Analysis and mitigation of torsional vibration of PM brushless DC drives with direct torque controller

In this paper, the stator torsional vibration of a direct-torque-controlled permanent magnet brushless DC drive is investigated. To facilitate the analysis, a new method to measure the torsional mechanical transfer function of the drive system by utilizing the cogging torque as a constant excitation source is presented. Experimental investigation reveals that the torsional vibration in the prototype brushless DC drive is mainly attributed to cogging torque and commutation torque ripple, and it consists of harmonics at frequencies multiple of 6f_e, with different amplitudes affected by the magnitude of torque harmonics and the structural transfer function of the stator-mounting assembly. Alternate torque-ripple compensative strategies are studied and their effectiveness in minimizing torsional vibration is validated experimentally.