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silicone rubber coating applied to insulators in rd located near Long Island Sound has depolymerized and become putty-like after six years of service. Coating samples were taken from high creep insulators which were energized and weathered, weathered but not energized, and neither weathered or energized (warehouse storage). The unweathered and unenergized coating did not appear to degrade. The weathered but unenergized coating had degraded measurably but was still in an operational state. The energized and weathered coating has become putty-line and lost all physical integrity. This suggests aging by a combination of chemical, photochemical (weathering), and electrochemical mechanisms.

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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.