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How do I check a DC motor for shorts?

Updated February 21, 2017

DC or direct current motors are the most efficient electrical motors on the market today. Understanding the basic layout of a DC motor can go a long way in the troubleshooting and repair of the motor. There are two major components of an electrical motor called the stator and the rotor. Generally a DC motor uses a permanent magnet and a set of electrical wires called windings. The permanent magnet is like any magnet and is attached to the outside stationary "stator". The stator lies just under the outside round cover of the motor body. The rotor is attached to the motor shaft and the windings provide the electrical magnetic counterforce to move the rotor in either a clockwise or counterclockwise rotation. The electrical power for the windings is conducted through carbon brushes. By following a basic procedure, you can check a DC motor for shorts.

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  1. Disconnect the power source from the DC motor. You should expose the bare wire leads that conduct the electrical power into the motor.

  2. Remove the DC motor from any mechanical attachment to the machine with the end wrench set. Depending on the machine type, the bolt heads can be either Metric or SAE type bolts. You must be able to freely move the motor shaft (rotor) to thoroughly conduct the motor tests.

  3. Attach the alligator clip leads to each of the motor power input wires.

  4. Turn on the volt ohmmeter and switch it to the "Ohms" setting. Touch the lead ends together and the meter should read a direct short or zero ohms.

  5. Attach the other end of the alligator clips to the leads of the volt ohmmeter. You should receive an immediate reading in ohms of approximately 10 to 100 ohms on the meter. This wide range takes into account for the carbon brush connection to the rotor windings.

  6. Rotate the motor shaft very slowly in either direction. The ohm reading on the meter should change as you rotate the shaft. This insures that the carbon brushes are making contact through the commutation plate of the rotor.

  7. Remove one of the alligator clips from the power wires and touch it to the motor frame. You must find a metal part on the motor case to see if the motor has a short to ground. If the meter registers a wide-open or infinite ohms the motor does not have a direct short to ground.

  8. Rotate the motor shaft as in step 6. Again the meter reading should be infinite ohms or a wide open. If this is the case the motor is fine and perhaps the power source is the problem. Check the power wiring for any short conditions. If the motor does indicate a short or zero ohms you must remove the end case cover that conceals the motor commutation plate and carbon brushes.

  9. Remove the end plate with the screwdriver. The long screws can be either a Philips or slotted head screw. The two screws will go through the case and into the shaft end of the motor case. Remove the screws completely from the motor.

  10. Pull the end cover off carefully and inspect the wiring that feeds the carbon brushes. The bare copper braid wire may have touched the case. If it has, carefully pry the braid wire from the case and retest the motor for a short as in step 8.

  11. Pull the rotor from the stator portion of the motor, if the ohm test still registers a zero ohm or short reading between the windings and the motor frame.

  12. Inspect the condition of the rotor windings. Chances are the windings maybe scorched from over heating or the protective varnish has been damaged. This will cause the DC motor to be shorted to the case from conduction through the shaft into the bearings and then the outside case. If the windings are damaged in any way, the motor will have to be rewound.

  13. Tip

    Most often the exposed metal braid wire that feeds the carbon brushes is the culprit for causing a short condition. Never insulate these wires with tape as they tend to become very warm under operation. The tape could melt creating more problems than the short itself.

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Things You'll Need

  • End wrench set (Metric or SAE)
  • Two alligator clip leads
  • Volt Ohmmeter
  • Screwdriver Philips or slotted

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