Comparative Study of Yokeless Stator Axial-Flux PM Machines Having Fractional Slot Concentrated and Integral Slot Distributed Windings for Electric Vehicle Traction Applications

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.