Effects of rotor skew on the performance of brushless doubly-fed induction machine

Brushless doubly-fed induction machines (BDFIM) have great potential as variable-speed generators in larger-scale wind turbines. Undesired space harmonics exist because the special rotor needs to couple both two stator windings which with different pole-pair numbers and different frequencies. These rich space harmonics lead to a bigger torque ripple comparing to the normal induction machine. This paper makes use of 2D multi-slice finite element (FE) method to study the effect of the rotor skew on the performance of the BDFIM in the cases without and with saturation. The results show the torque ripple is reduced significantly by skewing the rotor both in those two situations. But the rotor skew has negligible influence on the harmonics due to the saturation effect. Furthermore, the rotor skew has little influence on the total amount of losses. However, based on the simulation results, it redistributes the core losses along the axial direction in the BDFIM.