Activation and electro‐dynamic dampers, key technologies for the operation of superconducting magnetic bearings

Abstract

Purpose

The purpose of this paper is to focus on superconducting magnetic bearings (SMB). SMB for high‐speed rotors are contact free and offer inherently stable operations thus they are best qualified for the support of horizontally aligned rotors of turbo machines for gas‐compressors and expanders, e.g. special attentions have to be concentrated on the force activation of the SMB without dislocating the rotor from the aligned position.

Design/methodology/approach

For the activation of cylindrically shaped SMB‐designs, appropriate units with movable superconductor parts have been developed. They permit the maintenance of the rotor together with the field excitation unit in the aligned un‐displaced position. The eddy currents in the conducting cylinder of an EDD are induced by spatial fluctuations of the field and thus have been determined by transient calculations. The mechanical oscillation of the rotor was considered by a step‐wise displacement of the damper‐plate.

Findings

As the rotors of both the machine and the SMB operate best with reduced clearance to the stators, the shaft cannot be displaced to activate the force of horizontally aligned superconducting bearing assemblies. Thus, for cylindrical, co‐axial SMB‐designs the stator is shaped as two half shells embracing the SMB‐rotor. For the force activation the following procedure has to be carried out within the Dewar without displacing the shaft: at first the half shells are retreated from the rotor (warm HTSC) and after the cooling they are moved against the inner part of the warm bore thus generating the forces to compensate the weight and disturbances of the rotor. In case of planar‐cylindrical SMB‐designs, which are specially suited for extreme high speed applications, the bearing stators consist of a planar cylinder plate of HTSC‐bulks. The force activation is realised by lifting and descending the Dewar with the HTSC parts as a whole independently from the position of the rotor. The radial forces of the EDD and their partitioning in components which contribute to the damping‐ and to the spring‐force have been determined for different frequencies up to 160 Hz. To achieve accuracies in the percent range, the values of the time steps have to be well adapted to the electro dynamic conditions as oscillation frequency and conductivity.

Originality/value

Only the presented activation devices with movable HTSC stator parts enable the application of SMB even for horizontally aligned high‐speed rotors with reduced radial clearance. The recently developed fully integrated EDD secure a safe run of the rotor even during the speed up – passing the eigenfrequency in particular.

Citation

May, H., Hoffmann, J., Canders, W. and Palka, R. (2009), "Activation and electro‐dynamic dampers, key technologies for the operation of superconducting magnetic bearings", COMPEL - The international journal for computation and mathematics in electrical and electronic engineering, Vol. 28 No. 1, pp. 188-203. https://doi.org/10.1108/03321640910918986