Thrust Control via Rotor Skewing in Integrated Twin Bearingless Motor

This paper presents a practical method for achieving full five-degree-of-freedom (5-DOF) levitation control using a twin bearingless motor architecture. Bearingless motors inherently produce both torque and radial magnetic suspension forces, but conventional designs lack axial force capability. To address this limitation, this work proposes to introduce rotor skewing to generate axial thrust via torque-current excitation. A twin motor configuration with equal and opposite rotor skew enables net torque production through common-mode current excitation, while differential torque currents produce net axial force. Theoretical modeling is presented to highlight the electromagnetic mechanisms responsible for skew-induced axial force, and the design space is analyzed to maximize current-to-thrust efficiency. Finite element simulations quantify the effects of rotor skew angle on torque, radial force, and axial thrust production. A control scheme is presented which decouples torque and axial force through coordinated current excitation across both motor halves. Simulation results confirm that substantial axial force can be generated with minimal degradation of torque and radial bearing performance. A candidate design achieves axial force exceeding 5x the rotor weight under full differential excitation, requiring less than 20% of rated current to achieve axial levitation. These findings demonstrate the feasibility of skew-based axial force generation in twin bearingless motors, offering a compact and efficient solution for applications with moderate axial load demands.