How to Check for a Bad DC Electric Motor

DC (direct current) electric motors are used in many applications. The majority of these motors are used in automobiles for moving windows and passenger seating. Many times these motors are placed in concealed locations that can make troubleshooting the electrical integrity a bit challenging. Removing these motors for checking them may require the manufacturer's instructions for installing or replacing the motors, but performing a check on the DC motor's integrity can be executed by following some basic procedures.

Remove the electric motor from its mounted position and all electrical power that operates the motor. This may entail following a manufacturer's list of instructions for physically removing the electric motor from its current location.

Test the motor's continuity with the volt ohmmeter. Switch the meter to the “ohms” position and place the red and black leads into the meter's connection. The red lead should be connected to “ohms” and the black lead into the “common” connection point. Test the meter by touching the two leads together; the meter should read zero ohms or full continuity.

Touch the leads of the meter to the two motor leads. The meter should read a low resistance in the area from 10 to 30 ohms. If the meter reads an infinite ohms reading or an open circuit, rotate the end shaft of the motor. The meter should give various ohm readings as the motor shaft is rotated. This type of ohm reading will indicate that the motor is good and the problem resides with the electrical circuit itself and not the DC motor. If the meter still reads as an open circuit after the shaft has been rotated, the conducting brushes may be bad.

Use the screwdriver and remove the carbon brushes from the end of the motor. The carbon brushes will be located under the two plastic end caps. These end caps are generally on the opposite end of the motor from the metal driveshaft. The carbon brushes will be attached to a spring. This spring will have a copper braided wire that runs through the centre of the spring. The copper wire will be electrically connected to the carbon brushes.

Inspect the carbon brushes for any cracks or breaks on the surface. The area of the carbon brush that rides against the copper conductor or commutator inside the motor should be smooth in the finish and curved. This curved portion of the brush will fit or ride along the curved surface of the conductor or commutator. Any broken wire or spring on the brush assembly will cause the motor to fail and not operate. If both brushes are in good condition, the problem may reside with the copper commutator itself.

Use the screwdriver to remove the rear end cap on the electric motor. There are two long screws that run the length of the motor, and once these screws are removed, the end plate can be pulled from the outside motor assembly.

Inspect the copper plates that make up the commutator assembly. There should be a distinct opening between each wedge of copper plate that conducts the electrical power to the windings on the motor shaft. Be sure to check for any broken wires from the commutator plates and the motor coil windings. Also check for any burnt varnish on the motor coils, as this could indicate a failure of the insulation between the coiled wires. This type of damage can only be repaired by replacing the motor windings.

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