Investigations Into Control Strategies and Integrated Charging Solutions for Switched Reluctance Motor-Driven Electric Vehicles

Abstract

Electric vehicles (EVs) are increasingly recognized as a vital solution to mitigate greenhouse gas emissions and air pollution in the transportation sector. However, challenges such as limited driving range, higher upfront costs, and dependence on rare earth minerals hinder their widespread adoption. One promising solution lies in the Switched Reluctance Motor (SRM), distinguished by its simple structure devoid of permanent magnets or rotor conductors. This design offers inherent advantages such as efficient operation in high temperatures, robust fault tolerance, and minimal rotor losses, rendering SRMs a compelling choice for EV propulsion. Furthermore, the integration of integrated on-board chargers (IOBCs) adds another layer of efficiency and functionality to EV systems. By seamlessly combining motor driving and charging functions, IOBCs not only promise cost savings and reduced complexity but also offer increased power density. Notably, with bidirectional capability, IOBCs enable EVs to both charge from external sources and supply power back to the grid. This bidirectional functionality holds immense significance for diverse operating scenarios such as grid-to-vehicle (G2V), vehicle-to-grid (V2G), vehicle-to-load (V2L), and vehicle-to-home (V2H). This thesis explores the promising prospects of utilizing SRM drives in EVs and investigates the integration of IOBC systems. Acknowledging challenges such as higher torque ripple and lower power density associated with SRM motors for EVs, the research emphasizes the need for further research and enhancement in this domain. With a primary focus on enhancing efficiency, reducing costs, and increasing the overall power density of EV systems, the study aims to address key obstacles hindering the widespread adoption of EVs while advancing sustainable transportation solutions. This thesis also presents a precise simulation model of SRM for analyzing and conducting simulation studies on the proposed methods. This necessitates the characterization of the SRM, for which various methods are available in the literature, ranging from analytical approaches to electromagnetic Finite Element Analysis (FEA) tools. While FEA-based models

offer high accuracy, this thesis initially details the procedure for developing a MATLAB Simulink model derived from the FEA-based characterization of the SRM prototype which is used in further studies. Additionally, obtaining a linearized SRM model is crucial for controller design. This thesis details the procedure for obtaining the linearized SRM model. Subsequently in the thesis, a novel SRM drive utilizing a Miller converterfed SRM motor for medium-power applications with a single current sensor is proposed to reduce cost and improve performance. The literature offers a variety of control strategies for SRM drives, yet for medium-power applications, those equipped with a dedicated position sensor and employing a fixed control strategy based on a linearized transfer function hold particular relevance. Also, there is a notable absence of a comprehensive control development procedure in the existing literature. Moreover, there lies an opportunity to reduce costs by minimizing the number of current sensors necessary for controlling the SRM drive. The thesis offers a comprehensive control development, dynamic simulation, analysis, and experimental validation of the same. Speed and current controllers are designed using the K-factor method, and the efficacy of the proposed drive is rigorously evaluated across various operating modes in MATLAB Simulink. Additionally, a hardware prototype is developed and the digital control algorithm is implemented on the DSP microcontroller TMS320F28379D based on the designed controllers to further assess drive performance. The results obtained validate the robustness and dynamic performance of the proposed cost-effective SRM drive with single current sensor across variable speed, variable torque, and constant power modes of operation. In the literature, the single-pulse control (SPC) of SRM is discussed for high-speed applications, operating at the fundamental switching frequency to reduce core and switching losses effectively. However, its suitability for wide speed control is limited due to significant torque ripple. To address this, extending the constant conduction period with v/f control can mitigate torque ripple by facilitating torque sharing during winding commutation. Nevertheless, this extended conduction angle may result in negative torque production and a lower torque-to-ampere ratio. This research conducts an analysis of conventional SPC, conventional v/f conii trol, and the impact of a wider conduction angle on torque ripple and efficiency through simulation studies. Moreover, it investigates the effect of accelerated demagnetization on torque ripple and efficiency in v/f controlled SRM drives with a wider conduction angle. This thesis introduces a novel bidirectional dual-port ´Cuk converter-fed SRM drive with accelerated demagnetization capability. In the proposed ´Cuk converter-fed SRM drive, the magnetization voltage controls the speed and provides a decoupled higher demagnetization voltage, thus accelerating demagnetization. This facilitates reduced switching losses and core losses similar to those in SPC, while ensuring reduced torque ripple with a wider conduction period and a higher torque ampere ratio due to accelerated demagnetization. Additionally, the proposed system incorporated IOBC capability. In G2V mode, the converter is reconfigured to adopt a two-stage, non-isolated charger topology, comprising a diode bridge followed by a ´Cuk converter in the reverse direction. Detailed control development and analysis are presented in the thesis. The proposed SRM drive with IOBC capability is validated through simulations and an experimental prototype. It demonstrates robust performance across various modes of operation and offers cost reduction and increased overall power density. Existing literature identifies two types of IOBCs for SRM-based electric vehicles: one leveraging machine windings as inductors and the other not. While utilizing machine windings offers advantages like higher power density, it also introduces risks such as instantaneous torque pulsation. To ensure reliable utilization of machine windings as filter inductors, maintaining zero instantaneous torque (ZIT) during charging modes is imperative. This thesis extensively investigates the utilization of machine windings as filter inductors, highlighting potential benefits in terms of power density and cost reduction. In the literature, the issue of common-mode voltage in non-isolated IOBCs for SRM based EVs have not received much attention. Additionally, the SRM based IOBCs proposed in the literature have no bidirectional capability limiting the operating modes. Bidirectional capabilities for IOBCs enable versatile operating modes by allowing EVs to feed power back to the grid or other electrical loads. Additionally, the literature lacks detailed qualitative analysis of IOBCs using machine

windings as filter inductors, including the impact of even harmonics from unequal inductances in positive and negative cycles. This thesis introduces a IOBC system with bidirectional capability for a 4-phase SRM drive based EV. In the proposed drive outlined in the thesis, the converter is reconfigured as a two-phase interleaved totem pole PFC converter, utilizing machine windings as filter inductors. This configuration enables the suppression of common-mode voltage or current and higher efficiency compared to previously reported IOBCs, without the need for additional magnetic contactors. During charging, four stable ZIT positions are identified, ensuring reliable utilization of machine windings as filter inductors, thereby mitigating wear and tear induced by torque pulsations. The thesis provides a detailed analysis of instantaneous torque during charging modes, supported by FEA and MATLAB simulation results to validate ZIT. This versatile IOBC demonstrates bidirectional capability, accommodating various operational modes such as G2V, V2G, V2H, and V2L. The performance of the proposed system is further validated through an experimental prototype, affirming the advantages of the proposed IOBC, which utilizes SRM windings as filter inductors, promising versatile operating modes with bidirectional capability and reduced common-mode voltage. In conclusion, this thesis presents a comprehensive exploration of utilizing SRMs in EVs and the integration of IOBCs to enhance efficiency and functionality. Through simulation studies and experimental validation, novel solutions for SRM drives and IOBC systems for SRM based EVs are proposed, addressing challenges such as torque ripple, efficiency, and cost. The results demonstrate robust performance and validate the advantages of the proposed systems, paving the way for advancements in sustainable transportation solutions.