Inductance, a good thing to test

So. You’ve done a megger test and all is fine. You tried ohmeter across phases of motor leads and find 0.05 on all, so discover you cannot tell much of value by an ohmmeter test other than an open winding. What next?

If you are in a maintenance department at a company and have at least a small budget, or you are your own repair guy, a $ 50 inductance meter can be your friend!

Consider: Ohm reading on a typical 10+ hp motor will be LESS than 1 ohm in most cases: Unless you have a wheatstone bridge ohm detector device, your typical vom will have more than that resistance in its leads and connectors!

Consider: Inductance on the other hand can be 50mh on a 10hp motor, and still 2-3mh on a 100+ hp motor. A cheap inductance meter will read significantly down to less than 0.1mh!

Just check inductance across all 3 windings and compare answers. If any reading is significantly different than the others, it is a good indication that that winding has an issue. A lower reading on one phase is usually a partially shorted winding. I have found more than 1 motor with partially shorted windings this way.

Since you probably won’t know the motor’s rated inductance anyway, a cheap inductance meter should do fine since you are ony looking for relative differences. And with most motors having inductance 10 or 100 times higher than what even the cheapest meters will register, it gives good diagnostic data with a low cost tool.

By Mike Kilroy

Mike, thank you for the great primer reminding us in the detailed area of testing and troubleshooting these tricky areas.

Another cheap test that can be used on any small induction motor is to apply a small DC voltage to each phase and compare the current on all 3 phases. A 12V power supply is plenty, but heck even a battery would do. Also look for stray voltage on the case, which signals damaged insulation on the wire.

Note that when you do this, the rotor will leap abruptly to some position and lock there. Best not to have anything attached.

For anyone looking for a much more comprehensive discussion of test techniques, you can’t do much better than Rosenberg’s Electric Motor Repair (Reinhardt Press). It’s out of print, so it is best to look for it in used book stores like Alibris and Abe.