Electrical Machines and Drives for Electric, Hybrid

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INVITED PAPER

Electrical Machines and Drives for Electric, Hybrid, and Fuel Cell Vehicles Induction and switched-reluctance machines can provide the needed characteristics, but permanent magnet brushless machines offer a higher efficiency and torque density. By Z. Q. Zhu, Senior Member IEEE, and David Howe

ABSTRACT | This paper reviews the relative merits of induction, switched reluctance, and permanent-magnet (PM) brushless machines and drives for application in electric, hybrid, and fuel cell vehicles, with particular emphasis on PM brushless machines. The basic operational characteristics and design requirements, viz. a high torque/power density, high efficiency over a wide operating range, and a high maximum speed capability, as well as the latest developments, are described. Permanent-magnet brushless dc and ac machines and drives are compared in terms of their constant torque and constant power capabilities, and various PM machine topologies and their performance are reviewed. Finally, methods for enhancing the PM excitation torque and reluctance torque components and, thereby, improving the torque and power capability, are described. KEYWORDS

|

Brushless drives; electric vehicles; electrical

machines; hybrid vehicles; induction machines; permanentmagnet machines; switched reluctance machines

I. INTRODUCTION
Electrical machines and drives are a key enabling technology for electric, hybrid, and fuel cell vehicles. The basic characteristics which are required of an electrical machine for traction applications include the following [1]–[3]. • High torque density and power density. • High torque for starting, at low speeds and hill climbing, and high power for high-speed cruising.

Manuscript received June 10, 2006; revised November 11, 2006. The authors are with the Department of Electronic and Electrical Engineering, University of Sheffield, S1 3JD Sheffield, U.K. (e-mail: Z.Q.Zhu@sheffield.ac.uk; D.Howe@sheffield.ac.uk). Digital Object Identifier: 10.1109/JPROC.2006.892482

Wide speed range, with a constant power operating range of around 3–4 times the base speed being a good compromise between the peak torque requirement of the machine and the volt-ampere rating of the inverter. • High efficiency over wide speed and torque ranges, including low torque operation. • Intermittent overload capability, typically twice the rated torque for short durations. • High reliability and robustness appropriate to the vehicle environment. • Acceptable cost. In addition, low acoustic noise and low torque ripple are important design considerations. On an urban driving cycle, a traction machine operates most frequently at light loads around the base speed. Therefore, in general, it should be designed to operate at maximum efficiency and minimum acoustic noise in this region. Typical torque/power-speed characteristics required for traction machines are illustrated in Fig. 1. Induction machines (IM), switched reluctance machines (SRMs), and permanent-magnet (PM) brushless machines (Fig. 2) have all been employed in traction applications, and can be designed to exhibit torque/power-speed characteristics having the form shown in Fig. 3. In the constant torque region I, the maximum torque capability is determined by the current rating of the inverter, while in the constant power region II, flux-weakening or commutation phase advance has to be employed due to the inverter voltage and current limits. In region III, the torque and power reduce due to the increasing influence of the back-electromotive force (back-EMF). However, the power capability and the maximum speed can be enhanced without sacrificing the low-speed torque capability by employing a dc–dc voltage booster [4], a technique which is employed in the Toyota 0018-9219/$25.00 Ó 2007 IEEE



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Proceedings of the IEEE | Vol. 95, No. 4, April 2007

Zhu and Howe: Electrical Machines and Drives for Electric, Hybrid, and Fuel Cell Vehicles

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