Force Error Angle Reduction by Flux-Strengthening Motor Winding of Permanent Magnet Bearingless Motor

This paper proposes a novel technique for reducing force error angles in bearingless permanent magnet motors. Two-axis actively positioned bearingless motors generate a radial suspension force and torque with the suspension and motor windings, respectively. To achieve stable magnetic suspension, the suspension force should be precisely controlled. In contrast, undesired forces are generated due to the rotor radial displacement and the magnetic coupling between the motor and suspension fluxes. The force causes severe force error angles. Consequently, rotor vibrations occur, in 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. 3D-FEM analysis is used to illustrate the principle and present the calculation result. Furthermore, a prototype machine was fabricated, and it was demonstrated that the proposed method successfully reduced the force error angle in the experiment.