SOURCE OF INDUCTION-MOTOR BEARING CURRENTS CAUSED BY PWM INVERTERS

The recent increase of bearing damage in induction machines driven by transistorized inverters has spurred activity on possible causes relat ed to PWM inverters. This paper looks into a typical power conversion system from this perspective. It identifies the existence of the commo n mode voltages produced in all types of converters. A hypothesis is t hen proposed to explain the bearing current problem, In particular, it is shown that in an inverter-motor system the common mode voltages ge nerated by a PWM inverter, which are attributed to PWM switching harmo nics, couple through parasitic capacitances from stator windings to th e rotor body and then returns through the motor bearings to the common ly grounded stator case as a closed loop circuit. The hypothesis is ve rified by experimental measurement of common mode coupling currents an d true bearing currents in a specially modified induction motor. Solut ions are then provided to suppress the bearing currents.