One-Digital-Sampling Commutation Method for Low Inductance Brushless DC Motor

In a coreless stator brushless DC motor, ripple-less torque control is a challenging issue. Especially, due to its low inductance, conventional commutation methods that exclude the effects of the stator resistance and the digital sampling are less effective and even detrimental to minimizing the torque ripple. To overcome the limitations, this paper proposes a new commutation method for low inductance brushless DC motor. The proposed method synchronizes the period of the commutation region with one digital sampling period and it also considers the effect of stator resistance. Three cases of the proposed method, depending on the relation between the sampling period and the motor electric constant, are introduced. In addition, the back electromotive force is measured offline and the time delay due to the full-digital system is also compensated for the accurate duty ratio calculation. The validation of the proposed method is demonstrated by the experiment and the results show that the torque ripple is reduced by 18% compared to the conventional method.