To counteract the effects of centrifugal force on internal blades or vanes positioned in gaps (31) between adjacent claws (22, 23) of a claw-pole rotor (R) of a claw-pole alternator, a plastic sleeve (33) is fitted over the field winding (20) and has cooling fins or vanes or blades (32) projecting radially therefrom, at an inclination with respect to the axis of rotation of the rotor, and located between the inclined faces of the claw poles. The end portions (36, 37) projecting beyond the sleeve may be straight (FIG. 4) or the end portions (38, 39) may be curved (FIG. 5). Preferably, a curable casting compound is cast within the sleeve and secures the winding (20) in position, the sleeve being fitted in form of a circular bandage over the winding.