Dan Biby learned it all the hard way. An ordeal concerning several inverter-controlled pump motors with bearing damage made him a believer in the need for shaft grounding. Killer Shaft Voltages Although destructive currents can occur in any motor, they are more common in motors controlled by inverters, also known as variable frequency drives (VFDs). VFDs can save 30% or more in energy costs, but, they induce motor-shaft voltages that damage bearings. In fact, the cost of repairing or replacing failed motor bearings can wipe out any savings that a VFD yields and severely diminish the reliability of an entire system. Contractors and consulting-specifying engineers (CSEs) end up with unhappy customers, who soon discover that most warranties do not cover electrical bearing damage. This typically leads to finger pointing and the CSE and end user getting stuck with repair costs.
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