Performance Evaluation of a Homopolar Bearingless Motor for Ultrahigh Speed Applications

Abstract

This study evaluates the performance of a homopolar bearingless motor driven at 100 000 r/min with a mechanical output power of 120 W. The rotor has two salient iron core parts assembled into the shaft in tandem. The stator comprises two units located axially in tandem and an axially magnetized permanent magnet that is sandwiched between these two stator units. This magnet provides three-dimensional magnetic bias flux to polarize the rotor core parts, resulting in single polarity of one rotor core; thus, this type of motor is called homopolar motor. Each stator has two separated three-phase windings for the motor rotation and magnetic suspension, and the bearingless motor examined in this study has active suspension control with four-degrees-of-freedom (radial and tilting motions). The rotating part consists of only iron cores, and hence, it has the advantage of a rigid robust structure. Additionally, no angle detection is needed for the active magnetic suspension control. Therefore, the homopolar bearingless motor is suitable for high-speed applications. The conventional homopolar bearingless motors have an eight-pole rotor, but the proposed structure has a six-pole rotor to reduce the driving electrical frequency. Although this six-pole homopolar bearingless motor has 20% suspension force variation and 4.5° of force error angle, the rotor is driven up to 100 000 r/min without touchdown. This study presents the structure and principle of the proposed homopolar bearingless motor and the experimental results of the load test.