Circuit arrangement for the automatic de-excitation of a hysteresis motor

In a circuit arrangement including a hysteresis motor having a ferromagnetic rotor and a three phase stator winding, capacitors connected to said stator windings for providing compensation for the reactive power intake of said motor, and a static frequency converter including the series connection of a rectifier circuit, a d.c. intermediate circuit and a controlled inverter circuit, said static frequency converter having its input connected to a three phase mains and its output connected to said stator windings for supplying power to said motor whereby the rotating field produced by the stator windings magnetizes said rotor and thus produces a permanently magnetic excitation which continues in said rotor, even after disconnection of said motor from said three phase mains, in the form of residual magnetic poles;

• the improvement comprising circuit means for the automatic de-excitation of said motor, said circuit means comprising in combination:
• voltage monitoring means for monitoring the voltage of said mains at the input of said frequency converter and for producing an output signal indicative of an interruption in the mains voltage due to malfunction or end of operation;
• a buffer circuit for said d.c. intermediate circuit of said static frequency converter, said buffer circuit including a storage capacitor which is connected to the input of said controlled inverter circuit and which discharges and transfers its stored energy with decaying voltage to said stator windings of said motor via said controlled inverter circuit when the output voltage of said rectifier is interrupted;
• and means responsive to said output signal from said voltage monitoring means for automatically increasing the output frequency f.sub.0 of said inverter circuit by a predetermined amount .DELTA.f, whereby the residual magnetism in the rotor is quenched.