Abstract: The synchronous motor has a pick-up (C) measuring the position of its rotor and a power-varying circuit (VP) operating as a current generator. A power-varying circuit (VP) is controlled by a programmed microprocessor (MP) receiving information specifying the desired speed for the motor (M), information representative of the difference between the current speed and the desired speed for the motor, information from said pick-up relating to the position of the rotor of the motor, and stored information in order to enable it to force zero phase difference between the current I delivered by the power-varying circuit and the potential difference U across the terminals of the motor windings, thereby minimizing the installed power required of the power-varying circuit in order to run the synchronous motor and power-varying circuit at constant power. A control system operates the motor at constant power.