A Smooth Torque Control Strategy for Brushless DC Motor in Braking Operation

In braking operation, when the unipolar modulation pattern is adopted by brushless DC motor (BLDCM), there exists not only the commutation torque ripple, but also a problem concerning controllability of braking torque. This paper conducts a research into eight unipolar modulation patterns and proposes a smooth braking torque control strategy for BLDCM. First, the controllability principle of braking torque is theoretically analyzed under different modulation patterns. Then, this paper derives and compares the speed ranges of each pattern where the braking torque is controllable and the commutation torque ripple can be reduced. On this basis, according to the torque control performance of each modulation pattern in different speed ranges, an optimized combination modulation method is developed in full-speed range. During the whole braking process, the proposed control strategy ensures that the braking torque is controllable, and meanwhile the commutation torque ripple can be reduced, thus obtaining good braking torque control performance. The effectiveness of the proposed strategy is verified by experimental results.