Due to the high-power density and compact structure, axial-flux permanent magnet (AFPM) machines have gradually received much attention with a view to researching breakthroughs in the next generation electric drive technology for electric vehicles in the recent decades. The AFPM machines with factional slot concentrated winding (FSCW) and yokeless stator, namely yokeless, and segmented armature (YASA) motors, have drawn much attention for its high-power density and potential manufacturability due to the concentrated winding and modular stator core configuration. However, the significant rotor loss resulting from the abundant armature reaction harmonics in FSCW machines imposes a great challenge to the rotor heat dissipation, especially when the pursuit of higher speed has become the trend for electric vehicle applications. On the other hand, distributed winding is widely used in high speed radial flux permanent magnet (PM) machines due to its low armature reaction harmonics. In order to figure out the advantages and disadvantages of various winding arrangement and rotor configuration of AFPM for electric vehicle applications, the comparative study of four AFPM machines with various winding configurations and rotor PM arrangements are comprehensively conducted in this article. First, the design and primary optimization of the four AFPM machines are conducted for the electric vehicle requirement specifications. Then, a comprehensive three-dimensional finite-element analysis (FEA) is employed to compare the electromagnetic performance including torque/power density, efficiency, and flux-weakening capacity. Furthermore, the guideline of winding selection of AFPM machines for electrical vehicle is given. Finally, a yokeless stator AFPM prototype with ISDW configuration is manufactured and tested to verify the validity of the FEA results, as well as confirm the comparison conclusion.

This paper investigates the influence of static and dynamic rotor eccentricities on electromagnetic performance, including air-gap flux density, back electromotive force (EMF), electromagnetic torque, cogging torque, and unbalanced magnetic force (UMF), of a 2-pole/3-slot permanent magnet (PM) motor considering eccentricity ratio, eccentricity angle, and rotor initial angle. Both finite element analyses and experiments show that in a 2-pole/3-slot PM motor, the static rotor eccentricity leads to unbalanced magnitudes and phase angles of fundamental back-EMFs of three phases but does not change the harmonic contents, while the dynamic rotor eccentricity will not cause unbalance in three phase back-EMFs but affects the harmonic contents of back-EMFs and their phase angles, which causes asymmetric positive and negative half-periods and corresponding even harmonics of phase back-EMF waveform. In addition, both static and dynamic rotor eccentricities have negligible influence on the average torque, but result in large torque ripple, cogging torque, and UMF.

In this paper, the stator cooling system of an axial flux permanent magnet (AFPM) motor with integer-slot distributed winding (ISDW) for high speed high power electric vehicle traction application is discussed. The aim of the research is quantitively figuring out the power /power density boundaries of this type of axial field PM motor with the adoption of proposed oil-immersed cooling system consisting of multi-cooling channels. Firstly, the structure-cooling design considerations are provided. Then, a comparative study of stator thermal performance of high-power axial field PM motor with various cooling method is conducted with three-dimensional (3-D) computation fluid dynamics (CFD) analysis. The oil circuits inside the stator are further optimized to achieve a better cooling effect with the focus on the improvement of oil circuit layout configuration. Finally, a full-scale immersed oil-cooled prototype motor with 120 kW output peak power and 15,000 r/min are manufactured and experimented. The long-time stable operation at high speed verifies the reliability of stator structure design. The thermal performance is verified with measured results of end-winding temperature rise under continuous conditions.