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A High Torque Density, Direct Drive In-Wheel Motor for Electric Vehicles

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A High Torque Density, Direct Drive In-Wheel Motor for Electric Vehicles A HIGH TORQUE DENSITY, DIRECT DRIVE IN-WHEEL MOTOR FOR ELECTRIC VEHICLES Chukwuma Junior Ifedi School of Electrical and Electronic Engineering Newcastle University A thesis submitted for the degree of Doctor of Philosophy June, 2013 Abstract The use of in-wheel motors, often referred to as hub motors as a source of propulsion for pure electric or hybrid electric vehicles has recently received a lot of attention. Since the motor is housed in the limited space within the wheel rim it must have a high torque density and efficiency, whilst being able to survive the rigours of being in-wheel in terms of environmental cycling, ingress, shock, vibration and driver abuse. Part of the work of this PhD involved an investigation into different slot and pole combinations in order determine a superior machine design, within given constraints based upon an existing in-wheel motor drive built by Protean Electric. Finite element analysis and optimisation have been applied in order to investigate the machine designs and achieve the optimum combination. The main work of this PhD, presents a high torque dense machine employing a new method of construction, which improves the torque capability with a smaller diameter, compared to that of the existing Protean in-wheel drive system. The machine is designed with an open slot stator and using magnetic slot wedges to close the slots. Having an open slot stator design means the coils can be pre-pressed before being inserted onto the stator teeth, this improves the electrical loading of the machine as the fill factor in the slot is increased. The electromagnetic impact of the slot wedges on the machine design has been studied, also a method of coil pressing has been studied and the impact upon coil insulation integrity verified. To ensure adequate levels of functional safety are met it is essential that failures do not lead to loss of control of the vehicle. Studies on a fault tolerant concept which can be applied to the design of in-wheel motors are presented. The study focuses on the ability to sustain an adequate level of performance following a failure, while achieving a high torque density. A series of failures have been simulated and compared with experimental tests conducted on a Protean motor. Finally a prototype is constructed and tested to determine the true level of performance. The prototype is compared to a new motor built in-house by Protean and achieves an improved level of performance. i Acknowledgements First and foremost I would like to thank my supervisor Prof. Barrie Mecrow for his guidance throughout the course of this research, he not only assisted in getting me a scholarship from the Power Electronic Drives and Machine group and provided me the opportunity to work with Protean Electric, he was also able to impart a lot of knowledge and critical thinking. I appreciate the input from my second supervisor Dr. Glynn Atkinson, who helped me during the early stages of building my finite element models and solving some problems I encountered during my work. This work would not have been possible without support from Protean Electric, they provided me with sponsorship and the research idea, which became the outcome of this work. I would like to specially thank Simon Brockway, Gerry Boast, Chris Hilton, Dragica Kostic-Perovic, Al Fraser and Gunaratnam Sooriyakumar who provided me with the industrial supervision and help with undertaking the test on my prototype machine. I thank Chris Manning for helping me with the mechanical aspect of my work such as coil winding and pressing, stator assembly and coil testing, just to mention a few. Also I will like to thank Jack Noble, Graham Ewart, Allan Wheatley, Luke Coates and Stuart Baker for when they had to step in and assist in some activities at the mechanical workshop. I also will like to acknowledge Daniel Smith and Jamie Washington for how much I made their lives miserable, as they never turned me down every time I went to them with questions. Also to acknowledge Min Zhang, Christopher Spargo, Andrew Wechsler, Bassim Jassim, Alamoudi Yasser and Maher Algreer for the times we had discussions when trying to solve certain problems. To save the best for last, I give huge thanks to my mother, brother and sisters for their support during my studies, and most especially my father, words are not profound enough to express how grateful I am for his financial support to see me through my educational life, my endeavours in life to date have been possible due to his presence. ii Contents Abstract ............................................................................................................................................. i Acknowledgements ......................................................................................................................... ii Contents .......................................................................................................................................... iii List of figures ................................................................................................................................ viii List of tables ................................................................................................................................. xiii Acronyms and symbols ................................................................................................................ xiv Chapter 1 Introduction ................................................................................................................ - 1 - 1.1 Background ................................................................................................................. - 1 - 1.2 Electrical machine design study ................................................................................. - 2 - 1.2.1 Machine losses .................................................................................................... - 2 - 1.2.2 Winding configuration ........................................................................................ - 2 - 1.2.3 Machine field weakening .................................................................................... - 4 - 1.2.4 Cooling methods ................................................................................................. - 5 - 1.4 Methodology ............................................................................................................... - 6 - 1.5 Thesis overview .......................................................................................................... - 6 - Chapter 2 Electric Vehicles and their Associated Drive Train Design ....................................... - 8 - 2.1 Background ................................................................................................................. - 9 - 2.2 Electric vehicles, past and present ............................................................................ - 10 - 2.3 Overview and topologies for EV drive train ............................................................. - 11 - 2.3.1 Hybrid electric vehicles .................................................................................... - 13 - 2.3.2 Pure electric vehicles ........................................................................................ - 14 - 2.3.3 In-wheel electric vehicles ................................................................................. - 15 - 2.4 Designing an electric motor for EV .......................................................................... - 16 - 2.4.1 Vehicle and road load equations ....................................................................... - 17 - 2.4.1.1 Vehicle weight .............................................................................................. - 17 - 2.4.1.2 Aerodynamic drag force ............................................................................... - 17 - 2.4.1.3 Rolling resistance force ................................................................................ - 18 - 2.4.1.4 Gradient force ............................................................................................... - 18 - 2.4.2 Electric motor ................................................................................................... - 18 - 2.5 Survey of motor types suitable for EV ..................................................................... - 21 - iii 2.5.1 Permanent magnet machine .............................................................................. - 22 - 2.5.1.1 Radial flux machine ...................................................................................... - 23 - 2.5.1.2 Axial flux machines ...................................................................................... - 24 - 2.5.1.3 Transverse flux machines ................................................................................. - 24 - 2.5.1.4 Flux switching machines .............................................................................. - 26 - 2.5.2 Induction machine ............................................................................................ - 26 - 2.5.3 Switched reluctance machine ........................................................................... - 27 - 2.6 Un-sprung mass ........................................................................................................ - 29 - 2.7 Drive cycle ................................................................................................................ - 31 - 2.8 Summary ..................................................................................................................
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