Abstract：The Yokeless and segmented armature (YASA) axial flux machine has a wide range of applications in electric vehicles, aerospace, and industrial robotics due to its flat and compact structure, high operating efficiency, and high power density. However, the yokeless structure of its stator core leads to high cogging torque and torque pulsations. Additionally, the high power density and poor heat dissipation conditions at the stator end of such machines hinder further improvement of electromagnetic performance. The choice of the rotor topology, cooling method, and design of the cooling structure directly affect the electromagnetic performance of the machine. Firstly, the rotor topology directly affects the harmonic content in the air gap magnetic field. The equivalent magnetic circuit of the magnetic pole radial combined YASA machine is analyzed, the power equation under the rotor topology is derived, and the weakening mechanism of the cogging torque is studied. Then, based on the three-dimensional finite element electromagnetic simulation calculation, the effects of three rotor topologies— surface-mounted, Halbach array, and magnetic pole radial combination—are compared in terms of magnetic field distribution, air gap flux density, no-load back EMF, and torque characteristics. To further enhance the electromagnetic performance of the YASA machine, a double-ring water-cooling structure is proposed to be embedded in the stator support. Based on the accurate calculation of machine loss, the temperature rise characteristics of the machine under natural cooling and double-ring water-cooling conditions are calculated using the computational fluid dynamics (CFD) method. The temperature field and fluid field of the machine under different current conditions are then analyzed. Finally, the electromagnetic and thermal characteristics analysis results, along with the effectiveness of the proposed cooling structure, are verified through experiments. The following conclusions can be drawn. (1) When the auxiliary magnetic pole angle is 0<γ<T, the waveform of the cogging torque in the first half cycle is negative. When T<γ<2T, the waveform of cogging torque in the first half cycle is positive. The magnetic pole radial combined YASA machine utilizes the superposition of cogging torque to mitigate it by adjusting the main and auxiliary magnetic pole angles in the inner and outer layers. (2) The electromagnetic performance of the traditional surface-mounted type is the worst. The performance of the magnetic pole radial combined type and the Halbach array YASA machine is basically the same in terms of magnetic field distribution, air gap flux density, and no-load back EMF. Compared with the Halbach array, the pole radial combined YASA machine significantly reduces the cogging torque and torque ripple at the expense of some electromagnetic torque. (3) The double-ring water-cooling structure embedded in the stator support can significantly reduce the temperature rise of the stator core and windings. At the same time, the problem of excessive temperature rise at the inner and outer ends of the windings has been effectively solved. When the maximum temperature of the windings under water-cooling and naturally cooling conditions is kept the same, the current density can reach 16.2 A/mm2, which is 2.9 times that of naturally cooling conditions. Consequently, the torque density and power density of the machine can be significantly increased.
高鹏, 任红兴, 王晓远, 王力新, 赵晓晓. 磁极径向组合式定子无磁轭模块化轴向磁通电机磁热特性分析[J]. 电工技术学报, 2025, 40(24): 7969-7983.
Gao Peng, Ren Hongxing, Wang Xiaoyuan, Wang Lixin, Zhao Xiaoxiao. Magnetic and Thermal Characteristics Analysis of Magnetic Pole Radial Combination Yokeless and Segmented Armature Axial Flux Machine. Transactions of China Electrotechnical Society, 2025, 40(24): 7969-7983.
[1] Gadiyar N, Van Verdeghem J, Severson E L.A review of axial flux permanent magnet machine tech- nology[J]. IEEE Transactions on Industry Appli- cations, 2023, 59(4): 3920-3933. [2] 王晓光, 尹浩, 余仁伟. 轴向磁通无铁心永磁电机多层矩形扁线绕组涡流损耗解析计算及优化[J]. 电工技术学报, 2023, 38(12): 3130-3140. Wang Xiaoguang, Yin Hao, Yu Renwei.Analytical calculation and parameter optimization of eddy current loss for coreless axial flux permanent magnet synchronous machine with multilayer flat wire winding[J]. Transactions of China Electrotechnical Society, 2023, 38(12): 3130-3140. [3] 刘细平, 朱治国, 陈栋, 等. 基于转子永磁分段优化的轴向磁通永磁电机涡流损耗分析与试验[J]. 电工技术学报, 2024, 39(12): 3616-3629. Liu Xiping, Zhu Zhiguo, Chen Dong, et al.Analysis and experiment of eddy current loss of axial flux permanent magnet motor based on rotor segment optimization[J]. Transactions of China Electrotech- nical Society, 2024, 39(12): 3616-3629. [4] 武岳, 张志锋. 轴向磁通永磁电机扁铜线交流铜耗的混合解析计算及抑制[J]. 电工技术学报, 2023, 38(24): 6609-6618. Wu Yue, Zhang Zhifeng.Hybrid analytical calcu- lation and suppression of AC copper loss of flat copper wire in axial flux permanent magnet motor[J]. Transactions of China Electrotechnical Society, 2023, 38(24): 6609-6618. [5] 关涛, 刘大猛, 何永勇. 永磁轮毂电机技术发展综述[J]. 电工技术学报, 2024, 39(2): 378-396. Guan Tao, Liu Dameng, He Yongyong.Review on development of permanent magnet in-wheel motors[J]. Transactions of China Electrotechnical Society, 2024, 39(2): 378-396. [6] Aydin M, Zhu Z Q, Lipo T A, et al.Minimization of cogging torque in axial-flux permanent-magnet machines: design concepts[J]. IEEE Transactions on Magnetics, 2007, 43(9): 3614-3622. [7] Aydin M, Gulec M.Reduction of cogging torque in double-rotor axial-flux permanent-magnet disk motors: a review of cost-effective magnet-skewing techniques with experimental verification[J]. IEEE Transactions on Industrial Electronics, 2014, 61(9): 5025-5034. [8] Tiegna H, Amara Y, Barakat G.Study of cogging torque in axial flux permanent magnet machines using an analytical model[J]. IEEE Transactions on Mag- netics, 2014, 50(2): 7020904. [9] Jamali A S, Ardebili M.Cogging torque reduction in axial-flux permanent magnet wind generators with yokeless and segmented armature by radially segmented and peripherally shifted magnet pieces[J]. Renewable Energy, 2016, 99: 95-106. [10] 左曙光, 吴双龙, 吴旭东, 等. 轴向磁通永磁同步电机转矩解析模型和转矩优化[J]. 电工技术学报, 2016, 31(23): 46-53. Zuo Shuguang, Wu Shuanglong, Wu Xudong, et al.Analytical model and optimization of torque of an axial flux permanent magnet synchronous motor[J]. Transactions of China Electrotechnical Society, 2016, 31(23): 46-53. [11] Xiao Linyuan, Li Jian, Qu Ronghai, et al.Cogging torque analysis and minimization of axial flux PM machines with combined rectangle-shaped magnet[J]. IEEE Transactions on Industry Applications, 2017, 53(2): 1018-1027. [12] 武岳, 张志锋, 张冉. 表贴式轴向磁通电机梯形削极分段结构研究[J]. 电机与控制学报, 2024, 28(3): 84-95. Wu Yue, Zhang Zhifeng, Zhang Ran.Trapezoidal shaping segmented structure of surface mount axial flux permanent magnet motor[J]. Electric Machines and Control, 2024, 28(3): 84-95. [13] Zhao Jilong, Wang Qing, Han Qingfeng.Reducing torque ripple through innovative configuration of permanent magnet based on air gap field modulation theory in a novel axial flux reversal permanent magnet machine[J]. IEEE Access, 2024, 12: 54901-54912. [14] 王晓远, 尹春霞, 李天元. 新型Halbach阵列盘式永磁电机的磁场解析[J]. 电机与控制学报, 2024, 28(2): 21-31. Wang Xiaoyuan, Yin Chunxia, Li Tianyuan.Magnetic field analysis for axial flux permanent magnet motor with novel Halbach array[J]. Electric Machines and Control, 2024, 28(2): 21-31. [15] Liu Kai, Yin Ming, Hua Wei, et al.Design and analysis of Halbach ironless flywheel BLDC motor/ generators[J]. IEEE Transactions on Magnetics, 2018, 54(11): 8109305. [16] 魏嘉麟, 王又珑, 温旭辉, 等. 300 kW航空高速永磁发电机分析与设计[J]. 电机与控制学报, 2023, 27(9): 63-72. Wei Jialin, Wang Youlong, Wen Xuhui, et al.Analysis and design of 300 kW aerospace high-speed PM generator[J]. Electric Machines and Control, 2023, 27(9): 63-72. [17] 徐磊, 朱孝勇, 张超, 等. 磁极径向组合轴向磁场永磁电机转矩品质分析与优化设计[J]. 中国电机工程学报, 2021, 41(6): 1971-1983. Xu Lei, Zhu Xiaoyong, Zhang Chao, et al.Torque quality analysis and optimization design of axial field machine with radial combined permanent magnet poles[J]. Proceedings of the CSEE, 2021, 41(6): 1971-1983. [18] Yang Lu, Yang Kai, Sun Songjun, et al.Study on the influence of a combined-Halbach array for the axial flux permanent magnet electrical machine with yokeless and segmented armature[J]. IEEE Transa- ctions on Magnetics, 2024, 60(3): 8101005. [19] 孙明灿, 唐任远, 韩雪岩, 等. 高频非晶合金轴向磁通永磁电机不同冷却方案温度场分析[J]. 电机与控制学报, 2018, 22(2): 1-8, 23. Sun Mingcan, Tang Renyuan, Han Xueyan, et al.Temperature field analysis of a high frequency amorphous alloy axial flux permanent magnet machine with different cooling schemes[J]. Electric Machines and Control, 2018, 22(2): 1-8, 23. [20] 韩雪岩, 张华伟, 徐昕, 等. 基于计算流体力学的非晶合金轴向磁通永磁电机冷却系统设计[J]. 电工技术学报, 2017, 32(20): 189-197. Han Xueyan, Zhang Huawei, Xu Xin, et al.Design of cooling system for amorphous alloy axial flux permanent magnet motor based on computational fluid dynamics[J]. Transactions of China Electro- technical Society, 2017, 32(20): 189-197. [21] Lai Junquan, Li Jian, Xiao Tianzheng.Design of a compact axial flux permanent magnet machine for hybrid electric vehicle[J]. IEEE Transactions on Industrial Electronics, 2021, 68(8): 6630-6639. [22] Cao Junci, Zhou Boyu, Li Dong, et al.Analysis of heat transfer and cooling structure of dual-waterway parallel axial flux permanent magnet synchronous motor for electric vehicles[C]//2022 7th Asia Con- ference on Power and Electrical Engineering (ACPEE), Hangzhou, China, 2022: 992-997. [23] Chang Jiujian, Fan Yanen, Wu Jinglai, et al.A yokeless and segmented armature axial flux machine with novel cooling system for in-wheel traction appli- cations[J]. IEEE Transactions on Industrial Elec- tronics, 2021, 68(5): 4131-4140. [24] Li Jian, Lu Yang, Cho Y H, et al.Design, analysis, and prototyping of a water-cooled axial-flux permanent- magnet machine for large-power direct-driven appli- cations[J]. IEEE Transactions on Industry Appli- cations, 2019, 55(4): 3555-3565. [25] Zhang Bin, Seidler T, Dierken R, et al.Development of a yokeless and segmented armature axial flux machine[J]. IEEE Transactions on Industrial Elec- tronics, 2016, 63(4): 2062-2071. [26] Polikarpova M, Ponomarev P, Lindh P, et al.Hybrid cooling method of axial-flux permanent-magnet machines for vehicle applications[J]. IEEE Transa- ctions on Industrial Electronics, 2015, 62(12): 7382-7390. [27] Wang Xiaoyuan, Zhang Guodong, Gao Peng.Analysis and optimization of cogging torque for axial flux machine with Halbach permanent magnet array[C]//2022 25th International Conference on Electrical Machines and Systems (ICEMS), Chiang Mai, Thailand, 2022: 1-6. [28] Roy R, Ramasami S, Chokkalingam L N.Review on thermal behavior and cooling aspects of axial flux permanent magnet motors-a mechanical approach[J]. IEEE Access, 2023, 11: 6822-6836. [29] Gao Peng, Zhao Xiaoxiao, Gu Yuxi, et al.Coupled electromagnetic and thermal analysis of an axial flux permanent magnet synchronous machines with printed circuit board winding[J]. International Journal of Applied Electromagnetics and Mechanics, 2020, 63(3): 449-464. [30] Liu Ye, Wei Ziqiang, Ma Kang, et al.Improved thermal management and analysis for high-power heat pipe cooled FSCW PMSM for HEV auxiliary power unit application[J]. IEEE Transactions on Energy Conversion, 2024, 39(2): 1154-1167. [31] 于占洋, 胡旭阳, 李岩, 等. 新型强迫风冷散热结构在高功率密度外转子表贴式PMSM上应用分析[J]. 电工技术学报, 2023, 38(24): 6668-6678. Yu Zhanyang, Hu Xuyang, Li Yan, et al.Application analysis of novel forced air-cooled in outer rotor surface-mounted PMSM with high power density[J]. Transactions of China Electrotechnical Society, 2023, 38(24): 6668-6678. [32] Zhu Gaojia, Li Longnü, Mei Yunhui, et al.Design and analysis of a self-circulated oil cooling system enclosed in hollow shafts for axial-flux PMSMs[J]. IEEE Transactions on Vehicular Technology, 2022, 71(5): 4879-4888. [33] 陈前, 周亚南, 徐高红, 等. 混合转子永磁电机双风道冷却系统设计[J]. 电气工程学报, 2024, 19(2): 1-8. Chen Qian, Zhou Yanan, Xu Gaohong, et al.Design of dual air duct cooling system for hybrid rotor permanent magnet motor[J]. Journal of Electrical Engineering, 2024, 19(2): 1-8. [34] 王晓远, 杜静娟. 应用CFD流固耦合热分析车用高功率密度电机的水冷系统[J]. 电工技术学报, 2015, 30(9): 30-38. Wang Xiaoyuan, Du Jingjuan.Design and analysis of water cooling system for HEVs high-power-density motor using CFD and thermal technology[J]. Transa- ctions of China Electrotechnical Society, 2015, 30(9): 30-38.
2025, Vol. 40 
