Design and Optimization of a Switched Reluctance Motor for a Two-Wheeler and Three Wheeler Application

Abstract

Among the available traction motors, switched reluctance motors (SRMs) are gaining a lot of attention as a potential choice to propel electric vehicles (EVs) since they are magnet-free, and mechanically robust with a longer constant power range. Although SRM has several benefits, high torque ripple causing abnormal noise and vibrations has proven to be a major hindrance to its wide-scale use. Moreover, SRMs possess a lower torque density as compared to other permanent magnet-based motors. A motor solution with a higher torque density is highly preferred for in-wheel (IW) EV applications due to limited space inside the wheel hub. The performance of an SRM is commonly assessed using a variety of electromagnetic performance metrics such as starting torque, torque density, efficiency, and torque ripple. The electromagnetic performance metrics are highly sensitive to the type of SRM topology, their design, and excitation control parameters. Any modification to these excitation control or design factors has a distinct effect on the performance metrics, with varying degrees of potential benefit and drawback. As a result, within the design and development framework of an SRM, numerous optimization techniques have been widely employed to optimize these design and excitation control parameters. This thesis comprehensively investigates the sensitivity of the electromagnetic performance metrics with the dimensions of the geometric design variables for an SRM. The influence of the dimensions of the various geometric design variables such as rotor diameter, pole arc angles, and yoke thicknesses on the electromagnetic performance metrics such as average torque and torque ripple has been analyzed using static two dimensional (2D) electromagnetic finite-element analysis (FEA). The reason for the change in static characteristics due to variation in reluctance between SRM designs has not been detailed so far in the literature. This is addressed in the present work by the magnetic equivalent circuit (MEC) model that simplifies the design analysis. Results indicate that stator pole reluctance needs to be given due importance while studying the influence of rotor diameter. Also, it is imperative to set an adequate thickness of the stator and rotor yokes to minimize the effect of saturation on the performance. Rotor diameter and stator pole arc angle have a pronounced influence on the average torque and torque ripple while the influence of rotor pole arc angle and yoke thicknesses was relatively less. v Further, previously published research articles on SRM optimization intended to be used for EV applications have mostly focused on the optimization of their design and control variables only at the rated conditions. In EV applications, the load operating points (LOPs) of a traction motor are dynamic and spread widely across the torque speed envelope. To enhance their overall performance, it is vital to include them in the design optimization process. Therefore, in this thesis, a novel procedure for implementing the multi-objective design optimization (MODO) of an SRM based on a driving cycle has been demonstrated for an Electric rickshaw (E-rickshaw) application. Higher starting torque, and torque density with reduced electromagnetic losses throughout the driving cycle are established as the design objectives, subject to practical restrictions on current density and slot fill factor. The design objectives have been accurately evaluated through transient finite element analysis (FEA) and a computationally efficient SRM drive model (developed in MATLAB/Simulink) with consideration of the excitation control parameters. Kriging models have been constructed to reduce the computation cost of FEA during the optimization process. Then, a nondominated sorting genetic algorithm II (NSGA II) based multi-objective optimization coupled with the constructed Kriging models is conducted to generate a Pareto-optimal set. An optimal design that offers the best balance between the design objectives is selected from the Pareto-optimal set and the dimensions of corresponding design variables are used to build a prototype. Finally, the static and dynamic performance of the SRM prototype are experimentally evaluated and validated with the FEA simulations. Working conditions and design restrictions are more challenging for IW motors (typically used in electric two-wheeler applications) since a reducer is not utilized. Among the available SRM topologies, Multi-teeth (MT) SRM topology is a promising candidate for IW applications since it retains the inherent simplicity and cost effectiveness offered by traditional SRM designs. However, the design of IW-MTSRM topologies and their electromagnetic performance have not been explored sufficiently. In this thesis, a design formula governing the selection of the number of MT and rotor poles for MTSRMs has been proposed. Using this, a novel four-phase 8/18 IW MTSRM is derived. The characteristics of the 8/18 IW-MTSRM SRM are numerically compared with a conventional 8/10 SRM based on magnetic characteristics, vi efficiencies, and steady-state operation for the complete torque-speed range for an electric scooter (E-scooter) application. Results indicate that the 8/18 MTSRM has a higher peak torque capacity, torque density, superior drive cycle efficiency, and reduced torque ripple.