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 niaxiinum and a RMS value, but experiencing a pulse wave voltage with significant harmonics and transients. Tlie elTects of the overvoltage due to switching, rate of rise and switching frequencies on the perforniance of insulation are not clear. In the work presented here, the enects of some critical factors which characterize the power inverters (voltage, frequency, rate of rise, duty cyclc, etc.) are investigated to help to understand the failure mechanism of wire insulations under such conditions. Conventional magnet wire insulation system have relatively low life espectancy under conditions of liigli frequency pulses at high voltage and temperature. Tlie mechanism of insulation failure under these conditions can be either localized pinhole-type or inassive destruction depending on the voltage and frequency applied. The fast rate of rise combined with high repelitive frequency accelerates wire insulation degradation.

The inverter pulse resistant magnet wire, Tbermaleze Q', was developed and introduced to the market in September 1995. Since then, laboratory accelerated motor tests as well as field tests have been performed in many different inverter-fed motor applications. It has been proved to be a greatly improved product for inverter-fed motor applications. However, an accelerated lab test on twisted pairs at elevated temperatures indicated that the current version of this wire has its limitations in high temperature applications. Therefore, an improved version is developed. The dielectric performance of this improved wire in comparison to current Quantumshield will be presented and discussed in this paper.

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