The main circuit of induction furnace is parallel resonant type. The main circuit is half bridge inverter series resonant type. Most of the systems used in our country are developed by absorbing foreign advanced technology. Through circuit analysis and digital simulation, it is concluded that the current of induction coil is w waveform. Adjusting the trigger cycle of thyristor can achieve the effect of AC PWM, which makes the current frequency of induction coil change, so as to realize power conversion Rate adjustment.

1. Inverter main circuit

The series inverter circuit can be divided into full bridge inverter circuit and half bridge inverter circuit. The half bridge inverter circuit is shown in Figure 1. , C: to compensate the capacitor, R and L are the resistance and inductance of the induction coil. , C: it is a thyristor that has been charged to half of DC voltage 1 (take V, conduction as an example). When it is on, there are two parts flowing through the induction coil: one is through V1, l, R and C. The second is: through V -, l, ruler direction

C: The oscillation frequency of the two circuits is the same. T in Figure 2. Moment, capacitor C. Discharge completed, i.e. u.. =0, while C. When charging to DC voltage, so T-hour

At the moment, the current flowing through the circuit inductor is the largest. Under the effect of the commutation inductor, the two loops flowing through the inductor continue to flow. One is to supply the capacitor C. Reverse charging, the second is to capacitor C. Continue to charge until the time t: when the energy in the inductor is released, i.e. I. The voltage is negative U.. Under the action of capacitor C - and CZ voltage, both circuits discharge through D tube, and the current direction is from left to right until t. At this moment, the discharge current becomes zero, at this time, C charges forward 1 to Si Fu, and triggers Y: tube lying], and repeats the above process

2 zero input response of RLC series circuit

Suppose u, = AEZ, the eigenvalues of equation (1) are (a) two unequal negative real roots; (b) a pair of conjugate complex roots whose real parts are negative; (c) a pair of equal negative real roots when R, l and C choose different parameters

Since the half bridge inverter works at resonance, the corresponding equation has a pair of conjugate complex roots, so we only discuss the R process According to the trigger control principle of thyristor series resonant half bridge inverter circuit, after the zero crossing synchronous signal of induction coil current, the digital trigger circuit delays a certain commutation control angle port to send out commutation trigger signal. The voltage on the resonant capacitor lags behind the current 90. As shown in Figure 4. For a capacitor, the voltage at both ends of the capacitor lags the current by 90. When the over current is less than 1 UDC. In this way, the expression (2) of the capacitor voltage can be further expressed as that the output power of the system will be reduced when the insulation is needed. When the angle of mouth increases, the trigger pulse will move backward and the initial voltage of the capacitor will decrease, so that the amplitude of the resonant voltage will decrease and the resonant frequency will also decrease. And then forward charging to UD, i.e. t. Do not touch the thyristor V immediately during the operation. As shown in Figure 6, the delay time is 4. Give V at t: time. When the trigger pulse of induction furnace moves forward, the commutation frequency increases, the initial commutation voltage on the capacitor increases, and the resonant voltage amplitude increases. The load current in the interval is in the opposite direction, which is opposite to the direction of the voltage uan. At this time, the energy is fed back to the power supply by the load, rather than the power supply providing energy for the load. Therefore, it should be shortened as soon as possible

In Figure 5, C. The forward charge has not arrived yet. In this way, the output power is increased and the efficiency of the system is improved

When

Select C to configure the parameters of. =CZ = 900e-6f; l-0.57e-4h; UD = 1 200V, the main circuit of half bridge inverter is simulated at 250Hz, and the simulation circuit is shown in Figure 7

Simulation of 4 half bridge series inverter circuit

According to 5kg

Get the current waveform through the induction coil of the furnace, capacitance C. C: voltage waveforms at both ends are shown in Fig. 9

5 Conclusion

1) In half bridge inverter medium frequency power supply, each group of current flowing through induction coil is connected by sine wave anti phase. As long as the connection time is changed, the trigger pulse period is changed, the power regulation can be realized

2) The current w waveform multiplies the frequency of the induction coil. When two adjacent groups of waveforms are continuous, the frequency is 250Hz, and the frequency of a single sine waveform added to the induction coil is actually 500Hz

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