Magnetic Bearing Sizing for Flexible Rotors

Abstract

Magnetic bearing load capacity in flexible rotor systems must be adequate to accommodate external loads acting on the rotor. The first part of this paper develops the theoretical basis for computing the bearing load capacity requirements of flexible rotors subject to bounded external harmonic loads and strict motion constraints. The second part of this work illustrates the application of the theory to a specific example: a fairly simple three disk flexible rotor subject to mass unbalance loads. Load capacity requirements are computed for the example shaft at its first three free-free forward whirl critical speeds. Based on the bounds obtained, two bearing design cases are examined: one with 45 N load capacity and the other with 223 N load capacity. The performance of the rotor is not improved with the higher capacity bearing and neither is capable of adequately constraining the rotor at its second critical speed. It is concluded that this shaft cannot be operated above its second free-free critical speed without a midspan bearing.