Analysis of the circuit principle of ZX5-630 welding machine

The circuit schematic diagram of this electric welding machine is shown in the accompanying drawing.

1. Main circuit of welding machine
The main circuit of welding machine includes power transformer B1, thyristor elements SCR1~SCR6, balancing inductor L1, filter inductor L2 and shunt FL. The secondary of transformer is connected with thyristor element to form double reverse star rectifier circuit with balancing inductor (this rectifier circuit can output larger welding current). Filter inductor can reduce the pulse component in welding current and make the rectifier circuit run in inverter state, simplifying the trigger circuit. The current signal proportional to welding current is taken out from shunt and the external and dynamic characteristics of welding machine are controlled through negative feedback.

2. Control circuit

1. Synchronous circuit link: The trigger synchronous pulse signal is composed of a three-phase synchronous step-down transformer Bl, step-down resistors Rl~R3, and voltage-stabilizing diodes DWI~DW6. When +A, +B, and +C are respectively in the positive half cycle, the voltage-stabilizing diode of each phase is zero (common head) and a three-phase positive square wave voltage can be taken out. When -A, -B, and -c are respectively in the negative half cycle, the voltage-stabilizing diode of each phase is zero (common head) and a three-phase negative square wave voltage can be taken out. The square wave voltage is then differentiated through capacitors Cl, C2, C3, and R4, and the positive and negative zero-crossing pulse voltages of the three-phase voltage are taken out at both ends of R4. The synchronous signals of the positive zero-crossing and reverse zero-crossing points of the three-phase voltage are output by D7~Dl0 respectively, and the transistors T3 and T4 are controlled to conduct in turn. During the conduction period of T3 or T4, the phase-shifting capacitor C6 or C7 in the trigger pulse generating circuit is discharged, and the charging current is bypassed by the transistor to form a synchronous point.

2. Given voltage link: The dual power supply output with center tap from the secondary winding of step-down transformer B2 is bridge rectified by D14-D17, filtered by capacitors C8 and Cl0, and then output +15V and -15V as given voltage by three-terminal voltage regulators 7815 and 7915 respectively to provide stable power supply. The voltage is divided by W3 (grid compensation potentiometer) and R16, and then output by the moving contact of W3, and then sent to the inverting input terminal of operational amplifier F by W6 (current trimming potentiometer), W7 (given voltage), and W8 (zeroing potentiometer 1. By adjusting the size of W7, the output voltage of the operational amplifier can be changed, that is, the size of the phase shift angle can be changed, that is, the conduction angle of the thyristor can be controlled, and finally the output current of the welding machine can be adjusted.

3. Phase shift trigger link: It mainly consists of single junction transistor phase shift trigger circuit composed of phase shift control transistors T3, T4 and shift capacitors C6, C7, single junction transistors T1, T2 resistors R9, R13, etc. The output voltage of pin ⑥ of the operational amplifier is used to control the phase shift of the charging and discharging speed of C6 and C7 by 13 and T4. For example, when the synchronous transistor T5 is turned on, the potential of F⑥↓→the emitter junction voltage of T3↑→the current of T31e↑→the charging of capacitor C6 accelerates↑→the phase shift angle moves forward↑→the conduction angle of the thyristor increases↑→the welding current increases: on the contrary, it decreases, which plays a role in regulating the welding current.

4. Current negative feedback link: Current negative feedback is to draw the voltage signal If from both ends of the shunt PL and supply it to the inverting input terminal of the voltage comparator F through the resistor R32, and the positive pole of the given voltage Ug is introduced into the inverting input terminal of F through the potentiometer W7. Then the difference between us and the feedback voltage flowing through the resistor R32 is compared by F, and the negative voltage is output as the phase shift control voltage. When the welding current increases, the shunt If becomes more negative, so that the output voltage of F increases, that is, the conduction of T3 and T4 is weakened, the charging current of C6 and C7 is reduced, the phase shift angle is increased, and the welding output DC voltage is reduced, and the falling external characteristic is obtained.

5. Arc striking link: When no-load, the welding feedback voltage Uf is relatively high. After the resistor R26 and the potentiometer W10 divide the voltage, the voltage stabilizing diode D19 breaks down and conducts. The collector voltage Ue of the transistor T7 is pulled down. When the arc is struck, the welding rod and the workpiece rub (short circuit) and the arc is struck. Uf drops sharply, D19 and T7 are cut off, and the Ue potential rises to 15V. The capacitor C13 and the potentiometer W9 perform differentiation, and a pulse voltage with a wider width is generated on W9 to increase the given voltage, so that the arc striking current increases and the arc is easy to strike. When the arc voltage is established, the arc striking is completed. The arc striking current can be adjusted appropriately by adjusting W9.