In order to solve the problems of ordinary mill motors are usually started with a star - delta starter motor while increasing the capacity of the way , and the normal operation of the motor load is less than the rated motor load , causing big horse car phenomenon, energy wastage . A star - delta starter mode when the load carried by the motor is large, resulting in very difficult when the motor starts , often after several start, the ball after the formation of a certain coherence to start up . And starting current is very large, the impact of serious harm to the grid , sometimes occur burned contactors, circuit , motor and so on. Impact and mechanical parts , reduce the life of the equipment. Normal production to bring some threats and undue economic losses. Using slip speed motor start difficult problem to solve , but the low slip speed efficiency , equipment maintenance for large users of energy wastage caused by maintenance problems and other issues can not be solved mill reality.