Space charge accumulation in coating materials for motor windings under DC high voltage

Recently, the switching frequency of motors for automotive and railway cars are increased according with the innovation of power semiconductor technology. When the switching speed is increased, so called “inverter surge voltage” is increased, then the coating insulating materials are degraded due to the high voltage phenomena like partial discharge, dielectric heating and space charge accumulation. Since the degradation of the insulating materials can lead to failure of motors, it is necessary to investigate the degradation for safety motor drive. While many researches of partial discharge affected on the insulating degradation of the coating materials for windings have been reported, the investigations of the degradation by the space charge accumulation have not been reported yet. Therefore, in this report, we have investigated the space charge behavior in polyamide-imide (PAI) and polyester-imide (PEI), which are usually used as the coating insulating materials for motor windings, under DC high voltage using PEA(Pulsed Electro-Acoustic method) method. It is found that the space charge formation depends on the polarity of applied voltage, and it is also found that the moisture in the sample promotes the space charge accumulation.