Optimization of Outer-Rotor Hybrid Excitation FSM for In-Wheel Direct Drive Electric Vehicle M.Z. Ahmad and E. Sulaiman Research Center for Applied Electromagnetics Universiti Tun Hussein Onn Malaysia Batu Pahat, Johor, Malaysia zarafi@uthm.edu.my, erwan@uthm.edu.my T. Kosaka Dept. of Electrical and Computer Engineering Nagoya Institute of Technology, Nagoya, Japan kosaka@nitech.ac.jp Abstract—Research on flux switching machines (FSMs) has been an attractive topic recently due to tremendous advantages of robust rotor structure, high torque, and high power capability that suits for intense applications. However, most of the investigations are focusing on inner rotor structure which incongruous for direct drive applications. In this paper, the design optimization and performance analysis of 12Slot-14Pole hybrid excitation flux switching machine (HEFSM) with outer-rotor configuration are conducted for in-wheel direct drive electric vehicle (EV). Similar with conventional inner-rotor HEFSMs, two magnetic flux sources of permanent magnet (PM) and field excitation coil (FEC) have extra advantage of variable flux control capability, while the outer-rotor configuration has ability to provide much higher torque and power density suitable for in- wheel EV drives. Based on some design restriction and specification, design refinements are conducted on the initial design machine by using deterministic optimization approach. The final design machine has achieved maximum torque and power density of 335.08Nm and 5.93kW/kg, respectively, slightly better than inner rotor HEFSM and interior permanent magnet synchronous machine (IPMSM) design for EV. Keywords—Flux switching machine; outer-rotor; in-wheel direct drive; electric vehicle I. INTRODUCTION In 21st century, global warming is among a major issue discussed all over the world by scientists and government agencies. The factors that contribute to global warming are greenhouse effect due to human activities, heated by solar radiation, and geomagnetic variation [1]. As reported in [2], the burning of fossil fuels are the main contributors for global warning issue by means of conventional internal combustion engine (ICE) vehicles. Due to the price of fossil fuel is keep rising year by year, lots of researcher and industries are looking for electric vehicles (EVs) as the most possible solution in transportation [3]. Thus, it can be concluded that commercialization of EV will reduce dramatically the air pollution and will meet the national energy strategy to seek for an energy future that would be secure, efficient, and environmentally sound [4]. Generally, the propulsion system for conventional EVs consists of batteries, electric motors with drives, and transmission gears to wheels. This configuration led for torque and power loss on the transmission system and resulting less electric motor’s efficiency. In addition, it consumes a lot of space to locate the transmission and gearing system in vehicle’s cabin that increase the total vehicle weight. Therefore, in-wheel direct drive motor is an alternative mode for EV where the transmission system that composed of gearing, belting and mechanical system in conventional EV can be eliminated. Thus, the transmission losses are minimized, and the operation efficiency and reliability are improved [5]. Moreover, more batteries can be installed in the space that would be occupied by the transmission, which help to increase the driving range per charge. On the contrary, due to the elimination of gears, the system needs to produce the total torque directly into the wheel shaft with higher torque and power density as compared with conventional EV [6,7]. It is essential that electric motors with high torque density capability are commonly used for heavy applications such as in aerospace and automotive area [8]. Previously, permanent magnet (PM) brushless machines are widely used for these heavy applications due to their advantages of high torque density, high power density, wide speed, wide constant power, and high efficiency capabilities [9,10]. Nevertheless, due to the main flux source of PMs are located on the rotor, the machines are suffer from demagnetization effects and eddy current loss in the rotor. Moreover, the difficulty to remove heat from the rotating part is among the main issue on this kind of electric machine. Almost a decade, flux-switching motors (FSMs) has been an attractive research topic due to their several advantages of higher torque density and efficiency. With all active components such as PM, DC field excitation coil (DC FEC), and armature coil located on the stator, the machine becomes extremely robust in which only single piece of rotor iron is employed. Various applications of FSM have been reported, ranging from wind power generation, automotive, aerospace, power tools and etc [11-14]. Generally FSMs can be classified into three groups, namely permanent magnet (PM) FSMs, hybrid excitation (HE) FSMs, and field excitation (FE) FSMs. Both PMFSMs and FEFSMs have only single excitation flux source which come from PM and FE coil, respectively, while in HEFSM there are two magnetic flux source which come from PMs and FECs [15]. The Ministry of Education Malaysia and Universiti Tun Hussein Onn Malaysia.  l -))) 