When an electric motor fails to start, runs intermittently, runs hot, or continually trips its overcurrent device, there my be a variety of causes. Sometimes the problem lies within the power supply, including the conductors of the branch circuit or motor control.
- General Inspections
- Earth Continuity and Resistance Test
- Power Supply Test
- AC Motor Winding Continuity Test
- AC Motor Winding Resistance Test
- Insulation Resistance Test
- Running Amps Test
General Inspections
Do the following for the three-phase motor:
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Check the motor’s appearance. Search for burned, body damage, or fan or shaft cooling.
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Manually rotate the motor shaft to check the condition of the bearing. Look for smooth, free rotation of the shaft. If the rotation of the shaft is free and smooth, it may be in good condition, otherwise consider replacing, repairing or removing the shaft.
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The motor name plate, as with all checks and inspections, provides valuable information to help assess the motor’s true health. Examine carefully the name plate and compare the values of the running amp test (see below) with the value of the name plate.
Earth Continuity and Resistance Test:
Measure the friction between the motor frame (body) and earth with a multimeter. A balanced engine should be reading less than 0.5 ohms. Any value greater than 0.5 ohms indicates a motor problem. Further problem-solving may be needed.
Power Supply Test:
For three phase motors, the expected voltage between each of the three phase supply lines for a 230/400V system is 230V to neutral and 400V. Test that a multimeter is used to apply the correct voltage to the motor.
Ensure the power supply terminal is in good condition. Check the terminal connection bar (U, V, and W). The connection type is either Star (Y) or Delt for three-phase motors.
AC Motor Winding Continuity Test:
Check the consistency of motor winding from stage to phase (U to V, V to W, W to U) using a multimeter. Unless winding is OK, each phase to phase must be continuous. If the continuity test fails in any particular phase, the engine will probably be burned.
AC Motor Winding Resistance Test:
For phase-to-phase terminals (U to V, V to W, W to U), test the motor winding resistance or ohms reading using a multimeter or ohmmeter. The ohms reading for each winding must be the same (or nearly the same). Know the three steps have the same or almost the same windings.
Insulation Resistance Test:
The loss of an electric motor’s insulation resistance is one of the first signs the motor is about to fail. For a three-phase motor, isolation resistance is usually measured by means of an insulation tester or megger between each motor winding or phase and between each motor stage and motor frame (earth). Set the insulation resistance tester’s voltage level to 500V.
Check motor frame (earth) from phase to phase (U to E, V to E, W to E). Minimum test value of motor insulation resistance is 1 Meg Ohm (1 MΩ).
Running Amps Test:
Check the full load amps (FLA) with an appropriate meter or preferably a clamp-on meter with the nameplate FLA when the motor is running. Deviations from the rated FLA could lead to the engine being tested.
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