1 u2032 l 1 u25cf u03b4i 1 u s 8 l 2 i 2 u2212 i 2 u2032 l 2 u25cf u03b4i 2 u s 9 l 3 i 3 u2212 i 3 u2032 l 3 u25cf u03b4i 3 u b u2212 u s 10 the values u03b4i 1 u03b4i 2 u03b4i 3 can be determined by means of the measuring devices 13 to 15 in the three equations 8 to 10 the inductivities l 1 l 2 and l 3 as well as the potential u s at the star point are then unknown from the three equations 8 to 10 the ratios l 1 l 2 l 1 l 3 l 2 l 3 can be determined while eliminating u s which each represent a dimension for the position of the rotor within half a magnetic period u b l 3 1 1 l 1 1 l 2 u25cf u03b4 i 3 11 consequently four equations 8 to 14 are available for determining the unknown values l 1 l 2 l 3 and u s each of the inductivities l 1 l 2 l 3 may serve as a dimension for the position of the rotor within half a magnetic period it is understood that for obtaining several equations from which u s can be eliminated and l 1 l 2 or and l 3 can.