Electrical Troubleshooting Quiz - September 13, 2016

A 50-hp motor that drives a production machine must be replaced every eight to 15 months. It always gets an exact replacement. The plant engineer wants you to find out why this motor fails so often. The mechanics have already gone through everything to do with the gearbox, and they even beefed up the motor base and pedestal. Another team performed a laser alignment as part of this project. Part of the gearbox assessment involved measuring the torque required to drive the load; measurements were taken on both the output and input of that gearbox. The motor is a Design B, and its torque curves easily enclose the torque on its output shaft.

So what’s the next step?

First, determine the usual failure mode. If, for example, it’s usually winding failure then perhaps the winding insulation is not of sufficient temperature rating. On the other hand, if you’re seeing repeating bearing failure then the cause may be undesired current flowing through the bearings instead of flowing through the bonding conductor to the equipment grounding system.

Did the mechanics’ torque analysis cover motor starting? If the motor starts across-the-line under full load, that’s going to dump a lot of heat into those windings. If the motor already has a VFD, use a power analyzer on the motor side to assess harmonics and power factor.