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?


Greasing Electric Motor Bearings

Over the past years, we have been researching the problem of grease and electric motor bearings. The major problems are the type of grease, the proper application of the grease, and the frequency of application. This is the result of our own research involving grease manufacturers, bearing manufacturers, motor manufacturers, our industry's technical association, and our own experience in our facility.

Probably the least understood part of the problem is the grease itself. Without going into great detail, grease is approximately ninety percent oil and ten percent thickener. The oil does the lubricating; the thickener keeps the oil in place. The problem arises when you mix greases which have different thickeners. The most common thickener, or base, used in today's electric motor bearings has a polyurea base. The most common grease used by maintenance departments has a lithium base. Polyurea and lithium don't like each other. If you mix the highest quality polyurea based grease with the highest quality lithium based grease, the result can be a severe reduction in the effectiveness of the base. The result is that your grease can become pure oil and flow into the motor, leaving you with no bearing lubrication. This explains why we sometimes see motors which are full of oil, the bearings have failed, and the customer says there is no oil anywhere near that motor.

Most bearing manufacturers build their bearings in plants all around the world. Some have codes that define the grease that is in their bearings. But sometimes, these codes are hard to decipher. We specify polyurea based grease in all the bearings we use in motor rebuilding. This brings us in line with the motor manufacturers. The most readily available brands of polyurea grease are Exxon/Mobil Polyrex, Chevron SRI, Shell Dolium BRB, and Rykon Premium #2. Of these, Exxon/Mobil Polyrex is by far the most recommended brand.

Many customers ask when and how to grease their bearings. Our recommendation is to clean the grease fitting and remove the grease relief plug. With the motor running, pump in new grease until clean grease comes out of the relief port. Leave the relief plug out until grease stops coming out; this may take several minutes or more. Reinstall the relief plug. Your bearings will have the proper amount of grease in them. When you see grease coming out of the end bell around the shaft, you have put in far too much grease. The proper fill is about half of the volume of the bearing cavity, allowing for expansion. More motors fail from too much grease than too little. "A couple shots a week" is not a good policy, especially if the relief plug is never removed.

So, how often should you grease your motor bearings? This is where you will find a wide range of answers. We have seen manufacturers that say never grease a shielded bearing, the most common type supplied in new motors. Other manufacturers give varying intervals. Some take duty cycle into consideration, and some don't. Even though this may not be the answer you want to hear, we believe each application has its own timetable. Our suggestion is to develop a schedule based on the condition of the grease that comes out of the relief port. If the first grease that comes out looks exactly like the grease you are putting in, you can extend your greasing interval. If a solid glob of old grease that has to be, like the constipated mathematician, worked out with a pencil, comes out, you need to shorten your interval. This may result in a chart with greasing intervals that vary from motor to motor. As long as you do it properly, you could set your interval based on the "worst case" motor in your plant. This would require more grease, but it would minimize bearing related motor failures.

In today's manufacturing plants, we see a lot more repairing/replacing and a lot less maintaining of electric motors. Hopefully, this approach will help you save down time and cut repair costs.