EFFECT OF PWM INVERTERS ON AC MOTOR BEARING CURRENTS AND SHAFT VOLTAGES

This paper investigates ac induction motor shaft voltage problems, cur rent flow thru motor bearings and electric discharge current problems within bearings when operated under both pure sinewave and pulse width modulated (PWM) inverter sources, Recent experience suggests that PWM voltage sources with steep wavefronts especially increase the magnitu de of the above electrical problems, leading to motor bearing material erosion and early mechanical failure, Previous literature suggests th at shaft voltage-bearing current problems under 60 Hz sinewave operati on are predominantly electromagnetically induced. It is proposed that under PWM operation these same problems are now predominantly an elect rostatic phenomenon. A system model to describe this phenomenon is cha racterized and developed. Construction and test of a new electrostatic shielded induction motor (ESM) verifies this model and is also a poss ible solution to the bearing current problem under PWM operation.