SU1731513A1 - Open-circuit voltage limiter - Google Patents

Abstract

The invention relates to a welding technique, in particular to devices for reducing the no-load voltage of welding transformers used for manual arc welding. The purpose of the invention is to increase the reliability of the device while maintaining high speed. The device contains an RC circuit with a diode key element connected to a current transformer. The presence of these elements ensures reduced interference in the entire operating frequency range and eliminates the passage to the control nodes. The output of the RC circuit is connected to the inverting input of the comparator. The output of the comparator is connected to the inputs of two series-connected optocoupler thyristors. The input of the second optocoupler thyristor, in addition, is associated with the output of the second comparator. The non-inverting input of the second comparator through the second RC circuit is connected to the output of the first optocoupler thyristor. The use of a second RC circuit and optocouplers improves the reliability of the device. 2 Il.

Description

The invention relates to a welding technique, in particular to devices for reducing the no-load voltage of welding transformers used for manual arc welding.

A no-load voltage limiter for a welding transformer is known, which contains a thyristor switch included in the power supply circuit of the welding transformer, a switch control unit, an operating voltage source connected to the welding transformer, a power supply unit, a current transformer in the welding circuit, an RC circuit to create time delays, two thyristor optocouplers, resistors and a diode.

The disadvantage of the limiter is the unreliability of operation, which is caused by sensitivity to temperature. The sensitive threshold element in the circuit is an optocoupler, whose parameters depend on temperature. In addition, the sensitive threshold element is connected to the current transformer output via nonlinear elements (diode bridge), whose parameters also depend on temperature. The absence of an RC circuit with a diode key element in the circuit makes the circuit poorly protected from interference and interference due to leaks in the welding circuit.

A device is known for limiting the voltage of the hollow stroke of a source of power sl

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no welding arc containing a thyristor switch in the supply circuit of the welding transformer, switch control unit, operating voltage source connected to the welding transformer, power supply unit, current transformer in the welding circuit, threshold element, time delay element, RC circuit, resistors and diode. The disadvantage of the device is low reliability of operation, due to the absence of an RC circuit with a diode key element, the dependence of sensitivity on temperature. Due to the absence of an RC circuit with a diode key element, the constant charge time of the capacitor of the integrating element is less than the constant discharge time, which leads to an accumulation of consequences. This phenomenon leads to the fact that in the presence of constant pickups or leaks in the welding circuit, the voltage at the output of the integrating element gradually increases to the trigger voltage of the threshold element and keeps it in the on state. There is a false operation of the thyristor switch, and then - its shutdown with a time delay. Due to the charge of the coupling capacitor of the threshold and integrating elements, the circuit becomes insensitive to the state of the welding circuit, i.e. the excitation of the arc is impossible. Since in practice, pickups and leaks in the welding circuit do not exist for a short time, but for a long time, such a device becomes practically inoperative. The dependence of sensitivity and, accordingly, excitation reliability on temperature is due to the presence in the communication circuit of the output of the current transformer and the threshold element of the diode limiter, whose parameters depend on temperature.

An ABS welder’s safety automaton is known, which contains protective and thyristor switches included in the supply circuit of the welding transformer, control units of the protection and thyristor switches, a source of operating voltage connected to the welding transformer, a power supply unit, a current transformer in the welding circuit, two threshold element and RC circuit for creating time delay.

A disadvantage of the known automaton is the low reliability of operation, due to the dependence of sensitivity on temperature, the effect of interference and leaks in the welding circuit on the reliability of operation (frequent false disconnections), the dependence of the time delay from temperature. the inertia of its pickup of the useful signal, which ultimately affects the reliability of the excitation of the arc. The dependence of the sensitivity on temperature is caused by the dependence of the parameters of the low-frequency amplifier included in the communication circuit of the output of the current transformer with the input of the sensitive threshold element and the parameters of the threshold element made on the basis of a Schmitt trigger,

0 is the temperature. The effect of temperature on the parameters of the threshold element also affects the response time of the element of the activation time delay. At short durations of short circuit

5, the signal at the input to the control unit of the thyristor switch does not have time to be picked up by the time delay element due to its inertia. This leads to instability of the excitation of an arc. The presence of a low-frequency amplifier, in which the amplitude-frequency characteristic is uneven, and the absence of a communication circuit of a sensitive threshold element with a current transformer output

5 RC circuits with a diode key element make the circuitry circuit sensitive to interference and leakage in the welding circuit, which leads to frequent false disconnections of the protection switch. All of these factors make the machine unreliable.

The purpose of the invention is to increase the reliability of the device while maintaining high speed.

FIG. 1 shows a block diagram of a device for limiting the voltage of the no-load; in fig. 2 is a graph of voltage changes at the input and output of the second RC circuit with a diode key element.

The device consists of a protective switch 1, a thyristor switch 2; included in the supply circuit of the welding transformer 3, the control unit 4 of the protection switch 1, the control unit 5 of the thyristor switch 2,

5 of the operational voltage source 6 associated with the welding transformer 3, power supply unit 7 of all nodes and circuit elements, current transformer 8 in the welding circuit, threshold elements 9 and 10 in the form

0 comparators, the first RC circuit 11 to create a time delay consisting of a resistor 12 and a capacitor 13, a second RC circuit 14c with a diode key element 15, whose input is connected to the output of the current transformer 8, the output to the inverse of the threshold element 9 , thyristor optocouplers 16 and 17, to the output of the comparator 9, through a resistor 18, is connected the anode of the input of the optocoupler 16, the cathode of which is connected to the anode of the diode 19, the cathode of which is connected through a resistor 20 to the output of the second threshold element

-comparator 10 and with the anode of the input of the second optocoupler 17, the cathode of the input of the optocoupler 17 is connected to the negative bus of the power supply unit 7, to which the cathode of the output of the first optocoupler 16 and the capacitor 13 of the first RC circuit 11 are connected, the middle point of this circuit 11 is connected to the input of the second the comparator 10 and through the resistor 21 to the anode of the output of the first optocoupler 16. The output of the second optocoupler 17 is connected to the input of the control unit 5 of the thyristor switch 2. The unit 4 is connected to the output of the current transformer 8 and the welding circuit. Additionally, in FIG. 1, the electrode 22 and the workpiece 23, the resistor 24 and the capacitor 25 of the RC circuit 14 are labeled.

The device works as follows.

Standby idling mode. Security switch 1 is enabled. Power supply 7 is energized. The operating voltage source 6 creates a small voltage (less than 12 V) in the welding circuit. The capacitor 13 is charged through the resistor 12 from the power supply unit 7.

Excite arc. When the electrode 22 is closed with the part 23, a small current flows through the current transformer 8. The output of the current transformer 8 is a voltage that is applied to the input of the second RC circuit 14. The output of this circuit 14 is the voltage that is applied to the inverse of the comparator 9. If this voltage is greater than Uon.1, applied to the direct input of the comparator, then the output voltage of the voltage applied to the circuit is: output: resistor 18

- the input of the first optocoupler 16, diode 19, the input of the second optocoupler 17. Both the optocouplers are unlocked. The capacitor 13 is discharged through the resistor 21 and the thyristor of the optocoupler 16. At the same time, the thyristor of the second optocoupler 17 sends a signal to the control unit 5 to turn on the thyristor switch 2. The latter is turned on, giving full voltage to the welding transformer 3. In the welding circuit, all conditions are created to energize the arc. When a capacitor 13 is discharged at the output of the second switch 10, a voltage is applied that is applied through a resistor 20 to the input of the optocoupler 17, keeping it in the on state. Resistors 18 and 20 limit the current through the LEDs (inputs) of optocouplers. In this state, the circuit is before the end of welding.

Idling. When an arc breaks, the current in the welding circuit disappears. At the same time, the voltage at the output of the current transformer 8 disappears, then at the outputs of the second

PC-4 chains 14 and comparator 9. Optothyristor 16 is locked. Capacitor 13 starts charging through resistor 12. A shutdown time delay is created. Once

the voltage on the capacitor 13 will become equal to or higher than the reference voltage Uon.2-; at the output of the comparator 10, the voltage disappears, which leads to the disappearance of the current through the LED of the optocoupler 17. The latter is locked. At the output of control block 5, the signal disappears and the thyristor switch 2 is locked. The system goes into standby mode.

The operation of the scheme in the presence of interference. With

A large capacity of the welding circuit or large leaks through the insulation at the input of the RC circuit 14 creates a voltage with a pulse duration tn longer than the pause time tn. In the absence of a diode key element 15

the voltage at the output of the RC circuit 14 gradually increases and at time tt it becomes equal to Uon.1, which leads to the appearance of voltage at the output of the comparator 9 as in the arc excitation mode.

Since the arc does not occur, and the control unit 5 is turned on, the control unit 4 of the protection switch 1 with a time delay of 1-1.5 s turns off the protection switch 1. A false shutdown will occur. In the presence of a diode key element 15, the constant charge time of the capacitor 25 of the RC circuit 14 will be much longer than the constant time of its discharge. Therefore, during the time tn, the capacitor 25 has time to discharge and the voltage at the output of the RC circuit 14 does not have time to reach the value Uon.1.

The graph (Fig 2) shows the voltage curves at the input (curve 1) and output (curve 2) of an RC circuit 14 with a diode key

element 15 for different times t.

In the time period from t 0 to t ti, under the action of a positive half-period of the input voltage, a charge occurs

capacitor 25 to Del when UBx and out. In this period UBx willow. Due to the large time constant r, the voltage at the output slowly increases, reaching at the time t ti the maximum value equal to UC1.

In the period from ti to g, the voltage Uvih UBX. The capacitor 25 is discharged. Since the time constant is constant T | , curve 2 almost merges with

curve 1. At time t t2 Oout O, a UBX 0. In the period from t, 2 to xs, the voltage UBX oo, that means over-discharge of the capacitor 25. At time t, s, the capacitor 25 is charged negative

the half period of the input voltage is up to the highest value when UBx and OO. The constant time of this period is ti.

In the time period from t3 to 14, the voltage lUexl SvIh I, the capacitor 25 is discharged with a constant time t. At the time point te UBX - Uef, ix Uc2, the capacitor discharges and 1) Output 0. The voltage UBX 0.

In the time period from ts to te, the capacitor 25 is charged with a positive half-cycle of input voltage. The constant charge time is m TI. At time te uBX willow UC2, and UC2 Uci. This suggests that as the time increases, the noise immunity of the device increases as the voltage difference AU Uon.1 and the inputs at the inputs of the comparator 9 increases (the condition for triggering the circuit A and Uv Uon.1 0).

Thus, the presence of a negative half-wave input voltage leads to an increase in the noise immunity of the circuit. A similar phenomenon occurs when there are only negative half-waves of the input voltage.

The proposed implementation of a communication circuit between a current transformer 8 and a sensitive threshold element — comparator 9 makes the circuit insensitive to temperature changes. The full discharge of the capacitor 13 of the RC circuit 11 after the appearance of the useful signal makes it possible to provide a stable excitation of the arc when the electrode 22 is briefly touched to the part 23. All this allows to increase the reliability of the device and provides high reliability of the arc excitation and noise immunity.

Claims (1)

Invention Formula 0 five 0 five 0 five A no-load voltage limiting device containing protective and thyristor switches included in the power supply circuit of the welding transformer, switch control units, operating voltage source connected to the welding transformer, power supply unit, current transformer in the welding circuit, two threshold elements first RC circuit, characterized in that, in order to increase reliability in operation while maintaining high speed, a second RC circuit, two diodes, two thyristor optons, three resistors, and threshold elements are made on comparators, the second RC circuit with the first diode connected in parallel to the resistor of this circuit is connected between the output of the current transformer and the inverting input of the first comparator, to the output of which, through the first resistor, is connected to the anode of the first optocoupler, whose cathode is connected to the anode of the second diode, the cathode of which is connected via a second resistor, the output of the second compiler and the anode of the input of the second optocoupler, the first RC circuit is connected by a resistor to a positive capacitor, to the negative busbars of the power supply, and It points to the direct input of the second comparator and through the third resistor to the anode of the first optocoupler, the cathode of the second optocoupler and the cathode of the first optocoupler are connected to the negative power supply bus, and the output of the second optocoupler is connected to the thyristor switch. T and s yy / 4Vom CZb / 5 3V ir Ljfl Ј .Y 2 yy Vom Oh /// 0  .71.7У-1 7i / 2 i MI j i 1 u / з i l- j L. / 6 U about ± yu ff J cli 20 u 22 one 23 QEID .U