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.

I. Introduction

Permanent Magnet Brushless DC (PM BLDC) motor has been widely used in electric vehicles, robots and several industrial equipment thanks to its distinctive characteristics such as high efficiency, high torque density and simple control technique. Especially, the attention toward the coreless stator PM BLDC has continuously been increasing as it has no iron loss, magnetic saturation, or cogging torque [1]–[3]. From these advantageous properties, the total torque and its ripple are only determined by the magnetic torque, which is the product of phase current and back electromotive force (EMF). Meanwhile, since the magnetic flux of coreless stator PM BLDC does not pass through the core which has high permeability, its inductance is lower than conventional motors. Therefore, a tiny error in the control parameter could produce a large current ripple which induces the torque ripple [4]. Consequently, the precise control of the phase current that matches with back EMF becomes necessary.