Starting Performance Research of a High-power Middle-voltage Induction Motor Soft Starter Based on the On-off Transformer

The main circuit of the low-voltage three-phase induction motor soft starter almost is based on a adjusting-voltage circuit of SCR. Because of the restriction of SCR withstand-voltage for the middle-voltage (3 KV~10 KV) induction motor, although it is solved by SCR connection in series, SCR parameters will change after using some period of time, which make SCR performance of voltage-equalizing lower. The paper introduces a new type middle-voltage soft starter based on the on-off transformer. The transformer's primary winding is connected in series with the motor's stator. The secondary winding is connected with a pair of anti-parallel SCR. The voltage of the primary winding is continuously changed by controlling the firing angle of SCR, then the voltage of the motor's stator is continuously changed and the soft start is realized. At the same time, the problem of SCR withstand-voltage is solved. The starting performance of the soft starter is fully analyzed by experiment in the paper. The control method of eliminating the torque pulsation is produced and the soft starting control of the constant current is realized. According to the test on site, the high-power middle-voltage induction motor can start steadily and reliably with the new type soft starter