Reasons for casting enterprises to choose series Intermediate frequency furnace or parallel  Intermediate frequency furnace

The d Intermediate frequency ference between a series  Intermediate frequency furnace and a parallel  Intermediate frequency furnace is due to the d Intermediate frequencyferent oscillation circuits they use. The former uses L, R and C in series, and the latter uses L, R and C in parallel. As a foundry company, should I choose a series  Intermediate frequency furnace or a parallel  Intermediate frequency furnace? Although this needs to be based on your own actual situation, you must first understand the similarities and d Intermediate frequencyferences between the two, and their respective advantages and disadvantages before you can make the right choice.

A simple comparison between a series  Intermediate frequency furnace and a parallel  Intermediate frequency furnace has the following d Intermediate frequency ferences:

1. The load circuit of the series  Intermediate frequency furnace has low impedance to the power supply and requires a voltage source. Therefore, the rect Intermediate frequencyied and filtered DC power supply terminal must be connected in parallel with a large filter capacitor. When the inverter fails, the surge current is large, and protection is d Intermediate frequency ficult.

The load circuit of the parallel  Intermediate frequency furnace presents high impedance to the power supply, which requires power from a current source, and a large reactor must be connected in series at the end of the DC power supply. However, when the inverter fails, because the current is limited by the large reactance, the impact is small and it is easier to protect.

2. The input voltage of the series intermediate frequency furnace is constant, the output voltage is a rectangular wave, and the output current is approximately a sine wave. The commutation is performed after the thyristor current passes through zero, so the current always leads the voltage φ angle.

The input current of the parallel intermediate frequency furnace is constant, the output voltage is approximately sine wave, the output current is rectangular, and the commutation is performed before the voltage on the resonant capacitor crosses zero. The load current is always ahead of the voltage by a φ angle. This means that both are working in capacitive load.

3. The series intermediate frequency furnace is powered by a constant voltage source, in order to avoid that the thyristors of the upper and lower bridge arms of the inverter are turned on at the same time. As a result of short circuit of the power supply, the switch must be turned off before switching on and then turned on. That is, there should be a period of time (ta) in which all thyristors are in the off state. At this time, the stray inductance, that is, the induced potential generated on the lead inductance from the DC terminal to the device, may cause damage to the device, so it is necessary to select a suitable device surge voltage absorption circuit. In addition, during the thyristor off-time, in order to ensure the continuous load current and protect the thyristor from the high voltage on the converter capacitor, fast diodes must be connected in anti-parallel across the thyristor.

The parallel  Intermediate frequency furnace is powered by a constant current source. In order to avoid large confusing potentials on the filter reactance I.d, the current must be continuous. That is, it must be ensured that the inverter thyristors of the upper and lower bridge arms are first turned on and then turned off during commutation, that is, all the thyristors are in an on state during the commutation period (t). At this time, although the inverter bridge arm is straight through, because L is large enough, it will not cause a short-circuit of the DC power supply, but the long commutation time will reduce the system efficiency, so it is necessary to shorten t, that is, reduce the value of Lk.

4. The working frequency of the series intermediate frequency furnace must be lower than the natural oscillation frequency of the load circuit, that is, a proper ta time should be ensured, otherwise the commutation failure will be caused by the straight through of the upper and lower bridge arms of the inverter.

The working frequency of the parallel  Intermediate frequency furnace must be slightly higher than the natural oscillation frequency of the load circuit to ensure that when there is a suitable back pressure, t will be caused, otherwise the commutation between thyristors will fail; The reverse voltage will be too high, which is not allowed.

5. There are two ways to adjust the power of the series intermediate frequency furnace: change the DC power supply voltage Ud or change the trigger frequency of the thyristor, that is, change the load power factor cosφ.

Generally, the power adjustment method of the parallel inverter can only change the DC power supply voltage Ud. Although changing cosφ can also increase the inverter output voltage and power, the allowable adjustment range is small.

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