Imagine that a parts handler shut down, and another electrician responded to the call. He found that one of the overload protection strips for the main drive motor was blown and replaced it.
When he went to start the motor, however, he blew the three fuses supplying the motor. Concluding the motor was bad, he replaced it. The same thing happened to the replacement motor. Suspecting low voltage, he measured each phase, which came in at 477, 479, and 476. Without needing to do voltage imbalance calculations, he ruled out that problem.
Next, he placed a power analyzer on the supply. This showed a slight distortion in the waveform and a power factor of 94 — nothing that would explain the problem with the two motors. Puzzled, he called his supervisor and his supervisor then called you.
What should you do?
First, check for excess triplen harmonics. This is a common power quality problem that the electrician missed. However, that would just delay solving this issue. Now, put three key facts together:
- The original failure was due to overload.
- The motor won’t start, and neither will its replacement. Any attempt blows the fuses.
- This is a parts handler.
The problem is probably in the load. Likely, a part fell into the drive mechanism and jammed it up. This would have caused a running motor to blow its overload protection by overheating those strips. But starting a motor takes five times the running current, so the fuses blow in that situation.
Removing the part should allow the motor to start. To prevent a recurrence, determine how that part got into the mechanism, and install a screen or other barrier.
