A converter includes a coil of axis XX′, a multi-pole magnet, arranged in such a way that the magnetic polarities of two adjacent magnetised zones are oriented so that they are anti-parallel, a ferromagnetic field frame comprising a frame and a main section, the field frame further comprising a main air gap located on the main section, and a first lateral air gap and a second lateral air gap which are located on the frame, the main air gap, the first lateral air gap and the second lateral air gap being arranged in such a way as to form a guide member in which the multi-pole magnet is inserted by sliding, and in such a way as to allow a magnetic coupling of three successive magnetised zones with, respectively, the first lateral air gap, the main air gap, and the second lateral air gap.