Electrical aging test and repetitive partial discharge inception voltage on random wire wound winding insulation

The electronic switching converters used to supply squirrel cage motors can cause, during the normal operation, some critical problems leading to the early failure of the stator winding insulation. In fact if the electrical motors are supplied by switching electronic converters, the stator insulating system is subjected to voltage waveform having high frequency content (e.g. PWM); this kind of voltage waveforms can lead to an uneven distribution of potential along the windings and to the presence of overvoltages at the motor terminals. If these overvoltages have an amplitude higher than the partial discharges inception voltage, partial discharges activity incepts leading to the erosion of the organic enamel used to electrically insulate the wires of motor stator windings. During this research work the stators of the induction motors have been subjected to partial discharges measurements and electrical aging tests with the aim to find a possible relationship among the repetitive partial discharges inception voltage amplitude, the time to breakdown and the kind of motor insulation failure. In order to perform the electrical aging tests, a modified surge tester has been used: the tester applied unipolar positive voltage pulses at the terminal of the stator motor windings. The correlation between the repetitive partial discharge inception voltage measured before and after conditioning have been studied.