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Ring motors are electric machines that are typically characterized by having a hollow rotor / stator, a small difference between the inner and outer radii, and a large outer diameter relative to the axial length. The hollow portion of the ring motor allows integrating loads, such as an aerial or marine propeller, enabling power-dense systems. This paper reviews integrated ring motor designs from literature across different applications. Based on this review, first, design trends and performance parameters are identified and compared with conventional radial flux machines. Next, the bearing challenges posed by the unique form-factors of these machines are identified and approaches to realize bearings are presented. Finally, a research outlook is presented that identifies the benefits of applying multi-physics optimization, additive manufacturing, and bearingless machine technology to realize improved integrated ring motor designs. 

This paper proposes and develops a new analytic bearingless machine model that incorporates multiple airgap harmonic field interactions and has several advantages. The model can be used to address levitation performance requirements by developing force/torque regulation methods to precisely calculate commands to current regulators. This allows relaxing constraints during the design stage and has the potential to enable consideration of higher performance bearingless machines. Furthermore, analogous to torque enhancement in conventional electric machines, the proposed model can be used to identify options for suspension force enhancement in bearingless motors by controlling multiple magnetic field harmonics. This paper provides a detailed derivation of the model and shows how it can be used to improve force regulation accuracy and enhance force capacity. The paper finds that by controlling four airgap harmonic fields, instead of the typical two harmonics, force capacity can be increased by approximately 40%. Hardware measurements using a 10-phase bearingless induction machine validate the proposed model and force capacity increase.