DC welding. Scheme, description

ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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DC welding. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / welding equipment

The advantages of DC welding machines over their "alternating current counterparts" are well known. This includes soft arc ignition, the ability to connect thin-walled parts, less metal spatter, and the absence of non-welding areas. There is not even an annoying (and, as it turned out, harmful to people) cod. And all because there is no main feature inherent in AC welding machines - intermittent arc burning when the sinusoid of the supply voltage flows through zero (Fig. 1).

DC welding
Rice. 1. Graphs explaining the process of welding on alternating (a) and direct (b) current

Turning from graphs to real designs, it should also be noted: in AC machines, to improve and facilitate welding, powerful transformers are used (the magnetic circuit is made of special electrical iron with a steeply falling characteristic) and a deliberately high voltage in the secondary winding, reaching up to 80 V, although for 25-36 V is enough to support the arc burning and metal deposition in the welding zone. We have to put up with the prohibitively large mass and dimensions of the apparatus, increased power consumption. By reducing the voltage transformed into the secondary circuit to 36 V, it is possible to lighten the weight of the "welder" by 5-6 times, bring its dimensions to the size of a portable TV while improving other performance characteristics.

But how to ignite an arc with a low-voltage winding?

Rice. 2. Schematic diagram of a DC welding machine

The solution was to introduce a diode bridge with a capacitor into the secondary circuit. As a result, the output voltage of the modernized "welder" was increased by almost 1,5 times. The opinion of experts is confirmed in practice: when the 40-volt DC barrier is exceeded, the arc easily ignites and burns steadily, allowing even thin body metal to be welded.

The latter, however, is easily explained. With the introduction of a large capacitance into the circuit, the characteristic of the welding machine also turns out to be steeply falling (Fig. 3). The initial increased voltage created by the capacitor facilitates the ignition of the arc. And when the potential on the welding electrode drops to U2 of the transformer (working point "A"), a process of stable arc burning with metal deposition in the welding zone will occur.

DC welding
Rice. 3. Volt-ampere characteristic of the "welder"

A "welder" can be assembled even at home, based on an industrial power transformer 220-36 / 42 V (these are usually used in systems for safe lighting and powering low-voltage factory equipment). After making sure that the primary winding, containing, as a rule, 250 turns of insulated wire with a cross section of 1,5 mm2, is intact, the secondary ones are checked. If their condition is not important, everything (with the exception of a working network winding) is deleted without regret. And in the vacated space, a new secondary winding is wound (until the "window" is filled). For the recommended 1,5 kVA transformer, this is 46 turns of a 20 mm2 copper or aluminum bus with good insulation. Moreover, a cable (or several insulated single-core wires twisted into a bundle) with a total cross section of 20 mm2 is quite suitable as a bus.

The choice of electrode cross-section depending on the power of the transformer

DC welding

The rectifier bridge can be assembled from semiconductor diodes with an operating current of 120-160 A by installing them on heat sinks-radiators 100x100 mm. It is most convenient to place such a bridge in one housing with a transformer and a capacitor, bringing to the front textolite panel a 16-ampere switch, a peephole of the "On" signal light, as well as "plus" and "minus" terminals (Fig. 4). And to connect to the electrode holder and the "ground" use a piece of a single-core cable of the appropriate length with a copper cross section of 20-25 mm2. As for the welding electrodes themselves, their diameter depends on the power of the transformer used.

Rice. 4. Homemade DC welding machine

And further. When testing, it is recommended, by disconnecting the device (10 minutes after welding) from the network, to check the thermal conditions of the transformer, diode bridge and capacitor. Only after making sure that everything is in order, you can continue to work. After all, an overheated "welder" is a source of increased danger!

Of the other requirements, it is worth noting, I think, that the welding machine must be equipped with a spark-protective mask, gloves and a rubber mat. The place where welding work is performed is equipped with fire safety requirements. In addition, it is necessary to ensure that there are no rags or other combustible materials nearby, and the connection of the "welder" to the network must be carried out in compliance with electrical safety rules through a powerful plug connector of the electrical panel at the entrance to the building.

Author: V.Konovalov

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