Abstract: In an alternator for use in an automobile, a passage for ventilating air around slip-rings is formed on a rear cover that covers a rear end of the alternator where the slip-rings are disposed. A brush device for supplying exciting current to a rotor through the slip-rings is mounted around the slip-rings, and the axial rear end of the brush device is covered with the rear cover. A space enclosed by the brush device and the rear cover is formed around the slip-rings, and air in the enclosed space is ventilated through the ventilation passage when the alternator is driven. The slip-rings heated through operation are cooled down, and brush powder in the enclosed space is exhausted.

Abstract: An AC generator includes a rotor composed of a pair of generally cylindrical pole cores each having a plurality of claw poles. The claw poles have first and second chamfers disposed respectively on both leading and trailing sides thereof. A ratio b/a between a circumferential width a of the first chamfer and a circumferential width b of the second chamfer is in a range from 2.5 to 5.0, and a chamfered angle .theta.r.degree. of the second chamfer is in a range from 60.degree. to 75.degree..