In modern vehicles, the demand for on-board electric energy generation is rapidly growing in order to satisfy new requirements for passengers comfort and safety. This growth, which is predicted to reach 6 kW in the very next years, is forcing manufacturers to explore new solutions concerning the whole electrical system inside the vehicle. In particular, novel topologies of alternators are under research since the currently used Lundell type (claw-pole) alternator is reaching its power limits. In this paper an original torque dependant mechanical device is proposed in order to extend the speed range of an axial flux PM machine. This device displaces the rotors realizing a mechanical flux weakening with constant output power in generation. Simulations and experimental tests on a prototype realized in the lab are fully discussed.