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 motors, it can be very difficult to complete any repairs or maintenance on them without having to disassemble whatever it is that you are powering. That is why once you have gained access to your DC electric motor you should always quickly check it to see if it has broken down and needs to be replaced.
Begin by removing the DC motor from its bracket, making sure you have also removed any sources of electrical power that could accidentally cause it to start spinning. You may need to follow the manufacturer’s instructions to do this, as some motors are too tightly locked in position and could pose a risk of electrocution.
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 put the red and black wires into their connections (the red wire should be connected to the “ohms” and the black wire to the “common” point). Test that the meter is working properly by touching these two wires together; the display should read zero ohms (or full continuity).
To test your DC motor, touch the ohmmeter leads to the motor leads. The meter display should indicate low resistance (somewhere between 10 and 30 ohms), but if it reads infinite ohms or an open circuit, you should rotate the shaft at the end of the motor. The ohmmeter should give different readings as this shaft is rotated (which is an indication that the electric motor itself is fine, but that there is a problem with the electrical circuit. They have gone bad).
Use a screwdriver to remove the brushes from the end of the electric motor (you can find them under the plastic caps on the opposite end of the motor from the driveshaft). Carefully inspect the brushes for any 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 fine, then the problem may be with the commutator.
Take the screwdriver again and use it to remove the rear cover of the DC electric motor (removing the two screws that run the length of the motor). Inspect the plates that included the switch assembly; there should be an opening between each one. If you notice any broken wires or burned varnish, the switch has failed and its damaged parts will need to be replaced.