Electrical Troubleshooting Quiz, May 15, 2012

Losses of plant air capacity have caused production issues. The plant air system is supplied by four compressors, each turned by a 400-hp motor. One of these motors kept blowing its overloads, and the plant engineer had this motor replaced.

The new motor worked fine for a few weeks, and plant air was not a problem. But then it, too, started blowing its overloads. The other motors aren't having this problem, so you don't suspect it's related to the supply power. Testing with a DMM shows no voltage imbalance. Setting the DMM to record high/low over a three-day period produced no anomalous readings.

What's next?

The DMM is a great tool, but it isn't going to give you enough power information for a proper analysis. Use a power analyzer for that.
Have your motor shop do an autopsy on the failed motor. The fact that it's drawing enough additional current to blow its overloads but not tripping its breaker is significant. You need to look for chronic, rather than acute, causes of excess current.

Some possible culprits:

• Low power factor at the motor. If the motor has a PF correction capacitor, test it.
• Load problems. Has anyone looked at the compressor itself? Conduct a full PM on it, including lubrication replacement.
• Mounting problems. Put a vibration analyzer on this motor, and make sure you capture data during the compressor loading phase. If there's a mounting deficiency (e.g., cracked base), then the motor sees the additional movement as extra load.