Electrical Testing

Tip of the Week: Monitor Your Motors for Single Phasing

To detect power quality problems, make sure you have a system in place or monitoring motor conditions

Power quality problems can greatly shorten the life of a motor, primarily because of the excess heat resulting from them. For this reason, adding the feeder for each critical motor onto your power monitoring system is never a bad idea. If you don’t have a power monitoring system but are looking into that, plan from the start to monitor critical motors.

We read quite a bit about voltage imbalance, and how even a small amount can cut motor life dramatically. We read less about another problem, and its effects on motors can be even more dramatic.

The problem is called “single phasing,” and it means the loss of one phase in a three-phase system. A power monitor can alert you to this event, should it happen. In a complex distribution system, determining the cause can be difficult.

Photo credit: stalkerstudent/iStock/Thinkstock

A manufacturing plant in Tennessee had a single-phasing problem that ran for several weeks while maintenance flailed about for a solution. It was hiding behind a backfeed situation combined with a blown fuse on the supply transformer. This made it appear as if normal voltages were present on all three phases.

This facility didn’t have power monitoring or even rudimentary phase loss alarms. As the motor failure rate steadily rose, the plant engineer suspected power problems. But the maintenance department didn’t own much in the way of test equipment.

Fortunately, the plant engineer contacted a qualified testing firm (whose president would later serve as the president of NETA). Of course, this firm had a full range of electrical test equipment and knew how to use it.

Using (among other tools) portable power analyzers, the testing firm was able to see exactly what was going on. They methodically tracked the problem back to the supply transformer. Correcting the backfeed problem and replacing the open fuse solved the motor failure epidemic.

Good motor maintenance involves far more than grease changes and vibration monitoring. But that had been the total program for this plant, and the cost in lost production was astronomical. If your motors matter, make sure you have in place a system for monitoring conditions such as single phasing. Many VFDs come with protection against this problem, so choose a VFD with that feature if one is available for your application.

Hide comments


  • Allowed HTML tags: <em> <strong> <blockquote> <br> <p>

Plain text

  • No HTML tags allowed.
  • Web page addresses and e-mail addresses turn into links automatically.
  • Lines and paragraphs break automatically.