Feedback control system for the impedance control of an electric arc furnace

A feedback control system having a controlled system gain for controlling the impedance of an electric arc furnace containing material to be melted comprising:

• at least one electrode having a free end located within the electric arc furnace, wherein the distance from said free end of said at least one electrode to the material defines an arc gap;
• a hydraulic electrode actuator coupled to said at least one electrode and including a supply of hydraulic fluid for adjusting the position of said at least one electrode to change the arc gap;
• a control valve coupled to said hydraulic electrode actuator for controlling the supply of hydraulic fluid;
• a valve actuator coupled to said control valve for controlling said control valve;
• a controller coupled to said valve actuator and including a correcting stage, a difference-forming stage having a first and second inputs and a final stage to provide a corrected controller gain for controlling said valve actuator;
• an actual impedance signal generator coupled to said first input for transmitting a signal representing an actual impedance of the feedback control system to said controller;
• a set point signal generator coupled to said second input for transmitting a signal representing a desired impedance value to said controller, said controller comparing the actual impedance to the desired impedance to calculate said controlled controller gain;
• and
• a correcting signal generator coupled to said correcting stage for transmitting a correction signal representing the controlled system gain, wherein said correcting stage adjusts said controlled controller gain in inverse proportion to said controlled system gain to calculate the corrected controller gain, whereby fluctuations in dynamic responses of the feedback control system are suppressed.