Compared with traditional surfaced-mounted permanent magnet (SPM) machine, the consequent- pole permanent magnet (CPM) structures show a higher permanent magnet (PM) utilization ratio and comparable torque capability. Thus, the various PM arrangements are developed and studied in recent years. However, the corresponding topology evolution and the underlying air-gap field modulation (AGFM) mechanism are unrevealed. To present an in-depth understanding of their torque production principle, the topology evaluation and their AGFM effect in the surface-inset CPM machines were investigated and compared.
Firstly, topology evaluation of the CPM machines with different PM arrangements was illustrated by adopting PM unit concept. According to their relationship of the rotor modulated teeth and PM pole-pair number, the CPM machines with different pole-pair PMs in a PM unit was developed. Different from conventional CPM configurations share with same PM pole-pair and rotor modulated teeth, the unconventional CPM machines have the unequal number of the PM pole-pair and rotor modulated teeth. Some topological evolution rules can be summarized as: (1) When n (≥2) pole-pair of PMs are included in a PM unit, n typical non-traditional CPM structures can be formed. (2) An unconventional CPM machine with n (≥2) pole-pair of PMs in a PM unit, the maximum number of the rotor modulated teeth is n-1. (3) Because the S pole (or N pole) and the adjacent N and S poles in a PM unit can be replaced by the rotor iron teeth, there are two types of non-traditional CPM structures can be formed when the rotor modulation tooth is 1 in a PM unit.
To identify the different PM arrangement on their air-gap magnetic field, torque contributions of the various air-gap field harmonics were quantified by Maxwell Stress Tensor method (MSTD). The simulation results show that various air-gap field modulated harmonics are responsible for torque generation in three typical CPM machines, which implies that the pronounced AGFM effect can be obtained in conventional and unconventional CPM cases. Besides, compared with other modulated field harmonics, the |Zr-Zs| (5th), 2Zr (8th) and |Zr+Zs|(13rd) contributes a relatively higher torque components in conventional CPM machine. However, apart from the |Zr-Zs| (5th), 2Zr (8th) and |Zr+Zs|(13rd), the Zm (2nd)、|Zm+Zs| (7th) and |Zm-2Zs| (16th) are also participating in torque production. Although more air-gap field harmonics are responsible for torque generation in unconventional CPM structure, its total torque proportion of the modulated field harmonics is lower than the conventional CPM case. This is mainly due to the relatively lower amplitudes of the Zr (4th) air-gap flux density in unconventional CPM machines. To confirm the abovementioned analysis, a CPM machine prototype was manufactured. Some tested results on the open-circuit back-EMF and electromagnetic torques were measured and compared with simulations, which verifies the effectiveness of the theoretical and simulated analyses.
Some conclusions can be drawn from the simulation and experimental analyses: (1) Because of the unequal number of the PM pole-pair and rotor modulated teeth, the unconventional CPM machines exhibits a relatively lower Zr order air-gap flux density and hence resulting a lower torque capability. (2) Although various air-gap field harmonics can be modulated in unconventional CPM structure, the total torque proportion of the modulated field harmonics is less than 2 %, which indicates that the unconventional CPM machine can be seen as a typical traditional PM synchronous machine with only one working harmonic for torque generation. (3) Due to the higher torque proportion of the air-gap field modulation harmonics, the conventional CPM machines show a pronounced AGFM effect than unconventional counterparts.
李亚, 周庆林, 丁石川, 李伟, 杭俊. 表嵌式交替极永磁电机拓扑演化及气隙磁场调制效应研究[J]. 电工技术学报, 2023, 38(12): 3188-3198.
Li Ya, Zhou Qinglin, Ding Shichuan, Li Wei, Hang Jun. Investigation of Topologies Evolution and Air-Gap Field Modulation Effect on Surface-Inset Consequent-Pole PM Machines. Transactions of China Electrotechnical Society, 2023, 38(12): 3188-3198.
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