Torsional natural frequencies in the superconducting turbogenerator

In the design of superconducting AC generators the inner and outer rotors aie supported on thin torque tubes. This type of rotor structure introduces a torsional natural frequency, whereby the inner and outer rotor of the generator move in opposition, the frequency of which is influenced by the rotor electromagnetic structure. In this paper an analysis of the torsional natural frequencies within the turbine and generator is presented using eigenvalue techniques. The analysis is used to investigate the influence of the rotor structure on the frequency and damping of both the inter-rotor torsional mode and the rigid-body hunting oscillation mode. A simple formula for damping of rotor-hunting oscillations is presented. The eigen-value technique is also used to examine the influence of generator loading condition on the two oscillation modes.