Study on accumulation mechanism of space charge in covering insulating material for motor windings

In this study, we examined the accumulation mechanism of the space charge in a covering insulating material of motor windings under high voltage based on measurement results of space charge distributions. 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. For further improvement of the efficiency of the motor, the voltage of square wave applied to motors should be increased. However, as the result of it, the partial discharge (PD) may generate between the motor windings and stator. In a study of the PD, with respect to the partial discharge inception voltage (PDIV), it is concerned that a space charge accumulation in the covering insulating material for motor windings strongly affects the decrease of PDIV. Furthermore, with the covering insulating material for motor windings is thinning by the PD, it is concerned that an electrical breakdown may also be affected by the space charge accumulation in the covering insulating material because the accumulation emphasizes the local electric field in the material. However, there are few reports for the space charge accumulation property in the covering insulating material for motor windings. Therefore, we have investigated it using Pulsed Electro-Acoustic (PEA) method. In this paper, we examined the mechanism of the space charge accumulation by comparing the space charge accumulation property in polyamide-imide, which is used as an actual covering insulating material for motor windings, to that in polyimide, which is a typical engineering plastic.