Measurement of space charge distribution in coating material for motor wingings under square wave voltage

In the automotive industry, a reduction in CO₂ and low fuel consumption is required from the view of environment. According to such demand, electric vehicle using an inverter fed motor is developed and diffused. However, partial discharge is generated between motor winding, when the higher voltage is applied to the winding. Thereby, it may induce a fatal breakdown in the cover insulating. Judging from a published report, it is said that a decrease of partial discharge inception voltage are concerned by space charge accumulation in the insulating layer. However, only few studies about the space charge accumulation characteristic in the insulating layer of the windings has been carried out in the past. In this study, we especially focused on the space charge accumulations in polyamide-imide (PAI) under square wave voltage using Pulsed Electro-Acoustic (PEA) method. It is found that, a negative hetero charge accumulated near the anode immediately after DC voltage application. Moreover, even in the short circuit condition, negative charge remained near the anode. It means that the space charge accumulates in the bulk of PAI film within a short time of the DC voltage application. Therefore, even in the inverter drive system, excessive voltage application may affect the performance of the dielectric material. Therefore, in this study, we investigated the effect of the square wave voltage application to the film on the space charge accumulation.