Electrical Testing
Restoring Phase Balance in Motor Branch Circuits

Restoring Phase Balance in Motor Branch Circuits

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.

Photo credit: Smoczyslaw/iStock/Thinkstock

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.

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