Abstract: This alternator comprises an assembly of rotors (2), including a common shaft (3) on which are fixed a first rotor (4) having permanent magnets and a second, homopolar rotor (5), this assembly being disposed in a common stator whose power windings (7) surround the two rotors (4,5). An excitation coil (6) is disposed in confronting relation to the homopolar rotor and is supplied with direct current or alternating current as a function of the speed of rotation of the rotor assembly. In this way there is produced a magnetic flux which is added to that produced by the permanent magnets or is subtracted therefrom so as to maintain the output of the alternator constant.