Design of Bearingless Motors to Unload Static Forces from Foil Bearings

This paper explores the development of electric motors that generate static unloading forces to enable gas foil bearing implementation in large-scale turbomachinery. Foil bearings are increasingly being considered as a bearing solution for high speed motor shafts in hermetic environments. However, they experience wear during startup and shutdown, preventing their use with large turbomachine shafts. To overcome this challenge, the paper leverages bearingless motor technology to develop electric machines capable of controlling radial forces to reduce the effective shaft weight at low speeds. The paper presents a detailed investigation into design requirements that large, hermetic turbomachines impose on the electric machine and bearings. It then evaluates the feasibility of utilizing various bearingless motor design topology features to meet these requirements. Since the electric machine rarely needs to create radial forces (only at startup/shutdown), the aim is to ensure the machine retains maximum performance when it is not producing radial forces. A multi-objective FEA-based optimization is presented to characterize the design space, demonstrating that with careful design, it is feasible to develop high-performance electric machines that add capability to unload their shaft weight without compromising power density and efficiency.