Today, direct current (or DC) electric motors are used in a wide range of applications, such as moving windows and seats in your car. Due to the hidden nature of these engines, it can be very difficult to complete any repairs or maintenance on them without having to take apart whatever it is that is powering it. That’s why, once you’ve managed to get to your DC electric motor, you should always give it a quick checkup to see if it has “broken down” and needs to be replaced.
Start by removing the DC motor from its mount, making sure you’ve also removed any sources of electrical power that could cause it to accidentally start spinning. You may need to follow the manufacturer’s instructions to do this, as some motors are very stuck in position and could present an electrocution hazard.
You can then test the continuity (or connection) of the electric motor by connecting it to a voltmeter. Make sure the meter is in the “ohms” position, then attach the red and black leads to their connections (the red lead should be connected to the “ohms” and the black lead to the “common” point). Test that the meter is working properly by touching these two wires together; the display should show zero ohms (or full continuity).
To test your DC motor, touch the ohmmeter leads to the motor leads. The meter display should indicate a low resistance (between 10 and 30 ohms), but if it reads infinite ohms or an open circuit, you need to rotate the motor’s final shaft. The ohmmeter should give different readings as this shaft is rotated (which is an indication that the electric motor itself is fine, but there is a problem with the electrical circuit. If the meter continues to read as an open circuit, the conductive brushes may have gone “bad”.
Use a screwdriver to remove the brushes from the electric motor end (you can find them under the plastic caps on the opposite end of the motor from the drive shaft). Carefully inspect the brushes for signs 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 broken wires or springs, the motor will fail. If the brushes seem okay, 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 length of the motor). Inspect the plates that include the commutator assembly; there should be an opening between each. If you notice any broken wires or burned varnish, the switch has failed and its damaged parts will need to be replaced.