Critical factors for early failure of magnet wires in inverter-fed motor

Recently, more AC motors are being controlled with pulse width modulated variable speed drives (known as inverters). However, the premature failure of wire insulation in these inverter-fed motors has been observed. The wire insulation in the inverter-fed motor is no longer experiencing a traditional sine-wave voltage which is a steady state condition with a maximum and a RMS value, but experiencing a pulse wave voltage with significant harmonics and transients. The effects of the overvoltage due to switching, rate of rise and switching frequencies on the performance of insulation are not clear. In the work presented here, the effects of some critical factors which characterize the power inverters (voltage, frequency, rate of rise, duty cycle, etc.) are investigated to help to understand the failure mechanism of wire insulations under such conditions. Conventional magnet wire insulation systems have relatively low life expectancy under conditions of high frequency pulses at high voltage and temperature. The mechanism of insulation failure under these conditions can be either localized pinhole-type or massive destruction depending on the voltage and frequency applied. The fast rate of rise combined with high repetitive frequency accelerates wire insulation degradation.