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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. 

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 studies the force creation capabilities of active magnetic bearings (AMBs) and bearingless motors from the perspective of multiple airgap space harmonics/pole-pairs. This approach is analytic-based and is useful in explaining the underlying physics of the machine and conducting force capacity analysis for different numbers of phases/poles. The presented per unit (p.u.) model makes the force capacity results applicable to any motor dimensions and peak airgap field value. An explanation of the force capacity in bearingless motors is provided when only two harmonics are controlled (which is the typical approach in bearingless motor literature) and the relationship between torque, force, and magnetizing field values is identified. Using this relationship, optimal magnetizing field values for maximum torque-force capability are identified, which is useful to consider when designing a bearingless motor. This paper extends the force capacity analysis to bearingless motors with multiple (more than two) controllable space harmonics and proposes that force enhancement can be achieved through the control of the magnitudes and angles of these harmonics. Results show that potential force enhancement of over 40% in bearingless machines can be achieved when controlling four airgap harmonics as opposed to two harmonics. These results suggest that being able to control multiple harmonics can yield high performance designs. 

A generalized multi-phase (MP) combined winding design procedure for bearingless machines is proposed and devel- oped. Using this procedure, new bearingless motor windings can be designed and conventional motor designs with MP windings can be transformed into bearingless motors by simply modifying the phase currents. The resulting MP winding is excited by two current components – one responsible for torque creation and an- other for suspension force creation. By applying the appropriate Clarke transformation, independent control of force and torque can be achieved. Although there are numerous papers in the literature studying bearingless machines with MP windings and their advantages, this is the first paper to provide a formal design procedure that can be applied to any MP winding configuration. The proposed approach can be used to realize popular winding designs, including concentrated- and fractional-slot windings, and is applicable to all radial-flux bearingless machines. The paper uses the Maxwell stress tensor to formulate the force/torque model for the MP combined winding and uses the results to derive design requirements. A sequence of winding design steps is proposed and used to design example MP combined windings. Experimental validation is provided using a six-phase bearingless induction machine prototype. 

This paper proposes and develops a new and exact analytic electric machine model that has several potential advantages. The model can be used to address levitation performance requirements by developing exact 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 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 density. 

A multi-phase (MP) combined winding design procedure for bearingless machines is proposed and developed. Using this procedure, new bearingless motor windings can be designed and conventional motor designs with MP windings can be transformed into bearingless motors by simply modifying the phase currents. The resulting MP winding is excited by two current components – one responsible for torque creation and another for suspension force creation. By applying the appropriate Clarke transformation, independent control of force and torque can be achieved. Although there are numerous papers in the literature studying bearingless machines with MP windings and their advantages, this is the first paper to provide a formal design procedure that can be applied to any MP winding configuration. The proposed approach can be used to realize popular winding designs, including concentrated- and fractional-slot windings. The paper uses the Maxwell stress tensor to formulate the force/torque model for the MP combined winding and uses the results to derive design requirements for the MP combined winding. A sequence of winding design steps is proposed and used to design example MP combined windings.