Impedance and phase balancing of mains-frequency induction furnaces

The impedance presented to the supply by coreless and channel induction furnaces used for melting and holding metals is not constant. Many such furnaces are designed for single-phase working and their power supplies often include components to enable a balanced load to be taken from a 3-phase system. Owing to the varying furnace impedance, such balancing components are designed to be variable and are often operated by the furnaceman. Variation in furnace impedance is largely due to the condition of the furnace-lining refractory for a given metal charge, and it is useful to the foundry user to monitor this condition by assessing changes in impedance. The basic action of the induction furnace and approximate formulas for the calculation of furnace impedance, together with methods of phase balancing, are described in an attempt to assist furnace users in monitoring the condition of their plant. Examples of calculation and the use of a furnace-condition diagram are given. However, the formulas used are intended to serve as a guide and by no means do they fully describe the complex electromagnetic conditions which exist, normally requiring solutions to magnetic-flux problems in three dimensions.