The market penetration of electric vehicles (EVs) is going to significantly increase in the next years and decades. However, EVs still present significant practical limitations in terms of mileage. Hence, the automotive industry is making important research efforts towards the progressive increase of battery energy density, reduction of battery charging time, and enhancement of electric powertrain efficiency. The electric machine is the main power loss contributor of an electric powertrain. This literature survey reviews the design and control methods to improve the energy efficiency of electric machines for EVs. The motor design requirements and specifications are described in terms of power density, efficiency along driving cycles, and cost, according to the targets set by the roadmaps of the main governmental agencies. The review discusses the stator and rotor design parameters, winding configurations, novel materials, construction technologies as well as control methods that are most influential on the power loss characteristics of typical traction machines. Moreover, the paper covers: i) driving cycle based design methods of traction motors, for energy consumption reduction in real operating conditions; and ii) novel machine topologies providing potential efficiency benefits. INDEX TERMS Electric machine, electric vehicle, efficiency, power loss, design parameters, control methods, driving cycle.

In this paper, a mathematical model of a twelvephase flux-switching permanent magnet (FSPM) machine designed for high-power wind power generation is established and investigated. Firstly, the winding inductances in both stator reference frame and dq-axes reference frame are analyzed. It should be noted that, the characteristics of the twelve-phase winding inductances are different from that of its three-phase counterpart. The largest mutual-inductance is much smaller than half of the self-inductance and most mutual-inductances are negligible, which brings the benefit of improved magnetic isolation between phases and causes the twelfth order inductance matrix to be sparse. Thereafter, an accurate inductance model is established, taking account of the magnetic coupling between adjacent threephase winding sets. Finally, the flux-linkage equation, the voltage equation and the power/torque equation are derived in sequence. The effectiveness of the proposed mathematical model are verified by comparing the theoretical results with 2D finite-element analysis (FEA)-based predictions. The work in this paper lays an important foundation for the control strategy of the twelve-phase FSPM machine. Index Terms-Flux-switching permanent magnet (FSPM), mathematical model, twelve-phase, wind power generation.Manuscript

In this paper, the 12-phase switched flux (SF) permanent magnet (PM) (SFPM) machine and three surfacemounted PM (SPM) machines designed for direct-drive wind power generation are comparatively analyzed. Firstly, feasible stator-slot/rotor-pole combinations for symmetrical 12-phase winding layout are investigated for both machine topologies. Secondly, the key design parameters of the PM generators including the split ratio and stator teeth width ratio are optimized by finite element (FE) analysis, to achieve a high phase fundamental EMF per turn and a low cogging torque, both of which are desired by the direct-drive wind power generator. Thirdly, electromagnetic performances including air-gap field, cogging torque, static torque, inductance, output voltage and its regulation factor, output power and efficiency of the generators are compared. A 10 kW 24-slot/22-pole SFPM prototype is built and tested to validate the FE predicted results.

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