Automatic frequency control of an induction furnace

This paper was the overall winner of the 1999 IEE/SAIEE Young Engineer and Student Written Paper competition. A miniature high frequency induction furnace has been developed for the melting and alloying of small quantities of metal for laboratory and jewellery applications. The induction heating load forms part of a resonant circuit whose frequency is determined by the electrical characteristics of the work-piece placed inside the heating coil. The dynamic nature of the induction-heating load is such that it causes a shift in the natural resonant frequency of the load circuit. Automatic frequency control of the power source enables continuous operation at the resonant frequency of the tank circuit, resulting in a high conversion efficiency and maximum power transfer to the load at all times. The control technique implemented is called a resonance locked-loop. The implementation of a resonance locked-loop has the advantage of extending the applications of the induction furnace to the heating and melting of almost any metal.