How To Check a DC Motor

Unplanned downtime lasts an average of four hours and can cost companies about $2 million.

The good news is that if your issue is with a DC motor, troubleshooting one is pretty easy. That said, you should know what to do before something goes wrong so you can get back up and running as soon as possible.

Read on, and we’ll tell you how to check a DC motor if you run into problems.

1. Disconnect the Motor From the Power Source and Prep the Wire Ends for Diagnostics. 

Use a screwdriver to remove the set screws from the wire connections running between the motor and battery. Once removed, disconnect the wires so the motor can’t get power.

If necessary, expose the ends of the wires so you can use them to complete the test circuit. 

2. Detach the Motor From Its Machine Housing. 

Remove the DC motor from its machine housing so the rotor can move freely during diagnostics. Depending on the motor type, you’ll either have metric or SAE bolts, so make sure you have the right tools for the job. 

3. Prepare the Volt-Ohm Meter for Testing. 

Set the volt-ohm meter to its ohms setting. Plug the red volt-ohm meter lead into the ohms point, and the black lead into the ground point. Attach the alligator clip leads to each motor power input wires. Touch the two leads together and ensure the volt-ohm meter reads zero ohms. 

4. Run the Test: How to Check a DC Motor for Defects.

Touch the exposed wires of the motor with the volt-ohm meter leads. Black goes to black and red goes to red. Now, read the screen and determine the results:

10 to 100 ohms: This is a low resistance range and indicates there is nothing wrong with the motor.

Infinite ohms: This means there is an open circuit and requires further testing.

5. Rotate the Motor’s End Shaft and Take Note of the New Readings. 

Slowly rotate the motor’s shaft.

If the meter readings change while the shaft is in motion, the motor is good, but there’s a problem with the circuit.

If the meter readings still show an open circuit, there’s an issue with the motor. In most cases, you’ll find it’s one of three issues:

  1. A short in the ground
  2. Bad conductive bushings
  3. A failed commutator

6. Test for a Short in the Ground. 

Find a metal part of the frame’s motor and attach the black alligator clip to it. If the meter shows an open circuit, there is a short in the ground.

7. Remove and Inspect the Bushings. 

Locate the motor bushings under the plastic end caps on the other side of the driveshaft. Use a screwdriver to remove the bushings and then inspect them.

Look for cracks or breaks across the surface of the bushings. There should be a smooth curve where the bushings sit against the commutator.

If you find no defects, the problem is usually the commutator.

8. Remove and Inspect the Commutator. 

Remove the two screws that run the length of the motor and take off the rear end cap. Inspect the plates that make up the commutator assembly.

You should see an opening between each plate and no broken wires or burnt varnish. If you find anything else, the commutator has failed, and you’ll need to replace the damaged parts. 

Do You Need a New DC Motor? 

We’re glad we could share information on how to check a DC motor for failure. If you’ve conducted your tests and found that you need a new DC motor, Kor-Pak can help with that!

We have an array of Surplus DC Mill Motors to choose from. Each motor comes with an electrical warranty and has been rebuilt to prime working condition. Plus, you’ll get to pick from top brands like GE and Westinghouse.

We’ll repair your current motor and exchange it for a reconditioned one for a price you can’t beat.

Review this GE Table Motor Guide for more information about our offerings or contact us anytime with questions.

Posted in Industrial Equipment.