correctly measured by instruments that only show the mean value.

The third harmonic contributes to heating of the motor windings. This is taken into account by the motor manufacturer so that the r

ated load is not compromised. Therefore motor protective devices for direct starting in delta mode should always be set to the motor rated operational current (= current in feeding lines).

In delta mode in a star-delta starter the motor protective device is connected in series to the motor windings (Fig. 3.3-7). If it is normally set to 0.58 · I

it may release prematurely due to the

additional harmonics. In these cases the actual r.m.s value of the windings current should be measured and the setting of the protective device should be increased by the percentage of the

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r.m.s. value of the

harmonic current. This applies for motor protective devices such as bimetal relays, whose trip characteristic is based on the r.m.s. value of the current.

Electronic motor protective devices frequently use measuring principles that differ fro

m the above (for example based on the peak value of the current). In these cases, the settings adjustment must be made on the basis of practical tests.

3.3.4 Uninterrupted star-delta starting (closed transition)

With this circuit (Fig. 3.3-14 and Fig. 3.3-15) the decay of the motor speed during star-delta switching is avoided and the subsequent current peak is kept small.

Before the star contactor is opened, a fourth (transition) contactor K4M makes the motor circuit via resistances in delta. This means over (

Fig. 3.3-13) and the motor speed hardly falls. The delta contactor K2M then establishes

the final stat

e of operation and drops out the transition contactor K4M.

As the normal star-delta circuit, this circuit is only suitable for starting with small load torques.

that the motor current is not interrupted during switching-

Fig. 3.3-13 Oscillogram of star-delta starting with closed transition

Fig. 3.3-14 Star-delta starter for closed transition

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Fig. 3.3-15 The four switching steps of the closed transition star-delta – circuit A Starting in star – connection B Switching-over: Star and transition contactors are closed C Switching-over: Delta circuit via transition contactor and resistors D Operation in normal delta circuit

Rating of starters

Main contactor K1M 0.58 · I Delta contactor K2M 0.58 · I Star contactor K3M 0.58 · I

e e e

Transition contactor K4M 0.27 · Ie (typical value, varies with R1) Overload relay F1 0.58 · I

Transition resistor R1 (0.35...0.4) · Ue/Ie The factor should be selected from the stated range so that a standard resistance value results.

Unlike in the normal star-delta circuit, the star contactor in the starter for closed transition has the same rating as the main and delta contactor. This is f

or two reasons:

- The K3M star contactor must break the star current of the motor and of the transition

resistances. A current of approx. 1.5 · I

flows in the transition resistors. Therefore a corre-

spondingly higher contact rating is required.

- The closed transition star-delta circuit is often used in plants with higher frequencies of

operation, in which also a longer ele

ctrical life span is required.

The resistors are only loaded for a maximum of 0.1 seconds (short-time duty). However in most cases only the continuous load cap resistors the continuous load capacity P

acity of the resistors is known. For wired ceramic-tube

required for selection can be calculated by help of the

following approximation formulas:

≈ U

/(1200 · R) [W] for max. 12 operations/h

≈ U

/(500 · R) [W] for max. 30 operations/h

Notes

In a star-delta circuit with closed transition, no excessive switching current surge can be produced.

With large in

ertial masse

for clockwise or counterclockwise rotation (see Section

s, it should also be ensured that the motor is correctly wired

3.3.2), to prevent damage by torque

surges.

3.3.5 Amplified star-delta starting

With a large load torque, an adequate speed is not achieved in the normal star connection because of the reduction of the starting torque of the motor (see Fig. 3.3-5). A larger motor torque can be achieved with amplified star-delta also increases with the motor torque (see Tab. 3.1-1).

Two starting methods are possible:

- Mixed star-delta starting

- Part winding star-delta starting For both methods, motors with suitab

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le windings tappings are required.

starting. That being said, the starting current

Mixed star-delta starting

In this case the motor windings are usually divided into two equal halves. On starting, one half of the part-windings is de current in star-connectio

lta-connected, the other is star-connected (Fig. 3.3-16). The starting

n is approx. (2 ... 4) · I

. This generates a correspondingly larger

starting torque.

Fig. 3.3-16 Mixed star-delta starting Circuit diagram and connections of motor coils during starting (Y) and in operation (∆)

3.3.6 Part-winding star-delta starting

In this case, too, the motor windings are subdivided. In the star connection, only the main winding – one part of the entire winding – is used (Fig. 3.3-17). The starting current in star connection is (2 ... 4) · I

, depending on tapping, from which a larger breakaway torque results.

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Fig. 3.3-17 Part-winding star-delta starting Circuit diagram and connections of the motor windings during starting (Y) and in operation (∆)

Ratings of the starter components

With the exception of the star contactor, contactors and motor protective devices have the same ratings as with the “normal” star-delta circuit (

· I

selected for (0.5 ... 0.58)

because of the larger starting current.

see Section 3.3.1). The star contactor should be

Notes A sufficiently long switching interval for transition from star to delta operation should

be ensured, in accordance with Section 3.3.1. A closed transition star-delta connection in accordance with Section 3.3.4 is possible in both cases. With very large load-torques it may even be necessary. The transition resistor an

d the transition contactor should be rated as described there.

The rules in accordance with Section 3.3.2 apply for the connections.

3.4 Auto-transformer starting

3.4.1 Circuit and function

An auto-transformer starter makes it possible to start squirrel-cage induction motors with reduced starting current, as the voltage across the motor is reduced during starting. In contrast to the star-delta connection, only three motor leads and terminals are required. On starting, the motor is connected to the tappings of the auto transformer; transformer contactor K2M and star contactor K1M are closed. The motor starts at the voltage reduced by the transformer, with a correspondingly smaller current.

By this means the feeding current in comparison to direct starting would be reduced by the square of the transformer voltage ratio; nevertheless, it also covers the relatively high transformer losses. Depending on the tapping and starting current ratio of the motor, the starting current lies at (1 … 5) · I the square of the voltage across the windings. Auto-transformers usually have three available taps in each phase (for example 80 %, 65 %, 50 %), so that the motor starting characteristic can be adjusted to the load conditions.

is in most cases noticeably higher, as it

. In contrast, the motor torque falls with

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If the motor has reached 80 ... 95 % of its rated speed (depending on the desired reduction of the current surge after switching-over), the star contactor K1M on the transformer is opened. Now the transformer part-windings act as chokes. The motor voltage is only reduced by the chokes below the supply voltage and the motor speed does not fall. The main contactor K3M closes via auxiliary contacts of the star contactor and applies the full supply voltage to the motor. For its part, the main contactor K3M drops out the transformer contactor K2M. The entire procedure is thus uninterrupted.

Fig. 3.4-1 Auto-transformer starter with uninterrupted switching over (Korndörfer starting m ethod)

3.4.2 Rating of the starter

The main contactor K3M and the motor protective device F1 are selected according to the motor rated operational current I

. Transformer contactor and star contactor are only briefly closed

during starting. Their rating is determined by the required contact breaking capacity, as they must reliably cope with any unforeseen disconnection during start up. The star contactor also operates with every start-up during switching-over. The values of the rated operational currents for the transformer contactor K2M, depending on the start time and starting current, are between (0.3 … 1) · I

, for the star contactor between (0.45 … 0.55) · Ie.

3.5 Starting via chokes or resistors

The series-connected chokes (Fig. 3.5-1) or resistors (Fig. 3.5-2) reduce the voltage at the motor and hence also the starting current. The starting torque is reduced by around the square of the current.

3.5.1 Starting via chokes

At rest the motor impedance is small. Most of the supply voltage drops across the seriesconnected chokes. The breakaway torque of the motor is therefore strongly reduced. With increasing speed, the voltage across the motor increases because of the fall of the current consumption and the vectorial voltage distribution between the motor and the reactance connected in series. Hence the motor torque also increases. After the motor start-up, the chokes are shorted by the time-delayed main contactor K1M and the starting contactor K2M is dropped out.

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Fig. 3.5-1 Motor starting via series-connected chokes

3.5.2 Starting via resistors

The basic circuit diagram is the same as described in Section 3.5.1, only that the chokes are replaced by lower-cost resistors.

Fig. 3.5-2 Motor starting via series-connected resistors

With this method, the starting current can only be slightly reduced, as the motor torque falls with the square of the voltage and the voltage across the motor, other than with starting via chokes, only increases slightly with increasing speed. It is more advantageous to reduce the seriesresistance during starting in steps. This reduces the voltage across the resistor and increases that across the motor. The expenditure on hardware is thereby significantly larger.

A simpler solution are enclosed electrolytic resistors with a negative temperature coefficient. Their ohmic resistan

ce decreases automatically during starting because of heating by the

starting current.

3.6 Stator resistance soft starting

3.6.1 Circuit and function

This method is used with relatively small induction motors with squirrel-cage rotors to achieve a soft starting effect. The starting torque is reduced because an ohmic resistance is connected in the supply line of one phase (Fig. 3.6-1). This means that the motor is asymmetrically supplied, resulting in phases without series resistance. Modern solutions make use of controlled power semiconductors instead of resistors.

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a more gentle, surge-free motor start-up. The motor current is not reduced in the two

Fig. 3.6-1 Stator resistance soft starting for gentle motor starting

Note

A motor protective device without differential release must be used, as it would otherwise operate during start-up.

3.7 Pole-changing motors

3.7.1 Speed change by pole changing

The number of poles determines the rated speed in asynchronous motors at a given supply frequency. If the stator windings are designed for two or more different pole numbers, the speed can be changed in just as many steps by switching-over.

The Dahlander circuit that with only one winding and six terminals supports two pole numbers and hence speeds in the ratio 1:2 is two steps are thereby in a certain relationship to each other, depending on the circuit version. The Dahlander winding is divided into individual windings groups, stepped according to the smaller pole pitch. When current flows through each windings group in the same direction, the higher pole number is generated, and when the current direction is reversed in each second windings group, the lower pole number is generated. By repetition of the same windings arrangement from pole to pole, a very good windings symmetry is achieved.

A special type of Dahlander circuit is the PAM circuit (pole amplitude modulation). In PAM motors, an asymmetry of the field har the resulting pole numbers are in ratios other than 1:2 (e.g. 6/4-pole or 8/6-pole).

Motors in PAM circuits, like those in Dahlander circuits, only have six terminals. For winding types, the same versions of the external circuit diagram can be used. An additional star point contactor is always required for the YY stage, in addition to the two feeding contactors of both steps.

Pole changing can also be achieved by regrouping the windings branches. This is kn phase mixing or phase modulation. In this case the winding along the periphery is divided into coil blocks. Depending on the number of these blocks, double or multi-stage pole changing can be performed. Three terminals are required per speed level.

With pole changing by phase modulation, the connection diagram provided by the motor manufacturer should be consulted w Either only one feeding contactor (for example YYY/YYY circuit) is required per stage or in addition a supplementary bypass contactor (for example ∆/∆∆ circuit).

In Tab. 3.7-1 and Tab. 3.7-2 a summary is provided of the most common arrangements and circuit layouts of stator w

indings for pole-changing motors.

especially economical. The rated powers and torques of the

monics is accepted and the windings are grouped so that

both

own as

hen selecting the external circuit layout and the switchgear.

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Tab. 3.7-1 Pole-changing motors with 2 speeds

Tab. 3.7-2 Pole-changing motors with 3 or 4 speeds

3.7.2 Ratings of starters for pole changing

Pole-changing motors often have, especially at lower speeds, considerably less favorable efficiency and power factors (cos φ) than standard motors. The intake current is therefore usually higher than that assigned to the corresponding power in the selection tables. Therefore the feeding contactors of the individual steps (Fig. 3.7-1) for all arrangements and circuit layouts (separate windings, Dahlander, PAM, phase modulation circuits) should n to the rated operational powers, but according to the rated operational currents specified by the motor manufacturer. Selection is in accordance with utilization category AC-3; for steps with inching operations, the supply contactor should be suitable for AC-4.

The star point contactor of the YY-step (K3) in Dahlander circuits, carries, depending on the circuit variation, exactly or approximately half the current of the feeding

= I

eK3

eYY

/2 [A]

Selection is always in accordance with AC-3;

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ot be rated according

contactor for this step:

The star point contactor in a PAM circuit, because of the asymmetrical phase currents and of the harmonic content, should have the same rating as the feeding contactor of the YY step.

The rating of contactors in phase modulation circuits is based

on the specifications of the motor

manufacturer with respect to the rated operational current.

Fig. 3.7-1 Circuit diagram for motors in Dahlander or PAM circuits

For all arrangements and circuit layouts, a separate motor protective device should be provided for the thermal overload protection of the motor in each step that is adjusted to the respective rated operational current.

As a change of current direction occurs in Dahlander, PAM and phase modulation circuits when the pole number is cha

nged in a section of the windings, a de-energized interval is necessary during switching over to prevent unacceptably high switching current surges. If the making delay of the feeding contactors of both steps is smaller than 20 ms, electrical interlocking must be performed with a switching interval (approx. 30 … 50 ms). When switching over between two separate windings, the currents that are produced are only in the range of the starting currents that the switchgear can easily cope with.

Note

Normally with multi-spee

d motors, a common short-circuit protection is provided for all steps, that is rated according to the largest rated operational current. It must be checked whether this short-circuit protection is also permissible for the selected feeding contactor of the smaller step. Otherwise a larger contactor should be selected.

3.7.3 Rating of the starter for steps with star-delta starting

If a winding is designed for six terminals, star-delta starting can be provided in this step. Instead of the feeding contactor, a star-delta contactor combination is required. This is rated according to the rated operational current of the relevant step.

A reduction of the starting current can also be achieved in star-delta starting in Step l with the Dahlander circuit Δ/YY (Step l: Y- Δ contactors (Fig. 3.7-2).

; Step II: YY). The circuit can be realized with only four

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The contactors are rated according to the rated operational currents I For the contactors K3 and K4, the higher value applies (Tab. 3.7-3).

Contactor Function Load

K1 K2

K3 Delta contactor

K4 Star contactor

Tab. 3.7-3 Rating of starters for steps with star-delta starting

Feeding contactor Feeding contactor

and

star contactor

and

star contactor

Step I Step II

(Y-Δ) (YY)

eII

0.58 · I

and

0.5 · I

eII

Ca. 0.33 · I and

0.5 · I

eII

(Step l) or I

(Step lI).

eII

Fig. 3.7-2 2-step star-delta starter for motors in Dahlander circuit (with nine terminals), star-delta starting in Step I

3.8 Starting wound-rotor motors

With slip-ring motors (wound-rotor motors) the starting current can be limited to (1,1 ... 2,8) · Ie with high load torque and at extended starting times. This means that heavy-duty starting is also possible with supplies with poor loading capacities. See also Section 1.7.1.1.

Slip-ring motors have rotor windings, whose three ends extend over the slip rings. When resistors are (

Fig. 3.8-1). The starting resistors in each rotor phase are shorted stepwise by contactors during

start-up. (Fig. 3.8-2). In automatic starting arrangements, the contactors of the individual starting steps are controlled by adjusta switched dependent of speed by centrifugal switches.

connected in the rotor circuit, the starting current and hence the torque are affected

ble time relays. In so-called Combi-motors, the rotor resistances are

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2.0

1.8

av-acc

1.6

1.4

1.2

T/T

1.0

0.8

0.6

0.4

0.2

0.0

0.0 0.2 0.4 0.6 0.8 1.0

n/n

Fig. 3.8-1 Torque characteristic on starting of a slip-ring motor

… T4 Torque characteristics of the individual starting steps

Mean starting torque

av-acc

A starter for a slip-ring motor can be equipped with one or more steps. On the one hand, this allows the starting torque to adjust to the working machine and, on the other, the current peaks to the supply conditions.

Half-load starting Full-load starting

Number of resistance steps 2 3 4 2 3 4 Max. starting current I Min. starting torque T

max/Ie

min/Te

Starting time s 4 … 60

2.2 1.7 1.3 2.8 2.3 1.8

0.5 0.5 0.5 1 1 1

Tab. 3.8-1 Example of application: Starting motors under load

Note

With slip-ring motors, also the speed can be controlled with the resistors in the rotor circuit (resistance or slip contr

ol; e.g. for crane motors). This requires a correspondingly designed

control circuit and ratings of the contactors and resistors for variable speed operation.

Fig. 3.8-2 Circuit diagram with three-step, automatic shorting of external rotor resistors

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Ratings of the starter (start-up mode see Tab. 3.8-2)

The stator contactor K1M (feeding contactor) is selected, corresponding to the rated operational current I

of the motor under utilization category AC-2. A distinction is made in rotor contactors

between step contactors (K3M, K4M) and the final stage contactor (K2M). The rotor contactors only have to connect and conduct the current briefly. Their poles are usually delta-connected. The final stage contactor (K2M) must be designed for continuous duty AC-1; the loading is

0.58 · I

. The step contactors (K3M, K4M) operate in starter circuits in short-time duty mode.

e rotor

They can therefore be rated for this short-term loading or according to their making capacity.

Rating for starting with

half-load full-load heavy duty

Stator contactor K1M

av rotor/Ie rotor

e AC-2

0.7 1.4 2.0

e (stator)

Rotor contactors (poles connected in delta) Final stage contactor K2M I

Step contactors

2 steps K3M 0.20 · I 3 steps K3M, K4M 0.18 · I 4 steps K3M … K5M 0.15 · I

Overload relay F1

e AC-1

0.58 · I

e (stator)

e rotor

0.58 · I

0.35 · I

0.30 · I

0.25 · I

e (stator)

e rotor

0.58 · I

0.50 · I

0.43 · I

0.35 · I

e (stator)

Max. starting time per step 15 s 12 s 12 s Max. frequency of operation (starts per hour) 120/h 30/h 12/h

e (stator)

e rotor

av rotor

Rated operational current of the motor (stator) Rated operational current of the rotor Mean rotor current during starting

Tab. 3.8-2 Factor

s for the rated operational currents of the motor for contactor selection according to the AC-2

catalog values

Permissible rated voltage for the rotor contactors

As the rotor contactors are only under voltage briefly during starting, in accordance with IEC 60947-4-1, 5.3.1.1.2 the rated

operational voltage of the rotor U

may exceed the rated isolation voltage U

of the contactor by 100 %. Contactors for 690 V may

(rotor standstill voltage)

therefore be used up to a rotor standstill voltage of 1380 V.

e rotor

3.9 Electronic soft starters

Soft starters serve for a continuous adjustment of the starting characteristic of three-phase asynchronous motors to the requirements of the load by controlling the voltage across the motor and enable for an optimum integration of the drives in process control by means of various complementary functions.

While when star-delta starters are used, the starting torque and starting current can be fix reduced to ar range. It should be noted that the motor torque of a soft starter falls with the square of the voltage and current reduction. With the same starting current as with a star-delta starter in star connection (= 1/3 I

for star-delta. See also Section 1.7.1.3.

AΔ

With the conventional starting procedures such as direct on line starters, starting transformers or star-delta starters, the transients. Each switching procedure also means a rapid current change (transient current peaks) and hence generates high torque peaks in the motor. Electronic equipment with power

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ound a third, with electronic soft starters the reduction can be set within a wide

), with a soft starter the motor torque falls to 1/9 TAΔ in comparison to 1/3

AΔ

motor, supply and the entire drive chain is loaded by switching

semiconductors can prevent these transient effects and reduce the loading of power supply and drive.

The following features and options are characteristic in the use of soft starters:  Extended setting range of the starting characteristic or selection of various starting character-

istics for an optimum adjustment to the requirements of the working machine

 Infinite variable characteristic of current, voltage and torque. No transient current peaks  Motor connection with only three lines with control in the motor supply lines  Increased rated power of the soft starter (factor 1.73) with control in the windings circuit and

motor connection with six lines

 By-passing of power semiconductors after motor start to reduce the permanent losses  Limited number of starts per hour depending on starting conditions and thermal specifica-

tions of the

soft starter

 Extended coasting to stop and braking of drives  Crawl speed for positioning  Diagnostic and early warning functions such as overload, underload, locked rotor etc.  Integration in a communication network  Integrated (motor) protection functions  Current harmonics during the starting time by phase control  Drives with soft starters require for maintenance work on the motor a series disconnecting

device (for example disconnector sw

Soft starters are available in a variety of different designs, ea

itches, circuit breakers with isolating function).

ch with specific technical characteristics. For the selection of a device for a specific application the technical literature of the manufacturer and its technical support have to be observed. See also IEC 60947-4-2 [5] and [17]. For specific aspects of high efficiency motors see 1.7.1.2.1.

3.9.1 Voltage ramp versus current limitation

The basic mode of operation of soft starters is to control the voltage across the motor by phase control. Usually the phase control is performed in 3 phases and in both current half-cycles by means of triacs or antiparallel connected thyristors. Economical solutions use controlled semiconductors in only two or even only one (1) phase. The resulting asymmetries create disadvantages with respect to the available torque related to the current consumption and for example an increasing loading of the motor bearings because of torque asymmetries. The 1phase controller corresponds to the stator-resistance starting circuit (see Section

The voltage across the motor can for example be controlled by

 a (selectable) voltage ramp or  a fixed (reduced) voltage (quasi current-limiting)  in relation to a feedback variable such as

o motor current (current limitation) or o speed (start following a speed characteristic)

Depending on the method chosen, t produced ( current limitation, large

Fig. 3.9-1). When starting with a voltage ramp and especially when starting with

acceleration torques in the range of the breakdown torque are generated

ypical torque and speed characteristics for starting are

towards the end of the starting period.

3.5).

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Current at direct start

Current at soft start with voltage ramp

Current at soft start with current limit

T/T

Direct start

Current limit

Voltage ramp

Load

n/n

Fig. 3.9-1 Current and torque characteristics for starting

In the following a more detailed discussion of the characteristics of various available soft starter functions is presented.

3.9.2 Voltage ramp

The voltage across the motor is linearly increased during a settable time, starting from an adjustable initial value (Fig. 3.9-2). The starting current and the starting torque, and hence the accelerati tic of the load. This method is especially suitable for load-free start-ups and for working machines with increasing torque requirement at increasing speed (drives with larger inertial masses, fans etc.).

Percent Voltage

Fig. 3.9-2 Soft start with voltage ramp

on, adjust themselves in accordance with the voltage ramp and the torque characteris-

100%

Initial Torque

Start

Time (seconds)

Run

For drives with variable loading at the start – for example processing machines that normally start up in a load-free condition, but which can be under load due to a fault – soft starters with two voltage ramps are available (Fig. 3.9-3). The initial voltages and starting times of ramps are separately adjustable an

d hence can be adapted to both operating states. It is possible to

switch between both ramps as required.

Percent Voltage

100%

Initial T orque #2

Initial T orque #1

Ramp #2

Start #1

Ramp #1

Start #2

Time (seconds)

Run #1

Run #2

Fig. 3.9-3 Soft starter with changeable voltage ramp for various loading states at start.

3.9.3 Kickstart

Many drives have a high breakaway torque at rest, because for example bearings surfaces may generate high initial friction. This requires a short period of increased starting voltage at the

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Rockwell Automation Low-Voltage User Manual

Specifications and Main Features

Frequently Asked Questions

User Manual

1 Load characteristics and utilization categories

1.1 Utilization categories simplify the selection of devices

1.2 Electrical heating devices

1.3 Lamps and illumination equipment

1.3.1 Incandescent lamps

1.3.1.1 Halogen lamps

1.3.2 Discharge lamps

1.4 Transformers

1.5 Reactive power compensation and switching of capacitors

1.5.1 Reactive power compensation

1.5.1.1 Individual compensation

1.5.1.2 Group compensation

1.5.1.3 Central compensation

1.5.2 Switching of capacitors

1.5.2.1 Switching-on single capacitors

1.5.2.2 Switching of long, screened lines

1.5.2.3 Switching capacitors of central compensation units

1.6 Control circuits, semiconductor load and electromagnetic load

1.7 Three-phase asynchronous motors

1.7.1 Principle of operation

1.7.1.1 Slip-ring motors

1.7.1.2 Squirrel-cage induction motors

1.7.1.2.1 High efficiency motors

1.7.1.3 Influence of the voltage across the windings

2 Switching tasks and selecting the appropriate switchgear

2.1 Electrical equipment complying with standards and matching the application requirements

2.2 Basic switching tasks and criteria for device selection

2.2.1 Device types

2.2.1.1 Disconnectors (isolating switches)

2.2.1.2 Load switches

2.2.1.3 Switch disconnectors

2.2.1.4 Circuit breakers

2.2.1.5 Supply disconnecting devices

2.2.1.6 Supply disconnecting EMERGENCY STOP devices

2.2.1.7 Summary supply disconnect and EMERGENCY STOP devices

2.2.1.8 Fuses

2.2.1.9 Devices for thermal protection

2.2.1.10 Contactors

2.3 Parameters for the correct selection and sizing

2.3.1 Rated isolation voltage Ui

2.3.2 Rated operational voltage Ue, rated operational current Ie and utilization category

2.3.4 Short-circuit withstand capacity and short-circuit protection

2.3.4.1 Joule integral I2t

2.3.4.2 Cut-off current ID

2.3.4.3 Rated short-time withstand current lCW

2.3.4.4 Current limiting protective equipment

2.3.4.5 Coordination of electrical equipment

2.3.4.5.2 Coordination with respect to the operability after a short-circuit

2.3.4.6 Short-circuit switching capacity

2.3.4.6.1 Rated short-circuit making capacity Icm

2.3.4.6.2 Rated short-circuit breaking capacity Icu and Ics

2.3.5 Thermal protection

2.3.5.1 Ambient temperature

2.3.5.2 Operational overcurrents, heavy-duty starting

2.3.6 Life span

2.3.6.1 Prospective service life

2.3.6.2 Mechanical life span

2.3.6.3 Electrical life span

2.3.7 Intermittent and short-time duty, permissible frequency of operation

2.3.7.1 Intermittent duty and relative ON-time

2.3.8 Rated frequency and harmonics

2.3.9 Safety clearances

2.3.10 Mounting position

2.3.11 Protective separation

2.3.12 Site altitude

2.3.13 Shock and vibration

2.4 Specific application conditions and switching tasks

2.4.1 Parallel and series connection of poles

2.4.1.1 Parallelling

2.4.1.2 Series connection

2.4.2 AC switchgear in DC applications

2.4.3.1 Effect of the supply frequency on the thermal load

2.4.3.2 Effect of the supply frequency on the switching capacity

2.4.3.4 Switchgear used with soft starters

2.4.3.5 Switchgear for use with frequency converters (inverters)

2.4.4 Application of four-pole switchgear devices