Magnetic Bearing Actuator Design for a Gas Expander Generator

A magnetic bearing system for a high-speed cryogenic gas expander generator is presented. The expander represents a new class of such devices that, in contrast to the earlier technologies, makes use of the expanding gas energy by converting it into electrical power. The magnetic bearing and generator constitute a functionally complete unit that can be used in a variety of gas expanders utilizing different expander wheels and operating at various speeds. In order to obtain a compact design and increase rotor first bending frequency, the axial magnetic bearing is designed to have different pole areas on different sides of the thrust disk. This results in different load capacities in different directions, matching operational conditions of the unit. Regardless this anisotropy, the bearing features linear force vs current dependence. Radial and angular suspension of the rotor is achieved using two patented homopolar PM-biased radial bearings. The paper discusses design and analysis of these bearings. In spite of the three dimensional nature of the electromagnetic problem, combination of analytical solution and 2D FEA was found to be an adequate tool for the bearing analysis. Theoretical predictions of the bearing performance have been found to be in a good agreement with experimental data.