Preventive maintenance in induction motors

Preventive, Which test we use on induction motors on 380 V?
I realized that just insulation resistance is enough.
But, the books and others literatures show another tests like: Polarization index, ohmic resistance, unbalanced current and others less ordinary like: loop test. I like to be efficient and simplify the job for techs. I'm afraid because, maybe, I should be flexible a lot.

Q: What to you make in your company?

In general, the most common failure mode (>> 50%) of low voltage induction motors is bearing failure. Assuming that no special circumstances apply, i.e. no mission critical system, I would recommend to limit preemptive maintenance to vibration monitoring. This can either be done by using accelerometers, i.e. actual measuring the vibration spectrum. Keeping continuous logs of these vibration measurements will give you a good idea of when to expect a failure (and may even allow you to spot motors with broken rotor bars). Vibration monitoring can also be done in a more crude way using your hands and ears, if you do for example a weekly check of your motors.

Electric preemptive tests don't pay off - according to my experience. Winding failures are rather rare and if that happens you have to replace/rewind the motor anyway. Furthermore, none of the mentioned tests will be able to tell you that an electric failure will happen in the near future. A motor may check out perfectly OK today and fail electrically tomorrow. (Online partial discharge testing would perform better in this respect, but is most likely to complex and costly.)

In my experience, it is more efficient to make sure that you - in the unlikely case of an electric failure - minimize the downtime instead.
Which means make sure that you have a replacement motor at hands (or a reliable source able to deliver one fast), tools available and the technicians trained in replacing them. At least this approach worked very well for me, but of course, this depends a lot on your specific work environment.
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