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Due to their fundamental operational characteristics, rotating machines induce an electrical potential on their shafts. Shaft grounding brushes are installed to mitigate the effect of this voltage. Ineffective mitigation systems result in this voltage seeking alternate paths through other metal components such as bearings, seals or gear trains. This results in arcing which erodes metal surfaces and leads to failure of these components. Damage is often mis-diagnosed, and the components are routinely replaced prematurely
Shaft voltage occurs in electric motors and generators due to leakage, induction, or capacitive coupling with the windings of the motor. Currents due to shaft voltage causes deterioration of motor bearings but can be prevented with a grounding brush on the shaft, grounding of the motor frame, insulation of the bearing supports, or shielding. Shaft voltage can be induced by non-symmetrical magnetic fields of the motor or generator. External sources of shaft voltage include other coupled machines, and electrostatic charging due to rubber belts rubbing on drive pulleys
Every rotor has some degree of capacitive coupling to the motor's electrical windings but the effective inline capacitor acts as a high-pass filter, so the coupling is often weak at 50–60 Hz line frequency. But many Variable Frequency Drives (VFD) induce significant voltage onto the shaft of the driven motor, because of the kilohertz switching of the insulated gate bipolar transistors (IGBTs), which produce the pulse width modulation used to control the motor. The presence of high frequency ground currents can cause sparks, arcing and electrical shocks and can damage bearings. Grounding the shaft by installing a grounding brush device on the shaft of the electric motor provides an alternate low-impedance path from the motor shaft to the motor case. This method channels the current away from the bearings. It significantly reduces shaft voltage and therefore bearing current by not allowing voltage to build up on the rotor.
