Performance Analysis of a Bearingless Axial-Force/Torque Motor

The Bearingless Axial-Force/Torque Motor (AFTM) is a single-phase air-core coil motor, which can generate axial forces and drive torques concurrently using a common stator winding system. The end-windings of the concentric, nonoverlapping stator coils create axial directed Lorentz-forces, provided that a non-vanishing common-mode current component is present. Due to the three dimensional, highly integrated and compact actuator geometry, the device modelling and control design is a challenging task. Also, the performance metering for such an actuator is not obvious at all. This article concentrates on how the electromagnetic and electromechanic properties of the bearingless drive depend on the geometric design parameters. A coordinate transformation is proposed to decouple the control variables axial force and drive torque. On the basis of a dynamic system model, a simplified control strategy is used to highlight the influence of the actuator design on the closed-loop system dynamics and its characteristic time constants. The elaborated steady-state axial force and drive torque boundaries, based on the definition of the copper losses, consider the thermal limitations of the actuator.