Abstract: A rotary electrical machine such as a motor vehicle alternator comprises a stator and a rotor. The stator has at least one armature winding in at least one pair of stator slots. The rotor includes means for selectively establishing closed magnetic circuits which pass around the turns of the armature winding or windings. The means for setting up closed magnetic circuits comprise at least two permanent excitation magnets which set up two magnetic fluxes having components in opposite directions according to the direction of displacement of the rotor; and, between each pair of successive magnets, at least one excitation winding having two wires and being arranged to produce in an adjustably way two flux components which are such as to oppose the fluxes set up in the magnets of the pair. The wires of the excitation windings are located in rotor slots, each of which lies between two successive rotor poles.