How To Check Whether Your DC Electric Motor Has Gone "Bad"
electric resistance

These days, direct current (or DC) electric motors are used in a wide range of applications, such as the moving windows and seats in your car. Because of the concealed nature of these motors, it can be highly difficult to complete any repairs or maintenance on them without having to pull whatever it is powering apart. This is why, once you have managed to get to your DC electric motor, you should always give it a quick check to see whether it has gone “bad” and needs to be replaced.

Begin by removing the DC motor from its mount, ensuring that you have also removed any source of electric power that could accidentally cause it to begin turning. You may need to follow the manufacturer’s instructions to do this, as some motors are very much wedged into position and could pose a risk of electrocution.

Next, you can test the electric motor’s continuity (or connection) by attaching it to a volt ohmmeter. Ensure that the meter is in the “ohms” position, then place the red and black leads into its connections (the red lead should be attached to the “ohms” and the black lead to the “common” point). Test that the meter is working properly by touching these two leads together – the screen should read zero ohms (or full continuity).

To test your DC motor, touch the leads of the ohmmeter to the leads of the motor. The meter’s screen should indicate a low resistance (somewhere between 10 and 30 ohms), but if it reads an infinite ohms or an open circuit you should rotate the end shaft of the motor. The ohmmeter should give different readings as this shaft is rotated (which is an indication that the electric motor itself is good, but that there is a problem with the electrical circuit. If the meter is still reading as an open circuit, the conducting brushes may have gone “bad”.

Use a screwdriver to remove the brushes from the end of the electric motor (you can find them under the plastic end caps at the opposite end of the motor to the drive shaft). Carefully inspect the brushes for any sign of cracks or breaks in the surface – the area of the brush that sits against the conductor or commutator should be smooth and curved. If there are any broken wires or springs, the motor will fail. If the brushes appear fine, then the problem may be with the commutator.

Take the screwdriver again and use it to remove the rear end cap of the DC electric motor (by removing the two screws that run the motor’s length). Inspect the plates that comprise the commutator assembly – there should be an opening between each. If you notice any broken wires or burnt varnish, the commutator has failed and its damaged parts will need to be replaced.