The main production line was revamped last year. One of the enhancements was putting all of the motors on variable speed drives under PLC control. This has sharply reduced mechanical problems, but all is not roses. The drives unexpectedly stop for no apparent reason; no amount of tweaking the settings has solved this. And the number of motor failures has gone up, instead of having gone down as anticipated.
What could the problem be traced back to?
It could be harmonic interaction, if there are many drives and the total motor load is a significant share of the total load on the plant’s electrical distribution system. If the drives are harmonics-corrected, you can rule this out. If they aren’t harmonics-corrected, consider replacing the larger ones with new drives that have that feature and power factor correction.
To verify the existence of what is most likely cause of this problem, measure voltage between a drive’s case and its supply neutral. These should be at the same potential. If you measure more than 1/10th of a volt, a bonding connection is missing. Often, that’s because someone made a ground (earth, see Art. 100 definition) connection instead (look for a ground rod).
Check the shielded isolation transformer to ensure these points are bonded together: shield, core, secondary neutral, and equipment grounding conductor.
