Rockwell Automation Low-Voltage User Manual

Low-Voltage Switchgear and Controlgear

Technical Document

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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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