Abstract: In an alternating current synchronous servomotor, a disk-shaped rotor (20) is arranged between two stator halves having each a coil with leads extending in the radial direction. The rotor (20) comprises an even number of flat permanent magnets (57) forming an annular series around a hub (55) and of which the flat sides form the north and south poles (N,S) alternating in the series. The circonference of the hub (55) presents planar surfaces (55A) separated by edges (55B). A planar delimitation surface (57A) of one of the permanent magnets (57) bears on each of said peripheral surfaces (55A). A bandage (66) surrounding the delimitation surfaces (57B) of all the permanent magnets (57), which are radially directed to the outside, secures the latter to the hub (55) and connects them by means of the planar surfaces (55A, 57A), mechanically to the hub against rotation.