Effect of Thermal Treatment with High DC Stress on the Conductivity in Imide-Based Insulating Materials

This paper evaluates the leakage current properties of polyimide and polyamide-imide with two different pretreatments. Recently, with the spread of electric vehicles, there has been a demand to maintain the performance of insulating materials used for the winding coating of motors in these vehicles under high temperatures and high electric fields. However, it has already been clear that when high DC voltages are applied to some polymeric insulating materials under high temperatures for a long time, dielectric breakdown occurs, and it is assumed that the breakdown is due to distortion of the internal electric field caused by the accumulation of space charge and an increase in leakage current. However, the insulation degradation phenomena leading to dielectric breakdown have not yet been fully understood. This study evaluated the current leakage characteristics of polyimide and polyamide-imide films under three different conditions: untreated, thermally treated, and electrothermal treated, using the direct current integrated charge method (DCIC-Q(t)). The results show no significant change was observed in polyimide. Meanwhile, polyamide-imide showed insulation degradation due to the electrothermal treatment.