A tap-changer for oil-filled electric transformers comprising: a guide element (2) provided with a plurality of electric contacts (3) and a slide (4) slidable along the guide element (2) to change the electric connection between two or more of the electric contacts (3); moving means (5) of the slide (4) comprising a driving group (6) having at least one rotatable portion (8) and defining a plurality of discrete positions of the slide (4) with respect to the guide element (2), each one corresponding to a different electric connection between the electric contacts (3). The moving means (5) comprising: a ring-nut (20) susceptible to be put in rotation by the driving group (6) and whose side surface has two or more flat surfaces (21); a core (23) counter-moved with the side surface of the ring-nut (20), the portion (24) of the core (23) faced towards the ring-nut (20) having a flat surface (25) susceptible to be arranged one-to-one with the flat surfaces (21) of the side surface of the ring-nut (20) when the slide (4) and the guide element (2) are placed in correspondence of one of the discrete positions.
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