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Citation information: DOI 10.1109/TIA.2015.2397173, IEEE Transactions on Industry Applications 978-1-4799-5776-7/14/$31.00 ©2014 IEEE Reduced Rare-Earth Flux Switching Machines for Traction Applications Tsarafidy Raminosoa, Ayman El-Refaie, Di Pan, Kum-Kang Huh, James Alexander, Kevin Grace, Stefan Grubic, Steven Galioto, Patel Reddy GE Global Research Electrical Machines Laboratory Niskayuna, New York, USA t.raminosoa@ieee.org ; elrefaie@research.ge.com Xiaochun Shen Ensco PLC 5847 San Felipe, Suite 3300 Houston, TX 77057, USA xshen@enscoplc.com Abstract—There has been growing interest in electrical machines that reduce or eliminate rare-earth material content. Traction applications are among the key applications where reducing cost and hence reduction of rare-earth materials is a key requirement. This paper will assess the potential of different variants of flux-switching machines that either reduce or eliminate rare-earth materials in the context of traction applications. Two designs use different grades of Dysprosium- free permanent magnets and the third design is a wound-field variant that does not include permanent magnets at all. Detailed analysis of all three designs in comparison to the required set of specifications will be presented. The key opportunities and challenges will be highlighted. The impact of the high pole-count/frequency of the flux-switching machines will also be evaluated. Experimental results for one of the designs with Dysprosium-free permanent magnets will also be presented. Index Terms—Flux Switching Machine, Traction, Hybrid Vehicles, Electric Vehicles, Permanent Magnets, Dysprosium, Rare-Earth. I. INTRODUCTION Significant efforts are currently under way in the US and around the world to develop environmental friendly transportation. Development of electric and hybrid vehicles is a key part of this global effort [1], [2], [3], [4], [5], [6], [7], [8], [9]. In order to ensure marketability of hybrid or electric vehicles, their cost has to be reduced while keeping their performances high. One impactful method would be to reduce the electrical machines cost. One key way of achieving this is by trying to reduce or eliminate the use of rare-earth materials which have been experiencing significant increase/fluctuations in their prices. Many electrical machines in currently commercialized hybrid vehicles use high-temperature high energy product Neodymium permanent magnet (PM) materials [3]. Those magnets have a certain amount of Dysprosium (Dy) in their composition in order to enhance resistance to demagnetization. But automotive applications require cost effective material for mass production and Dy is known to be the most expensive heavy rare earth in conventional Neodymium (Nd) magnets. Hence, permanent magnets not using or reducing the use of Dy are expected to be very cost effective but are more prone to demagnetization than their conventional counterparts [10], [11]. The use of Dy-free permanent magnets brings a significant design challenge to minimize demagnetization risk due to the combined effect of temperature and armature reaction. Dy-free permanent magnets can be used in many other electrical machine topologies. For example, their use in various IPM configurations are examined in [10] and [11] for traction applications. Nevertheless, flux switching machines feature a particular magnetic circuit configuration that favors Dy-free magnets in terms of demagnetization risk [12]. Permanent Magnet Flux Switching Machine (PMFSM) is an emerging option for traction applications [13]. A PMFSM has the permanent magnets located in the stator and uses a variable reluctance rotor. The modulation of the permanent magnet field by the rotating variable reluctance rotor emulates a rotating PM field in the airgap. The stator features a concentrated winding creating several harmonic rotating fields. One of them matches the number of poles of the emulated PM rotating field and interacts with it to create torque. Hence, from the control standpoint, a PMFSM behaves in the same way as a conventional PM machine and can be vector controlled [14]. In flux switching machines, the armature reaction field is mainly perpendicular to the magnetization direction of the permanent magnets [15]. Therefore the permanent magnets are less exposed to the demagnetizing armature reaction field compared to conventional PM machines. For that reason, PMFSMs were identified as a potential application of Dy-free permanent magnets with lower thermal stability but keeping the high energy density of Nd magnets. This paper presents two PMFSMs using Dy-free Nd magnets to achieve the US Department of Energy (DoE) 2020 targets for traction motors for hybrid/electric vehicles. The key requirements are 55kW peak power for 18 seconds and 30kW continuous power over a speed range going from 2800 rpm to 14000 rpm. The set of specifications the machine is designed for are