Dan Biby learned the hard way. An ordeal concerning several inverter-controlled pump motors with bearing damage made him a believer in the need for shaft grounding.
Although destructive currents can occur in any motor, they are considerably more common in motors controlled by energy-saving inverters, also known as variable frequency drives or simply drives. VFDs can save 30% or more in energy costs, but whether used to control a motor’s speed or torque, they often induce motor-shaft voltages that damage bearings.