Electrical vibration appears in a vibration spectrum as a non synchronous peak or peaks. This is to say that electrical vibration frequency is not an integer multiple of shaft rate. Rolling element bearing tones and vibration from other machines are also non integer multiples of shaft rate. There are occasions in which one wishes to be certain that vibration is electrical and not coming from another source. This is especially true in VFD's which can create some frequencies in the vibration spectrum that look very much like bearing wear patterns. One way to do this is to monitor the machine in real time, with a live spectrum on the screen of the data collector, and then shut the machine off. Any vibration that is electrically related will disappear immediately from the vibration spectrum while shaft related vibration peaks will gradually move to the left of the screen as the machine slows down. External vibration will remain where it was. This is a quick and easy way to distinguish electrical vibration from mechanical vibration, but of course one needs to be able to shut the machine down in order to take this test.

Tip Provided by: Jason Tranter, Managing Director, Mobius Institute

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