Several motors supplied by the same MCC blow their overloads at about the same time, but the occurrence seems random. Everything will be normal for weeks, then suddenly, all of the motors have open overloads.
Over the past year, each of the motors has passed a battery of tests performed by the motor repair shop your plant has been using for many years. Voltage checks show normal supply voltage. Alignment checks have all come up “within spec” and there’s no excess vibration.
Where should you look next?
The voltage checks will reveal a chronic low voltage or voltage imbalance condition. They will not reveal a periodic one. Put a power analyzer on the supply to several of motors. After the next episode, you can look in the history of the power analyzer to see what power event(s), if any, occurred coincidental to an overload condition.
Something like low voltage might not be the cause, but merely a symptom of another problem. For example, the PLC control doesn’t allow for the thick slurry that develops when a particular process is run. Changing the addition rate may solve this problem.
Being able to time the power signature is paramount to tracking down the actual cause.
