Megger mtr105 User Manual

Guide to low voltage motor testing

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Contents

Introduction 5

Motor types 6

DC Motor 6

DC Motor Types – Series; Shunt; Compound; Permanent Magnet 6

DC Motor Types - Series 7

DC Motor Types - Shunt 8

DC Motor Types - Compound 8

DC Motor Types - Permanent Magnet 9

Advantages of a DC motor 10

AC Motor 10

AC Motors Types 10

Star Conﬁguration 11

Delta conﬁguration 12

Star vs Delta conﬁguration 12

Why perform Motor Tests? 13

What problems create the need for a test? 13

Testing and Diagnostics 14

Trending of test data 17

Megger Baker MTR105 – The Tests 18

Insulation resistance 19

Spot or Timed insulation resistance test 20

Polarisation Index (PI) 21

Dielectric Absorption Ratio (DAR) 22

Temperature correction 24

Guard Terminal 25

Three phase test 28

Voltmeter 28

Phase rotation 28

Continuity 28

Diode test 29

2 Guide to Motor testing

Contents

Digital Low Resistance Ohmmeter (DLRO) 29

Motor Direction of Rotation test 30

Inductance 32

Capacitance 32

Temperature measurement 32

Megger Baker MTR105 Overview 33

Description 33

Features 33

Applications 34

Safety 34

Insulation resistance tests 34

Voltmeter 35

Continuity (Resistance) tests 35

DLRO Four wire Kelvin low resistance 35

Motor Direction of Rotation test 35

Inductance, Capacitance and Resistance meter (LCR) 36

Temperature 36

Display 36

Guard Terminal 36

Storage and download of results 36

Instrument software updates 36

Speciﬁcations 37

Guide to Motor testing 3

Contents

This is the ﬁrst in a series of information booklets that will support electrical testing in the

benchmarking, maintenance and repair of rotating machines. This guide uses the speciﬁc tests

of the Megger Baker MTR105 to demonstrate the importance and application of these tests for

low voltage machines rated up to 2300 volts, as described in IEEE Standards which are listed at

the end of this document.

4 Guide to Motor testing

Introduction

Electric motors are made up of a multitude of components that when combined and assembled

into a motor have to endure extreme electrical and mechanical operating stress as well as

varying environmental conditions during their service life. To prevent premature failure, regular

testing is necessary to ensure reliable operation and, importantly, to extend the motor’s service

life. Electrical testing usually consists of Insulation Resistance (Meg Ohm MΩ) tests and a Low

Resistance tests (Milli-Ohm mΩ). These tests are essential in determining the health of a motor,

however, not all faults or early detection of potential faults can be detected with these tests.

Performing a suite of different test types where each test provides a ‘piece of the puzzle’ helps

to build up to a clearer picture which is essential in determining the of the health of the electric

motor.

Guide to Motor testing 5

Motor types

There are two main types of motors “ac” and “dc”. A Direct Current (dc) motor has direct

current connected to the windings and rotor (armature) to produce rotation. An Alternating

Current (ac) motor has alternating current connected to the stator (stationary windings). In

both types this produces rotation on the rotor (armature) through a magnetic ﬁeld.

DC Motor

DC Motor Types – Series; Shunt; Compound; Permanent Magnet

A simpliﬁed representation of a DC motor showing a single loop of the armature

Magnetic poles

Coil

Commutator

Fig 1: Simpliﬁed DC motor

6 Guide to Motor testing

Motor types

Commutator

Brush holder

Brush

Brush holder spring

Motor enclosure

Cover

Fig 2: A typical representation of a DC motor showing an exploded view of the

brush components and with multiple loops making up the armature.

DC Motor Types - Series

DC Supply

Field

Armature

Fig 3: Series excited DC motor

An electric motor powered by direct current where the ﬁeld windings are connected in series

to the armature windings.

Guide to Motor testing 7

Motor types

DC Motor Types - Shunt

DC Supply

Field

Armature

Fig 4: Shunt excited DC motor

An electric motor powered by direct current where the ﬁeld windings are connected in parallel

to the armature windings. This allows both coils to be powered by the same source.

DC Motor Types - Compound

DC Supply

Field Field

Armature

Fig 5: Cumulatively compound excited DC motor

The dc compound motor is a combination of the series motor and the shunt motor. It has a

series ﬁeld winding that is connected in series with the armature and a shunt ﬁeld that is in

parallel with the armature

8 Guide to Motor testing

DC Motor Types - Permanent Magnet

Motor types

Armature

Supply

Field

magnet

Armature

Field

magnet

Fig 6: Permanent magnet DC motor

A DC Motor whose poles are made of permanent magnets is known as a Permanent Magnet

DC (PMDC) Motor. The magnets are radially magnetized and are mounted on the inner

periphery of the cylindrical steel stator. The stator of the motor serves as a return path for the

magnetic ﬂux.

Guide to Motor testing 9

Motor types

Advantages of a DC motor

Used in applications where only a dc voltage source is available. The motor speed is easily

controlled by change to the applied voltage. They are mainly used where high torque is

required at low speed and constant high torque is needed at variable speed ranges.

AC Motor

AC Motors Types

 Single phase (shaded pole; split-phase; capacitor start; capacitor-run; capacitor-start and

run).

 Three phase

Advantages over dc – used in all other applications and due to the brush-less design less

maintenance is required.

Advantages of three phase over single phase motors – more energy efﬁcient and have no

capacitors or centrifugal switches to drive or maintain

This guide will focus on three phase AC motors as they represent the majority of motors in

use today. The diagram shows a typical construction of a three phase motor with an open

conﬁguration i.e. the motor is not conﬁgured for Star or Delta conﬁguration and all three

phases are isolated. See ‘Star conﬁguration’ and ‘Delta conﬁguration’ for additional details.

Stator Coil

Rotor

Phase B

Phase A

Phase C

Phase A

Motor

Fig 7: 3 phase motor

10 Guide to Motor testing

Phase B

Phase C

Phase A

+ V

60 120 180 240 300

-V

Phase B Phase C

360

1 Cycle

Star Conﬁguration

In a STAR conﬁguration the three phases are connected together to form a neutral point.

 Line current is equal to the Phase current

 Allowed supply voltage is higher (than Delta)

 The phase voltage is 1/√3 of the line voltage

 Voltage per phase is lower (than Delta)

 Lower inrush current

 Less power

Motor types

Fig 8: Star connection of motor winding

Guide to Motor testing 11

Motor types

Delta conﬁguration

In a DELTA conﬁguration the opposite ends of the three phases are connected together where

the end of a phase is connected to the start of another phase.

 The line voltage is equal to the phase voltage

 Allowed supply voltage is lower (than Star)

 The line voltage is equal to the phase voltage

 Voltage per phase is higher (than Star)

 Higher inrush current

 More power

Fig 9: Delta connection of motor winding

Star vs Delta conﬁguration

Delta is typically used where a high starting torque is required. Star is used where a low starting

current is required.

12 Guide to Motor testing

Why perform Motor Tests?

Early detection of faults during manufacture of new motors is vital. This is important at both

component and assembly levels. Detection of faults, for in service motors, as soon as they

begin to develop results in a reduction of ‘downtime’ and reduced repair costs.

Early detection and correct diagnosis of developing faults will help determine the condition

of in-service equipment. We can then predict when maintenance should be performed or

arrange routine maintenance, i.e. a time-based preventive maintenance schedule, to provide

sufﬁcient time for the planned controlled shut down of the affected process. Both predictive

and preventative maintenance can reduce ﬁnancial losses, maintain production levels and avoid

catastrophic consequences.

What problems create the need for a test?

When motors and generators are new, the electrical system should be in a very good condition.

Additionally rotating machine manufacturers have continually improved the quality of their

products. Nevertheless, even today, motors and generators are subject to many changes to

conditions which can cause these products to fail, i.e. mechanical damage, vibration, excessive

heat or cold, dirt, oil, corrosive vapours, moisture from processes, or just the humidity of the air.

In varying degrees these factors are at work and as time goes on, combined with the electrical

stresses that exist, create a harsh environment for daily operation. As pin holes or cracks

develop, moisture and foreign matter penetrate the surfaces of insulation, providing a low

resistance path for leakage current.

Once started, the different enemies tend to aid each other, permitting excessive current

through the insulation. Sometimes the drop in insulation resistance is sudden, as when

equipment is ﬂooded. Usually, however, it drops gradually, giving plenty of warning, if tested

periodically. Such tests permit planned reconditioning before service failure. If not tested

periodically, a motor with poor insulation, for example, may not only be dangerous to touch

when voltage is applied, but also be subject to burn out. What was good insulation has

become a partial conductor.

Guide to Motor testing 13

Testing and Diagnostics

Electrical testing and diagnostics can be segmented to two main categories:

 Static (De-Energised) Electrical Testing –

 When the supply to the machine is isolated, electrical tests are carried out to ﬁnd

faults or to provide data which can be used as a benchmark or trended over time.

 Dynamic (Energised) Electrical Testing –

 This will include live testing, analytics and compliment static testing

Although static tests are discussed, there are three important dynamic tests which have been

included, these are supply voltage, frequency and phase rotation.

The industrial and utility market is driven by the simple need to keep production moving

without interruption. There are many other reasons to test rotating machines, these include:

 Safety - people and property

 Compliance with standards and regulations

 Reduce downtime

 Save money / time – plan downtime to repair or replace

 Save energy

 Maintain service to the end user

 Maintain critical services

 Maintain performance / productivity

 Trend data to predict failure

 Lifetime (or end of life) planning

 Research, development, design, prototyping

 During and after manufacture

 On receipt

 Prior to installation

 Commissioning

 Maintenance

 After servicing

 Fault ﬁnding in situ

 Fault ﬁnding on the bench

 During the repair process

14 Guide to Motor testing

+ 30 hidden pages