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Medium-frequency electric furnace The regulator section is equipped with four regulators: an intermediate-frequency voltage regulator, a current regulator, an impedance regulator, and a reverse commutation angle regulator.

The voltage regulator and current regulator together form a conventional current-voltage dual closed-loop system. Throughout the entire startup and operation phases, the current loop remains continuously active, whereas the voltage loop operates only during the operation phase.

The impedance regulator's operation, viewed from the input side, is entirely in parallel with that of the current regulator. The only difference lies in the fact that the negative feedback loop of the impedance regulator has a higher gain than that of the current regulator. Furthermore, while the output of the current regulator controls the DC voltage at the output of the rectifier bridge, the output of the impedance regulator governs the proportional relationship between the intermediate-frequency voltage and the DC voltage—that is, the phase angle of the inverter’s power factor.

Regulator The operating process of the circuit can be divided into two scenarios.

One scenario occurs when the DC voltage has not yet reached its maximum value. Due to the relatively large reverse feedback effect of the impedance regulator, the setpoint of the impedance regulator becomes smaller than the feedback signal, causing the impedance regulator to operate in a limiting state. In this case, the corresponding inverse commutation angle is zero. At this point, the impedance regulator can be considered inactive, and the system functions entirely as a standard voltage-current dual-loop system.

Another scenario is when the DC voltage has already reached its maximum value, causing the current regulator to enter limiting mode and effectively stop functioning. As a result, the output of the voltage regulator increases, yet the feedback current remains unchanged. For the impedance regulator, once the feedback current signal falls below the set current value, the impedance regulator exits limiting mode and begins to operate, adjusting the O-angle reference value of the inverse commutation angle regulator. This causes the output intermediate-frequency voltage to rise further, and the DC current also increases accordingly, eventually reaching a new equilibrium. At this point, only the voltage regulator and the impedance regulator remain active. The resistance R continues to increase until it reaches the maximum inverse commutation angle 0°—at which point the inverse commutation angle regulator ensures that the inverse commutation bridge can operate stably at that specific 0° angle.