Computationally Efficient Calculation of Skew Effects in Brushless Doubly-Fed Induction Machines

Brushless doubly-fed induction machines (DFIMs) have great potential as variablespeed generators in wind turbines. Undesired space harmonics exist because the special rotor needs to couple to two stator windings with different pole-pair numbers and different frequencies. These undesired space harmonics could lead to noise, vibrations and low power quality. Applying skew benefits to overcome these drawbacks. Previously, a 2D multi-slice finite element (FE) method was applied to study the effects of skew which was time consuming. In the stage of initial design, it is not efficient to use such a model to predict how much the average torque and the torque ripple would be reduced by skewing slots. This paper makes use of normal 2D FE results and applies skew factors in post-processing to investigate the influence of rotor skew. It also proves that to apply skew factors appropriately, not only the space-harmonic order needs to be considered, but also the time-harmonic order. The proposed method can give an approximate prediction of skew effects with limited computing time. The results also indicate that skewing the rotor slot over one stator slot pitch could be a good choice to minimize the torque ripple in a brushless DFIM with a nested-loop rotor.