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Electric welding. Adjustment of welding current in the source for semi-automatic welding with a thyristor regulator. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / welding equipment

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Voltage regulation sources with stepwise adjustment of welding voltage and current is carried out by switching the taps of the welding transformer using special jumpers or switches.

As practice shows, this approach usually does not allow choosing the optimal welding mode, and also does not guarantee a constant result when changing the parameters of the welding circuit, power supply, or when working with various protective gas mixtures.

Increase in the number of switching steps allows to improve the operational properties of the source, but at the same time it is necessary to use complex and cumbersome multi-position switches, the winding units of the source are greatly complicated. On the one hand, this increases its cost, and on the other hand, it greatly reduces its reliability.

For a long time there have been and are used various ways of smooth adjustment of welding voltage and currentusing moving windings, magnetic shunts or magnetic amplifiers.

But such methods do not have fundamental advantages, because they imply:

• more complex and expensive transformer design;
• the presence of special adjusting electromagnetic or mechanical units.

In addition, such options are more often suitable for sources that have a falling external characteristic, and are not entirely suitable if the external characteristic should be gently falling or hard. For such sources, for a long time there was no worthy alternative to sources with contact switches.

Ensuring the continuity of the welding current

The chance to change the status quo and replace contact switches with non-contact ones came in 1955 with the manufacture of the thyristor, the first switching semiconductor device with sufficient power for use in welding sources. The use of thyristors made it possible to obtain a smooth adjustment of voltage and current, as well as to abandon moving mechanical parts, which increased the reliability of welding sources.

Let's consider a welding current source having smooth adjustment of welding voltage and current.

Thyristor as a key element has two states:

• open;
• closed.

Closed thyristor does not conduct current, but in open - conducts. Since the thyristor is capable of conducting current in only one direction, it is often called semiconductor controlled valve (Silicon Controlled Rectifier, SCR).

Unlike a diode, a thyristor, in addition to the anode and cathode, has an additional control electrode: passing a current through it, you can turn the thyristor into an open state. Unfortunately, in order for the thyristor to switch to the closed state, it is not enough to remove the control signal from the control electrode. To do this, it is necessary to reduce to zero the current flowing through the thyristor. This makes it not a fully controlled semiconductor device.

However, this circumstance does not interfere much if the thyristor is used in AC circuits. In this case, zeroing and current polarity reversal occur twice during the period. Therefore, the thyristor can be turned off naturally at the end of each AC half cycle.

Since the thyristor does not have intermediate states of conduction, the current or voltage can only be adjusted by changing the time of its open state t_u (Fig. 18,13).

18.13. The principle of voltage and current regulation using a thyristor

This type of regulation has both advantages and disadvantages. To pluses refers to the fact that the thyristor has a very high resistance in the closed state and very low - in the open state. Therefore, insignificant power is dissipated on it, which makes it possible to build highly efficient thyristor controlled sources.

К cons refers to the fact that the consequence of the operation of the thyristor controller is the "biting" of fragments of the sinusoid and an increase in the duration of pauses t_n in the output voltage.

The use of a full-wave controlled rectifier (Fig. 18.14) ensures more efficient use of the transformer, eliminates one-sided bias of the transformer core, and also reduces the duration of pauses tn between pulses.

Rice. 18.14. Voltage and current regulation with a full-wave controlled rectifier

However, even in this case, especially for the minimum welding current, the pauses in the output voltage are significant. To maintain the arc during these pauses, it is necessary to use a more efficient choke than in a welding source with an uncontrolled rectifier. And here we are faced with mutually exclusive requirements, which were discussed earlier.

С one sideto ensure the continuity of the welding current, it is necessary to increase the inductance of the inductor. WITH other side, in order to obtain the required rate of rise of the short-circuit current, the inductance of the inductor cannot be increased above a certain value, which is guaranteed not to satisfy the first requirement.

In the previous chapter, to meet these requirements, we used an additional source of make-up current. In this case, this solution is not suitable, because due to the operation of the controlled rectifier, the voltage balance will be disturbed. Therefore, a current commensurate in magnitude with the main current will be taken from the make-up source. That is, when you try to reduce the current using a controlled rectifier, the missing current will flow into the welding circuit from the make-up source.

This problem can be solved using two winding choke L1, L2 (Fig. 18.15). The inductances L1 and L2 are interconnected through throttle ratio

Let us consider in more detail the principle of operation of this throttle. Let's say one of the thyristors of the controlled bridge is open. In this case, the arc current I(V3), which is simulated by a voltage source V3 with an internal resistance of 0,05 ohms, flows through the inductor winding L1, which has a slight inductance of 0,3 mH (Table 18.1).

At the moment when the voltage V3 exceeds the instantaneous voltage of the AC voltage source VI, the previously opened bridge thyristor will close, and the load current I (V3) will begin to flow in the circuit D5, L2, L1, V3. Since the magnetically coupled inductors L1 and L2 are connected in series, in this case the load current will decrease in K = K_TP + 1 times, and the inductance will increase in K² time.

Hack and predictor Aviator. Unlike current, which decreases linearly, inductance increases quadratically.

This means that the resulting inductance of the inductor will be able to maintain a continuous load current for a longer time. This is confirmed by the load current graph I(V3) (Fig. 18.15). It follows from this graph that the arc current is continuous and in the worst case (when the source produces a minimum welding current of 60 A) does not fall below 10 A.

Choke inductance L₁ can be selected using the data in the table. 18.1. In our case L₂ = 0,3 mH. In turn, the inductance L₂ also cannot have arbitrary values, but is determined by the transformation ratio, which is usually expressed only as an integer.

Rice. 18.15. Using a two-winding choke to maintain continuous current during voltage pauses

Therefore, for the transformation coefficients K_TP = 1; 2; 3; 4; 5... the secondary winding of the inductor will have an inductance = 0,3; 1,2;

Hack and predictor Aviator. The greater the transformation ratio, the higher the winding inductance L₂ and the longer the inductor will be able to maintain current in the voltage pause.

However, with an increase in the transformation ratio, the overall dimensions of the throttle also increase. Therefore, it is necessary to select the minimum possible transformation ratio in the simulator, which guarantees that at the minimum welding current, the current in the voltage pause does not fall below 10 A.

In this case, this condition is satisfied for K_TP \u5d 3. From the corresponding time diagram of the load current I (V10), it can be seen that the minimum value of the load current does not fall below 132 A, and the amplitude reaches XNUMX A. That is, if the amplitude value of the current reaches the specified value, then energy is accumulated in the inductance Lx , sufficient to maintain the current in the voltage pause.

If, with a further increase in current, the inductor core saturates, then this will not worsen its operation in a pause, but will allow to reduce overall dimensions. The use of a saturable choke will also stabilize the operating current in the secondary (L₂) inductor winding at level I_L2 = 13 A.

Otherwise, this current would be proportional to the load current. Maximum operating current primary (L₁) of the inductor winding corresponds to the maximum welding current I_L1 = I_{sv max} = 180 A.

The choke is wound on a W-shaped tape core made of steel 3411 (E310). The primary winding of the inductor contains 18 turns of an insulated copper bus with a cross section of 36 mm2. The secondary winding of the inductor contains 90 turns of copper wire in enamel insulation with a diameter of 1,81 mm. It is necessary to insert non-magnetic spacers 1 mm thick into the gaps of the throttle core (total non-magnetic gap 2 mm).

Pic. 18.16. Timing diagrams of current in the windings of a two-winding choke

Rice. 18.17. Source model designed to record the trajectory of the remagnetization of a non-linear choke

Taking advantage of the fact that SwCad can model non-linear inductances, let's create a source model with a non-linear choke (Fig. 18.17). According to the calculation results, the non-linear inductance setting line is as follows:

Test node removing the remagnetization loop is built on two current sources - G1 and G2, controlled by voltage, which are used to measure and normalize the displayed parameters.

The transfer coefficient of the controlled current source G1, which provides the output voltage of the integrator, equal to the induction, can be calculated by the formula:

The calculated value of the transfer coefficient must be written in the Value line of the setup menu of the controlled current source G1.

Transfer coefficient of controlled current source G2, providing an output current equal to the intensity in the core of a non-linear transformer, can be calculated by the formula:

The calculated value of the transfer coefficient must be written down in the Value line of the settings menu of the controlled current source G2.

In the horizontal axis settings, in the Quantity Plotted line, instead of the time parameter, enter the I(G2) parameter. Vertically display the voltage at the output of the integrator by clicking on the right terminal of the capacitor C1 (Fig. 18.18).

Rice. 18.18. Choke core remagnetization trajectories for minimum (a) and maximum (b) welding current

On fig. 18.18 shows the trajectory of the magnetization reversal of the core of a non-linear inductor. At the minimum welding current (Fig. 18.18, a), the inductor core is on the verge of saturation. With an increase in current, the core is saturated (Fig. 18.18, b).

Author: Koryakin-Chernyak S.L.

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