In this paper, a high-speed Halbach-array PM rotor for ironless stator axial-flux permanent magnet (AFPM) machine is proposed and researched with consideration of the multi-physics constraints, including the electromagnetic characteristics, mechanical strength and rotor dynamics. Considering the severity of attractive force between twin rotors, the complex repulsive force among permanent magnet (PM) segments of Halbach-array and centrifugal force, the new hybrid sleeve and PM fixation method are proposed to improve the rotor mechanical strength. Then, the rotor stress is analyzed by numerical analysis and 3D finite element analysis (FEA). Furtherly, the sleeve thickness and interference fit are optimized by 3D FEA. The rotor dynamics and bearing arrangement are analyzed to validate the effectiveness of the high speed Halbach-array PM rotor. A 50kW, 9000r/min ironless stator AFPM machine prototype with Halbach-array PM rotor is manufactured and tested. The measured results show that the output power is up to 53.8kW at 9000r/min. The feasibility of high speed Halbach-array PM rotor and accuracy of analysis method are validated by both simulated and measured results.

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.

Ironless stator axial-flux permanent magnet (AFPM) machines provide many attractive electromagnetic performances such as high efficiency in high-frequency mode and strong short-term overload ability, which make them more competitive in flywheel energy storage applications. However, designing high-torque-density AFPM machines, particularly their ironless stators, is with significant challenges and needs great efforts. In this paper, a high-strength oil-immersed cooling stator with non-overlapping concentrated winding is proposed for high-power ironless stator AFPM machine applications. The paper explains in detail the design, analysis and manufacturing process of the proposed oil-immersed cooling stator. With the three-dimensional finite-element analysis (3-D FEA), a 9000rpm/50kW 12-slot/10-pole prototype for a flywheel energy storage system is successfully designed and developed using the proposed high-strength oil-immersed cooling ironless stator. The experimental test results demonstrate the effectiveness of the cooling design and mechanical strength.

This paper presents integrated design and analysis of a new axial-flux permanent magnet (AFPM) machine, which consists of an ironless stator with double-layer concentrated non-overlapping windings and twin external rotors with Halbach-array PMs. A general sizing equation based on fundamental theory, easily adjustable for ironless stator AFPM machine with concentrated windings, is derived to obtain preliminary parameters. Then, by using three dimensional finite element analysis (3-D FEA), the optimization of a primary machine are implemented. A precise and fast calculation method based on 3-D FEA model is proposed to confirm the eddy-current loss for ironless stator winding. By making tradeoff between DC current loss and eddy-current loss of Litz wire for winding, the efficiency is further improved for high frequency operation. Despite using Halbach-array PMs, a certain thickness of rotor yoke is recommended to increase air-gap flux density. Finally, a prototype machine is developed, experimented and evaluated. The test results reveal the strong overload and heat dispersal capacity of proposed stator ironless AFPM machine with the maximum efficiency of 98.5%. Measured performances of the manufactured prototype verify the validity of the proposed analysis method and FEA results.

Due to the larger diameter of effective air-gap for torque production, outer-rotor permanent magnet (PM) machines exhibit the distinctive advantages, such as high torque density and large rotor inertia. This article presents a compact outer-rotor permanent magnet starter/generator (OR-PM S/G) to match diesel engine for extended-range hybrid electric vehicle. In order to solve the serious temperature rise problem resulting from the copper losses under constrained slot space in the inner stator, a new winding indirect liquid cooling technique by inserting heat pipes in the center of slots is proposed. It facilitates a path from the center of the coils to the cooling medium with a higher thermal conductivity, thus increasing the tolerable current density of coils. By optimizing the geometry and dimensions of heat pipe and coil, the temperature rise in slots are achieved within an acceptable limit according to the computed results of FEA and computational fluid dynamics (CFD). In order to validate the cooling design, a 30kW OR-PM S/G prototype with rectangular-shaped PM and the proposed indirect liquid cooling windings is manufactured and experimented. The power capability and efficiency of the proposed OR-PMS/G are validated by the measured results with calculation results. A good agreement between the predicted and measured temperatures indicates the validity of indirect cooling method for concentrated winding with heat pipe.

In this paper, the implementation of a single-stator double-rotor fractional-slot concentrated-winding (FSCW) axial-flux permanent-magnet (AFPM) machine are presented to verify a simple and reliable structure design and oil-immersed cooling of the stator with modular soft magnetic composites (SMC) core. Due to the yokeless and segmented stator, the process of embedding the windings into the stator teeth will probably become simpler. By preforming rectangular copper wire, the shorter end-winding length and higher fill factor can be achieved. In order to eliminate eddy-current losses and manufacturing costs, the fiberglass epoxy is used for the supporting components of the fixation for modular SMC stator and PMs, which is a non-magnetic and non-conductive material. Hence, the robustness of stator and rotor are taken into account due to the stator modularity weakening effect on strength. Meanwhile, a fully sealed stator with oil-immersed cooling is proposed to enable the current density with 20 A/mm2 in the armature windings for stable operation. The electromagnetic property, including in magnetic field distribution, torque capacity and flux-weakening ability, are performed based on the three-dimensional (3-D) finite element analysis (FEA), as well as the mechanical performance. The coupled thermal and computational fluid dynamics (CFD) model is built to predict the flow and temperature distribution inside stator with oil immersed forced cooling. The construction and experimental verification of a full-scale 30kW prototype are elaborated to verify the predicted performances in terms of torque density, structural strength and cooling.