Ripple Analysis and Reduction of a Dual-PM Direct-Drive Machine by a Harmonic Current Injection Approach

The dual permanent magnet machine (DPMM) possesses the characteristics of high torque and low speed which is suitable for direct-drive equipment. Because of the low magnetic reluctance of this motor, the torque is very sensitive to dimensional tolerance. The torque ripple in a synchronous motor may result in noise and vibration. This research analyzes the torque ripple in several different scenarios on the precise model and model with partial random mechanical tolerances. Through derivation calculation and simulation analysis, it is found that the impact of mechanical tolerance on torque ripple can be predicted. It is proven that the impact of mechanical tolerances on torque ripple is significant through analysis. The torque harmonics brought by tolerances is a major part of torque ripple, and its impact is inevitable in the motor entity. In order to reduce the torque ripples, this paper proposes a harmonic current injection (HCI) method. HCI, as a feedforward compensation of the current loop, generates torque harmonics purposefully to counteract torque ripple. A DPMM with a split-tooth structure is simulated and tested in this paper to demonstrate the analysis of torque fluctuations and the effectiveness of HCI.