Assessing AC motor dielectric withstand capability to reflected voltage stress using corona testing

The dielectric withstand capability of standard low voltage AC induction motors when subjected to steep fronted dv/dt waveforms of PWM voltage source inverter pulses is investigated. Inverter IGBT device switching generates this arduous dv/dt which results in reflected wave transient peak voltage magnitudes of twice to three times bus voltage with rise times is low as 0.05-0.2 μs imposed on the motor terminals. Although these transients can be definitively measured, they stress the standard motor dielectric insulation system in a region previously unspecified and untested. Thus, an allowable peek repetitive surge dielectric withstand voltage vs. rise time curve is not readily available from motor manufacturers. Experimental results of tapped stator windings under inverter operation offer insight on reflected wave stress internal to the motor. The effect of peak repetitive surges on motor insulation is discussed with predominant focus on corona and partial discharge failure mechanisms. Test results correlate well with field failure rates and show three distinct classes of voltage withstand capability in the motor industry today