Abstract: The present invention relates to an alternating-current-synchronous-servomotor having a disk-shaped rotor (55, 56) which is located between two stator halves. The rotor comprises an even number of flat permanent magnets pieces (56) having magnet field lines extending parallel to the shaft of the rotor. The permanent magnet pieces (56) form an annular like series including interstices extending around a hub (55) supported on the shaft. The magnet pieces are made of a magnetically non-conductive material and have flat sides acting as magnetic pole surfaces (N, S) which extend in parallel planes to which the axis of the shaft extend perpendicularly. The permanent magnet pieces (56) have limiting surfaces located radially inwards, which are supported each on one of peripheral planes of the hub (55). Said peripheral planes and said limiting surfaces comprise cooperating means (57, 58, 59, 60) for fixing said magnet pieces (54) on said hub (55).