Author: Zhu, Jingwei
Title: Magnetic topology analysis of the switched reluctance motor for torque improvement
Degree: M.Phil.
Year: 2018
Subject: Hong Kong Polytechnic University -- Dissertations
Electric vehicles
Reluctance motors
Department: Department of Electrical Engineering
Pages: xiv, 118 pages : color illustrations
Language: English
Abstract: Various motors have been investigated for electric vehicle (EV) application, namely the brushed and brushless DC motor, induction motor, permanent magnet (PM) motor. Each of the above motors, however, have its own deficiency. The switched reluctance motor (SRM) has superior features of simple, robust structure, low cost and better fault tolerance. Therefore, it a better candidate for EV application. However, limitation of the SRM is relatively lower torque density in comparison with the PM motor. In this thesis, two topologies of in-wheel motors based on switched reluctance principle are proposed to enhance the torque density. The first topology combines the idea of multi-tooth per stator pole and more rotor poles than stator teeth together. The magnetic principle of the proposed topology is analyzed initially for torque improvement. Then, the structure determination is made, including number of teeth per stator pole, number of rotor poles and number of phase, on the basis of higher torque output and efficiency. Moreover, the motor topology is optimized by genetic algorithm (GA) and equivalent magnetic circuit (EMC) model, while taking the design constraints for SRMs into consideration. The EMC model is utilized to obtain the value of self-inductance based on the calculation of different magnetic flux lines. The formula of fitness function, which focuses on specific torque and torque per unit of copper loss, can be defined from the calculation result of the self-inductance for GA optimization.
A new hybrid reluctance motor by inserting permanent magnets between adjacent stator poles of a multi-teeth SRM is put forward as the second topology. The operations are based on switched reluctance principle and the direction of winding configuration on adjacent phases is reversed corresponding to the magnetization direction of PMs. The function of PMs is not only to increase the flux quantity in the airgap, but also to regulate the flux density in the stator pole, reducing the saturation, to enhance the torque output. Then, the structural analysis is carried out to determine the shape of stator pole by using FEM, which concentrates on lower magnetic saturation. In addition, the optimization process is also on the basis of EMC model and GA method. Unlike SRMs, the EMC model of this topology takes the effect of PMs into consideration. For GA method, the objective function based on highest specific torque is obtained by EMC model calculation. With the design constraints for motors and some constant parameters considered, multi-variable optimization is implemented for the maximum value of objective function. After that, a motor with a smaller size and the same working principle of the hybrid reluctance topology is manufactured for high torque density experimental verification. The motor is an outer stator and inner rotor topology, designed and optimized by EMC model and GA method for the best torque performance. The static torque performance are measured and compared with FEM results. Besides, the torque-speed and output power-speed curves can be obtained from the experimental measurement. The experimental results demonstrate that the prototype can achieve high torque density, power output and efficiency.

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