Comparison of nested-loop rotors in brushless doubly-fed induction machines

The brushless doubly-fed induction machine (DFIM) has great potential as a variable-speed generator for wind turbine applications. This special machine has a richer space-harmonic spectrum due to its special nested-loop rotor construction compared with conventional induction machines. It may result in higher iron losses, higher torque ripple and more time-harmonics adding to the grid total harmonic distortion (THD). This paper applies the 2D finite element (FE) model to investigate several different nested-loop rotor constructions. It shows the outer loop makes more contribution to the torque while the inner loop plays a small role in the torque production. The most outer loop determines the overall THD level while the inner one has little influence on it. The THD could be reduced by increasing the number of the outer loops. More machine performances could be studied to derive more guidelines for designing the middle loops.