How to Check an Electric Motor Capacitor

Written by joey butler
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How to Check an Electric Motor Capacitor
Capacitors, also called condensers, improve the efficiency of electric motors. (condensadores image by Vanesa Boullosa Lopez from

The purpose of coupling a capacitor with an electric motor is to increase the efficiency of the electric motor. A capacitor stores power that would normally be wasted by an electric motor and redelivers that power during the negative power cycles of the motor. When a motor capacitor looses capacity, the motor can overheat or fail, as most motors are now designed and manufactured in such a way that they will not operate without a capacitor in place. You can ensure your motor is running safely and at peak efficiency by checking the motor's capacitor regularly.

Skill level:

Things you need

  • Voltage meter

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  1. 1

    Switch the circuit breaker powering the motor to the "Off" position.

  2. 2

    Remove the wires connecting the capacitor to the motor or power supply.

  3. 3

    Set your voltage meter to test for continuity. The continuity setting delivers a small DC voltage through the circuit being tested. You can use this small DC voltage to determine if the capacitor is functioning; however, you will not know the amount of capacitance.

  4. 4

    Place the two voltage meter leads on the two wire connection points on the capacitor. Leave the leads in place for about thirty seconds and then remove the leads. This charges the capacitor with a small voltage.

  5. 5

    Set the voltage meter to test for DC voltage. Place the two voltage meter leads on the two wire connection points on the capacitor. The voltage should spike rapidly then fall to zero. The voltage level is not important. You only want to know if the capacitor stored the DC voltage from the continuity tester and then released it when you checked for voltage. If no voltage spike is present, the capacitor is defective and should be replaced.

  6. 6

    Reconnect the wires to the capacitor and restore power.

Tips and warnings

  • Some voltage meters come with a setting for testing capacitance. If this is the case, you can simply disconnect power, remove the wires and test the capacitance of the capacitor using the voltage meter.
  • There is always a danger of electric shock when working with high voltage. Only certified electricians should attempt this procedure.

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