Design and Characteristic Analysis of a Wide-Narrow Stator Poles Axial Flux Switched Reluctance Machine
Yu Fengyuan1,2, Chen Hao1,2, Yan Wenju1,2, Liu Yongqiang1,2, Dong Linjie1,2
1. School of Electrical Engineering China University of Mining and Technology Xuzhou 221116 China; 2. Xuzhou Key Laboratory of New Energy Electric Vehicle Technology and Equipment Xuzhou 221116 China
Abstract：In recent years, switched reluctance motors (SRM) have been applied in electric vehicles and aerospace due to their simple and robust structure, good fault tolerance, large starting torque, and wide speed range. However, the low torque density and high torque ripple have limited their large-scale application. With the improvement of industrial manufacturing level, the axial flux motor (AFM) has gradually attracted attention due to its shorter axial length and higher power density. In this paper, a novel wide-narrow stator-pole axial flux SRM (WNS-AFSRM) is proposed and investigated. Firstly, the topology and operating principle of the WNS-AFSRM are introduced. The motor has a parallel double-stator structure in which the left and right stator discs maintain a short magnetic path by sharing one single rotor segment. The stator teeth are staggered between the wide and narrow poles, with concentrated windings wound on the wide stator poles and no windings on the narrow stator poles. Hence, better magnetic, thermal and mechanical isolation is achieved, and the reliability of the WNS-AFSRM is ensured. Secondly, the design flow chart is presented, the power equation is also given, and the initial structural parameters of the WNS-AFSRM are determined. The three-dimensional finite element method is employed to analyze the influence of six key dimensional parameters on the torque output capability. Then, four parameters with a considerable impact are selected, and the Taguchi algorithm is carried out to optimize these four key dimensional parameters to improve motor performance. Comparing the torque waveforms before and after the optimization shows that the maximum synthetic torque decreases from 34.21N·m to 31.01N·m, while the minimum synthetic torque increases from 19.46N·m to 21.71N·m. Thus, the torque ripple has been reduced by 39.9% from 63.1% to 37.9%. Besides, the average torque increases from 23.40N·m to 24.48N·m with an increment of 4.6%. Thirdly, the static and dynamic performance of the WNS-AFSRM is simulated, and the performance is compared with two other motors. The radial electromagnetic force is compared with a single-stator WNS-AFSRM under the same outer diameter. It is demonstrated that the proposed double-stator WNS-AFSRM structure could effectively reduce the unbalanced axial electromagnetic force of the rotor. Moreover, a comprehensive performance comparison is conducted with a radial flux SRM with the same wide-and-narrow stator-poles structure under the same outer diameter and axial length. It can be found that the inductance at the aligned position is almost equal for both motors, but the NWS-AFSRM has a lower unaligned inductance. In addition, the core mass, total mass and average torque per unit mass of the NWS-AFSRM are more significant than that of the NWS-SRM, verifying the increased torque density of the proposed NWS-AFSRM. Finally, the final dimensional parameters are given, and a 1.25kW prototype is manufactured. The measured static flux linkage and dynamic current waveforms verify the correctness of the finite element and simulation results. The following conclusions can be drawn from the simulation and experimental results: (1) The left and right stators maintain short magnetic path by sharing a common rotor segment, and the magnetic flux cancels each other at the unaligned position to obtain an appreciable maximum-minimum inductance ratio. (2) The influence of variations in vital dimensional parameters on the rated output torque performance is analyzed using the single variable method, and the multi-objective Taguchi optimization algorithm results in a 39.9% reduction in torque ripple and a 4.6% increase in average torque for the NWS-AFSRM. (3) The proposed double-stator NWS-AFSRM structure is compared with the single-stator NWS-AFSRM using the three-dimensional finite element method. The results show that the proposed structure could effectively reduce the unbalanced axial electromagnetic force and improve the reliability of the motor.
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2023, Vol. 38 
