Comprises an electromagnetic stabilizing groups are strips (M) feeding direction 100, transverse 100 'a plurality of first and second electromagnets (15, 16) aligned along a perpendicular to the plurality of first and second electromagnets (15, 16) are mutually arranged in a position mirroring the theoretical feeding plane (50); Electromagnets (15, 16) each of at least a first and a second pole (18 ', 18' ') the core (17) is provided; Each of the first and second pole (18 ', 18' ') of the first and second coil (3', 3 '') wound around the circumference; 'To generate, respectively, a coil (3 first and second magnetic fields (T, 7 "), respectively, 3' ') on the two power supply (4 to supply electric', 4 ''); It is connected to the core being made of a first second coil (3 ', 3' ') ferromagnetic material arranged to the each other to be magnetically independent of the at least one of the concentrator.