Dependence of duty ratio of square wave voltage on space charge accumulation property in motor winding coating insulating material

In the automotive industry, a reduction in CO
₂
and 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 widely used. However, partial discharge generates in motor winding, when the higher voltage is applied to the windings. Thereby, it may induce a fatal breakdown in the insulating coating layer of the windings. Judging from a published report, it was said that a decrease of partial discharge inception voltage (PDIV) was strongly related with space charge accumulation in the insulating layer. However, only few studies about the space charge accumulation characteristic in the insulating coating for the windings has been carried out in the past. In this study, we proposed a new space charge distribution measurement technique using Pulsed Electro-Acoustic (PEA) method to evaluate the space charge accumulation characteristics in such layer. Using this technique, we can observe the pace charge distribution in an insulating film during the application of square wave voltage of 50 Hz is applied to the film. Judging from the obtained results using the technique, we found that the space charge accumulated in the film by the square wave voltage application, and it might cause of the reduction of PDIV. In this experiment, the space charge measurements were carried out by applying the square wave voltage to the samples of polyimide (PI), polyamide-imide (PAI) and polyester-imide (PEI), those are commonly used for coating material. Judging from the observed results, the tendency of space charge accumulation depends on the kinds of the materials. Furthermore, it was found that the duty ratio may strongly affect the partial discharge generation when polyamide-imide was used for the insulating layer for the windings.