Failure mechanism of winding insulations in inverter-fed motors

The failure of magnet wires under repetitive pulses as seen in inverter-fed motors cannot be attributed to a single factor but is a result of the combined effects of partial discharge, dielectric heating, and space charge formation. Voltage overshoots produced by PWM drives may be above discharge inception voltage. Partial discharge may therefore be present in inverter-fed motors. In addition to partial discharge, pulses with a fast rise time and high frequency enable the insulation to generate local dielectric heating, which increases the local temperature. The degradation rate of the insulation is therefore increased. Furthermore, the fast rise and fall of pulses make it possible for space charges to accumulate in the winding insulation and on its surface.