Asynchronous operation characteristics and soft-starting method for the brushless doubly-fed motor

The brushless doubly-fed motor (BDFM) is versatile and can perform under several modes of operation. Other than the commonly used doubly-fed mode, BDFM can also operate in an asynchronous mode wherein the control winding of the motor is short circuited. The proposed study first analysed the mechanical characteristics of BDFM operation under this mode, whereby it was noted that there exist two induction torque components corresponding to different pole pairs within the motor. The motor, therefore, behaved as a conventional induction machine. Based on the above analysis and by introducing the concept of equivalent resistance of the machine-side inverter, an asynchronous soft-starting method for BDFM is proposed herein. By switching one side of the inverter bridge arms to the off state and adjusting the duty cycle on the other side, a controllable equivalent resistance is obtained and inserted into the control winding of BDFM. Consequently, the starting characteristic of BDFM can be improved to achieve soft start-up of the motor. Finally, experiments were conducted on a 30 kW BDFM prototype, of which, the results verify the accuracy of the torque analysis and improved performance of the proposed soft-starting method.