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Damaged Armature: Telltale Signs & Testing Methods

There are different parts to an electric motor: the stator, bearings, a series of belts or gears, a commutator, and last but not least, a rotor or armature.

The two; rotor and armature, are similar yet completely different. The former is a part of an electric motor which rotates, able to have bars which conduct current, may be wounded, or just remain as a rotor. Whereas the latter consists of bars which conduct current and brushes that opens up an electrical path for the current.

While both parts are equally vital in their own way to a motor, we’ll be focusing on the armature in this article. With a damaged armature, the efficiency of your motor can be heavily affected. Read on to find out the negative impact caused upon your motor and specific ways on how to check on an armature’s condition.

Signs of a bad armature

  • Worn commutators: This happens due to the friction of carbon brushes against the commutator surface. Eventually, this will gradually affect the condition of the brushes as well, causing it to wear down rapidly.
  • Burnt-out armature: It may be the result of several issues like poor airflow, overloading, stalling, earthing, insulation breakdown, regulator failure, and so forth. If a burnt-out armature is the problem, you will have to rewind the armature.
  • Earthing: It occurs when part of the winding connects with the metal core of the armature. Usually, this is the case when insulation breaks down, either through overheating or fatigue on the edge of a slot due to constant cooling, heating, and spinning.

Ways to check for a damaged armature


Basically, it is an electrical device used for detecting shorted coils in motors. What it does is make a magnetic flux which causes a shorted armature to put a current into the feeler. If your armature is in bad shape, the feeler will start vibrating in tune to the current produced.

Inspection plug brushes

One common occurrence which is usually seen in a damaged armature is the number of attempts needed to get the motor to turn on. At first, two or three attempts are sufficient to turn it on, but in time to come, it will not be able to be switched on entirely.

Once you take a look at your inspection plugs and see that the brushes have been damaged, then it’s highly likely that the armature is the culprit. To double-check and see if that is really the case, simply install new brushes and see if they will be worn out and damaged too.

Specific testing methods

According to Grosschopp, there are several ways we can check the condition of the armature before we decide to a complete electric motor overhauling. Below, we’ve roughly summarised the different testing methods which you can try out.

180° resistance test

With an ohm/voltmeter, you can take a look at the resistance value of the series windings linked between the two commutator bars of every coil.

Afterwards, adjust the meter to Ohms and then take the measurements of the resistance from the two commutator bars, specifically 180° away from one another. Then, rotate the armature and take down the resistance value between every set of two bars on the commutator.

While it is not possible to determine the armature’s exact resistance value, every measurement should amount to the same number. If you notice that the resistance amount differs from one another greatly, there may be an issue in the windings.

To be precise, a decrease in resistance value could mean that there may be a shorted wire within the coil. While a sudden increase in resistance value may mean that the wire is broken or burned through, causing interruption to the circuit.

Bar to bar resistance test

Just like the previous test, you have to check if every measurement is around the same value.

The only difference between this test and the previous test is you’ll be checking the measurement of one coil, not the resistance of every one of the coils combined, pair by pair between the two bars; which explains the much lower resistance value.

The conditions of a damaged armature remain the same as well; keep a lookout for any drastic increase or decrease in resistance value.

Commutator bar resistance test

The last test you can do is take down the resistance value of each commutator bar to the iron armature stack.

Using force, press the motor armature stack directly onto the armature shaft so you can take the measurements from the armature shaft. Even then, in certain situations, the armature will be insulated from the armature stack. In the event that such a case occurs, you will need to measure from every commutator bar to the iron armature stack directly instead.

What you should look out for here is for any signs of electrical continuity to the armature shaft and/or armature stack. If you do, it’s an indication of a damaged armature.

Now, we hope that you’ve clearly understood how to check for a damaged armature through these tests. Once, the armature does not pass any of these tests – you may have to consider getting a rewind, replacement, or upgrade. Similarly, just like any of your other equipment, such as your generator, you should do electric generator servicing whenever necessary to keep your equipment at tip-top condition.