A rotating part that includes at least one generator unit having at least one coil, at least one permanent magnet and two pole shoes having pole surfaces facing radially outward is provided, The non-rotating part has an arc-shaped saddle adaptor of ferromagnetic material arranged with a radial distance to the pole surfaces. The saddle adaptor is configured to close a magnetic circuit passing via the pole shoes through the coil in a rotational position where the saddle adaptor overlaps with the pole shoes of the generator unit.