PURPOSE: To increase the thermal capacity of a damping winding by employing a hollow damping winding and filling the hollow section with liquid refrigerant having large specific heat.

CONSTITUTION: A hollow damping lever 11a and a damping ring 11b are communicated each other. Then a pipe 12 communicated with two damping rings 11b coupled to the opposite end sections of the damping lever 11a to provide a flow path, a cylinder 13 is arranged in the way. The cylinder 13 contains a movable chip 14 and a compression spring 15 for supporting the movable chip 14. When the magnetic pole comes to a standstill, the movable chip 14 is brought to a position shown on the drawing by means of a compression spring 15 while when the magnet pole is rotating, the movable chip 14 is subjected to centrifugal force and moved in the direction of an arrow while resisting the spring 15 force thus compressing the compression spring 15. In other words, the movable chip 14 moves in the direction of an arrow to a position where it is balanced with the compression spring 15 force, when it is subjected to centrifugal force. Liquid refrigerant moves through the damping lever 11a and the damping ring 11b at a heating position corresponding to the moving length of the movable chip 14. Consequently the thermal capacity of damping wiring is increased.