Fault Finding – Three Phase Induction Motors

ProblemPossible CauseTestsSolution
Motor will not start.Fault with supply. Motor or load locked up. Wrong connections in control circuit.Check for correct voltage at motor terminals. Make sure motor and load are free to turn. Check to ensure contactors operate.Fit new fuses, reset circuit breakers, etc. Remove clamps, locks, etc. Sort out control circuit.
Supply or Started trips out at start.Wrong or loose connections. Motor overloaded. Intertia of load to high. Low Voltage due to volt drop in cables Overload or circuit breaker incorrectly set or sized.Check all lugs are properly crimped or soldered, and connections are tight. Check load performance data against motor performance data. Measure voltage at motor terminals while motor starting. Check settings of overload and circuit breaker and allow for starting currentFix up connections. Change motor for correct size. Change cables for correct size. Correct setting of overload or breaker or change.
Motor starts but has no torque. Motor does not reach full speed or takes a long time to accelerate.Incorrect connection. Delta wound motor connect in star. Star/Delta starter staying in Star. Inertia of load to high. Motor overloaded. Low voltage due to volt drop in cables.Check connection diagram and nameplate data. Check load performance data against motor performance data. Measure voltage at motor terminals while motor startingSort out and correct connections. Check timer and starter control circuit. Change motor for correct size. Change cables for correct size.
Motor Overheating.Motor overloaded. Ineffective cooling. Excessive ambient. Wrong connection. Delta wound motor in star. Motor “Single Phasing”. Wrong voltage or frequency. Supply voltage unbalanced.Check load performance data. Check fan and air flow and temperature of air. Look for build up of dirt. Check connection diagram and nameplate data. Check volts and amps in all three phases. Check nameplate Measure phase to phase voltage accuratelyFix problem with load or fit larger motor. Clean motor. Sort out cooling of air temp. and flow. Sort out connections. Restore supply to all phases. Correct voltage or frequency. Balance supply or accept unbalance.
No load amps in excess of Full load ampsIncorrect connection Star wound motor connection Delta. Voltage in excess of nameplate. Motor supplied for different voltage or frequency.1 & 2. Check connection diagram and nameplate data.

3. Measure voltage at motor terminals.
4. Compare supply voltage and frequency to nameplate.
1 & 2. Sort out and correct connections at motor terminals.
3. Correct supply voltage
4. Change motor for correct voltage and frequency
Mechanical Noise or Vibration. Noisy bearings. Bearings overheating.Thrust from load or misalignment. Damaged bearings, too much grease, no grease, or foreign matter in grease. Rotor pulling or foreign matter in air gap. Out of balance load, coupling or pulley. Excessive belt pull. Motor foundations not rigid.1. Check gap between coupling halves and alignment.
2 & 3. Turn shaft slowly by hand and feel for roughness or stiffness. Check for bent shaft or fan rubbing.
4. Run motor disconnected from load and then with pulley or coupling removed.
5. Run motor without belts.
6. Check design and construction foundations
1. Re-align couplings
2 & 3. Clean bearing housing, change bearings and repack with fresh grease.
4. Fix up out of balance items
5. Loosen belt tension
6. Increase strength of foundations
Motor amps in excess of nameplate full load amps on loadMotor overloaded. Low supply voltage. Wrong voltage or frequency. Wrong Connections. Motor ‘Single-Phasing’. Supply voltage unbalanced. Motor Speed not matched to load.1. Check load and performance data.
2. Measure voltage at motor terminals
3. Check nameplate.
4. Check nameplate
5 & 6. Check volts and amps in all three phases.
7. Measure motor speed and check load speed requirements.
1. Fix problem with load or fit larger motor.
2. Fix problem, maybe with larger cables.
3. Correct voltage or frequency.
4. Sort out and correct.
5 & 6. Restore balanced supply to all three phases.
7. Change motor for correct motor speed.
Excessive electric noiseWrong connections. Wrong voltage. Motor ‘Single-Phasing’.1. Check connections
2. Check voltage with nameplate
3. Check volts with amps in all three phases.
1. Fix up connections
2. Correct voltage.
3. Restore supply to all three phases.
Unbalanced amps in different phases when motor loadedUnbalanced supply voltageMeasure phase to phase voltage accuratelyBalance supply or accept unbalance
Motor runs in wrong directionWrong connections.Watch shaft rotationSwap two phases of supply.
Fault Finding – 3-Phase Induction Motors
3-Phase Induction Motor

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