Life tests on twisted pairs in presence of partial discharges: influence of the voltage waveform

The stator windings of electrical motors fed by means of inverter drives are subjected to switching voltages that constitute a heavy electrical stress for the winding insulation systems. Such stress together with the uneven distribution of potential along the windings and the possible presence of voltage peaks, due to resonance and reflection phenomena, often leads to premature failure of Type I winding insulation. In this study the electrical aging of enamelled insulating wires in presence of PDs has been investigated considering the applied voltage waveform as the core of the degradation processes together with the voltage amplitude. The experimental activity consisted in life test campaigns on twisted pair specimens. The specimens have been stressed applying five voltage waveforms until the total breakdown; some of the waveforms are designed in order to reproduce the output of a PWM drive. The times to breakdown relevant to different voltage amplitudes have been collected and lifetime plots for each voltage waveform have been computed and traced. It has been demonstrated that life time values are greatly affected by the particular shape of the test voltage. Furthermore, the obtainable life curves relevant to the pulsed waveform simulating the PWM drive conditions have to be traced by means of two straight lines having different slopes.