How Do I Determine If a Single Phase Motor Is Bad?

General Troubleshooting

• 1

  Turn the motor off and wait for it to stop spinning. Grasp the shaft and try to move it back and forth. Turn the shaft by hand and listen for a grinding or grating noise. A motor with bad bearings often displays excessive movement in the shaft and may make noise when the shaft turns. Bad bearings also make it difficult to turn the shaft, and the shaft grinds to a halt quickly rather than coasting to a gentle stop.

• 2

  Turn on the motor and observe it. If it does not turn, but it hums, wrap a piece of rope tightly around the pulley or shaft and pull it to spin the shaft. If the motor starts, the problem is a bad start capacitor, an easy and inexpensive repair.

• 3

  Turn on the motor. A motor that reaches full speed, but has difficulty maintaining speed, may have a bad run capacitor. Turn the motor's circuit breaker off, remove the capacitor and discharge it by shorting the terminals with a screwdriver. Test the capacitor with the multimeter's microfarad or capacitance setting. A reading that is lower by 20 percent or more than that shown on the side of the capacitor indicates the capacitor needs replacement, which is another easy, inexpensive repair.

• 4

  Look for a reset button. A motor that tripped the thermal protection circuit or internal circuit breaker won't start until the button is pushed back in. Push the button and try to start the motor. If the motor runs for a while, but trips its thermal protection breakers, it might be overloaded or have other problems.

• 5

  Turn off power to the motor at the disconnect switch or unplug the motor. Remove the cover from the motor's wiring box and look for loose connections. Fix any connections that appear loose and try starting the motor again. Loose connections that prevent a motor from starting make it look defective, but fixing the connections solves the problem.

• 6

  Remove the wiring nuts from the two wires that supply power to the motor and take the supply wires off the motor wires. Do not let the supply wires touch other wires, each other or anything metal. Set the multimeter to measure voltage in the range shown on the motor's metal nameplate. Clip one meter lead to each power wire, and turn on power to the motor or press the start switch. If the meter reading does not fall in the correct range, the problem lies in the power supply and not the motor.

Check Motor Windings

• 7

  Make notes or label the wiring in the motor's wiring compartment to make it easy to restore the wiring. Remove the wire nuts from the two wires that connect the motor to the power supply and separate all the wires. It is not necessary to disconnect the green or bare ground wire. Set the multimeter to measure ohms at the lowest setting possible.

• 8

  Look at the motor's wiring diagram to determine which motor leads are associated with the windings. Manufacturers often include the wiring schematic inside the wiring compartment cover, or on a metal plate attached to the motor.

• 9

  Measure the resistance across a winding by touching one meter lead to each of the two wires associated with that winding. A winding is a piece of wire wound many times around a steel core, and, therefore, won't have much resistance. An open winding has a high resistance measured between its leads. An open winding indicates the motor needs repair or replacement.

• 10

  Touch one meter lead to a winding wire, and the other lead to the motor's case or ground terminal. A low or near-zero resistance indicates the winding is shorted to the case, and the motor needs replacement. Check both winding leads in this fashion for each winding. A near infinite resistance between the windings and the case indicates no short exists.