Printing press unwinder going too fast!

Unwinder going too fast

Recently I had a customer who said the unwind section on their printing press was going too fast. In fact, they had no control at all. When the section was started, the motor would try to go full speed. The maintenance person told me he tried adjusting various settings on the drive with no effect. I went to the plant to see what was going on. The DC field on the motor was okay, it had field current. Then we discovered there was a DC tachometer on the motor used for speed feedback to the drive. It would make sense if the tachometer wasn't providing feedback, then the drive would try to go 100% on . I hooked up my voltmeter to the DC tachometer output and we turned the unwinder by hand, no voltage on my meter. We thought we might have a bad tachometer so we looked at the brushes on the tachometer and they looked fine. We then noticed we could turn the shaft of the tachometer independent of the motor. That's it, we had a loose tachometer coupling. We opened it up and found more than a loose coupling, we had a completely disintegrated coupling. See picture. To make sure the tachometer was good we turned the shaft by hand and measured the voltage output. We replaced the coupling and started the line up without issues.

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