Design, analysis, and implementation of SRM drive for electric vehicles

Student thesis: Doctoral Thesis

Abstract

Automobiles are regarded as a major source of greenhouse gases which cause a serious environmental problem, hence researchers are oriented towards transportation electrification. Different types of electrical machines are available for electric vehicle (EV) applications mainly; the permanent magnet synchronous motor, squirrel cage induction motor, and switched reluctance motor (SRM). The SRM can be a suitable candidate for this function. However, high torque ripple and relatively low power density have been the justification for its non-adoption in EVs. This thesis focuses on tackling these challenges to pave the way for the SRM to penetrate the electrified transportation sector. The thesis considers the machine, the converter, and the control when facing these adoption challenges. In simplistic terms, the current into and out of an SRM (at phase current turn-on and turn-off) can be expressed by 𝑉=𝐿𝑑𝑖𝑑𝑑⁄. The objective is to force the current into and out of the machine as quickly as possible, that is, through maximizing the 𝑑𝑖𝑑𝑑⁄ term, thereby increasing the base-speed, whence output power. The inductive term L is related to the machine design and specifically low L is required (for V fixed) at only phase turn-on (higher L is required at the turn-off to maximize the machine co-energy). This task is accomplished by deploying rotor conducting screens which are basically non-magnetic electrically conducting materials such as aluminium and copper. The screens fill the rotor inter-polar air gaps. When the rotor rotates flux passes through the screen inducing voltage, whence current, which in turn generates flux opposing the original stator flux. As a result, the unaligned inductance is minimised which increases the conversion area and therefore more torque is developed. The material, thickness and shape of the screen are studied to select the optimal design that results in torque improvement. The effect on efficiency is highlighted, being an important factor in EVs. A procedure for calculating the effective value of unaligned inductance for screened motors using the flux tube method is presented. Finite element analysis results for different cases are given to validate the claims. Another torque improvement approach is to increase 𝑑𝑖𝑑𝑑⁄ (at both phase turn-on and turn-off) by effectively increasing the term V, in 𝑉=𝐿𝑑𝑖𝑑𝑑⁄, which is a converter parameter. A brief review of available power converters is presented A new asymmetric neutral-point diode-clamped converter (NPC) with inherent voltage boosting is presented. The inbuilt NPC clamping capacitors are used for both voltage level clamping and also as dc rail voltage-boosting capacitors to increase motor output power. A detailed design procedure for sizing the boost-capacitors is outlined. Simulation along with experimental results validate the proposed converter. Different torque ripple (TR) minimization strategies including direct instantaneous torque control, torque sharing function, and current profiling using artificial neural networks are assessed using MATLAB/Simulink. The effect of voltage-boosting on minimizing TR is investigated. A new torque control function (TCF) which fully utilizes the dc link voltage is proposed. It will be substantiated that the new TCF dramatically extends the zero TR speed range, reduces switching losses, and is applicable to any SRM with overlapping phase torques.
Date of Award4 Nov 2020
Original languageEnglish
Awarding Institution
  • University Of Strathclyde
SponsorsUniversity of Strathclyde
SupervisorKhaled Ahmed (Supervisor) & Barry Williams (Supervisor)

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