Under its maintenance care, the plant has 140 3-phase motors ranging from 5 hp to 50 hp. The plant also has large motors for its plant air system, but that system is under the care of a contractor.

The new plant engineer, who recently hired you, just finished a review of the plant’s motor failure 10-yr history. In the air compressor system, the last failure was seven years ago, and it was a vibration problem resulting from a slightly bent shaft.

In the balance of plant, however, it looks like no motor lasts more than a couple of years before needing to be replaced. In looking over the repair summary, he noticed “shorted winding” was nearly always the cause.

How might you determine the cause?

Since the compressor motors aren’t failing and that load is too small to justify its own service, you can rule out most power supply problems. However, check the feeders supplying the balance of plant motors and look for voltage imbalance, and then check a few motor supply branch circuits for the same.

The most likely cause is incorrect lubrication. If you’d had bearing failures, the causes would have probably included using incompatible greases. Adding too much grease or adding grease without opening the grease drain on the motor will result in grease being spun into the windings. As it heats up and liquifies, the grease will create short circuits across the windings even if the insulation isn’t damaged; that insulation is not designed for submersion in oil (the active component in grease).

Go out on motor lubrication PMs and observe how they are being performed. You will likely identify the root cause.