High torque density magnetically-geared switched flux permanent magnet machines

Due to the high price of rare-earth magnets, it is pertinent to develop high torque density permanent magnet (PM) machines with reduced magnet volume, which is the main purpose of this paper. This paper proposes novel high torque density magnetically-geared switched flux permanent magnet machines in which the stator is partitioned into two stators but with armature windings and PM excitations separately on each stator whilst the number of rotor segmented iron pieces is approximately twice or equal to the number of the stator teeth / poles. Therefore, the developed machines are double stator / single rotor machines. The armature outer stator is equipped with concentrated non-overlapping winding and identical to the stator of a fractional-slot PM machine, whilst the structure of the inner excitation stator has interior PMs with flux focusing, but structurally identical to the rotor of a conventional interior PM machine. The influence of design parameters is investigated at a fixed copper loss by a 2-dimensional finite element analysis method. Furthermore, the effect of different rotor and stator pole combinations is also analysed in detail. The electromagnetic performance of the proposed machines show that the odd rotor pole number machines are potentially better in terms of high torque and low torque ripple compared to the equivalent even rotor pole number machines under the same condition, albeit with potential unbalanced magnetic force which may be eliminated by doubling the stator and rotor pole numbers. Finally, the predicted electromagnetic performances of the proposed machines are validated by experiments on the fabricated prototype.