Practical Control Implementation of Mid-Point Current Injection Drives for Bearingless Motors

Bearingless motors are capable of creating both torque and radial forces, thereby unloading or eliminating bearings in an electric machine. The technology obviates the need for contact bearings and enables compact, oil-free, and low-vibration motor systems. Comparing all the winding configurations in bearingless motors, mid-point current injection (MCI) features distinctive characteristics that help convert an electric machine into a bearingless motor with the fewest modifications, but has been insufficiently developed in existing literature. This paper investigates control considerations to enable independent motor and suspension drives. To this end, the paper identifies the control challenges and proposes corresponding solutions. It is shown that with a proper control strategy, the cross-coupling voltages can be compensated in the suspension drive, mitigating potential force ripples in the bearingless motor. Additionally, this paper introduces a self-sensing control technique in the suspension drive that enables an encoder-less suspension operation. The simulation results show the effectiveness of the proposed control strategy and self-sensing technique in the suspension drive.