Derivative-Discontinuous Electroheat Control

In electroheat processes such as induction furnaces, arc furnaces, radiant furnaces, plasma torches, fluidized beds, etc., that are usually discontinuously controlled (because of the large power ratings involved) a great deal of confusion has existed as to the effect of derivative signals in the primary feedback loop. This has resulted in empirical design rules for the derivative feedback in controllers for such processes. An analysis of the effect of derivative feedback upon the performance indices of such electroheat processes is presented. Methods of evaluating these indices for various combinations of command and derivative feedback are given, and the boundaries are clearly defined for the process instability that results from excessive derivative feedback. It is shown that the well-known cycling characteristic display is inadequate for conveying to the practicing engineer the effect of derivative feedback. Two new displays have been derived that clearly show the effect of command and derivative feedback upon the process performance. Finally, the optimum derivative feedback conditions are given for the typical electroheat process. Mathematical derivations have been avoided, and the presentation is intended for the professional engineer engaged in industrial electroheat control.