Analysis of Force Capacity in Magnetic Bearings and Bearingless Motors from the Perspective of Airgap Space Harmonic Fields

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.