Small Electric Motor Repairby Josh WeberUpdated July 21, 2017
Troubleshooting a small electric motor that fails is most effective when you follow a step-by-step procedure that rules out potential causes and helps isolate defective circuits or components. As with all modifications on electric or electronic equipment, make sure that you unplug the motor before making any modifications or repairs.
Ensure Motor Has a Power Supply
Check outlet into which motor is plugged to be certain power is available. Check by plugging working lamp into outlet and turning on.
Check circuit breaker if outlet has no power. If circuit breaker is tripped "off," reset to "on" and plug motor back in.
Try to start motor. If motor fails to start, check circuit breaker. If breaker returned to "off' position, the problem is in the motor.
Check motor power cord by removing the two wires that connect to the motor and connecting these temporarily either by splicing together or connecting with a jumper wire. Connect the leads from the voltmeter individually to the two wall plug connectors. With the voltmeter set to RX 1, scale the meter bend and twist the power cord. A reading of 0 ohms indicates the cord is good. A higher reading indicates a short in the cord, and the cord must be replaced.
Check the motor power switch functioning. Disconnect the two wires leading from the switch to the motor and connect these to the leads of the voltmeter and with the meter set to the RX 1 scale, turn the switch "on" and then "off." The meter should jump from 0 ohms in the "off" position to high ohms in the "on" position. If there is no change in reading, the switch is bad and must be replaced.
Check Commutator and Brushes
Remove motor housing by loosening and removing housing screws, exposing field coil and commutator. Inspect commutator surface for dirt, oil, metal dust and indications of pitting or burning.
Clean and polish commutator using a fine grade of sandpaper held against the surface of the commutator as the commutator is rotated.
Inspect commutator for a single pitted or discolored (when compared with the others) bar, since this is an indication of an open circuit in the field coil.
Conduct commutator bar check by setting the voltmeter to the RX 1 scale and then touching the probes of the meter to adjacent bars on the commutator, gradually moving around the entire perimeter of commutator. The readings should be the same on all bar-to-bar checks. A high ohm reading indicates an open circuit; 0 ohms indicates a short circuit and the commutator is defective.
Check commutator brushes to ensure solid and even contact with the commutator.
Check brush springs to ensure that pressure applied to brushes is uniform and sufficient to hold brushes firmly against commutator. Replace any worn brushes and weak brush springs.
Conduct a Field Coil Check
Set voltmeter to RX 100 scale and clip meter probes to each of the two field coil leads (wires).
Replace coil if you get a high reading, indicating an open circuit and that the coil is defective.
Replace coil if there is a low reading between commutator checks, indicating a short circuit within the field coils.
NOTE: Ensure that the electric motor is unplugged before conducting any of the above tests.
Josh Weber is a retired industrial engineer. He has called on his engineering experience to write how-to articles for Associated Content, Demand Stuios and a business publication, "The Oyster Pointer." He is a graduate of The Virginia Military Institute and has a B.A. in economics and history.