PURPOSE: To increase effective magnetic flux interlinking with an armature winding by setting up a superconducting plate in the peripheral part of a field winding.

CONSTITUTION: A superconducting rotor, firmly fixing a field winding 1 and a superconducting conductor plate 9 to a field winding mounting shaft 2, is cooled by a liquid refrigerant 4 of liquid nitrogen or the like accumulated in the internal diameter side of a refrigerant vessel 3. Further for reducing intruding heat by radiation, a radiation shield 6 is set up between a normal temperature damper 7 and the refrigerant vessel 3. Here because even the superconducting conductor plate 9 in the peripheral part of the field winding 1 is cooled to a temperature in which a superconductive condition is obtained, magnetic flux, tending to traverse the superconducting conductor plate 9, is repulsed by a Meissner effect, and because magnetic flux 8, generated by the field winding 1, distributes also the harmonic component as shown in the drawing, effective magnetic flux, interlinking with an armature winding, increases.