How to Check a Motor Start Capacitor

Written by g.k. bayne
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Motor start capacitors help an electric motor overcome heavy loads. The capacitor does this by storing an electric charge much like a battery. Upon starting the motor, the capacitor releases this stored charge to the motor's windings and gives it a boost. This boost to the electrical power aids the motor's shaft in overcoming heavy loads such as air compressors, table saws and large fans. When an electric motor fails to start, a good place to begin troubleshooting is the start capacitor, which is generally mounted on top of the motor and is cylindrical in shape.

Skill level:
Moderately Challenging

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Things you need

  • Screwdriver
  • Needle-nose pliers
  • Volt ohmmeter

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Instructions

  1. 1

    Remove all electric power from the motor. Shut off the circuit breaker or pull the fuse that supplies power to the electric device.

  2. 2

    Pull the round cover from the capacitor using the screwdriver. Generally there are two screws that hold the thin metal cover to the top of the electric motor frame. Remove the power wires from the capacitor's connectors. Use the needle-nose pliers and pull the wires from the connectors one at a time.

  3. 3

    Switch the ohmmeter to the "Volts AC" position. Insert the black lead into the "Com" or common connector on the meter. Place the red lead into the "Volts" connector. Touch the leads, one each, to the capacitor's terminal connectors. In other words, touch the red lead to one terminal and the black lead to the other. Hold the leads in place to discharge the capacitor of any power. The meter may register some form of voltage value, but should soon dissipate down to zero volts.

  4. 4

    Switch the meter to the "Ohms" position on the meter's switch. Remove the red lead from the "Volts" connector and place it into the ohms connector.

  5. 5

    Touch the two leads to the two terminals of the capacitor, as in Step 3. The resistance reading should begin at zero ohms and then slowly climb in the resistance-reading display. This indicates that the capacitor is storing a small electric charge from the meter. The capacitor is good. The capacitor is bad and should be replaced if you get the following two test results: If the meter immediately shows a very high resistance reading, in which casee the capacitor has an open in the circuit. If the meter remains at zero ohms and does not climb in the reading.

Tips and warnings

  • Never check a capacitor that has not been discharged. The stored electrical power can damage the meter if read in the ohms position before the discharge process in Step 3 above.

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