On the frequency control in high efficiency induction heating system with resonant inverter

This paper is focused on the possibilities of implementing an auto-adaptive control, without a phase-locked loop circuit, for the operating frequency in an induction heating system with single-phase voltage source inverter and resonant L-LC load. Based on the phasor diagram of the equivalent resonant load under resonance condition, three possibilities have been identified to express an error to be canceled in order to obtain the operation at resonance frequency. This error is proposed to be the input quantity of a PI frequency controller. To minimize the switching losses in the voltage source inverter, an increasing of the control frequency over the resonance value is achieved by increasing the component associated to the inductive current in the error expression. The Matlab/Simulink implementation and simulation results for the heating process of a seamless pipe confirm the theoretical findings. The results of the real-time implementation of the control algorithm on an experimental setup based on dSPACE DS1103 controller board illustrate its effectiveness.