Reduction of Force Error Angle by Flux-Strengthening Control in Two-axis Actively Positioned Bearingless PM Motor

This paper proposes a novel technique for reducing force error angles in bearingless permanent magnet motors. Two-axis actively positioned bearingless motors generate the active radial suspension force and torque with the suspension and motor windings. The suspension force should be precisely controlled to achieve stable magnetic suspension. On the other hand, undesired forces are generated due to the rotor radial displacement and the magnetic coupling between the motor and suspension fluxes. The force causes serious force error angles. It results in rotor vibrations, and at the worst case, the rotor touches down on the stator. In this paper, the flux-strengthening control in the motor winding is proposed to reduce the force error angle. The principle and the calculation result are shown with 3D-FEM analysis.