Reliability increase of resonance induction heating units during the fault conditions

Abstract

Purpose. To develop an automatic protection system against fault operation conditions for the resonant inverter of the induction heating unit due to absence of load. Methodology. Resonant inverter load transfer function analysis using the first harmonic approximation, calculation of control parameters using numerical methods, computer simulation of induction heating unit. Experimental studies based on a smelter with microprocessor control in order to check the adequacy of the proposed model and protection system. Findings. A computer model of the induction heating unit is presented, the adequacy of which was verified by the experimental results of the induction melting unit. By using this model, circuit protection circuit of the resonant inverter against low load conditions is developed, which is based on the shift of the inverter operating frequency due to the introduction of an additional shift of the phase difference between current and voltage in the feedback loop. The methodology for calculating this phase shift was created using the load transfer function of the inverter (‘capacitor bank-inductor-part’ system) and numerical methods for solving equations. The above calculation method can be used for resonant inverters of any configuration and topology. According to the computer simulation of the induction heating unit operation during fault, the developed system limits the voltage amplitude within the safe operating region of the inverter reactive elements, while in normal operating conditions the protection system does not affect the operation of the unit. The protection system successfully keeps the amplitude of current and voltage in the oscillating circuit within safe limits during low loads. The created system is simple enough for its quick implementation in existing digitally controlled units and has high speed. Originality. Methodology to create voltage limiting system for resonant inverters of any configuration with frequency control has been developed. Practical value. Developed protection system allows to reduce induction heating process interruption by the user. Moreover, protection system increases fault tolerance of induction heating units. Structure of proposed protection system simplifies the modernization of existing units and the development of new models using such system.