The present invention comprises a rotor core and a cover disposed on the rotor core, wherein the cover includes a body part and a plurality of wing parts formed on the body part, the wing part is disposed between an outer boundary, which is a circular curve, and an inner boundary, which is a circular curve, the wing part includes an inner part and an outer part, the inner part forms a first inlet angle at a first point and forms a first outlet angle at a second point, the outer part forms a second inlet angle at a second point and forms a second outlet angle at a third point, the first point is positioned at the inner boundary, the second point is positioned at an intermediate boundary, which is a circular curve, disposed between the outer boundary and the inner boundary, and the third point can be positioned at the outer boundary.