Fundamentals of semi-automatic welding. Scheme, description

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Electric welding. Fundamentals of semi-automatic welding. Encyclopedia of radio electronics and electrical engineering

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Semi-automatic or, as it is also called, mechanized welding in protective gases, despite its technological complexity, has found wide distribution in everyday life and at work.

The advantages of this type of welding include:

the ability to weld sheet steel with a thickness of 0,5 mm;
slight sensitivity to rust and other base metal contaminants;
low cost compared to other types of welding.

The disadvantages include:

increased metal spatter compared to manual welding, which requires constant protection and cleaning of the burner nozzle;
more intense radiation of an open powerful arc.

Due to its advantages, semi-automatic welding is dearly loved by car service workers, where it has practically replaced all other types of welding.

On fig. 18.9 schematically shows a welding station for semi-automatic welding, which is equipped with a DC welding source 1, a feeder 2, a wire spool 3, a gas valve 4, a gas heater and dryer 5, a gas reducer 6, a gas cylinder 7. To the welded part 12 welding wire, current and shielding gas flow through the hose holder 8. There is a specialized channel 9 for the wire. Often the channel has an anti-friction coating to facilitate wire feeding. The wire is connected to the source through a special sliding copper tip 10.

Fundamentals of semi-automatic welding
Rice. 18.9. Welding station for semi-automatic welding

Typically, semi-automatic welding is used for welding steel and stainless steel, as well as for welding aluminum. Welding is performed with metal or flux-cored wire, which is fed into the welding zone by a special feeder.

To protect the welding zone from air oxygen and nitrogen, a shielding gas is used, which is carbon dioxide, argon, helium, or mixtures thereof. The most common is the welding of steel in a carbon dioxide environment or in a mixture of carbon dioxide and argon.

Welding is done DC reverse polarity (minus on the product). Relatively thin wire is used for welding 0,5-2 mm, due to which the current density increases to 110-130 A/mm2. For comparison, in manual welding with coated electrodes, the current density does not exceed 20 A/mm2.

Since the current density is increased, welding proceeds in the ascending section of the current-voltage characteristic of the arc. To match the current-voltage characteristics of the arc and the source, the latter must have a rigid external characteristic. In this case, due to the high current density and the rigid external characteristic of the source, the arc length is self-adjusting (Fig. 18.10).

Fundamentals of semi-automatic welding
Rice. 18.10. Arc self-regulation in semi-automatic welding

For example, when the arc is shortened, the welding current increases sharply, the electrode begins to melt more intensively and the arc length is restored. Accordingly, when the arc lengthens, the welding current decreases, the melting of the electrode slows down, and the arc length is also restored. That is, the length of the arc depends on the output voltage of the source and depends little on the wire feed speed. In turn, the welding current is proportional to the wire feed speed.

In order for the self-regulation mechanism to work, the welding source in case of a short circuit at the output must provide a rate of current rise at the level of dI / dt = 60-180 kA / s for a wire with a diameter of 0,8-1,2 mm.

The rate of current rise depends on the inductive component of the welding circuit, which is determined by the leakage inductance of the welding transformer and the inductance of the linear choke in the welding circuit.

If the welding source is powered by a single-phase network, then there is a conflict of requirements:

on the one hand, we must increase the inductance of the inductor in order to ensure the continuity of the current in the welding circuit;
on the other hand, we must reduce this inductance to provide the necessary current slew rate.

Due to the difficulty of resolving this conflict among amateur designers, there was a strong opinion that there is no good single-phase source for semi-automatic welding.

Of course, the use of a three-phase transformer with a three-phase rectifier makes it possible to obtain a direct current with a low level of ripple, which favorably affects the stability of the arc. In this case, it remains with the help of a small choke to limit the rate of current rise at the required level. True, in everyday life the presence of a three-phase network is more an exception than a rule. In addition, a three-phase source is more complicated and more expensive than a single-phase source.

In fact, not everything is so bad, and there is a way to correct the above disadvantage of single-phase sources: for example, the use of an additional make-up current source that maintains the arc during the absence of the main current. To maintain the arc, a current of about 10 A is sufficient. Possible options for organizing the make-up current are shown in fig. 18.11.

Fundamentals of semi-automatic welding
Rice. 18.11. Options for organizing the make-up current: a - option used in single-phase sources of the Selma company; b - alternative option

The options shown in fig. 18.11, have a similar principle of operation, so consider the option shown in fig. 18.11, a. This option is used in Selma single-phase sources. Here, the main and make-up current sources operate from a common single-phase transformer T1.

The load current comes directly from the VD1-VD4 rectifier bridge, and there is no error in this. As practice shows, to limit the rate of current rise, the leakage inductance of the welding transformer T1 is usually sufficient.

The make-up current source is assembled on diodes VD5, VD6, capacitor C1 and inductor L1. To feed the arc, the energy accumulated by the capacitor at the moments of mains voltage is used. The inductor L1 maintains the make-up current at the required level, preventing the capacitor C1 from discharging quickly.

The variant shown in Fig. 18.11b has less detail, but the SwCad simulation showed that this option provided only 30% of the make-up current compared to the option in fig. 18.11, a. We will not save on "matches" and will opt for the option shown in Fig. 18.11, a.

Author: Koryakin-Chernyak S.L.

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