Application Note

Three-phase motors have fewercomponents that may malfunc-tion than other motor types.Therefore, 3ø motors usuallyoperate for many years withoutany problems.

If a 3ø motor is the problem,the motor is serviced or replaced.Servicing usually requires that themotor be sent to a motor repairshop for rewinding. If the motoris less than 1 HP and more than5 years old, it is replaced. If themotor is more than 1 HP, but lessthan 5 HP, it may be serviced orreplaced. If the motor is morethan 5 HP, it is usually serviced.

Troubleshooting three-phase motorsThe extent of troubleshooting a3ø motor is dependent upon themotor’s application. If the motoris used in an application that iscritical to the operation or pro-duction, testing is usually limitedto checking the voltage at themotor. If the voltage is presentand correct, the motor isassumed to be the problem.Unless it is very large, the motoris usually replaced at this time soproduction can be resumed. Iftime is not a critical factor, fur-ther tests can be made in orderto determine the exact problem.

Three-phase motors

F r o m t h e F l u k e D i g i t a l L i b r a r y @ w w w . f l u k e . c o m / l i b r a r y

See Figure 1. To troubleshoot athree-phase motor, apply the fol-lowing procedure:1. Using a voltmeter, measure

the voltage at the motor ter-minals. If the voltage is pres-ent and at the correct level onall three phases, the motormust be checked. If the volt-age is not present on all threephases, the incoming powersupply must be checked.

2. If voltage is present but themotor is not operating, turnthe handle of the safetyswitch or combination starterOFF. Lock out and tag thestarting mechanism per com-pany policy.

ON

OFF

OFF

ON

2

5

34

6

1

TURN OFF ANDLOCK OUT POWER

L1

L2

L3

AND LOCKTURN OFF

LOADDISCONNECTON AND TRY

TURN POWER

T5

C

A B

T2

T8T4

T1

T7

T9

T6 T3

T3T6

T9

T7

T1

T4

T8

T2

B

A

C

T5

IF ZEROSHORTED

- COIL IS

- COIL ISOPEN

IF INFINITY

CHECK FOR OPENOR SHORTED WINDING

STARTING MOTOR

DUAL-VOLTAGE,DELTA MOTOR WYE MOTOR

DUAL-VOLTAGE,

INCORRECT

CORRECTIF NOT WITHIN 10%

- VOLTAGEOF RATED VALUE

- VOLTAGEOF RATED VALUEIF WITHIN 10%

OUT POWER

TROUBLESHOOTING THREE-PHASE MOTORS

AT MOTOR TERMINALSMEASURE VOLTAGE

Figure 1. Troubleshoot three-phase motors with an ohmmeter.

2 Fluke Corporation Three-phase motors

3. Disconnect the motor from theload.

4. After the load is disconnected,turn power ON to try restart-ing the motor. If the motorstarts, check the load.

5. If the motor does not start,turn it OFF and lock out thepower.

6. With an ohmmeter, check themotor windings for any opensor shorts. Take a resistancereading of the T1-T4 coil.This coil must have a resist-ance reading. If the reading iszero, the coil is shorted. If thereading is infinity, the coil is

opened. Since the coil wind-ing is made of wire only, theresistance is low. However,there is resistance on a goodcoil winding. The larger themotor, the smaller the resist-ance reading.

After the resistance of one coilhas been found, the basic elec-trical laws of series and parallelcircuits are applied. When meas-uring the resistance of two coilsin series, the total resistance istwice the resistance of one coil.When measuring the resistanceof two coils in parallel, the totalresistance is one half the resist-ance of one coil.

Troubleshooting guidesTroubleshooting guides for motorsstate a problem, its possiblecause(s), and corrective action(s)that may be taken. These easy-to-reference guides, while general in nature, may be usedto quickly determine potentialproblems and possible courses of action. See Figure 2.

Figure 2. Troubleshooting guides are used to determine problems and possible courses of action.

Troubleshooting Guide for Three-Phase MotorsProblem Possible Cause Suggested Test Tool Corrective Action

Motor will not start Wrong motor Most three-phase motors are dual-voltage. Check for proper motor connections. connections

Blown fuse or Basic electrical tester, Test the OCPD. If voltage is present at the input, but not the output of the OCPD, the fuse isopen CB DMM , clamp meter, blown or the CB is open. Check the rating of the OCPD. It should be at least 125 % of the

or megohmmeter motor’s FLC.

Motor overload on Allow overloads to cool. Reset overloads. If reset overloads do not start the motor, starter tripped test the starter.

Low or no voltage Basic electrical tester, Check the voltage at the motor terminals. The voltage must be present and within 10 % of theapplied to motor DMM or clamp meter motor nameplate voltage. If voltage is present at the motor but the motor is not operating,

remove the motor from the load the motor is driving. Reapply power to the motor. If the motor runs, the problem is with the load. If the motor does not run, the problem is with the motor. Replace or service the motor.

Open control circuit Basic electrical tester, Check for cleanliness, tightness, and breaks. Test the circuit starting with the incoming powerbetween incoming DMM or clamp meter and moving to the motor terminals. Voltage generally stops at the problem area.power and motor

Fuse, CB, or overloads Power not applied to Basic electrical tester, Measure voltage at each power line. Correct any power supply problems.retrip after service all three lines DMM or clamp meter

Blown fuse or open Basic electrical tester, Test the OCPD. If voltage is present at the input, but not the output of the OCPD, the fuse isCB DMM , clamp meter, blown or the CB is open. Check the rating of the OCPD. It should be at least 125 % of the

or megohmmeter motor’s FLC.

Motor overload on Allow overloads to cool. Reset overloads. If reset overloads do not start the motor, starter tripped test the starter.

Low or no voltage Basic electrical tester, Check the voltage at the motor terminals. The voltage must be present and within 10 % of theapplied to motor DMM or clamp meter motor nameplate voltage. If voltage is present at the motor but the motor is not operating,

remove the motor from the load the motor is driving. Reapply power to the motor. If the motor runs, the problem is with the load. If the motor does not run, the problem is with the motor. Replace or service the motor.

Open control circuit Basic electrical tester, Check for cleanliness, tightness, and breaks. Test the circuit starting with the incoming powerbetween incoming DMM or clamp meter and moving to the motor terminals. Voltage generally stops at the problem area.power and motor

Motor shaft does Disconnect the motor from the load. If the motor shaft still does not turn, the bearings arenot turn frozen. Replace or service the motor.

Motor overheats Motor is single Basic electrical tester, Check each of the three-phase power lines for correct voltage.phasing DMM or clamp meter

Improper ventilation Infrared temperature Clean all ventilation openings. Vacuum or blow dirt out of motor with low-pressure, dry,compressed air.

Motor is overloaded Basic electrical tester, Check the load for binding. Check shaft straightness. Measure motor current under operatingclamp meter or DMM conditions. If the current is above the listed current rating, remove the motor. Remeasure thewith clamp accessory current under no-load conditions. If the current is excessive under load but not when unloaded,

check the load. If the motor draws excessive current when disconnected, replace or service the motor.

Excessive harmonics Power quality Check for the presence of harmonics in the feeder supplying the motor, especially 5th harmonic analyzer which can generate heat rise.

3 Fluke Corporation Three-phase motors

This article is based on materialexcerpted from Electric MotorDrive Installation and Trou-bleshooting, TroubleshootingElectric/Electronic Systems,

2nd Edition, Power Quality Measurement andTroubleshooting, and Electrical Motor Controls,2nd Edition published by American TechnicalPublishers, Inc. To obtain information on relatedtraining products, visit the American Tech website at www.go2atp.com.

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