The difference between series intermediate frequency furnace and parallel intermediate frequency furnace_Energy saving induction heating equipment manufacturers

Which is better for series circuit induction melting furnace or parallel induction melting furnace?his is a problem that people who have just contacted the intermediate frequency furnace industry often ask. The difference between the series intermediate frequency furnace and the parallel intermediate frequency furnace is different from the oscillation circuit used. The former uses L (inductor), R (resistance) and C (capacitor) in series, the latter is L, R and C in parallel.

The induction heating power supply is divided into two types according to the compensation form, a parallel resonant (current type) power supply and a series resonant (voltage type) power supply. 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 the 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 power supply is required 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.

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 exceeds 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 crosses zero. The load current is also always ahead of the voltage φ angle. That is to say, both work in a capacitive load state.

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, it must be turned off first and then turned on when the current is changed. 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 the 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.

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.

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 change 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.