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Operating Principle of the Inverter Section

The inverter triggering section of this circuit employs a frequency-sweeping zero-voltage soft-start method. Due to the requirement for automatic frequency adjustment, although the inverter circuit operates in self-excitation mode and the control signal is derived from the load side, no additional starting circuit is needed in the main circuit. Consequently, there’s no need for pre-magnetization or pre-charge startup procedures, which simplifies the main circuit. However, this simplicity comes at the cost of a more complex control circuit.

The startup process is roughly as follows: Before the inverter circuit starts up, an externally triggered signal—whose frequency is higher than the resonant frequency of the tank circuit—is first used to trigger the inverter thyristors. Once the circuit detects the DC current in the main circuit, it begins to scan the frequency of the external trigger signal from high to low. As the frequency of the external trigger signal drops and approaches the resonant frequency of the tank circuit, the intermediate-frequency voltage gradually builds up and is fed back to the automatic frequency-control circuit. Once the automatic frequency-control circuit is activated, it stops scanning the frequency of the external trigger signal and takes over control of the inverter’s leading angle, enabling the equipment to enter steady-state operation. If the initial startup attempt fails—that is, if the automatic frequency-control circuit fails to lock onto the feedback signal from the intermediate-frequency voltage—then the external trigger signal will continue to scan down to the lowest frequency. When the repeated-start circuit detects that the external trigger signal has entered the lowest frequency band, it will initiate a restart, pushing the external trigger signal back up to the highest frequency and starting the frequency-scan cycle all over again until the startup is successful. The repetition-start cycle lasts approximately 0.5 seconds, and the entire startup process—from initial start-up to full-power operation—takes no more than 1 second.