Motor Vibration Or Noise

October 4, 2004
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Excessive or unusual motor vibration and noise can be symptomatic of a number of problems. In order to help you rule out some causes and figure out your best course of action, check for the following on HVACR motors.

Motor shaft alignment with the load:
This is only likely on direct-coupled mechanical loads. Check the equipment manual for alignment procedures.

Unbalanced voltage on three-phase motors:
Measure the voltage of each phase. The voltages on all three phases should be within one percent of the average. If this is not the case, contact an electrician.

Missing phase voltage on a three-phase motor:
This is a severe form of unbalanced voltage. In a nutshell, one phase has no voltage.

Check any fuses and reset the breakers. If the fuses and breakers are good, it could be the transformer. If that seems to be the case, contact a qualified electrician.

High voltage:
If motor voltage exceeds 10 percent of the nameplate rating, the motor can saturate magnetically. This will cause excessive motor hum and excessive temperature, which could shorten motor life. If the voltage is too high, contact the power company.

Starting switch fails to open:
Motors that have an internal switch to remove the starting winding after the motor has come to full speed will exhibit high current, high torque, and lower speed if that switch does not do its job.

The problem can be verified with a multimeter and screwdriver. The multimeter should be connected across the motor terminals when the motor is not connected to the power source. The actuator sleeve should be pushed inward with the screwdriver. The resistance should change. If it does not, replace motor.

Loose pulley or fan:
Make sure the setscrew is tight. Almost undetectable tightening of the setscrew can remove noise.

Defective or unbalanced load:
Check for obvious signs of damage, such as bent or dinged fan blades. Also, does vibration continue after the power is turned off? If so, replace the load.

Defective or loose mounts:
If the motor is mounted using rubber, the rubber can become fatigued over time. Check for tears or other signs of deterioration. If necessary, replace the mounts.

Mounting screws can also loosen over time. Make sure they are tight, and retighten if necessary.

Defective or misaligned bearings:

  • Grab the shaft. Check for side-to-side movement.

  • Listen for a load strike when the motor coasts down or starts up.

  • Listen for bearing noise by spinning the shaft.

  • Defective bearings can be caused by blows to the shaft on installation, or by an excessively vibrating load. Bearings can also wear out.

    If any of the above symptoms are present, replace the motor or item with the bad bearing. In some cases, it may be possible to replace just the bearing.

    Uneven air gap:
    If you hear a "wow-wow" noise when the motor starts, this is an indication that the motor has an uneven air gap. This will usually not affect motor life, but it could cause customer complaints. If that is the case, replace the motor.

    Motor winding grounded:
    If the motor is grounded, other performance parameters will also be affected, including amps and speed. The problem is best checked with megger or megohmeter. It can sometimes be found with a multimeter.

    As a last resort, if the motor ground connection can be removed safely and the noise disappears when the motor is run, the motor is grounded. Replace the motor.

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

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