Combined Winding Drives for Industrial-Scale Bearingless Motors

Bearingless machines designed with combined windings obtain higher performance than machines designed with separated windings. However, combined windings create challenges for the drive electronics, resulting in disproportionate increases to the drive ratings and cost when used to operate significant power machines. This paper is focused on developing a drive solution for bearingless machines that require large or continuous motor current, such as industrial-scale induction and reluctance machines. The paper investigates and compares the drive performance of the four drives capable of operating with independent magnetizing current, torque, and force control: the multiphase (MP), bridge and parallel dual-purpose no-voltage (DPNV), and middle-point current injection (MCI) drives. Equivalent circuit models are developed to facilitate the drive design for pole-specific bearingless induction machines (PS-BIM) and a new control approach is developed to solve performance issues in MCI drives. The paper finds that the MCI drive using the new control approach requires less drive current and is the most cost-effective solution for high torque current applications.