Electrical Troubleshooting Quiz, Aug. 27, 2013

What to do when key motors in your plant experience excess vibration.

Over the past few weeks, several key motors have experienced excess vibration. Due to production scheduling purposes, the repairs were done after the dayshift went home. In each case, the laser alignment system showed a particular motor was only slightly out of alignment. The second shift electricians would align it, then start it and monitor for vibration.

When dayshift production started, there were no problems with the repaired motor. But by afternoon, the vibration would start again. The plant manager suspects sabotage, but you have thread-lock on all of the adjustment fasteners and they haven't moved.

How can you find out what's going on here?

It's late August, and the heat is awful. Given that, and the fact the vibration problems become noticeable in the afternoon, the culprit is probably thermal expansion.

This expansion can change the distance between parts (e.g., the motor is too high relative to the load) enough that a "properly aligned when cold" motor vibrates severely at operating temperatures.

To see if this is the problem, use an alignment system that can repeatedly collect and then trend the alignment data. Do the same with the temperature at various points (e.g., motor shaft, motor case, motor base). If you see an alignment drift that corresponds to temperature rise, you have an expansion problem.

Your collected data will also provide the correct points for an alignment that offsets the drift (assuming you don’t choose to cool via forced air instead). You'll need to realign when the weather cools.

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