Comparison of switching schemes for brushless DC motor drives

Permanent magnet brushless dc (BLDC) motors are widely used in high performance applications, where torque smoothness is essential. In this paper, commutation torque ripple due to phase commutation of brushless dc motor are described for two commonly used switching schemes. In a conventional three-phase BLDC machine with 120° electrical conduction, commutation torque ripple occurs due to change over of load current from one phase to another at every 60 electrical degrees. Similar is the case for 180° conduction mode due to change of phase currents at 60° intervals. A detailed analysis shown in this paper reveals that, the torque ripple reduces for 180° switching mode at higher speeds due to reduction of both torque ripple magnitude and commutation time, compared to the 120° conduction mode of the inverter. Suitable computer simulations and experiments were performed to verify the results.