Development of a cryogenic induction motor for use with a superconducting magnetic bearing

We have constructed a cryogenic induction motor to turn the rotor of a superconducting magnetic bearing (SMB). Both the motor and the SMB are operated at liquid He temperatures. We give a model for the motor and present measurements of its operation. The rotation speed is very stable. Over 8 h it shows an RMS variation of only 0.005 Hz from a mean of 2 Hz. The speed variation within one period of rotation is 3% ± 1% implying that the angular position of the rotor can be determined to an accuracy of 1 deg. for all angles of rotation even if angular position is encoded only once every period. Friction and heat dissipation in this motor is dominated by eddy currents. We discuss the application of the motor to astrophysical polarimetry