Study of Star-Delta Soft Switch to Extend AC Motor High Speed Operation Region

In traction application, speed range of motor is an important index for motor drives. In AC motor control, the maximum speed of the motor is limited by the output voltage capability of the traditional three-phase inverter with star connenction of windings. Switching the star connection to delta connection in high-speed range can extend the speed range. In order to extend the speed operation region, star-delta switch proposed by many literatures relies on mechanical relay, which needs a dead zone of tens of milliseconds and seriously affects torque output. Besides, the traditional method will cause current overshoot during the switch transient process, decreasing the device security and reliability. Aiming at the defects existing in the star-delta hard switching, this paper proposes a star-delta soft switching method. Without adding extra power electronics devices, DC-bus capacitor is used to provide the path of zero axis current in the transient process, which helps to achieve the smooth torque output and zero current switch in the transient. Experiments have been done to validate the performance of the proposed method. The switching transient from star to delta connection in the motor drive can be much more stable than hard switching method.