New induction heating transformers

In the past, induction heating output circuits have been coupled to cyclic switching devices by means of air-core or ferritecore tuned transformers. This transformer was part of the tank circuit of an oscillator, which had 60 to 65% overall efficiency. This paper describes an arrangement embodying a solid-state inverter and an untuned output transformer, thus increasing the efficiency to 85-90%. Tuning of the induction load coil is accomplished by use of a separate capacitor on the secondary. Frequency range is 100 to 500 kHz and power outputs 1200 to 5000 volt-amperes. The untuned transformer is of the core type. It has a grain-oriented silicon-steel core, with a pair of coils, tapped to suit a wide range of output impedances without excessive losses. Exciting current needed from the switching devices had to be kept small to prevent surges during the cyclic commutation periods. The paper describes the design features which were used to obtain these unique results, with particular attention to the magnetic properties of the core.