Combined Windings for Bearingless Motors—An Overview

To create bearingless motors with high-performance electromechanical power conversion, recent research has demonstrated the need for combined windings where each stator coil can contribute to both torque and force simultaneously. This paper examines popular combined winding designs found in literature—multiphase, parallel and bridge dual-purpose no-voltage (DPNV), and mid-point current injection (MCI)—and shows that they are all equivalent from the machine perspective: the same stator slot current results in the same airgap fields to create the same torque/force output. From this, it is concluded that all winding "types" are actually the same fundamental multiphase winding, but connected to the drive electronics in different configurations. The paper presents mapping between the different drive configurations by using proposed winding function equivalencies. Finally, inductance matrices and equivalent space vector models are presented, and nuances in the modeling and control are highlighted for each drive configuration. These findings are summarized in a manner that allows a drives engineer to quickly apply state-of-the-art motor drive current regulation techniques to a bearingless machine connected in any of these popular winding configurations.