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.

This paper reviews bearingless motor technology for medium and high power applications. Historically, bearingless motors have resided in research settings and certain niche commercial applications. Significant literature has been dedicated to developing designs that feature highly-specific geometry, restricted by applications with a unique set of needs, oftentimes with low power requirements. The first part of this paper reviews existing and potentially new use-cases of magnetic levitation in transportation and industrial applications. The second part of the paper reviews bearingless motor technology for these applications, presents a comprehensive literature review of prototype test results, and highlights efforts to generalize and increase the power rating of bearingless motor technology. The final part of this paper derives sizing equations of different bearingless motor topologies, which are summarized as a design tool for developing industrial-scale bearingless motor technology.

This paper develops a magnetic equivalent circuit model suitable to the design and optimization of the synchronous ac homopolar machine. The ac homopolar machine is of particular interest in the application of grid-based flywheel energy storage, where it has the potential to significantly reduce self-discharge associated with magnetic losses. The ac homopolar machine features both axial and radial magnetizing flux paths, which requires finite element analysis to be conducted in 3-D. The computation time associated with 3-D finite element modeling is highly prohibitive in the design process. The magnetic equivalent circuit model developed in this paper is shown to be a viable alternative for calculating several design performance parameters and has a computation time which is orders of magnitude less than that of 3-D finite element analysis. Results obtained from the developed model are shown to be in good agreement with finite element and experimental results for varying levels of saturation.

A winding design approach is proposed to create a single motor winding, which is able to produce both radial force and torque. This approach can be used to design new bearingless motors, as well as to transform conventional motor designs into bearingless motors by simply modifying the winding end connections. The resulting winding has two sets of terminal connections, one for torque and one for suspension. The suspension terminals experience no motional-electromotive force when the rotor is centered, which means that the suspension drive can have a low voltage rating and that rotor vibrations can be passively dampened by simply short-circuiting the suspension terminals. Bearingless motors that use these so-called dual purpose no voltage windings can be designed to have higher torque density and lower losses associated with the magnetic suspension operation than traditional bearingless motors, which utilize separate torque and suspension windings. It will be shown that many popular winding designs, including fractional-slot and concentrated windings, can be realized as dual purpose no-voltage windings. The proposed approach applies to traditional $p\pm 1$ pole-pair bearingless motors, as well as the bearingless consequent-pole and ac homopolar motors. Fractional-slot motor winding theory is used to derive the new winding requirements and a generalized design procedure; example designs are explored through finite element analysis and experimental results from a hardware prototype of a bearingless ac homopolar motor.

A winding design approach is proposed for the bearingless ac homopolar and consequent pole motors that utilizes the same coils for both suspension force and torque production. This enables a “pure” bearingless motor design, where the same iron and copper are used for both magnetic bearing operation and torque production, and can result in more optimal machine performance. Separate terminal connections are provided for suspension and torque operation; the suspension terminals experience no motional-emf when the rotor is centered, thereby reducing the required voltage rating of the bearingless drive compared to other “dual purpose” winding designs. It is shown that only certain combinations of stator slots, phases, and poles yield suitable winding designs; most notably, the number of motor pole-pairs must be co-prime with the number of phases. A design procedure is proposed which allows the designer to simply modify end-connections of many conventional armature windings, including both integral- and fractional-slot windings; an example winding design is presented and analyzed using 3-D finite element analysis (FEA); and a hardware prototype has been constructed with experimental results included to validate the proposed design approach.

The bearingless version of the induction motor (IM) has unacceptable performance for high speed or significant power applications. This is due to design challenges that are unique to the bearingless IM, including fundamental topology differences, rotor current induced by the suspension field, and a lack of techniques that can rapidly and accurately model the machine. This article presents a complete investigation into the design topology, modeling, and optimization of bearingless IMs to identify high-efficiency, power-dense designs for a high speed industrial compressor system. Key differences in the design of the bearingless IM from that of the classical line-fed IM are explored and an analytic design approach is proposed. A pole-specific rotor and a combined stator winding are used to improve the machine performance. Computationally efficient finite element modeling techniques are proposed and evaluated based on their ability to accurately calculate bearingless IM design performance metrics. An optimization framework is developed around these advancements using the multiobjective evolutionary algorithm based on decomposition. This article exercises this framework to explore the design space of four different slot-pole combination bearingless IMs for a 50 kW, 30 000 r/min compressor. High performance designs are identified that achieve >96% efficiency with a torque density competitive with high performance servo motors that use contact bearings.