Harmonics study of nested-loop rotors in brushless doubly-fed induction machines

Abstract

Brushless doubly-fed induction machines (DFIMs) can be potential variable-speed generators for off-shore wind turbine applications. This special machine has two stator
windings with different pole-pairs. There is no direct magnetic coupling between two stator windings. A unique nested-loop rotor is designed to couple both stator windings. However, it results in lots of undesired space-harmonics in the airgap fields compared to conventional induction machines. There are few papers working on the design of nested-loop rotors. To design a better rotor, it is important to understand the complicated magnetic coupling in brushless DFIMs. This paper analyzes the space-harmonics and time-harmonics to provide a better understanding of the magnetic cross-coupling through the nested-loop rotor. The harmonic analysis is validated by FEM simulation and by taking measurements on a prototype in simple induction operation mode.