Space charge behavior in covering insulating material for motor windings under applied voltage of square wave

In this study, we observed a space charge accumulation behavior in the covering materials (Polyamideimide: PAI, Polyester-imide: PEI) for motor windings under applied voltage of square wave. In these days, the inverter-fed motor drive using pulse width modulation control is applied to many electric devices for industrial machinery, automotive, railway cars and so on, and it is effective to decrease a dissipation power. For further improvement of the efficiency of the motor, it is said that higher voltage is applied to motors. However, if the high voltage is applied to the motor winding, a partial discharge (PD) is generated between motor windings because of a so-called inverter surge voltage generation. The inverter surge voltage is induced by the mismatching of the impedance and it sometimes becomes twice of the applied voltage. The generated PD induces some degradation of the covering material, and then finally it makes a electrical breakdown in it. It is usually said that the PD inception voltage depends on the value of surge voltage and the kind of insulating material. Furthermore, it is also said a space charge accumulation in the insulating material affects on the PD inception voltage. However, at present, it has not been reported about the effect of the space charge accumulation in the covering material for motor windings on the PD inception voltage. Therefore in this study, we investigated the space charge accumulation property of PAI under square wave voltage by using Pulsed Electro-Acoustic (PEA) method. From the measurement results, it was found that the space charge accumulation in PAI under the square wave voltage with high frequency of 1 kHz.