Improved PWM-OFF-PWM to Reduce Commutation Torque Ripple of Brushless DC Motor Under Braking Conditions

This paper proposes an improved PWM-OFF-PWM of the brushless DC motor under braking condition which can reduce commutation torque ripple. Compared with other brake modulation methods, PWM-OFF-PWM can restrain non-commutation torque ripple attributed to non-conductive freewheeling, while incapable of suppressing commutation torque ripple at low speeds effectively. Therefore, to solve commutation torque ripple caused by the turn-off time being shorter than turn-on time, an improved PWM-OFF-PWM is proposed in this paper. Moreover, to describe the influence factors of commutation torque ripple, the formulas of the duty cycle of switching tube in the commutation period and that during non-commutation period is derived. We can conclude from mathematical analysis that when duty cycles of outgoing and incoming phases satisfy derived formula, the non-commutation current in commutation interval will be more stable, which can contribute to suppress the commutation torque ripple at low speed under PWM-OFF-PWM mode and to improve the operation stability of BLDCM. The correctness and feasibility of the proposed method can be proved by simulation and experimental results.