A rotor slot is asymmetrically shaped with respect to a straight line drawn from the central axis of the rotation of the rotor in a radial direction, and has a slot opening, the circumferential width of which is smaller than that of the rotor slot, at the top of the rotor slot, so that a loss generated in the rotor due to a carrier harmonic component can be reduced and the power factor can be improved, enabling a current generated in an armature winding to be suppressed and thereby improving efficiency in a rated operation.