cooling of an alternator driven by an internal combustion engine wherein a field core of the alternator is composed of claw - type magnet poles and a cylindrical core such that an oil passage is formed in the regions where the cylindrical core and the rotor shaft oppose each other and where the cylindrical core and the claw - type magnetic poles oppose each other . the oil passage is communicated at its one end with an oil source outside the alternator so as to introduce an oil into the oil passage during operation of the alternator so that a centrifugal force is applied to the oil by the rotation of the rotary field core thereby spraying the oil in the form of vapor or mist onto the stator coil .