Influence of Magnetic Eccentricity in Bearingless PM Synchronous Machines

Bearingless permanent magnet (PM) synchronous machines, used as high-speed drives, typically employ a simple rotor structure. Machines of smaller size are equipped with a solid shaft on which a permanent magnet ring is mounted, protected by a carbon-fiber sleeve. Larger machines often require an additional rotor sheet package and use permanent magnet shells instead of a ring. Since the permanent magnet exhibits a small yield strength, no shrink-fitting between the magnet ring and the shaft can be applied. Thus, the PM ring is required to be slightly bigger in inner diameter than the shaft in outer diameter, leading to a half-moon shaped air gap between magnet and shaft. Although this air gap is usually in the micrometer-range it can lead to a considerable rotational-frequent radial disturbing force. We discuss the in uences, such as the rotor diameter and the pole count, which contribute to this magnetic eccentricity problem. We also show the disturbing influence on the rotor position control by measurements on a built prototype.