Apparatus for controlling the distance of a melting electrode from the surface of the melted material in a vacuum arc furnace

In apparatus for controlling the arc distance between a melting electrode and the surface of melted material in a vacuum arc furnace, wherein the short-circuits caused by the droplets between the melting electrode and the surface of the melted material are used as a control criterion, said apparatus comprising means for determining the short-circuits occurring within a given period of time, the so-called droplet rate;

• an averager (80), coupled to said short circuit determining means, for averaging of said droplet rate;
• and controller means, coupled to said averager, for controlling said arc distance in dependence upon the average droplets rate;
• the improvement comprising reciprocal value forming means (81), coupled to said averager (80), for producing a signal representing the reciprocal value of the droplet rate signal produced by said averager;
• desired value forming means for producing a signal representative of the desired average time between to consecutive droplet short circuits;
• and means coupled to said reciprocal value forming means and to said desired value forming means, for producing a signal representative of the difference between said reciprocal value and said desired value, said controller means being responsive to said difference signal.