Your plant has long had an insulation resistance trending program for all critical motors and for most other motors above 20HP. With this program, motors that develop signs of insulation deterioration get replaced while they are still reliable. Very rarely does a motor actually fail while in service.
That is, until about six months ago. Now it seems an in-service motor fails every few weeks. Not only that, the number of motors tagged by the insulation resistance trending program has gone up tenfold. Even some of the replacements fail within a year.
The plant manager wants you to find out what’s going on. Where should you start?
Suspect transient voltages in the power distribution system. If you are also experiencing an increase in failures of electronics, this is almost surely the root cause. You could try to verify with a recording DMM, but it’s not going to give you the detailed power event picture you need. Monitor with at least one power analyzer, preferably several of them (each on a different feeder).
If you have a transient voltage suppression system (TVSS), is it multi-stage and do the devices still work properly? If you don’t have a TVSS, install one. When is the last time your lightning protection system was inspected and your grounding system tested?
Is there an electrically continuous path around the motors, through which undesired current can travel? Or must it travel through the motors?
