Motor Troubleshooting: Failure To Rotate

July 29, 2004
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As we mentioned at the end of last month's motor quiz, we are focusing on HVACR motor troubleshooting specifics. This month we will discuss troubleshooting if the motor isn't rotating; next month we will examine how to troubleshoot a motor if it isn't coming fully up to speed or accelerating.

What We Have Here Is A Failure To Rotate

The first tool you need to use is one of your most important: your ears. Is the motor humming?

If the motor hums, the problem could be:

  • A bad run capacitor - Check the capacitor or replace it with a known good one of the same microfarad (MFD) rating. Consider using a higher voltage rating.

  • A bad start capacitor - Check the capacitor or replace with a known good one of the same MFD rating. Look for oil leaks.

  • If it's in a rapid start-stop application, get a motor or capacitor designed for rapid starts and stops.

  • A bad starting switch - That is, if you spin the shaft, the motor will rotate. Replace the motor.

  • One winding is open - Again, if you spin the shaft, the motor will rotate. Replace the motor.

  • Bad bearings - You can move the motor shaft side to side. Replace the motor.

  • Improper connections - Verify that the motor connections are correct; check amps after you make any corrections, but be aware that you may need to replace the motor anyway.

    If the motor makes no sound, carefully measure power to motor.

  • No power to the motor could mean there is a blown fuse or circuit breaker. Verify that the fuse or circuit breaker size is correct. The size should be 3 X the nameplate amps for a fuse; 7 X the nameplate amps for an instantaneous breaker; 2.5 X the nameplate amps for a time delay breaker.

    If it's the correct size, replace the motor.

  • If there is power to the motor, and the motor is cool, check for open leads or an open winding. You may be able to see winding damage on inspection. It can be verified with your multimeter; you should read continuity between all of the leads.

    If there is no continuity, replace the motor.

  • If the thermal overload tripped, the motor will be hot to the touch. Wait for the motor to cool down! When the protector resets (or is reset), look for other symptoms.

    Next month Tech Tips will look at troubleshooting a motor that will not come up to speed or accelerate.

    For more information, click on the Emerson Climate Technologies logo above.

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