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(1) Selection of trigger mode
Conventional iron melting furnace trigger circuit triggers use analog control circuits and cannot overcome their inherent shortcomings. Compared with the analog type, the digital control circuit has the main advantages of stable output waveform and high reliability, but its disadvantage is that the circuit is more complicated, and the control precision is not high when the phase-shifting trigger angle is large. With the development of single-chip technology, the advantage of the control circuit composed of single-chip microcomputer is more obvious. In addition to the same advantages as the digital trigger circuit, the phase-shifting trigger angle is completed by software calculation. The trigger circuit has simple structure and flexible control. The influence of temperature drift is small, the control precision can be compensated by software, and the phase shift range can be adjusted by any adjustment.
(2) Control principle of phase shift trigger pulse
Phase control requires reference to the natural commutation point of the converter circuit. After a certain phase delay, the trigger signal is output to turn on the fuse furnace trigger circuit . In practical applications, the natural commutation point is given by the synchronization signal, and then synchronized by the synchronous voltage zero-crossing detection method in the CPU, and the phase shift calculation is completed by the CPU control software, and the trigger pulse is output according to the phase requirement.
The following figure shows the three-phase bridge type full-controlled rectification circuit. The timing of the trigger pulse signal output can also be determined by the MCU according to the level of the synchronization signal. When the MCU detects the A-phase synchronization signal, the output pulse timing is usually triggered by the phase-shift pulse. That is, a synchronous voltage signal and a timer are used to complete the calculation of the trigger pulse, which is feasible when the three-phase circuit is symmetric. Since the three phases are completely symmetrical, the phases are 120 ° out of phase with each other , the circuit is commutated every 60 o, and the timing of the commutation is known in advance. The method uses less single-chip resources, only one synchronization signal, the circuit is relatively simple, but the software design workload is large.
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