Evaluation of electrical insulation characteristics of polyamide-imide film using masurement results of space charge distributions with an applied square wave voltage

In the automotive industry, a reduction in CO2 and a low fuel consumption are required from the environmental point of view. According to such demand, electric vehicle using an inverter fed motor is developed and it used widely. However, partial discharge is generated in motor winding, when the higher voltage is applied to the windings. Thereby, it may induce a fatal breakdown in the insulating coating of the windings. Judging from a published report, it is said that a decrease of partial discharge inception voltage(PDIV) is related with space charge accumulation in the insulating coating. However, only few studies about the space charge accumulation characteristic in the insulating coating of the windings has been carried out in the past. In this study, we tried to apply a new space charge distribution measurement using Pulsed Electro-Acoustic(PEA) method to evaluate the insulating characteristics in such layers. In this measurement, the space charge distribution is measured with a square wave or a AC voltage with a contain frequency. By performing the measurement, we tried to analyze the mechanism of the space charge accumulation in polyamide-imide(PAI) film with applying the square wave and AC voltages. Also, we tried to investigate the influence of application of the square wave and AC voltage on insulation lifetime of PAI film. From the measurement results, it was found that the square wave voltage application was much affected to the PAI film degradation than the application of AC voltage.