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The aim of this paper is to present a solution for active output voltage regulation in permanent-magnet (PM) automotive alternators, through mechanical flux weakening. An axial-flux PM ironless machine is investigated, which is composed of two stators connected in series which can be shifted one with respect to the other, thus allowing a theoretically infinite constant power speed range. This paper focuses on the aspects related to layout, sizing, and control of the shifting mechanism, which are critical for accurate output voltage regulation. Experimental tests on a 2-kW prototype validate the capabilities of the proposed solution, both during steady-state and transient operating conditions.