| Induction heating power has been used widely in industrial areas for the advantages of protecting environment and saving energy. The control method for inverter is crucial to determine whether induction heating power can work safely and efficiently. Because of the drawbacks of the control methods currently used, such as complex circuitry, and difficult testing, they can not fit for the development of high frequency, high capacitance, and digitizing. So it is practically significant to improve the adaptability of inverter.The control circuit of parallel inverter is investigated in this paper. Based on the available documents at home and abroad on this subject, the operating principle, operating states, load character and basic principle of Phase-Looked Loop(PLL) are analyzed firstly, Which provide the theoretical guidance for the designing of parallel inverter.Afterwards, a novel control strategy is put forward. A Swept-frequency with separate and self-excitation switching is applied in starting circuit, which can look for resonant point automatically. A combination of PLL and overlapping angle following-up control strategy are employed to realize frequency tracking and keep weak capacitive constantly. A 50kW/30KHz parallel induction heating prototype was developed. Experiences were done several times in the range of 18KHz～50KHz, and different load resonant frequencies were tested to show a high successful start-ratio. The results of frequency and overlapping angle tracking also show the flexibility and efficiency of this control strategy.Finally, an all digital phase-locked loop(ADPLL) based on FPGA is put forward in order to fit for digitizing, This method used two counters with different modulus to implement a proportional-integral control strategy. Phase error was utilized to improve capture speed. Experimental results show that this ADPLL can not only implement frequency tracking substituting for analog PLL, but can also be applied to a wide-range of loads. |
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