Summary form only given. Recently, dual-permanent-magnet-excited (DPME) machine has been proposed for low-speed large-torque direct drive applications [1]. Unlike traditional PM machines, the DPME machine employs two sets of PMs, one on stator and the other on rotor. It relies on the field harmonics to achieve electromechanical energy conversion, and the so-called bi-directional field modulation effect (BFME) is artfully engaged to guarantee the effective coupling between the magnetic field excited by the armature windings and those excited by the two sets of PMs. It has been demonstrated that in coaxial magnetic gears the shape factors of the ferromagnetic segments have profound impacts on the field modulation effect, and the transmitted torque density [2]. Therefore, the purpose of this paper is to investigate the optimum design method for improving the BFME of DPME machine, so as to further improve the pull-out torque of this new type of machine. Fig.1 shows the cross section view of the initial DPME machine, there are 23 rotor PMs and 27 stator PMs. Both rotor PMs and stator PMs are radially magnetized, thus each PM and its adjacent iron tooth form a pair of magnet poles. The three-phase armature windings are deployed in the 24 slots on stator, and there pole-pair number is equal to 4. The shape factors investigated are the inner width and the outer width of the rotor teeth, the inner width and the outer width of the stator teeth, and the depth of the rotor teeth, as shown in Fig.1. The depth of the stator teeth is not taken into consideration since it will affect the deployment of the armature windings. By using finite element method, the calculated impacts of these shape factors on the pull-out torque are also shown in Fig.1. Finally, the optimum design solution can be obtained by using statistical techniques such as response surface methodology. Fig.2 (a) shows the cross section view of the obtained optimum design solution, and its flux linkage distribution - t no-load is shown in Fig.2(b). Comparison of the initial machine and optimum machine has been conducted. The back EMF waveforms and the pull-out torques at different current density are given in Fig.2 (c) and (d). The results demonstrated that the pull-out torque can be improved by 20 .3 % with the volume of PM used decreased by 8 .9 %.
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