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Axial flux permanent magnet machines (AFPM) are popular for applications that benefit from high torque density and an axially compact form factor, such as in-wheel traction drives. Although the radial flux permanent magnet machine (RFPM) and the AFPM work based on the same underlying principle, the differences in their geometry introduce complexities in analysis of the AFPM. In this paper, the different AFPM design variants, their sizing approaches, computationally efficient design optimization techniques, and manufacturing techniques reported in literature are reviewed. In addition to classical AFPM machines, emerging variants and research opportunities with potential to push the boundaries of electric machine technology are reviewed. These include bearingless AFPMs, magnetically geared AFPMs, and combined radial-axial flux machines.

Bearingless motors are electric machines capable of simultaneously creating both torque and suspension forces. A detailed investigation into the underlying physics governing magnetic force creation in these machines is conducted in this paper, resulting in a newly derived analytic bearingless machine force model. Several key insights on designing bearingless machines for optimum force and torque performance are obtained from this investigation, including designing bearingless machines to be compatible with field-weakening and with conductive rotors. Validation for these findings is provided numerically, using optimization and FEA, and experimentally with hardware test results as well. The analytical foundations of bearingless machine operation developed in this paper makes bearingless machine design more intuitive and will pave the way for further innovation in this burgeoning field.

In recent years, the electrification of industrial, urban and commercial mobility, e-mobility, has garnered significant attention due to a rapidly growing interest in reducing reliance on fossil fuels and global carbon footprint. Both industrial and traction applications desire to reduce manufacturing costs and modular manufacturing methods. In addition, electric machines that can maintain a wide constant power speed ratio (CPSR) are desired for traction applications. Internal permanent magnet (IPM) machines are widely used in the current industry to meet such wide CPSR requirements. However, achieving wide CPSR using an IPM machine requires an iterative design and validation process due to the co-dependency of crucial machine parameters on the same rotor design features.
Machines that use a combination of rotor types, i.e., hybrid rotor machines, are shown to be effective in producing wide CPSR designs. The arrangement of rotors in a dual rotor machine also gives two additional degrees of freedom in design, individual stack lengths and a relative angle between the rotors. While these degrees of freedom can be used to improve the performance of a hybrid rotor machine, they also introduce additional degree of complexity in analyzing and understanding the performance characteristics.
The focus of this research is to propose and develop analysis and modeling method that considers all available degrees of freedom and propose a design process to achieve wide CPSR using hybrid rotor machines. The proposed analytical model and evaluation method also enables a quick and cost-effective machine design process that can standardize rotor cross-section for the manufacturers while meeting a wide range of system requirements using combinations of rotor modules. In addition, since a dual rotor machine can use different combinations of rotor types and positions and exhibit the performance characteristics of a wide range of synchronous machine types, the developed analysis unifies the modeling and field weakening theory of synchronous AC machines.