In this paper, the modeling and control of reluctance-force-based magnetic suspension in cylindrical rotor, smooth air-gap bearingless motors are presented. The full suspension system dynamics, including both the destabilizing forces due to the motor field and the active magnetic suspension control forces, are modeled, and a transfer function of the bearingless motor suspension plant is derived. It is shown that the suspension system dynamics in a bearingless motor depend on the motor winding current amplitude. This requires the magnetic suspension controllers to address the changing system dynamics and to stabilize the suspension under different driving conditions. A controller design with its gains changing with the motor winding current amplitude is proposed. The derived model and the proposed controller design are verified by experiments with a hybrid hysteresis–induction type bearingless motor. It is shown that the derived mathematical model provides an effective basis for loop-shaping control design for the reluctance-force-based magnetic suspension systems in bearingless motors, and the proposed controller design can stabilize the rotor's suspension under varying excitation conditions.

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