The increasing use of variable frequency drives (VFDs) within the electric motor industry as a consequence of energy conservation targets has caused a significant increase in stray voltage accumulation within shafts of rotating machinery.
VFDs regulate the speed of the motor by converting alternating current (AC) voltage to direct current (DC) voltage, then back to a pulse width modulated AC voltage of variable frequency. The voltage change relative to the time induces a form of capacitance between the motor stator and the rotor, which in turn induces a voltage on the rotating shaft. Once this shaft voltage builds up it will divert to ground through the path of least impedance, which with no grounding protection is nominally through the bearings.
When a large enough current passes through the bearings it will arc between the ball bearings and the races. In bearings which are operating properly, there is a micro gap with the ball bearings floating on a ‘fluid film’ of lubricating substance. Once the build up of voltage overcomes the dielectric capacity of this lubricant it will cross over this micro gap causing an electric discharge machining (EDM) effect on the outer race, thus damaging the bearings which will eventually ultimately require replacement.
Various methods of mitigating shaft voltage build up have been suggested including use of shielded cable, grounded shafts, insulated bearings, insulated grease and the installation of a Faraday shield within the motor. Shaft grounding methods might employ the use of brushes or bushes which contact the shaft. Brushes are to clusters of string filaments and bushes are a solid block components.