Some Problems in Electric Furnace Operation

Based upon the theory that the reactance of a three-phase electric furnace does not depend upon the load but is a constant depending on the arrangement of the conductors, the general equations of the three-phase furnace are developed and discussed. It is shown that, dute to the fact that the main conductors carrying the current from the interlaced bus bars to the furnace are more or less in one plane, a transformer action is introduced between phase 1 and phase 3, taking power away from 1 and transporting it to S. The phase rotation being 1, 2, 3. This power may be considerable. When voltages between phases and currents are balanced the power delivered by the three electrodes is quite unbalanced, a fact that is very apparent in large furnaces, electrode 1 seeming dead while electrode 3 takes most of the power. Nevertheless, when only voltage, current and total power are concerned, very simple expressions can be used which are entirely satisfactory in practise. A few methods are suggested to reduce the unbalancing of the load. Finally the two-phase connection of the load is studied to show that the unbalancing of the currents which is inherent in this connection does not check with observations on the furnace under operation. The load must be considered as connected in star, the bottom of the furnace being the neutral. The electric energy is transformed into heat energy in three separate zones situated under the three electrodes.