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Series Intermediate-Frequency Furnace The difference between series and parallel intermediate-frequency furnaces lies in their oscillating circuits. In the former, L, R, and C are connected in series, whereas in the latter, they are connected in parallel. As a casting enterprise, should we choose a series intermediate-frequency furnace or a parallel intermediate-frequency furnace? Although this decision ultimately depends on our own specific circumstances, we must first identify the similarities and differences between the two types, as well as their respective advantages and disadvantages, before making the right choice.

 

This article focuses on Series Intermediate-Frequency Furnace With Parallel Intermediate-Frequency Furnace I made a simple comparison, and the main differences are as follows:

1

In a series-load circuit for medium-frequency furnaces, the power supply is required to have very low impedance and must provide energy in the form of a voltage source. Therefore, at the output end of the DC power supply—after rectification and filtering—a large filtering capacitor must be connected in parallel. When the inverter fails, the inrush current is high, making protection particularly challenging. In contrast, in a parallel-load circuit for medium-frequency furnaces, the power supply exhibits high impedance and thus requires a current source for power supply; consequently, a large resistor must be connected in series at the DC power supply end. However, when an inverter fault occurs, due to the high reactance of the current, the inrush current is relatively small, making protection much easier.

 

2

In a series-connected intermediate-frequency furnace, the input voltage remains constant, while the output voltage is a rectangular wave and the output current approximates a sine wave. Commutation occurs after the thyristor current crosses zero; therefore, the current always leads the voltage by an angle φ. In a parallel-connected intermediate-frequency furnace, the input current remains constant, the output voltage approximates a sine wave, and the output current is a rectangular wave. Commutation takes place before the voltage across the resonant capacitor crosses zero, and the load current also always leads the voltage by an angle φ. In other words, both types of furnaces operate under capacitive load conditions.

 

3

The medium-frequency induction furnace in this series is powered by a constant-voltage source. To prevent the thyristors in the upper and lower arms of the inverter from conducting simultaneously, which could lead to a short circuit in the power supply, it is essential to turn off one set of thyristors before turning on the other during commutation—that is, all thyristors must be turned off for a certain period of time (ta). During this period, the stray inductance—specifically, the induced voltage generated by the inductance from the DC terminals to the device leads—could potentially damage the devices; therefore, it is necessary to select an appropriate surge voltage suppression circuit for the devices. Furthermore, to ensure a continuous load current and to protect the thyristors from the high voltages generated by the commutation capacitors during the thyristor-off period, fast diodes must be connected in reverse parallel across the thyristors. The parallel-type medium-frequency furnace is a constant-current-source power supply; to avoid large potential differences across the filtering reactor, the current must remain continuous. In other words, during commutation, the thyristors in the upper and lower arms of the inverter must first turn on and then turn off—that is, all thyristors must be in the conducting state throughout the commutation period (t). Although the inverter arms are effectively short-circuited at this point due to the sufficiently large inductance, no short circuit of the DC power supply will occur. However, since the long commutation time reduces system efficiency, it is crucial to shorten the commutation period (t).

 

4

The operating frequency of a cascade intermediate-frequency furnace must be lower than the natural oscillation frequency of the load circuit—that is, it must ensure an appropriate turn-off time, t_a. Otherwise, direct conduction between the upper and lower arms of the inverter could occur, leading to commutation failure. For a parallel-type intermediate-frequency furnace, the operating frequency must be slightly higher than the natural oscillation frequency of the load circuit, so as to maintain an adequate reverse-blocking time, t_b. Otherwise, commutation between thyristors would fail. However, if the current is too high, the reverse voltage that the thyristors experience during commutation will become excessively high, which is unacceptable.

 

5

There are two methods for power regulation in series-connected intermediate-frequency furnaces: one is to vary the DC supply voltage UD, and the other is to change the firing frequency of the thyristors, thereby adjusting the load power factor. In parallel inverters, the power regulation method generally allows only variation of the DC supply voltage Ud. By adjusting the power factor cosφ, it is also possible to increase the output voltage of the inverter and boost the output power; however, the adjustment range is relatively limited.