0093-9994 (c) 2016 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information. This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/TIA.2017.2692198, IEEE Transactions on Industry Applications An Investigation of DC-Link Voltage and Temperature Variations on EV Traction System Design Nan Zhao, Nigel Schofield Department of Electrical and Computer Engineering McMaster University Hamilton, Canada zhaon5@mcmaster.ca, nigels@mcmaster.ca Rong Yang, Ran Gu McMaster Automotive Resource Centre McMaster University Hamilton, Canada yangr27@mcmaster.ca, gur4@mcmaster.ca Abstract— DC-link voltage and temperature variations are critical issues when designing an electric vehicle (EV) traction system. However, systems are generally reported at fixed voltage and temperature and may not therefore be fully specified when considering the variation of these parameters over full vehicle operating extremes. This paper presents an assessment of power- train options based on the Nissan Leaf vehicle, which is taken as a benchmark system providing experimental validation of the study results. The Nissan Leaf traction machine is evaluated and performance assessed by considering DC-link voltage and temperature variations typical of an automotive application, showing that the system lacks performance as battery state of charge decreases. An alternative traction machine design is proposed to satisfy the target performance. The vehicle power- train is then modified with the inclusion of a DC/DC converter between the vehicle battery and DC-link to maintain the traction system DC-link voltage near constant. A supercapacitor system is also considered for improved system voltage management. The trade-offs for the redesigned systems are discussed in terms of electronic and machine packaging, and mitigation of faulted operation at high speeds. Keywords—electric vehicles; traction system; DC-link supply; permanent magnet machines; magnet temperature I. INTRODUCTION The market for electric vehicles (EVs) is gradually growing due to energy and environmental issues associated with conventional combustion engined vehicles [1]. The EV traction system consists of electric traction machine(s), power conversion electronics and energy source(s). The energy source(s) provide a DC voltage or DC-link supply via power electronic converters, which in turn supplies or accepts energy to/from the traction machine. Since the energy density of electro-chemical batteries is significantly less than gasoline [2], high efficiency power-trains are a necessity to achieve reasonable vehicle driving range. The appearance of high-performance rare-earth magnets make brushless permanent magnet (PM) machines suitable candidates for EV traction, since they promise high energy conversion efficiency and high torque density within vehicular volume and mass constraints [3]. Brushless PM machines with interior permanent magnet (IPM) rotors have the widely quoted benefits of saliency torque contribution, less magnet mass, superior demagnetization withstand [4] and, as such, have been widely adopted in commercial hybrid electric vehicles (HEVs) such as the Honda Accord, Hyundai Sonata and Toyota Prius, and all-electric vehicles (EVs), such as the Nissan Leaf [5]. In EV traction systems, the battery provides a DC-link to supply DC power to the traction inverter that then converts DC to AC power to drive the traction machine, TM in Fig. 1 (a). The process is reversed for regenerative braking of the vehicle. DC-link voltage and temperature variations are critical issues when designing an electric vehicle traction system [6]. For brushless PM machines, ambient temperature variation impacts on the performance of the rotor PM material and hence machine operation, a feature that should be considered at the PM traction machine design stage. Additionally, when there is no control of the DC-link voltage, the DC-link voltage varies during vehicle loading cycles, which impacts on the performance of the traction machine and hence vehicle power-train, and also impacts on the inverter voltage and current requirements. Considering the DC-link voltage variation, existing literatures discuss the advantages, disadvantages and control strategies implemented when a DC/DC converter or a power buffer is connected between the battery and inverter, as discussed in Section II. However, the decision is not clearly defined and confused by individual application preference. In addition, the impact of voltage and temperature variations on the IPM traction machine torque and power performance has seldom been evaluated and hence solutions are not suggested. Further, traction system design and sizing based on the above issues has not been reported to-date. This paper therefore evaluates the performance of a benchmark battery EV traction system, by considering variations in DC-link voltage due to battery regulation, state-of-charge and ambient temperature for accepted automotive operation, since these will be the major contributions to the traction system performance envelope. DC-link voltage variation due to the battery internal temperature is not considered in this paper, where it is assumed that the battery thermal management maintains the