Independent Control of Torque and Radial Force in Bearingless Switched Reluctance Motors

Radial force and torque are the control objectives which determine the machine performance of levitation and rotation in a bearingless switched reluctance motor (BSRM). This paper proposes a control scheme for rotating and levitating a 12/8 BSRM. The motor torque and radial force are independently controlled with hybrid excitations in main windings and levitation windings. Firstly, the mathematical relationship between radial force and currents, which is utilized in this work, is derived with Maxwell stress tensor method. Then the proposed control scheme is analyzed. The average torque of each phase generated in levitation region equals to zero for its symmetry of the aligned position. Accordingly, the current calculating algorithm is deduced to minimize the magnitude of instantaneous torque in levitation region. The principle and realization of the proposed scheme are demonstrated with FE analysis. Experimental results show that the proposed scheme is effective for a stable levitation.