PURPOSE: To eliminate the need for the supply of exciting power during revolution by bringing a field winding for a synchronous machine to a superconductive state, supplying exciting power under the state of the stoppage of the synchronous machine and settling magnetic force on the rotor side at an initial stage.

CONSTITUTION: A rotor with a magnetic pole 4 is installed inside a stator frame 1 and a stator core 2. A field winding 5 on which a hollow superconductor, the inside of which is cooled by liquid nitrogen, is wound is mounted to the magnetic pole 4. A terminal 9 for initial excitation is set up to the field winding 5. Liquid nitrogen for cooling the field winding 5 is fed through a coupling 8 from the fixed side. Since the field winding 5 cannot be supplied with exciting power during revolution, exciting power is fed during the stoppage of a synchronous machine. Since the field coil 5 is composed of the superconductor, the currents are held.