Abstract

Rare-earth based permanent magnet synchronous motors (PMSMs) are increasingly being utilized for traction applications thanks to their high torque/power densities along with high efficiencies. The high power/torque density feature leads to a lower weight and more powerful machine. On the other hand, the high efficiency feature will lead to less overall power consumption and losses. However, PMSMs have the known drawbacks of high and unstable price per kg of the rare-earth PM materials which drove electric machine researchers to investigate alternative technologies for fully or partially limiting the reliance upon rare-earth PMs.
One of the promising solutions is by replacing the rare-earth PM by another abundantly available and rare-earth-free PM. However, full replacement of the rare-earth PM will lead to significant reduction in torque density. Depending on the shape of the demagnetization characteristics (DCs) of the rare-earth-free PM, it is possible to achieve a controllable PM working point throughout the machine operation. These machines are commonly known as Variable Flux Motors (VFMs). To cope up with the low torque density problem of VFMs, hybrid PM VFMs are being investigated. These machines include a blend of rare-earth and rare-earth-free PMs. In general, the combination of the two PMs improves the torque density and can have a strong effect on the de/remagnetization requirements. Therefore, the performance of a hybrid PM VFM relies mainly upon the position/size of the PMs. The design of the hybrid PM VFMs is similar to designing a rare-earth PMSM along with a VFM in a single rotor structure. The result is a machine that has higher torque density than VFM with limited flux regulation range. The range of flux regulation affects the high efficiency improvement gain in hybrid PM VFMs. Therefore, the PM flux linkage in these machines can not be reduced fully to zero. In addition, its demagnetization range is limited by the rare-earth PM DCs. Therefore, as these machines include mixed features of PMSM and VFM, the commonly adopted design methods for PMSM has to be modified this is
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because mixed performance metrics do exist for the hybrid machines. Meanwhile, it is important to specify a unified performance assessment criterion that can be applied to any VFM design which will enable fast yet comprehensive performance evaluation of VFM topologies.
In addition, a systematic PM sizing methodology is presented for deciding the dimensions of the non-rare-earth and rare-earth PMs in a magnetic circuit according to a specified position. Thereby, the scaling-up effects can be investigated by applying the sizing approach and the performance assessment template. Meanwhile, the effect of rare-earth PM irreversible demagnetization is investigated in detail.
Finally, a new hybrid PM VFM design concept is also proposed and applied to a spoke type machine showing superiority in terms of torque density and re/demagnetization performance. The proposed machine contains a hybrid PM magnetic circuit of series-hybrid nature made by a rare-earth PM along with a parallel-hybrid branch made of low-cost PMs of different dimensions and grades.