Testing Electric Motors

Testing electric motors in the field really only requires two meters - an ohmeter and a megohmeter.

First, the best place to test the motor is in the connection box with the leads disconnected. If that's not possible, test as close to the motor as possible. The more wire and electrical devices you have between you and the motor, the more you have to assume there are no problems with them. We all know what happens when we ASS-U-ME. I've seen many motors removed from service because someone had a cable problem outside of the motor.

To test the windings, use an ohmeter. Make sure any jumpers for voltage selection remain connected. For a nine lead, dual voltage motor connected for 480 volts, these would typically be the 4-7, 5-8, and 6-9 connections. Measure the resistance from 1-2, 2-3, and 3-1. The resistance readings should be between 1-2% of each other - in other words, BALANCED! There is no way to tell how much resistance you should get, but it is typically low. Realize that you are testing with a DC battery. Therefore, you are reading the resistance of the copper wire. In many cases, this will look like a short circuit. GOOD! It's when it doesn't look like a short circuit that you have problems. As long as the readings are low and balanced, you're ready to go to the next step. I can't tell you how many motors are brought to us because "the windings are shorted." Don't make that mistake.

To test the insulation system, use a megohmeter. This is commonly called a "megger", but that is a registered trade name belonging to Biddle, the long-time megohmeter manufacturer. Without permission from Biddle, I use "megger" as a generic name for my tester. Everybody recognizes it, and it takes less typing. When using a megger, make sure there is an open set of contacts between you and any upstream electronic device, such as a VFD, so you don't send 500VDC into some sensitive electronic components. Making sure that the voltage selection jumpers are connected, set your megger for 500VDC to test motors of a 230/460 nature. 1000 volts is not better - it's wrong! Connect one megger lead to the ground lug in the connection box, connect the other megger lead to any motor lead (we'll come back to that). Turn the megger's crank, or push the button if you have a digital tester. You should read hundreds of megohms. If you get a zero reading. Stop testing - the motor is shot. Many times I do this test first. If I get a bad reading, I don't have to do anything else.

The type of readings you can get vary widely. If you get under 5 megs, it's bad. From 5 through around 100 megs means your insulation is about to fail completely, or you have nasties living in you motor. The nasties can be water, oil, excess bearing grease, or anything else that will provide a high resistance path over the surface of the windings to ground. Many times, a motor with these readings can be steam cleaned, baked dry, and retested with marked improvement in the insulation readings. What we are trying to determine is if the insulation system is good. This consists of the enamel extruded on the magnet wire when it is made, the insulating paper we put in the motor slots and between phases, and the varnish that glues it all together. Don't let a dirty motor get in the way of proper readings.

OK, back to the reason we only need to connect the megger to one lead. The motor windings are connected to each other in either a wye or a delta configuration. This is an economical decision made by the motor manufacturer. No, it doesn't matter how the secondary of the upstream transformer is connected. Most American motors will be wye connected (wye not?) up to about 25HP. Above that, they are typically delta. Most IEC motors will be delta because of their way of voltage selection. That's for another post. The point is, the motor windings are connected phase-to-phase. Therefore, you can get from any point in the windings to any other point in the windings without leaving the motor. So, the test current generated by applying the megger's DC voltage can "find" a problem anywhere in the windings.

By the way, some motors are connected externally, not internally. This includes motors made for wye-delta or part-winding starting, among others. This requires a modification of our testing methods. Stay tuned, and we'll have another post covering motor connections in the future.

So here's your Final Exam:

To test the windings for continuity, use:
An ohmeter
A megger

To test the insulation, use:
An ohmeter
A megger

Did you pass?


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thanks for the tips man, i really enjoy reading your blog…so many great stuff…
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Anand Chikkodi said...

This is only for starters. Pls throw some light what are the test involved as per std and why it is done.


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How high should the test voltage be for a 3 phase, 575 Volt motor? I think that the megohmmeter has a 500 and 1000 Volt setting.

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