A novel technique for the determination of relative corona activity within inverter-duty motor insulation systems using steep-fronted voltage pulses

The frequent failures of induction motors being fed by IGBT-based variable speed drives due to high voltage spikes has led to an effort to both quantify the failure mechanism and develop an effective test procedure by which to assess the ability of a particular insulation system to withstand the high voltage transients which can arise in these applications. The process of corona degradation is discussed as the primary cause of these insulation failures. The concept of Corona Inception Voltage (CIV)-the voltage at which corona degradation begins in an insulation system-is presented, and the major factors contributing to the CIV of a motor's insulation system are reviewed. At this point, the task of determining a motor's CIV is discussed, and a new test procedure is introduced by the author. This procedure allows the determination of a completely wound motor's CIV when subjected to steep-fronted voltage waveforms similar to those generated by an IGBT-based variable speed drive. Several different motor insulation configurations are then evaluated with the new test procedure and compared to expected results. A practical use of the new procedure is then displayed through a comparison of the insulation systems of several currently available inverter-duty motors.