This control device is used in connection with a motor comprising a magnetic rotor and at least two magnetic stator circuits coupled with respective coils (b.sub.1,b.sub.2). A logic circuit (21) is arranged to alternatively feed drive pulses to both coils and to connect the coil which is not fed to a detection circuit (24, 25) the output of which is connected to the logic circuit in order to determine the duration of a feed pulse or to trigger a new feed pulse. This control device allows the current consumption of the motor as well as the locking torque to be reduced, since the servo control obtained insures the rotor positioning in case an important disturbance should occur.

A driving circuit for a stepping motor has a pair of emitter-connected PNP transistors with the common junction point of the emitters connected to a source voltage. A pair of source-connected static induction transistors each has its drain connected to the collector of one PNP transistor with the connection point being connected to one side of the drive coil of the motor. The leak current characteristic of the static induction transistors along their drain-to-source paths is chosen so that sufficient leak current flows when all four transistors are simultaneously in the OFF state to effectively dampen the motor.

The motor assembly comprises a bidirectional stepping motor having a coil, a stator provided with a central opening in which is mounted for rotation about an axis a rotor including a permanent magnet which is magnetized along an axis of magnetization, and positioning means for defining two rest positions of the rotor in which the axis of magnetization of the magnet is directed along an axis of static equilibrium. The stator is formed of two pole pieces which partly surround the rotor and are interconnected by two intermediate parts and by four isthmuses in such a manner that the intermediate parts are intersected by the axis of static equilibrium and that two isthmuses which are opposite each other in relation to the axis of rotation of the rotor are located in one direction making with the axis of static equilibrium an angle of between 30.degree. and 60.degree.. Two of these isthmuses which are located in the same direction have a width such that they are very rapidly saturated with magnetic flux when the coil produces in the stator, in response to a driving pulse, a magnetic field which enables the rotor to be made to turn in one direction, while the other two isthmuses have a width two to four times greater so that, when the coil generates a magnetic field which causes the rotor to turn in the other direction, the wide isthmuses, or, in this case again the narrow isthmuses become saturated, but less rapidly, so as to obtain a difference in the form of the current in the coil which enables a control device associated with the motor to determine the direction in which the rotor prepares to turn in response to a driving pulse and to avoid any unintentional reversal of the direction of rotation of the motor.

The motor according to the invention comprises a disc-shaped rotor with multipolar permanent magnetization along an annular area and a stator comprising a plurality of magnetic circuits (9, 10, . . . ) arranged so that the magnetic field is comprised within the two cylindrical face portions based on the magnetized annular area and located on either side of the rotor. Means for compensation of the harmonics of the current-free torque and of the current torque are provided, as well as the arrangement of magnetic pick-ups (25, 26). The invention applies to industrial synchronous motor which may operate as stepping motors.

A control system for a stepping motor comprises flowing an electric current to a plurality of phases in succession and driving the stepping motor, and flowing a loop current to a coil of the phase which is not concerned in the driving of the stepping motor when the deceleration area of the stepping motor is reached, thereby obtaining a braking force.