Electrical Troubleshooting Quiz, July 23, 2013

Perform these tests on motors in addition to Insulation resistance testing.

Suppose it's been a few weeks since you did the mixer repair we just looked at. You've been vindicated on the E-stop issue, and it's now been confirmed that the recipe does not create undue thickening. The motor, however, has gone through a few sets of overloads since your last visit. Two other techs have replaced the overloads, just as you did. Both conducted insulation resistance testing on the windings, and the readings were good.

The production superintendant wants this downtime problem solved. What should you do?

While it's a valuable part of your motor testing arsenal, insulation resistance testing can reveal some problems and not others. You need to perform other tests on this motor.

With the motor off, conduct:

·        Phase-to-phase voltage measurements with a DMM. You want less than 2% difference between any two phases.

·        Visual inspection of the equipment grounding (bonding) connections.

·        Visual inspection of the mounting system; look for warping and cracking.

While it's running, conduct:

·        Thermal imaging; pay special attention to the thrust bearing.

·        Vibration analysis under varying load conditions.

·        Power analysis on the input, using a portable power analyzer.

After you make any needed repairs, perform this battery of tests again. Once you can perform them in sequence with a "pass" for each one, leave the power analyzer attached while the motor runs. If the overloads open again, you want to capture any power quality anomaly preceding (or occurring during) that event. You will likely identify the culprit.

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