Tech Tips: How to Check for a Damaged Armature

Here are three quick checks you can perform with a volt/ohm meter to test a DC motor armature winding to determine if a motor armature is functioning properly.


Hi I’m Jim. I’m a design engineer here at Groschopp and I’m here with a Groschopp Tech Tip. For today’s Tech Tip, we’re going to cover how to measure an armature for broken or damaged windings. On this armature, we have a shaft and a commutator, which has bars. The commutator bars are connected to the armature winding and the winding is wound around the teeth on the lamination stack. This creates an electromagnetic effect, which interacts with the permanent magnets in the motor housing, causing the motor to turn. We also have an insulation system isolating all of these elements from ground.

At Groschopp, to check for damaged or shorted windings, we have three measurement techniques. The first is called the 180-degree test and as the name implies, we are going to measure the resistance of the windings on commutator bars 180 degrees apart. And we will be measuring all of the windings that are in series, looped all the way around from bars opposite each other. On this particular measurement, we’re reading about 0.6 ohm. The actual value isn’t important. What’s important is that every time we perform this measurement, going all of the way around, rotating our armature, that it stays consistent. If it varies radically, goes to zero, or open circuit, that would indicate a damaged winding.

The next test we’ll do is a bar-to-
bar test, which measures each individual loop. And again, as the name implies, the bars adjacent to each other, we measure 0.3 ohm, 0.4. Now, you may not know what your armature design is going to read. Again, it’s just important that they do not vary radically.

The last test is the bar-to-ground test. In this test we’re measuring the resistance of every bar to ground, in this case the armature shaft. And we never want to have continuity between any of the bars and ground. It’s always supposed to be open circuit.

If your measurements fail any of these, it’s very possible the armature has broken or damaged windings and will not function properly. This has been a Groschopp Tech Tip. If you’d like more information about fractional horsepower motors, please visit Groschopp.com.

Read our blog post about how to check an armature for damaged windings: https://www.groschopp.com/how-to-check-a-motor-armature/

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