This new type of transformer comprises a ferromagnetic drum-type core characterized in that the drum core has a plurality of holes or windows parallel to the drum longitudinal shaft to place the windings being the windows arranged close to the periphery of the drum symmetrically distributed at 360° of the circumference, each winding being parallel to the longitudinal shaft of the drum and each one of the windings crossing said longitudinal shaft. The core comprises two main components: a central body and an air gap filling system. The central body is formed by a plurality of silicon steel sheets, stacked one over the other, each of them has slots or spaces on its periphery thereof to place the windings and with an air gap filling system. Said filling system can be: wedge-shaped sheets, set of sheets extending parallel to the shaft of the core or a metal sheet wound around the central body.