A channel connecting a first opening formed on one side surface of a rotor and a second opening formed on the other side surface of the rotor is provided, and a first distance from a rotating shaft to the center of the first opening and a second distance from the rotating shaft to the center of the second opening are differentiated. A differential pressure is generated between the first opening and the second opening provided in the rotor by the differentiating the distances from the center of the rotating shaft to the first opening and the second opening, a flow of air is generated by the differential pressure from one surface or the other surface of the rotor to the other surface or the one surface, whereby the stator in proximity to the rotor may be cooled.