How to Troubleshoot an AC Electric Motor

We are surrounded by electric motors in our everyday lives and most of these are AC or alternating current motors. DC motors are another discussion.  This discussion is oriented towards motors used in commercial and industrial applications, though the concepts apply to most residential and appliance uses.  Though we use that power every day, most people don’t actually understand what goes into an electric motor – or how to troubleshoot one if it is acting up. AC electric motors are highly engineered electrically and mechanically, extremely reliable and seldom need service. A basic understanding of how electric motors work, and the simple things you can do, goes a long way to maintaining motor life.

What are the parts of an electric motor?

There are a few different things working together to make an electric motor run. These include:

Stator (or magnet)

As the name implies, the stator is a permanent, fixed magnetic piece that is attached around the edge of the motor case. The stator is not one solid piece, but is rather many thin metal sheets, or laminations. Using several metal sheets pieces instead of a single core improves energy efficiency.


The rotor is mounted on a shaft that spins at a speed determined by the motor design and motor control equipment. Conductors are usually in the rotor and they engage with the magnetic field generated by the stator. This action causes the motor shaft to turn.

Air gap

Between the rotor and stator is a necessary air gap. This gap is usually small – too large of a gap will cause performance problems in a motor.


The coils of a motor are wires called windings. They are designed to form their own magnetic poles, which facilitate a current.

Troubleshooting electric motor repair

If you have an electric motor that isn’t working, there are some steps you can take on your own to try to determine, and potentially fix, the problem.   These are the basic steps, but more complex testing is usually required to determine the serviceability of the motor.

Shut off power to the motor.

You don’t need it for diagnosis, so protect yourself by making sure it is disconnected.

Do a visual inspection of the outside of the motor.

There could be things on the exterior that are preventing the interior mechanisms from operating properly. Some items to look for include:

  • Dirt or debris being pulled through the motor windings.
  • Broken feet or mounting holes.
  • Dark paint in the center of the motor: this is an indication that excessive heat may have damaged the motor.
  • Moisture – water and electricity don’t mix and will damage a motor that is not designed for a wet or submerged application.
  • Aroma – the sense of smell will indicate very quickly of a motor problem.

Find the motor’s nameplate.

This should be easy to locate on the outside of the motor. It will have detailed information, including specifications for horsepower, voltage, full load current rating, RPM (revolutions per minute), torque design, ambient temperature rating, starting inrush current and other data.

Check the bearings of the motor.

The bearings are found on either end of a motor. If these are not connected properly, rusted or have gone “bad” for any other reason, the motor will not run effectively. Look and listen for any signs of rubbing, scraping or awkwardness in the motion of the rotor. It should spin easily and quietly.

Check the windings.

You can look for shorted wires leading to the frame. When there is a wire that has a short, it will trip up the circuit breaker or blow a fuse and make the motor stop working. With an ohmmeter, check for a short or continuity to ground with any of the motor leads.  Use of an ohmmeter that outputs 500 volts DC for a winding insulation to ground test is recommended.  Read the test instrument instruction manual first.

Use an ohmmeter.

This device will let you place test probes in appropriate jacks then see what resistance value you get back. In three phase motors, the line-to-Iine resistance readings should vary less than 2%.  In motors above 10 HP, the winding resistance can be lower than what the ohm meter function in a multimeter is able to read.  A very low reading ohmmeter may be required. Read up on how to use an ohmmeter or read the manual that came along with it.

For a single phase motor, visually inspect the capacitor.

This is usually housed inside a metal cover. If you look under the cover, you may see leaking oil, or holes, or even smell burning.

For a single phase motor, check the rear bell housing.

Make sure the start winding switch contacts are not welded shut or contaminated.

Check the fan.

These blades will be behind the metal guard on back of the motor. Also check to see that nothing is clogging the metal guard near the fan.

If you follow all of these steps and still can’t locate your problem, call a motor professional to take a look. If there is a way to fix the motor, it makes sense to do that before paying for a totally new one.

How to Troubleshoot an AC Electric Motor was last modified: July 20th, 2017 by admin

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Jerry is the owner of Sloan Electromechanical and is active in all aspects of the company. He is passionate about doing the work RIGHT and proposing the best product solution, hence the Sloan team is focused on aligning company values with client values. Please post your questions or comments and Jerry will respond. For a faster or confidential response, please contact Jerry directly 619-515-9691 or LinkedIn
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