Temp. control in furnace for magnesium treated cast iron - switching electrical heating rods cyclically in equally distributed groups depending on actual and desired values

摘要

A thermostat control loop connects up only one heating element or one group of element at a time for a short period depending on a comparison of actual and desired preset temp. A thermocouple produces a signal which is compared with that set by a desired value setter. If too high, a control unit cuts out the main contactor for the heating rod elements. If too low, a desired value generator is activated and its output is compared in a comparator with that of another thermocouple. This combines a uniform temp.distribution with a controlled heating-up and min. fluctuations of a set temp. The energy requirement is a min. and the mains supply is loaded symmetrically. The life of the heating rods is increased and no expensive variable voltage transformers are required.