Method for controlling an induction motor, and devices driven by such an induction motor

In the case of vector control, both with and without a rotation speed sensor, the setting accuracy for the rotation speed of an induction motor deteriorates in operation at a low rotation speed, as a result of fluctuation of the secondary resistance. The vector control according to the invention overcomes these difficulties and implements accurate setting of the rotation speed and of the vector, even in the vicinity of a rotation speed of zero. In this case, an alternating signal, whose frequency differs from that of the fundamental of an AC voltage supply for driving the induction motor, is superimposed on the supply AC voltage. A physical value (parasitic inductance) is determined, as is produced as a result of a difference between saturation states of the iron core of the motor which are caused by the fact that a motor current flows in addition to the AC voltage because of the superimposition, or the determination is carried out as a function of the motor voltage and the alternating current produced. The processes quoted above are repeated a plurality of times by varying the phase of the fundamental in order to obtain the position (the angle) of the magnetic flux of the fundamental of the motor as a function of a plurality of physical values obtained. The induction motor is then controlled as a function of the magnetic flux position signal.