Suitability of Bearingless Motor Windings for Non-Salient Rotor Displacement Self-Sensing

This paper investigates the suitability of rotor displacement self-sensing for bearingless motors. Classical separated windings are investigated along with more modern combined windings, where each coil contributes to both torque and force creation. Combined windings have been proven to be a critical component for realizing high performance bearingless motors for industrial applications. These applications are often highly cost-sensitive and require bearingless solutions to have minimal overhead compared to traditional motors. Self-sensing levitation control removes one of the primary costs: the radial position sensor. This paper enables research for eliminating these sensors for machines that use combined windings. Starting from first principles, winding functions are defined for several types of combined windings. The phase inductance matrix is computed including the effects of a non-centered rotor, and then transformed into decoupled torque and suspension force components. It is shown that combined winding bearingless motors can support self-sensing akin to the popular separated winding bearingless motor, albeit with added complexity; the multi-phase combined winding admits the most manageable solution. Hardware experiments validate the analytic models of windings.