Selection of electric drive for EVs with emphasis on switched reluctance motor

Abstract

Different electrical machines have been investigated to check their suitability for electric vehicle (EV) applications. The interior permanent magnet synchronous motor (IPMSM) and brushless DC (BLDC) motor are widely employed in EVs due to the use of permanent magnets (PMs), which provide high torque density and better efficiency. Therefore, PM machines are the preferred choice for applications where a high-efficiency motor is required. However, manufacturers are interested in finding an alternative to PM machines to avoid the issues related to rare-earth magnets. Induction machines (IMs), switched reluctance machines (SRMs) and synchronous reluctance machines (SyncRels) are gaining importance for EVs due to the lack of PMs. The simple and low-cost construction due to lack of PMs or windings on the rotor makes the SRM a potential candidate for EV applications in preference to PM machines and IMs. SRMs are more reliable in high-speed and high-temperature operation due to the absence of rotor excitation. The price volatility, environmental concerns and supply chain issues of rare-earth magnets present in PM machines have become a long-running problem in producing high-efficiency motors [1]. However, the absence of rare-earth magnets in SRMs is an advantage in this aspect. Rotor displacement and rotational stress limit the performance of PM motors in high-speed operation due to excessive centrifugal forces, which cause stress to develop in magnet slots and bridges. However, SRMs can be operated at high speeds due to the lack of slots and bridges in the rotor. The performance of PM machines deteriorates during field-weakening operation, whereas in SRMs, field weakening is a natural phenomenon at high speed. Moreover, SRMs have a wide constant power-speed range compared with PMs and IMs. IMs suffer from rotor copper losses due to the use of die-casting aluminium for rotor conductors, which limits high-temperature operation; usually, copper die-casting is used at high temperature, and this involves an expensive and challenging manufacturing process. Moreover, the independent torque production capability of each phase due to the electrical isolation of each phase endows SRMs with a fault-tolerant nature. In spite of all these advantages, SRMs suffer from high torque ripple, noise and vibration. However, torque ripple doesn't exclude the adaption of SRMs for EV applications; several significant contributions have been made towards torque ripple minimization by modifying rotor geometry and optimized control of phase current. The selection of the electric drive is the crucial step of EV design. This chapter deals with selection criteria and a comprehensive understanding of performance characteristics of different electric drives widely used in EV applications, including PM machines, IMs and SRMs. This chapter mainly deals with how the control principles have been evolved for SRMs to meet the requirements of EV adaption and briefly discusses the control methods established for performance enhancement, such as torque ripple minimization and torque to ampere ratio improvement. This chapter also deals with the adaption of direct torque control (DTC) to SRMs and analyses the reasons behind negative torque generation in phase. This chapter includes recent contributions towards the evolution of DTC over a period of time in terms of sector reorganization, voltage vector reformation and optimized voltage vector selection. This chapter also presents simulation studies of conventional and modified versions of DTC. © 2023 selection and editorial matter, Dharavath Kishan, Ramani Kannan, B Dastagiri Reddy and Prajof Prabhakaran; individual chapters, the contributors.