How do I test a three-phase electrical motor?

Updated April 17, 2017

There are lots of types and sizes of electric motors for different jobs. The common ones found running shop tools like table saws, lathes and other bench tools are induction motors, which typically range from ¼ to 5 horsepower. Single-phase electric motors are made to run on common house current. Three-phase motors run more smoothly, because the alternating current comes through in three phases that keep the flow steadier, but require special wiring or a phase converter.

Testing a Three-Phase Motor

Three-phase electric motors are reliable and can run for years without problems because they have no brushes and few moving parts. If a motor stops or won’t turn on, the problem could be coming from the power supply or the load, as well as from the motor itself. First, test the voltage at the terminals. Since the motor has three windings on its core, it will have three separate terminals to feed them. Attach a volt meter to each terminal, one at a time, to see if the voltage is at the right level on all three phases. If it’s not, look for problems in the power supply. If the voltage is right, you need to look further.

Make sure the starter is off or the safety switch is engaged so the motor does not start up unexpectedly while you’re working on it. Then disconnect it from the load and try turning the motor on again. If it starts, the load is the source of your troubles.

Checking the Coils

Check resistance in the coil windings, using an ohm meter. First disconnect or lock out the power. Begin by testing the T1-T4 coil. There should be some resistance if the coil winding is good. The bigger the motor, the less the resistance, but there should always be some. If there is no resistance (a reading of zero), the winding has a short circuit. A reading of infinite resistance indicates an open circuit. Once you have the resistance from the first coil, you can check the other two. If they are wired in a series circuit, the resistance of two coils should be double that of one alone. In a parallel circuit the resistance is half, if they are working properly. That should tell you which if any of the coils is the problem.

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About the Author

Manny Frishberg made his home on the West Coast for more than 30 years. He studied writing and journalism at Portland State University. His articles have appeared in Wired, "Discover," the "Puget Sound Business Journal" and dozens of other websites and magazines and has earned four writing awards from the Society for Professional Journalism.