With the gradual increase of the scale of modern industrial production, the matching production equipment is also developing towards integration, large scale and large scale. The motor power of large mechanical equipment is also increasing. High voltage grade, large capacity and high performance motors become the most important direction. For all kinds of large ac and dc motors used for transmission, such as rolling mills, auxiliary machines for power stations, blast furnace fans, railway traction, rail transit, ship power, drainage and irrigation pumps, the capacity of single machine is constantly expanding, and the variety is also constantly increasing. This also causes the motor production enterprises to close to the high voltage large and medium motor industry, in order to improve their competitiveness.