depending on the angular movement of the rotor 2 which it is desired to obtain which is predetermined as a function of the specific application the total theoretical number n of complete periods t of the ac current i required for feeding the motor 1 for driving it over such an angular movement is determined whether by calculation of experimentation depending on the order number x of these complete periods t in their total theoretical number n even on the direction of the controlled rotational movement a table is formed of the variations of the power supply for the motor 1 to be provided for theoretically complying with a predetermined speed reference c the instantaneous real number n 39 of complete periods of the ac current i supplying the motor 1 is counted from an initial endmost position depending on each value of x 39 of the instantaneous real number even on the direction of the controlled rotational movement the supply to the motor is matched to the value given by the table for the corresponding intermediate theoretical number m depending on the load driven by motor 1 a shift may appear between the actual position 9 of rotor 2 and position 10 obtained by counting the periods of the signal 7 causing its rotation in this case in order to take such a shift into account in accordance with the invention a signal g is elaborated representative of the so called 34 slipping 34 difference between the speed of synchronism v s of the motor and the actual rotational speed v of its rotor 2 depending on the level h of the slipping signal g and on the angular position.