The brushless doubly-fed reluctance machine has two sets of stator windings of different pole number linked by a reluctance type of rotor. The rotor modulates the stator MMF from one winding so that flux links the other winding creating mutual coupling. This is still a research machine and the design methodology is under development. Past works indicates that a ducted rotor with radial laminations is best suited to be the reluctance rotor. In this paper the geometry of the rotor is investigated. If the ducts are too thin then the reluctance ratio is low and the coupling is affected. If they are too wide then the reluctance path for the coupling flux is high and the machine will saturate quickly. It is found, using finite element analysis, that the duct ratio should be about 38% from the results in the paper. This is in line with previous work on synchronous reluctance machines that can also use ducted rotors (although they use axial lamination which are inappropriate for this application).
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