Automatic Frequency Control of a Parallel Resonant Inverter for an Induction Furnace

Main problem of an induction heating system is electrical and magnetic properties of a load changed due to the rising temperature. Consequently, resonant frequency of the load is also changed. The power transfer to the load decreases when the system cannot adjust the switching frequency to new resonant frequency. A phase locked-loop (PLL) frequency control system for the miniature induction furnace prototype is described in this paper. Automatic frequency control system of the power source capacitates continuous operation at the resonant frequency, resulting in high conversion efficiency because of zero voltage switching (ZVS) and maximum power transfer to the load at all times. Two work pieces, 13g of copper pipe and aluminum sheet, have been continuously heated by the 2kW parallel resonant inverter prototype. Experimental results, the inverter operates in resonant frequency at all the times therefore the maximum power is delivered to heated load throughout the heating time.