Brushless doubly-fed induction machines: Torque ripple

The brushless Doubly-Fed Induction Machine (DFIM) without its brush-gear and slip-rings seems interesting as successor of the normal DFIG in wind turbine drivetrains. However, the brushless DFIM magnetic field has a rich space-harmonic spectrum, which causes additional torque ripple. This paper focuses on torque ripple in the brushless DFIM. The causes and origin of torque ripple are discussed and an analytical model is developed that derives the torque ripple from the air-gap magnetic field distribution. Further, a finite element method for torque ripple calculation is presented. Both methods are used to calculate the torque response of a case study machine. Results are compared and the accuracy of the analytical model is validated with good result. The analytical model is then used to further analyse the torque ripple, resulting in exact torque ripple frequencies and additional insight in the cause of the torque ripple.