Motor Won't Come Up To Speed

August 31, 2004
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What do you check if an HVACR motor is failing to come up to speed or accelerate? Ask yourself the following:

  • Was the motor applied properly? In other words, does it have enough starting or acceleration torque? If it is a shaded-pole motor, you may need to use a permanent split capacitor (PSC) type. (This is particularly true for multi-speed applications that run on lower speeds.) If it is a split-phase motor, replace it with a capacitor-start unit.

  • Is the voltage too low at the motor terminals? Measure carefully! The motor should operate within 10 percent of its nameplate voltage. If you find voltage lower than 90 percent of what is marked on the nameplate, use a higher-hp motor or a motor with a service factor.

  • Is the motor overloaded? Check its amp draw. If the amp draw is greater than 110 percent of what is marked on the nameplate, you may need to replace the motor with a higher-hp motor or a motor with a service factor. If it is a belt-load system, consider using a smaller motor pulley or larger load pulley. If there is an adjustable pulley on the motor, adjust it for a smaller diameter.

  • Check the motor's connections; are they loose, damaged, or faulty? High-resistance connections at the terminal board can lower the voltage that the motor sees and can also cause severe overheating. Make sure that all quick connects are snug.

  • Is the belt tight (in cases where the load is connected with a belt to the motor)? Loosen the belt and retighten it to the manufacturer's specs.

  • Are there signs of rotor rub? Does the motor "bark" or "growl" when it is first energized? The bearings might be worn. Check for side-to-side play. If there is side-to-side play, the bearings are worn. Replace the motor.

  • Are the bearings dry? If so, the shaft will be difficult to turn; that means permanent damage usually has already taken place. Again, you need to replace the motor.

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