Space charge distribution measurement in corona discharged filler added polyimide films under DC stresses

A thin insulating layer is used for motor windings. In an inverter fed motor driving system, an inverter surge voltage generates and it leads a partial discharge on the surface of insulating layer. Since the partial discharge reduces a motor lifetime, it is necessary to investigate a generation mechanism of the partial discharge. In the mechanism of it, we have focused on an effect of space charge accumulation. In this study, we tried to measure the space charge distribution in a specially designed insulating material against the partial discharge to investigate the effect of space charge accumulation on degradation of the insulating material by corona discharge. The material is composed of three layers of the same thickness which are two surface layers including an inorganic filler and one middle layer without the filler. To observe the degradation of the material by corona discharge, the surface of the material was exposed to corona discharge of high voltage pulses using a spherical electrode with frequency of 1 kHz for 2 hours. After the corona discharge treatment, we applied a DC voltage to the material to observe a time dependence of the space charge accumulation in the material. As the results, a positive and a negative space charges were observed at positions of one thirds of the sample thickness from an anode and a cathode electrodes. The charges supposed to be accumulated at the interfaces of the layers. On the other hand, the corona discharged sample showed a complicated space charge behavior followed by a breakdown. We also found that the space charge accumulation behavior strongly affects the corona discharged insulating material.