A finite element post-processing for skew effects in brushless doubly-fed induction machines

Abstract

Brushless doubly-fed induction machines have great potential as variable-speed generators in wind turbines. Undesired space harmonics exist because the special rotor needs to couple both stator windings which with different pole-pair numbers and different frequencies. These undesired space harmonics lead to a bigger torque ripple compared with conventional induction machines. Previously, a 2D multi-slice finite element (FE) method was applied to study the effects of rotor skew on torque responses in brushless DFIMs. It results in a significant computing time because several 2D FE slices are coupled and calculated simultaneously in one model. It is not efficient to use such a model to predict how much average torques and torque ripples would be reduced by applying skewed slots at the beginning of design. This paper makes use of normal 2D FEM results and applies skew factors in post-processing to investigate the influence of rotor skew on the torque responses. The proposed method can give an approximate prediction of skew effects on torque responses with limited computing time.