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Electrical Troubleshooting Quiz - May 16, 2017

May 16, 2017
Getting to the bottom of an electrical motor failure problem.

A new plant engineer recently hired you to oversee electrical maintenance. She told you the motor failure rates here seems high compared to the last three plants she worked at.

The plant has dozens of 2 hp and fractional horsepower motors; these almost never fail. But it seems the plant is replacing a 10 hp, 20 hp, or 50 hp motor just about every week.

A sample motor maintenance PM procedure has the technician conducting vibration analysis, checking for voltage imbalance, conducting insulation resistance testing, checking the mounting bolts, and lubricating the motor. The plant engineer is adding thermography in the near future but otherwise considers the PM complete.

What might you look for to get to the bottom of this motor failure problem?

Smaller motors (typically up to 5 hp) usually have sealed bearings. The larger motors have lubrication fittings. It is your larger motors that are failing, so the cause is probably excess lubrication.

The bearing cavity in any motor should be less than half full. Otherwise, you get additional internal friction that can actually melt the grease and cause it to run out. Another problem from overgreasing is grease gets into the windings and shorts them out. If motors are failing from spun bearings or shorted windings, overgreasing is the likely culprit.

Also, don’t “check” mounting bolts. Replace mounting hardware and torque to spec, if the motor vibrates excessively.

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