Torque Density Improvement of a Dual-Rotor Axial-Flux PM Machine Based on Sizing Equation

Axial-flux PM (AFPM) machines are generally expected to provide higher torque density and compact axial length, compared with typical radial-flux PM machines. Different design constraints of the AFPM machine may result in quite different electromagnetic performance. However, so far studies to explore the torque density distribution characteristics of the axial-flux machine under different electromagnetic loadings and material property of core have seldom been found. This work is quite important for engineers to design high-performance AFPM machines. Therefore, in this paper, the torque density of one Torus-type AFPM machine, i.e., the dual-rotor single-stator configuration, under different power and speed ranges, magnetic and electrical loadings and pole numbers, are systematically explored through one modified sizing equation model, and some design guidelines are summarized for the high-performance AFPM design. Firstly, the structural and electromagnetic characteristics of the Torus-type PM machine are presented. Secondly, a modified sizing equation model is established to reveal the mapping relationship between the electromagnetic performance of the Torus-type machine and its structural and electromagnetic parameters. Thirdly, the effects of various design constraints on torque density of the AFPM machine are summarized. Fourthly, two numerical models including the AFPM machine and a radial-flux PM machine are developed to confirm the merits of the AFPM machine in terms of torque density. The research results show that the power level, rotary speed and current density of stator winding have significant impact on the torque density of the Torus-type AFPM machine among various design constraints. Besides, the axial-flux PM machine can increase the torque-volume density well, compared with the radial-flux counterparts with high magnetic-saturation core.