How to Test a Small AC Motor

Test the ground resistance of the motor if there are unusual electrical discharges that may result in you or others receiving small shocks. Attach the electrodes of a multi-meter between the motor's terminals and the motor body. Set the multi-meter to resistance and turn the motor on. If there is not a a high resistance value, measured in ohms or kilo-ohms, you may have a ground leakage. Also check the resistance of the AC motor windings by connecting the multi-meter's electrodes between both of the motor's terminals, which should equal to a low finite value in resistance, depending on the size of the motor.

Inspect the fuse or circuit breaker for the motor if it has been running as usual in the past, but is now failing to start at all. Replace any burned-out fuses and reset the circuit breaker for the motor. If the circuit breaker continues to trip, discontinue using the motor until you have performed a more extensive electrical test. Remove the starter and bench-test it by hooking it up to a car or deep cycle battery with jumper cable clamps. Be sure to ground the negative cable prior to attaching the other end of the negative cable to the battery.

Check the capacitor by using a multi-meter if the motor is failing to start or if it takes too long to accelerate. Set the multi-meter to RX100 scale and place the electrodes on the capacitor terminals. Properly working capacitors will register at zero ohms at first and then drift back toward a higher reading. If the capacitor reads at zero ohms and stays at the same reading, then that indicates a short circuit. A steady reading of higher ohms indicates an open circuit.

Listen to the motor for loud vibrations. There may be many causes for excessive vibration, including an improperly aligned load or a load that is out of balance. Remove the motor and inspect the motor shaft for any bends or cracks. Other problems may also rest in the motor bearings, which can be tested by turning the motor's drive shaft. If the rotation feels rough and makes noises, then the bearings either need replacement or new grease if the motor model uses grease fittings.