the direction of the blade axis 7 which points in the radial direction 16 the magnets 5 are faulted as permanent magnets or as electrically controlled magnets the magnets 5 are arranged in series as seen in the circumferential direction 17 which results in a first circular magnet row 18 in this case a number of n magnets 5 are provided in the circumferential direction 17 wherein n represents a positive whole number the magnets 5 are arranged in series with a regular spacing of u wherein u represents the circumference of the inner circumferential surface a second circular magnet row 19 comprising a plurality of magnets 5 is arranged behind the first circular magnet row 18 as seen in the axial direction 6 the second circular magnet row 19 comprises a plurality of magnets 5 which are arranged in series in the circumferential direction 17 the second circular magnet row 19 has magnets 5 which are arranged in series with a regular spacing of u furthermore a third additional circular magnet row 20 can be arranged behind the second circular magnet row 19 in the axial direction 6 this third circular magnet row 20 also comprises a plurality of magnets 5 which are arranged in series with a regular spacing of u so that the magnetic field is formed as homogenously as possible the second circular magnet row 19 is arranged in an offset manner to the first circular magnet row 18 the third circular magnet row 20 is in turn offset to the second circular magnet row 19 the offset of the third circular magnet row 20 in relation to the second circular magnet row 19 and the offset of the second circular magnet row 19 in relation to the first circular magnet.