PURPOSE: To make it possible to radiate heat from a magnet promptly for a short time during control, by forming a cylindrical rotating body with an opening with an air-flow receiving plate on one side and a bottom part with a plurality of air holes and an air blower on the other side.

CONSTITUTION: A blower 20 that rotates with a rotating body 1 has a fin with an opening in a forward rotating direction and a closed end on the other side. When the rotating body 1 rotates, cooling air flows into an air hole 1b to cool an electromagnet 4. The air hits on an air-flow receiving plate, flows into a cooling fin 1d in a rotating body 1 and is discharged outside into the open air. A semicircular bent part 30a is formed at an air-flow receiver 30 so that the cooling air from the blower 20 is thrust into the cooling fin 1d and discharged outside through centrifugal force of the cooling fin 1d. As a result, the heated air radiated from the electromagnet 4 is discharged outside in a short time.