Selection of series intermediate frequency furnace and parallel intermediate frequency furnace
The difference between the series intermediate frequency furnace and the parallel intermediate frequency furnace is different from the oscillation circuit used. The former is connected in series by L, R and C, and the latter is connected in parallel with L, R and C. As a foundry company, should we choose a series intermediate frequency furnace or a parallel intermediate frequency furnace? Although this needs to be based on your own actual situation, first of all, you must find out the similarities and differences between the two, and what are the advantages and disadvantages of each, and then you can make the right choice.
There are mainly the following differences:
1.The load circuit of the series intermediate frequency furnace presents a low impedance to the power supply and requires power supply from the voltage source. Therefore, a large filter capacitor must be connected in parallel to the rectified and filtered DC power supply. When the inverter fails, the surge current is large and protection is difficult. The load circuit of the parallel intermediate frequency furnace presents a high impedance to the power supply, and is required to be powered by the current source. A large reactor needs to be connected in series at the end of the DC power supply. However, when the inverter fails, the current is limited by the large reactance, and the impact is not large, which is easy to protect.
2.The input voltage of the series intermediate frequency furnace is constant, the output voltage is a rectangular wave, the output current is approximately sinusoidal, and the commutation is performed after the current crosses zero on the thyristor, 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 sinusoidal, the output current is rectangular, and the commutation is performed before the voltage across the resonant capacitor is zero. The load current is always ahead of the voltage by φ. That is to say, both work in a capacitive load state.
3.The series intermediate frequency furnace is powered by a constant voltage source, in order to avoid simultaneous conduction of the upper and lower bridge thyristors of the inverter. When the power supply is short-circuited, when switching, it must be turned off first and then turned on. That is, there should be a period of time (ta) to turn all thyristors off. The stray inductance at this time, that is, the induced potential generated from the DC terminal to the lead inductance of the device, may cause damage to the device, and thus it is necessary to select a surge voltage absorbing circuit of a suitable device. In addition, during the turn-off of the thyristor, in order to ensure continuous load current, the thyristor is protected from the high voltage on the commutating capacitor, and the fast diode must be anti-parallel across the thyristor. The parallel intermediate frequency furnace is powered by a constant current source. In order to avoid a large confusing potential on the filter reactance I.d, the current must be continuous. That is to say, it must be ensured that the upper and lower arm thyristors of the inverter are turned off first and then turned off during the commutation, that is, all the thyristors are in the conducting state during the commutation period (t). At this time, although the inverter bridge arm is straight through, since the L is large enough, the DC power supply will not be short-circuited, but the long commutation time will reduce the system efficiency, so it is necessary to shorten t, that is, reduce the Lk value.
4.The operating frequency of the series intermediate frequency furnace must be lower than the natural oscillation frequency of the load circuit, that is, it should ensure that there is a suitable ta time, otherwise the commutation failure will occur due to the direct connection of the upper and lower arms of the inverter. The operating frequency of the parallel intermediate frequency furnace must be slightly higher than the natural oscillation frequency of the load circuit to ensure that there is a suitable back pressure time t, otherwise it will cause the commutation between the thyristors to fail; but if the height is too high, the thyristor during the commutation The reverse voltage that is tolerated 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. The power adjustment mode of the parallel inverter can only be changed by changing the DC power supply voltage Ud. Changing the cosφ can also increase the inverter output voltage and increase the power, but the allowable adjustment range is small.
