How to Test If a Motor Winding Is Grounded

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Things You'll Need

  • Masking tape

  • Needle-nose pliers

  • Screwdriver set

  • Multimeter or ohmmeter

Always test motors for grounded windings before replacing them.
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A motor fails when its windings short to ground, short to each other, or open--a break in the winding's wire. The wire in the windings use an insulating covering as an isolation barrier. If the winding's insulation wears away near the motor's case, the bare-wire winding shorts to ground. Technicians should perform a series of tests on an electric motor during routine preventive maintenance or before replacing it; this includes testing the motor for grounded windings.

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Step 1

Disconnect the electricity to the motor. If the motor's appliance plugs into a wall receptacle, then remove the plug from the receptacle. If the motor operates an appliance that connects to a circuit breaker, like an air-conditioning compressor-motor, then turn off the appropriate circuit breaker.

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Step 2

Access the motor's electrical connections and its case. Each type of motor application requires its own type of access. For example, pull the back panel off of a refrigerator to reach the condensing fan motor or disconnect a ceiling fan to reach its motor.

Step 3

Label each motor wire with a masking tape tag and a pencil. Make notes on the tape identifying the wire's terminal. In most cases, wire terminals have identification labels stamped next to the terminal.

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Step 4

Disconnect all motor wires. If the motor wires use slide-on terminal connectors, then grasp the terminal connector with needle-nose pliers and pull it off the wire terminal. If the motor uses screws to hold the terminal connectors in place, then loosen the screw with the correct screwdriver, often a Philips-head, and remove the terminal connector.

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Step 5

Turn a multimeter or ohmmeter to its highest resistance setting. A multimeter has voltage, amperage and resistance settings. Ohmmeters read resistance.

Step 6

Touch one of the meter's probes to the motor's case. If the manufacturer painted the motor's case, then scrape the paint from a small area with a screwdriver and then place the meter probe on the bare spot.

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Step 7

Touch the meter's second probe to one of the motor's wire terminals. Read the meter. The meter should read infinite. If not, then the motor's windings are shorted to ground. Repeat this step for each wire terminal.

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