A motor vehicle alternator connected to an output shaft of an internal combustion engine mounted on a vehicle, comprising a rotor made of a permanent magnet for producing a magnetic field, an armature (stator) having a coil placed opposite to the permanent magnet rotor and a belt drive mechanism for connecting one of the permanent magnet and the armature to the output shaft of the engine to bring them into a relative rotation such that the armature relatively moves in the magnetic field produced by the permanent magnet. In the alternator, there is provided an axial displacement mechanism for displacing at least one of the permanent magnet rotor and the armature in an axial direction of the relative rotation, whereby a magnetic flux passing through the armature is changed. The axial displacement mechanism comprises a solenoid, a plunger housed in the solenoid to be movable when the solenoid is energized and a lever connected to the plunger at one end and to a carrier that carries the armature such that it pivots about a fulcrum to displace the armature in the axial direction of the relative rotation. Other types of the mechanism using a rack and pinion, etc., are used. The operation of mechanism is controlled such that a desired electric power is generated.