Test & Measurement

Restoring Phase Balance in Motor Branch Circuits

Jan. 13, 2016

Take these steps to address motor failures caused by a phase imbalance.

While troubleshooting the cause of a motor failure, you discovered a phase imbalance of 5% at the motor leads. Now what? You need to find and fix the cause, but where do you start? Begin by disconnecting the motor and then again checking voltages.

One way to do this safely (in the typical motor setup) is to open the local disconnect. Then remove the motor overload strips from the motor overload protection device. This will prevent the disconnected leads at the motor weatherhead from becoming energized. Now you can measure voltages on the supply side of the overload protection device.

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If the phase imbalance problem went away (does not exist on the supply), the problem is in either the cabling to the motor or in the motor itself. Testing the cabling with an insulation resistance tester can tell you if there’s a ground fault, but make sure you understand how to perform the testing (e.g., getting a temperature-corrected reading). Testing the cable with a hi-pot tester is also highly recommended.

If you don’t have either instrument, don’t think that your DMM is going to be useful for determining cable integrity or detecting a fault. The DMM operates at only 9V. Use the appropriate instrument.

If the cable checks out to be good, then the motor probably has insulation problems. You can use your insulation resistance tester to check the motor windings, but unless there is a significant fault, this testing isn’t likely to be conclusive. But by the process of elimination, at this point it’s probably the motor. Replace it or send it out for test and repair.

What if the problem did not go away when you took the motor out of the circuit? Then it’s in the supply. As with the motor cabling, you can test the supply conductors for faults.

Before you do that, look at the other branch circuits. Do all of them have voltage imbalance? Does it go away when you open the supply breaker for the circuit you’ve been working on?

Look at the load distribution. Often, lights (a single-phase load) are run off a three-phase panel. You may just need to redistribute the loads to restore phase balance. If you have a current clamp, you can use your DMM to see the actual loads.

About the Author

Mark Lamendola

Mark is an expert in maintenance management, having racked up an impressive track record during his time working in the field. He also has extensive knowledge of, and practical expertise with, the National Electrical Code (NEC). Through his consulting business, he provides articles and training materials on electrical topics, specializing in making difficult subjects easy to understand and focusing on the practical aspects of electrical work.

Prior to starting his own business, Mark served as the Technical Editor on EC&M for six years, worked three years in nuclear maintenance, six years as a contract project engineer/project manager, three years as a systems engineer, and three years in plant maintenance management.

Mark earned an AAS degree from Rock Valley College, a BSEET from Columbia Pacific University, and an MBA from Lake Erie College. He’s also completed several related certifications over the years and even was formerly licensed as a Master Electrician. He is a Senior Member of the IEEE and past Chairman of the Kansas City Chapters of both the IEEE and the IEEE Computer Society. Mark also served as the program director for, a board member of, and webmaster of, the Midwest Chapter of the 7x24 Exchange. He has also held memberships with the following organizations: NETA, NFPA, International Association of Webmasters, and Institute of Certified Professional Managers.