The converter described comprises a generator (2) having a rotor (3) driven by a spring (1) via a gear-train (4), and means (11) for electrically braking the rotor (3). In one form of embodiment of the converter, the mean rotational speed of the rotor (3) is adjusted to a predetermined set speed by a control circuit (12) which periodically cuts out the braking means (11) in response to a reference signal having a period equal to the ratio between a predetermined rotational angle of the rotor (3) and the set speed. The control circuit (12) cuts the braking means (11) in again after a length of time which varies in response to a signal generated by a circuit (26, 27) for comparing the real angular position of the rotor (3) at the beginning of each period of the refernce signal with the angular position it should be in when running at the set speed.The converter may for instance be used in a timepiece having hands (9) that are also driven by the spring (1).