Comparative study of biased flux permanent magnet machines with doubly salient permanent magnet machines considering with influence of flux focusing

• Different from DSPMMs in which PMs also are located in the stator back-iron, BFPMMs have flexible rotor-pole number and symmetrical bipolar phase flux-linkage as well as identical three-phase equilibrium.
• By introducing the inner-, outer-, and combined-type flux-focusing structures into BFPMMs, the torque densities are enhanced by 19%, 18.8% and 40.3% respectively while the PM utilization efficiencies are increased by 19%, 38.6% and 23.3% respectively.
• Compared with DSPMMs based on the inner-type flux-focusing structure, BFPMMs exhibit 18% higher average torque and 80% lower torque ripple.

Different from the doubly salient permanent magnet (PM) machines (DSPMMs), the stator/rotor pole number combination (Ns/Nr) in biased flux PM machines (BFPMMs) is flexible and Nr can be any integer except the phase number and its multiples. When Ns/Nr differ by one, 6-stator pole BFPMM exhibits bipolar phase flux-linkage and symmetrical phase back-electromotive force waveforms. Based on the optimal 6/7 Ns/Nr and the rated copper loss, the torque density of BFPMMs is enhanced by 19%, 18.8% and 40.3% respectively when employing the flux-focusing structures with inner-type, outer-type and combined-type. Meanwhile, the PM utilization efficiency is also enhanced. Moreover, based on the inner-type flux-focusing structure, the optimized 6/7 Ns/Nr BFPMM exhibits about 18% higher average torque and 80% lower torque ripple than the optimized 6/4 Ns/Nr DSPMM. Further, the unbalance between phases which is observed in DSPMMs is overcome in the BFPMMs. The analyses are validated experimentally on a prototype machine.