Microscopic view of aging mechanism of polyimide film under pulse voltage in presence of partial discharge

Inverter-fed motors are damaged due to pulse voltage. The situation leads to early failure of insulation. Polyimide is an important insulating material in the inverter-fed motors. Its insulation aging process and failure mechanism are different from traditional ac and dc voltage. In this paper, structure change of polyimide film aging under pulse voltage above partial discharge inception voltage (PDIV) is investigated through scanning electron microscopy (SEM), thermally stimulated current (TSC) and lifetime test. It reveals that the degradation of structure in polyimide propagates from surface to interior with continuous aging, creating three layers in section. In the TSC spectrum, two main well-resolved peaks, namely α and β, exist in the 20-200°C range. The configuration corresponding to α peak in polyimide is damaged more severely than the configuration corresponding to β peak and its degradation is accelerated with increase in frequency.