Do-it-yourself welding transformer. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / welding equipment

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A welding machine is a welcome purchase for any household. The advantages of manual electric welding are obvious and indisputable: ease of use, the widest range of applications, high performance and reliability of connections - and all this with the ability to work almost anywhere where there is an electrical network. Problems with the choice and acquisition of welding machines today, it seems, do not exist. A lot of household and professional industrial welding machines have appeared on sale. Vieingly offer their products and all kinds of handicraft workshops and craftsmen. Yes, but the prices for factory-made devices "bite", as a rule, several times, exceeding the current average monthly income. Basically, it is this sad discrepancy between one's own wealth and price that always forces many people to take up welding with their own hands.

In modern literature, you can find a lot of material on welding. In recent years, a number of articles devoted to the improvement and calculation of elements of welding transformers (ST) have been published in Radioamator, which undoubtedly indicates the interest of readers in this topic. I propose the most important thing: how and from what to make welding transformers at home. All the welding transformer circuits described below have been practically tested and are really suitable for manual electric welding. Some of the circuits have been worked out "among the people" for decades and have become a kind of "classic" of independent "transformer building".

Like any transformer, the ST consists of primary and secondary (possibly with taps) windings wound on a large magnetic core made of transformer iron. The mode of operation distinguishes the CT from a conventional transformer: it operates in an arc mode, i.e. at almost maximum power. And hence the strong vibrations, intense heating, the need to use large-section wire. The ST is powered from a single-phase network of 220-240 V. The output voltage of the secondary winding in idle mode (x.x.) (when no load is connected to the output) for self-made STs, as a rule, lies in the range of 45-50 V, less often up to 70 Q. In general, the output voltages for industrial welders are limited (80 VAC, 90 VDC). Therefore, large stationary units have an output of 60-80 V.

The main power characteristic of the ST is considered to be the output current of the secondary winding in the arc mode (welding mode). In this case, the electric arc burns in the gap between the end of the electrode and the metal to be welded. The gap is 0,5 ... 1,1 d (d is the electrode diameter), it is maintained manually. For portable structures, the operating currents are 40-200 A. The welding current is determined by the power of the ST. The choice of the diameter of the electrodes used and the optimal thickness of the welded metal depend on the output current of the ST.

The most common are electrodes with steel bars D3 mm ("troika"), which require currents of 90-150 A (usually 100-130 A). In skillful hands, the "troika" will burn even at 75 A. At currents greater than 150 A, such electrodes can be used for cutting metal (thin sheets of iron 1-2 mm can be cut at lower currents). When working with an electrode D3 mm, a current of 20-30 A flows through the primary winding of the ST (usually about 25 A).

If the output current is lower than required, then the electrodes begin to "stick" or "stick", welding their tips to the metal being welded: for example, the ST starts to work with a dangerous overload in short circuit mode. At currents greater than the allowable, the electrodes begin to cut the material: this can ruin the entire product.

For electrodes with an iron rod D2 mm, a current of 40-80 A is required (usually 50-70 A). They can accurately weld thin steel with a thickness of 1-2 mm. D4 mm electrodes work well at a current of 150-200 A. Higher currents are used for less common (D5-6 mm) electrodes and metal cutting.

In addition to power, an important property of ST is its dynamic response. The dynamic characteristic of the transformer largely determines the stability of the arc, and hence the quality of the welded joints. From the dynamic characteristics, one can distinguish steeply dipping and gently dipping. During manual welding, inevitable oscillations of the end of the electrode occur and, accordingly, a change in the length of the arc burning (at the moment of ignition of the arc, when adjusting the length of the arc, on irregularities, from hand trembling). If the dynamic characteristic of the ST is steeply falling, then with fluctuations in the length of the arc, slight changes in the operating current occur in the secondary winding of the transformer: the arc burns stably, the weld lies flat.

With a gently dipping or rigid characteristic of the ST: when the arc length changes, the operating current also changes sharply, which changes the welding mode - as a result, the arc burns unstably, the seam turns out to be of poor quality, it is difficult or even impossible to work with such a ST manually. For manual arc welding, a steeply falling dynamic characteristic of the ST is required. Sloping is used for automatic welding.

In general, in real conditions, it is hardly possible to somehow measure or quantify the parameters of the current-voltage characteristics, however, like many other parameters of the ST. Therefore, in practice, ST can be divided into those that weld better and those that work worse. When a CT works well, welders say, "Cooks soft." This should be understood as the high quality of the weld, the absence of metal spattering, the arc burns stably all the time, the metal is deposited evenly. All the CT designs described below are actually suitable for manual arc welding.

The mode of operation of ST can be characterized as short-term repetitive. In real conditions, after welding, as a rule, installation, assembly and other works follow. Therefore, the ST after working in the arc mode has some time for cooling in the cold mode. In the arc mode, the ST heats up intensively, and in the cold mode. cools, but much more slowly. The situation is worse when ST is used for cutting metal, which is very common. In order to cut thick rods, sheets, pipes, etc. with an arc, at a not too high current of a home-made transformer, it is necessary to overheat the ST too much.

Any industrially manufactured apparatus is characterized by such an important parameter as the operating time coefficient (PR), measured in%. For domestic factory portable devices weighing 40-50 kg, PR usually does not exceed 20%. This means that the ST can operate in the arc mode for no more than 20% of the total time, the remaining 80% it must be in the cold mode. For most home-made designs, the PR should be taken even less. We will consider the intensive mode of operation of the ST as such, when the arc burning time is of the same order as the break time.

Home-made CTs are performed according to different schemes: on P-, PU- and W-shaped magnetic cores: toroidal, with various combinations of winding arrangements. The SM manufacturing scheme and the number of turns of future windings are mainly determined by the core available - the magnetic circuit. In the future, the article will consider real schemes of home-made STs and materials for them. Now let's determine what winding and insulating materials will be needed for the future ST.

Given the high power, a relatively thick wire is used for the ST windings. Developing significant currents during operation, any ST gradually heats up. The heating rate depends on a number of factors, the most important of which is the diameter or cross-sectional area of ​​the winding wires. The thicker the wire, the better it passes current, the less it heats up and, finally, the better it dissipates heat. The main characteristic is the current density (A / mm2): the higher the current density in the wires, the more intense the heating of the ST. Winding wires can be copper or aluminum. Copper allows you to use 1,5 times the current density and heats up less: it is better to wind the primary winding with copper wire.

In industrial devices, the current density does not exceed 5 A/mm2 for copper wire. For self-made variants of ST, 10 A / mm2 for copper can also be considered a satisfactory result. With an increase in current density, the heating of the transformer is sharply accelerated. In principle, for the primary winding, you can use a wire through which a current with a density of up to 20 A / mm2 will flow, but then the ST will heat up to a temperature of 60 ° C after using 2 x 3 electrodes. If you think that you will have to weld a little, not quickly, and you still won’t find the best materials, then you can wind the primary winding with wire and with a strong overload. Although this, of course, will inevitably reduce the reliability of the device.

In addition to the section, another important characteristic of the wire is the method of insulation. The wire can be varnished, wound in one or two layers of thread or fabric, which, in turn, can be impregnated with varnish. The reliability of the winding, its maximum overheating temperature, moisture resistance, and insulating qualities strongly depend on the type of insulation (see Table 1).

Table 1

Note. PEV, PEM - wires enamelled with high-strength varnish (viniflex and metalvin, respectively), are produced with thin (PEV-1, PEM-1) and reinforced insulating layers (PEV-2, PEM-2); PEL - wire enameled with oil-based varnish; PELR-1, PELR-2 - wires enamelled with high-strength polyamide varnish, respectively, with thin and reinforced insulation layers; PELBO, PEVLO - wires based on PEL and PEV wires with one layer, respectively, of natural silk, cotton yarn or lavsan; PEVTL-1, PEVTL-2 - wire enamelled with high-strength polyurethane enamel, heat-resistant, with thin and reinforced layers of insulation; PLD - wire insulated with two layers of lavsan; PETV - wire enamelled with heat-resistant high-strength polyester varnish; PSD type wires - with insulation made of alkali-free fiberglass, superimposed in two layers with gluing and impregnation with heat-resistant varnish (in brand designations: T - thinned insulation, L - with a surface varnish layer, K - with gluing and impregnation with silicone varnish); PETKSOT - wire insulated with heat-resistant enamel and fiberglass; PNET-imide is a wire insulated with high-strength polyamide-based enamel. Under the insulation thickness in the table, the difference between the maximum wire diameter and the nominal copper diameter is taken.

The best is fiberglass insulation impregnated with a heat-resistant varnish, but it is difficult to get such a wire, and if you buy it, it will cost a lot. The least desirable, but the most affordable material for homemade products are ordinary PEL wires, PEV Dtsii. Such wires are the most common, they can be removed from the coils of chokes, transformers of obsolete equipment. Carefully removing the old wires from the coil frames, it is necessary to monitor the condition of their coating and additionally isolate slightly damaged areas. If the coils with wire were additionally impregnated with varnish, their turns stuck together, and when trying to disconnect, the hardened impregnation often tears off the wire's own varnish coating, exposing the metal. In rare cases, in the absence of other options, "homemade" wind the primary windings even with a mounting wire in vinyl chloride insulation. Its disadvantages: extra volume of insulation and poor heat dissipation.

The quality of laying the primary winding of the ST should always be given the greatest attention. The primary winding contains more turns than the secondary, its winding density is higher, it heats up more. The primary winding is under high voltage, with its interturn circuit or insulation breakdown, for example, through moisture that has entered, the entire coil quickly "burns out". As a rule, it is impossible to restore it without disassembling the entire structure.

The secondary winding of the ST is wound with a single or stranded wire, the cross section of which provides the required current density. There are several ways to solve this problem. First, you can use a monolithic wire with a cross section of 10-24 mm2 made of copper or aluminum.

Such rectangular wires (commonly referred to as busbars) are used for industrial MTs. However, in most home-made designs, the winding wire has to be pulled many times through the narrow windows of the magnetic circuit. Try to imagine doing this about 60 times with 16mm2 solid copper wire. In this case, it is better to give preference to aluminum wires: they are much softer, and they are cheaper.

The second way is to wind the secondary winding with a stranded wire of a suitable cross section in ordinary vinyl chloride insulation. It is soft, easy to fit, securely insulated. True, the synthetic layer takes up extra volume in the windows and prevents cooling. Sometimes for these purposes they use old stranded wires in thick rubber insulation, which are used in powerful three-phase cables. The rubber is easy to remove, and instead wrap the wire with a layer of some thin insulating material. The third way - you can make a secondary winding from several single-core wires approximately the same as the primary winding. To do this, 2-5 wires D1,62,5 mm are carefully pulled together with adhesive tape and used as one stranded wire. Such a bus of several wires takes up little space and has sufficient flexibility, which makes it easy to install.

If it is difficult to get the right wire, then the secondary winding can be made from thin, the most common wires PEV, PEL D0,5-0,8 mm, although this will take an hour or two. First you need to choose a flat surface, where you must rigidly install two pegs or hooks with a distance between them equal to the length of the secondary winding wire of 2030 m. Then stretch several tens of strands of thin wire between them without deflection, you get one elongated bundle. Next, disconnect one of the ends of the beam from the support and clamp it into the chuck of an electric or manual drill. At low speeds, the entire bundle, in a slightly taut state, twists into a single wire. After twisting, the length of the wire will decrease slightly. At the ends of the resulting stranded wire, you need to carefully burn the varnish and clean the ends of each wire separately, and then securely solder everything together. After all, it is desirable to isolate the wire by wrapping it along its entire length with a layer, for example, adhesive tape.

For laying the windings, fastening the wire, inter-row insulation, insulating and fastening the magnetic circuit, you will need a thin, strong and heat-resistant insulating material. In the future, it will be seen that in many SM designs, the volume of the magnetic circuit windows, in which several windings with thick wires must be laid, is very limited. Therefore, in this "vital" space of the magnetic circuit, every millimeter is precious. With small core sizes, the insulating materials should occupy as little volume as possible, i.e. be as thin and flexible as possible. The widespread PVC iso1,6-2,4 mm in a simple varnish insulating tape can be immediately excluded from use in heated sections of ST. Even with a slight overheating, it becomes soft and gradually spreads or is pressed through by wires, and with significant overheating, it melts and foams. For insulation and bandage, you can use fluoroplastic, glass ... and lacquered fabric keeper tapes, and between rows - ordinary adhesive tape.

Adhesive tape can be attributed to the most convenient insulating materials. After all, having a sticky surface, small thickness, elasticity, it is quite heat-resistant and strong. Moreover, now adhesive tape is sold almost everywhere on reels of various widths and diameters. Coils of small diameters are the best suited for pulling through narrow windows of compact magnetic cores. Two or three layers of adhesive tape between the rows of wire practically do not increase the volume of the coils.

And finally, the most important element of any ST is the magnetic circuit. As a rule, for homemade products, magnetic circuits of old electrical appliances are used, which before that had nothing to do with ST, for example, large transformers, autotransformers (LATRs), electric motors. The most important parameter of the magnetic circuit is its cross-sectional area (S), through which the magnetic field flow circulates.

For the manufacture of ST, magnetic cores with a cross-sectional area of ​​​​25-60 cm2 (usually 30-50 cm2) are suitable. The larger the cross section, the more flux the magnetic circuit can transmit, the more power the transformer has and the fewer turns its windings contain. Although the optimal cross-sectional area of ​​​​the magnetic core, when the medium power CT has the best characteristics, is 30 cm2.

There are standard methods for calculating the parameters of the magnetic circuit and windings for industrial MT circuits. However, for homemade products, these techniques are practically not suitable. The fact is that the calculation according to the standard method is carried out for a given power of the ST, and only in a single variant. For it, the optimal value of the cross section of the magnetic circuit and the number of turns are calculated separately. In fact, the cross-sectional area of ​​the magnetic circuit for the same power can be in a very wide range.

There is no connection between an arbitrary section and turns in standard formulas. For self-made CTs, any magnetic circuits are usually used, and it is clear that it is almost impossible to find a core with "ideal" parameters of standard methods. In practice, it is necessary to select the turns of the windings for the existing magnetic circuit, thereby setting the required power.

ST power depends on a number of parameters, which cannot be fully taken into account under normal conditions. However, the most important among them are the number of turns of the primary winding and the cross-sectional area of ​​the magnetic circuit. The ratio between the area and the number of turns will determine the operating power of the ST. To calculate the ST, designed for D3-4 mm electrodes and operating from a single-phase network with a voltage of 220-230 V, I propose to use the following approximate formula, which I obtained on the basis of practical data. Number of turns N=9500/S (cm2). At the same time, for a PT with a large area of ​​the magnetic circuit (more than 50 cm2) and a relatively high efficiency, it can be recommended to increase the number of turns calculated by the formula by 10–20%.

For STs manufactured on cores with a small area (less than 30 cm), it may be necessary, on the contrary, to reduce the number of calculated turns by 1020%. In addition, the useful power of the ST will be determined by a number of other factors: efficiency, secondary winding voltage, supply voltage in the network ... (Practice shows that the mains voltage, depending on the location and time, can vary between 190-250 V).

Equally important is the resistance of the power line. Being only a unit of an ohm, it practically does not affect the readings of a voltmeter with high resistance, but it can greatly dampen the power of the ST. The influence of line resistance can be especially pronounced in places remote from transformer substations (for example, cottages, garage cooperatives, in rural areas, where lines are laid with thin wires with a large number of connections). Therefore, initially it is hardly possible to accurately calculate the output current of the ST for different conditions - this can only be done approximately. When winding the primary winding, its last part is best done with 2-3 taps after 20-40 turns. Thus, you can adjust the power by choosing the best option for yourself, or adjust to the mains voltage. To obtain higher powers from ST, for example, to operate a D4 mm electrode at currents greater than 150 A, it is also necessary to reduce the number of turns of the primary winding by 20-30%.

But it should be remembered that with an increase in power, the current density in the wire also increases, and hence the intensity of heating of the windings. The output current of the ST can also be slightly increased by increasing the number of turns of the secondary winding so that the output voltage x.x. increased from the expected 50 V to higher values ​​(70-80 V).

Having included the primary winding in the network, it is necessary to measure the current x.x., it should not have much knowledge (0,1-2 A). (When the PT is connected to the network, a short but powerful current surge occurs). In general, current x.x. it is impossible to judge the output power of the CT: it can be different even for the same types of transformers. However, having studied the dependence curve of the current x.x. from the CT supply voltage, you can more confidently judge the properties of the transformer.

Ris.1

To do this, the primary winding of the ST must be connected through LATR, which will allow you to smoothly change the voltage on it from 0 to 250 V. The current-voltage characteristics of the ST in idle mode with different numbers of turns of the primary winding are shown in Fig. 1, where 1 - the winding contains little turns; 2 - ST operates at its maximum power; 3, 4 - moderate power ST. At first, the current curve gently, almost linearly increases to a small value, then the rate of increase increases - the curve smoothly bends upwards, followed by a rapid increase in current. When the current tends to infinity up to the operating voltage point of 240 V (curve 1), this means that the primary winding contains few turns, and it must be winded up (it should be borne in mind that the ST, switched on to the same voltage without LATR, will consume current approximately 30% more). If the operating voltage point lies on the bend of the curve, then the ST will produce its maximum power (curve 2, welding current of the order of 200 A). Curves 3 and 4 correspond to the case when the transformer has a power resource and an insignificant cold current: most homemade products are focused on this case. Real currents x.x. are different for different types of CT: most lies in the range of 100-500 mA. I do not recommend setting the current x.x. more than 2 A.

After getting acquainted with the general issues of manufacturing home-made welding transformers, we can proceed to a detailed examination of actually existing ST designs, the features of their manufacture and materials for them. Almost all of them I collected with my own hands or took a direct part in their manufacture.

Welding transformer on a magnetic circuit from LATRs

A common material for the manufacture of home-made welding transformers (ST) has long been burnt LATRs (laboratory autotransformer). Those who have dealt with them know well what it is. As a rule, all LATRs have approximately the same appearance: a well-ventilated round tin case with a tin or ebonite front cover with a scale from 0 to 250 V and a rotating handle. Inside the case there is a toroidal autotransformer, made on a magnetic circuit of a significant cross section. It is this core-magnetic circuit that will be needed from LATR for the manufacture of a new ST. Usually, two identical magnetic core rings from large LATRs are required.

LATRs were produced of various types with a maximum current from 2 to 10 A. Only those CTs are suitable for manufacturing, the dimensions of the magnetic circuits of which allow the required number of turns to be laid. The most common among them is probably an autotransformer of the LATR 1M type, which, depending on the winding wire, is designed for a current of 6,7-9 A, although the dimensions of the autotransformer itself do not change from this. The LATR 1M magnetic core has the following dimensions: outer diameter D=127 mm; inner diameter d=70 mm; ring height h=95 mm; section S=27 cm2 and weight about 6 kg. You can make a good ST from two rings from LATR 1M, however, due to the small internal volume of the window, you cannot use too thick wires and you will have to save every millimeter of window space.

There are LATRs with more voluminous magnetic core rings, for example RNO-250-2 and others. They are better suited for making CTs, but are less common. For other autotransformers similar in parameters to LATR 1M, for example, AOSN-8-220, the magnetic core has a larger outer diameter of the ring, but a smaller height and window diameter d = 65 mm. In this case, the window diameter must be expanded to 70 mm. The ring of the magnetic core consists of pieces of iron tape wound on top of each other, fastened at the edges by spot welding.

In order to increase the inner diameter of the window, it is necessary to disconnect the end of the tape from the inside and unwind the required amount. But don't try to rewind in one go. It is better to unwind one turn, each time cutting off the excess. Sometimes the windows of larger LATRs are also expanded in this way, although this inevitably reduces the area of ​​the magnetic circuit.

Both rings must be insulated at the start of CT fabrication. At the same time, pay special attention to the corners of the edges of the rings - they are sharp, they can easily cut the superimposed insulation, and then close the winding wire. It is better to apply some strong and elastic tape along the corners, for example, a dense keeper or a cambric tube cut along. From above, the rings (each separately) are wrapped with a thin layer of fabric insulation.

Next, the isolated rings are connected together (Fig. 2). The rings are tightly pulled together with a strong tape, and on the sides they are fixed with wooden pegs, also then tied with electrical tape, the core of the magnetic core for the ST is ready.

The next step is the most important - laying the primary winding. The windings of this ST are wound according to the scheme (Fig. 3) - the primary in the middle, two sections of the secondary - on the side arms. "Specialists" who know this type of transformer often call it "eared" in a kind of jargon because of the round "cheburashkin ears" protruding in different directions of the secondary winding sections.

The primary one takes about 70-80 m of wire, which will have to be pulled through both windows of the magnetic circuit with each turn. In this case, one cannot do without a simple device (Fig. 4). First, the wire is wound on a wooden reel and, in this form, is pulled through the windows of the rings without any problems. The winding wire can consist of pieces (even ten meters each) if you managed to get only one. In this case, it is wound in parts, and the ends are connected together. To do this, the tinned ends are connected (without twisting) and fastened with several turns of a thin copper core without insulation, then finally soldered and insulated. Such a connection does not crack the wire and does not take up a large volume.

The diameter of the wire of the primary winding is 1,6-2,2 mm. For magnetic circuits made up of rings with a window diameter of 70 mm, a wire with a diameter of not more than 2 mm can be used, otherwise there will be little space for the secondary winding. Contains a primary winding, as a rule, 180-200 turns at normal mains voltage.

So, suppose you have an assembled magnetic circuit in front of you, the wire is prepared and wound on a reel. Let's start winding. As always, we put a cambric on the end of the wire and draw it with electrical tape to the beginning of the first layer. The surface of the magnetic circuit has a rounded shape, so the first layers will contain fewer turns than the subsequent ones - to level the surface (Fig. 5).

The wire should be laid coil to coil, in no case allowing the wire to overlap the wire. Wire layers must be isolated from each other. (During operation, the MT vibrates strongly. If the wires in varnish insulation lie on top of each other without intermediate insulation, then as a result of vibration and friction against each other, the varnish layer may collapse and a short circuit will occur). To save space, the winding should be laid as compactly as possible. On a magnetic core of medium-sized rings, the interlayer insulation should be used thinner.

For these purposes, small rolls of adhesive tape are well suited, which easily pass through filled windows, and the tape itself does not take up extra volume. One should not strive to wind the primary winding quickly and in one go. This process is slow, and after laying hard wires, fingers begin to hurt. It is better to do this in 2-3 approaches - after all, quality is more important than speed.

When the primary winding is done, most of the work is done. Let's deal with the secondary winding. Let us determine the number of turns of the secondary winding for a given voltage. To begin with, we will turn on the already finished primary winding in the network. Current x.x. of this option, the ST is small - only 70-150 mA, the transformer hum should be barely audible. Wind 10 turns of any wire on one of the side arms and measure the output voltage on it.

Each of the side arms accounts for half of the magnetic flux created on the central arm, so here 0,6-0,7 V falls on each turn of the secondary winding. Based on the result, calculate the number of turns of the secondary winding, focusing on a voltage of 50 V ( about 75 turns).

The choice of material for the secondary winding is limited by the remaining space of the windows of the magnetic circuit. Moreover, each turn of a thick wire will have to be pulled along the entire length into a narrow window, and no "automation" here, alas, will help. I have seen transformers made on LATR 1M rings, into which craftsmen, with the help of a hammer and their own patience, pushed a thick monolithic copper wire with a section of twenty "squares".

Another thing, if you are new to this business, then you should not tempt fate, unwinding solid copper back is as difficult as winding it. It is easier to wind with aluminum wire with a cross section of 16-20 mm2. The easiest way is to wind the usual stranded wire 10 mm2 in synthetic insulation - it is soft, flexible, well insulated, but it will heat up during operation. It is possible to make a secondary winding from several strands of copper wire, as described above. Wrap half of the turns on one shoulder, half on the other (Fig. 3). If there are no wires of sufficient length, you can connect them from pieces - it's okay. Having wound the windings on both arms, it is necessary to measure the voltage on each of them, it may differ by 2-3 V - the slightly different properties of the magnetic circuits of different LATRs affect, which does not particularly affect the properties of the ST. Then connect the windings on the arms in series, but you need to make sure that they are not in antiphase, otherwise the output will be a voltage close to 0. At a network voltage of 220-230 V, the ST of this design should develop a current in arc mode of 100-130 A, with short circuit current of the secondary circuit up to 180 A.

It may turn out that it was not possible to fit all the calculated turns of the secondary winding into the windows, and the output voltage turned out to be lower than required. The operating current will decrease slightly. To a greater extent, lowering the voltage x.x. affects the ignition process. The arc is easily ignited at a cold voltage close to 50 V and higher, although the arc can be ignited without any problems at lower voltages. I happened to work with ST with x.x. 37 V on alternating current, and at the same time the quality was quite satisfactory. So if the manufactured ST has an output voltage of 40 V, then it can be used for work. Another thing is if you come across electrodes designed for high voltages - some brands of electrodes work from 70-80 V.

On rings from LATRs, ST can also be made according to the toroidal scheme (Fig. 6). This also requires two rings, preferably from large LATRs. The rings are connected and insulated: one ring-magnetic circuit with a significant area is obtained. The primary winding contains the same number of turns, but it is wound along the length of the entire ring and, as a rule, in two layers. The problem of the shortage of the internal space of the magnetic circuit window of such a CT scheme is even more acute than for the previous design. Therefore, it is necessary to isolate as thin layers and materials as possible. Do not use thick winding wires (recommended for primary winding D1,8 mm). In some installations, LATRs of especially large sizes are used; a toroidal ST can be made on only one ring of this type.

An advantageous difference between the toroidal circuit ST is a fairly high efficiency. For each turn of the secondary winding there is more than 1 V of voltage, therefore, the "secondary" will have fewer turns, and the output power is higher than in the previous circuit. However, the length of the turn on the toroidal magnetic circuit is longer, and it is unlikely that it will be possible to save on the wire here. The disadvantages of this scheme include the complexity of winding, the limited volume of the window, the impossibility of using a wire of large cross section, as well as the high intensity of heating. If in the previous version all the windings were separate and at least partially had contact with air, now the primary winding is completely under the secondary, and their heating is mutually enhanced.

It is difficult to use rigid wires for the secondary winding. It is easier to wind it with soft stranded or multi-core wire. If you correctly select all the wires and carefully lay them, then the required number of turns of the secondary winding will fit into the space of the magnetic circuit window, and the desired voltage will be obtained at the output of the ST. The arc burning characteristic of the toroidal CT can be considered better than that of the previous transformer.

Sometimes a toroidal ST is made from several rings of LATRs, but they are not placed on top of each other, but the iron strips of the tape are rewound from one to the other. To do this, first, internal turns of the strips are selected from one ring - to expand the window. The rings of other LATRs are completely unrolled into strips of tape, which are then wound as tightly as possible around the outer diameter of the first ring. After that, the assembled single magnetic core is wound very tightly with insulating tape. Thus, a ring-magnetic circuit is obtained with a more voluminous internal space than all the previous ones. In this you can fit a wire of considerable cross section, and make it much easier. The required number of turns is calculated from the cross-sectional area of ​​the assembled ring. The disadvantages of this design include the complexity of the manufacture of the magnetic circuit. Moreover, no matter how hard you try, you still won’t be able to manually wind the iron strips on top of each other as tightly as before. As a result, the magnetic circuit turns out to be flimsy. When the ST is operating, the iron in it vibrates strongly, emitting a powerful hum.

Sometimes the "native" windings of LATRs burn out only from one edge on the current collector or generally remain unharmed. Then there is a temptation to save yourself the extra effort and use a ready-made, perfectly laid primary winding of one ring. Practice shows that, in principle, this idea can be realized, however, the benefits of such an undertaking will be minimal. The winding LATR 1M has 265 turns of wire with a diameter of 1 mm. If you wind the secondary directly on it, then the transformer will develop exorbitant power for itself, quickly heat up and fail. After all, in reality, the "native" winding of the LATR can operate at low power - only for D2 mm electrodes that need a current of 50-60 A. Then a current of about 15 A should flow through the primary winding of the transformer.

For such a power, the primary winding of ST from one LATR should contain about 400 turns. They can be winded up by first varnishing the conductive path and isolating the native LATR winding. You can do it differently: do not wind the turns, but turn off the power with a ballast resistor included in the circuit of the primary or secondary winding. As an active resistance, you can use a battery of powerful wire resistors connected in parallel, for example, PEV50 ... 100, with a total resistance of 10-12 Ohms, included in the primary winding circuit. During operation, the resistors get very hot, to avoid this, they can be replaced with a choke (reactance). Wind the inductor on the frame of a 100-200-watt transformer with the number of turns 200-100. Although the CT will have a much better performance if the ballast resistor (hundredths of an ohm) is included at the output of the secondary winding. To do this, use a piece of thick high-resistance wire wound into a spiral, the length of which is selected experimentally.

In some devices, LATRs of especially large sizes were used, only on one ring from this one can be wound a full-fledged ST. In the designs described above, it was necessary to use two rings each: this was done not so much because of the need to increase the area of ​​\uXNUMXb\uXNUMXbthe magnetic circuit, but to reduce the number of turns, otherwise they simply would not fit in narrow windows. In principle, a cross-sectional area and one ring are sufficient for a ST: it would have even better characteristics, since the magnetic flux density would be closer to optimal. But the problem is that smaller area magnetic cores inevitably require more turns, which increases coil volume and requires more window space.

Welding transformer on a magnetic circuit from the stator of an electric motor

From LATRs, let's move on to the next common source of obtaining good magnetic circuits for ST. Often, toroidal CTs are wound on the material of a magnetic conductor taken from a failed large asynchronous three-phase electric motor, which are most common in industry. For the manufacture of ST, motors with a power close to 4kV•A and more are suitable.

The electric motor consists of a rotor rotating on a shaft and a fixed stator pressed into the metal housing of the motor, which are connected by two side covers, pulled together by pins. Only the stator is of interest. The stator consists of a set of iron plates - a round magnetic circuit with windings installed on it. The shape of the stator magnetic circuit is not entirely circular, on the inside it has longitudinal grooves into which the motor windings are laid.

Different brands of engines, even of the same power, may have stators with different geometric dimensions. For the manufacture of STs, those with a larger case diameter and a correspondingly shorter length are better suited.

The most important part in the stator is the magnetic ring. The magnetic core is pressed into a cast iron or aluminum motor housing. Wires that need to be removed are tightly packed into the grooves of the magnetic circuit.

It is better to do this when the stator is still pressed into the housing. To do this, on one side of the stator, all winding outputs are cut off at the end with a sharp chisel. On the opposite side, the wire should not be cut - there the windings form something like loops, for which you can pull the remaining wires. Using a pry bar or a powerful screwdriver, the bends of the wire loops are pry up and pulled out several wires at a time. In this case, the end of the engine housing serves as a stop, creating a lever. The wires come out easier if you burn them first.

You can burn with a blowtorch, directing the jet strictly along the groove. Care must be taken not to overheat the stator iron, otherwise it will lose its electrical qualities. The metal case is then easy to destroy - a few blows of a good hammer, and it will crack - the main thing is not to overdo it.

When removing the housing, you should immediately pay attention to the method of fastening the set of magnetic circuit plates. The plates can be fastened together in a single package, for example by welding, or simply placed in the body and clamped at the end with a lock washer. In the latter case, when the windings are removed and the case is destroyed, the unfastened magnetic circuit will crumble into plates. To prevent this from happening, even before the complete destruction of the body, the plate pack must be fastened together. They can be pulled together with studs through the grooves or welded with longitudinal seams, but only on one side - the outer side, although the latter is less desirable, since parasitic Foucault currents will increase.

If the motor core ring is bonded and separated from the windings and housing, then it is tightly insulated as usual. Sometimes you can hear that the remaining grooves of the windings must be filled with iron, supposedly to increase the area of ​​​​the magnetic circuit. In no case should this be done: otherwise the properties of the transformer will deteriorate sharply, it will begin to consume an exorbitantly large current, and its magnetic circuit will be very hot even in idle mode.

The stator ring has impressive dimensions: an inner diameter of about 150 mm, in which you can lay a wire of a significant cross section without worrying about space.

The cross-sectional area of ​​the magnetic circuit periodically changes along the length of the ring due to the grooves: inside the groove, its value is much smaller. It is this smaller value that should be guided by when calculating the number of turns of the primary winding (Fig. 7).

For example, I will give the parameters of a real-life CT made from the stator of an electric motor. For it, an asynchronous motor with a power of 4,18 kV•A was used with an inner diameter of the magnetic circuit ring of 150 mm, an outer diameter of 240 mm and a height of the magnetic circuit ring of 122 mm. The effective cross-sectional area of ​​the magnetic core is 29 cm2. The set of plates of the magnetic circuit was initially not fastened, so it had to be welded with eight longitudinal seams along the outer side of the ring. As we feared, the welds did not cause any pronounced negative consequences associated with Foucault currents. The primary winding of the toroidal ST has 315 turns of copper wire with a diameter of 2,2 mm, the secondary is designed for a voltage of 50 V. The primary winding is wound in more than two layers, the secondary is laid on 3/4 of the length of the ring. ST in arc mode develops a current of 180-200 A at a supply voltage of 230 V.

When winding the secondary winding of a toroidal MT, it is desirable to lay it so that it does not overlap the last part of the primary, then the primary winding can always be wound up or unwound during the final adjustment of the MT.

Such a transformer can also be wound with windings spaced apart on different arms (Fig. 8). In this case, there is always access to each of them.

Welding transformer from television transformers

All the designs of welding transformers described above have common drawbacks: the need to wind the wire, each time pulling the turns through the window, as well as the lack of magnetic circuit material - after all, not everyone can get rings from LATR or a suitable stator from an electric motor. Therefore, I designed and manufactured a CT of my own design, which does not require scarce materials. It does not have these disadvantages, and it is easy to implement at home. As a starting material for this design, a very common material is used - parts from television transformers.

In old domestic color TVs, large, weighty network transformers were used, for example, TS-270, TS-310, ST270. These transformers have U-shaped magnetic circuits, they can be easily disassembled by unscrewing just two nuts on the tie rods, and the magnetic circuit breaks into two halves. For older transformers TS-270, TS-310, the cross section of the magnetic circuit has dimensions of 2x5 cm, S=10 cm2, and for newer transformers TS-270, the cross section of the magnetopropod S=11,25 cm2 with dimensions of 2,5x4,5 cm. the window width of older transformers is several millimeters larger.

Older transformers are wound with copper wire, from their primary windings a wire with a diameter of 0,8 mm can be useful.

The new transformers are wound with aluminum wire. Today, this good in droves migrates to landfills, so problems with their acquisition are unlikely to arise. A few old or burned-out transformers can be bought inexpensively at any telerepair shop. Here are their magnetic circuits (together with their own frames), with minor alterations, can be used to manufacture ST. For ST, you will need three identical transformers from TVs, while the total area of ​​\u30b\u34btheir combined magnetic circuit will be 2-9 cm1,2,3. How to connect them together is shown in Fig. XNUMX, where XNUMX are magnetic cores with frames from television transformers. Three separate U-shaped cores are connected end-to-end to each other and pulled together with the same frame clamps. In this case, the parts of the metal frames protruding beyond the end must be cut off: on the central magnetic circuit on both sides, on the side ones - only on one inner side.

The result is a single large-section magnetic circuit, which is easy to assemble and disassemble. When disassembling television transformers, it is necessary to immediately mark the adjacent sides of the magnetic circuits so that during assembly the halves of different cores are not confused. They must fit in exactly the same position as they were assembled at the factory.

The window volume of the resulting magnetic circuit allows using a wire up to 1,5 mm in diameter for the primary winding, and a rectangular section of 10 mm2 or a stranded wire made from a bundle of thin wires with a diameter of 0,6-0,8 mm of the same section for the secondary bus. This, of course, is not enough for a full-fledged ST, however, it justifies itself in cases of short work, given the low cost of manufacturing this design.

The windings are wound on cardboard frames separately from the magnetic core. A cardboard frame can be made from a pair of "native" transformer frames, throwing out the side cheeks from one narrow side, and instead glue the wide cheeks together using additional strips of hard cardboard. When winding cardboard frames inside, it is imperative to tightly enclose several scraps of wooden planks, but not just one, otherwise the winding will tighten it and it will not come out back. Windings must be laid turn to turn, as tightly as possible. From the outside, after the first layer of wire and then every two, it is necessary to insert wooden inserts (Fig. 10) to ensure gaps and ventilation of the windings.

The secondary winding is best made from a 10 mm2 rectangular busbar, as it will take up the least volume. If you don’t have a bus, and you decide to make a secondary winding wire from a bundle of thin wires lying around, as described above, then be prepared for possible difficulties with its installation. In the case of a stranded wire of the secondary winding, it may turn out that it does not "fit" into the prescribed volume of the frame: mainly due to warping of the springy turns, but it is better not to pull them together, since the frame will collapse. In this case, you will have to completely abandon the cardboard frame.

The secondary winding must be wound on the magnetic circuit already assembled, with the primary winding coil installed, pulling each of its turns through the window. On a rigid magnetic circuit, a flexible wire can be pulled much tighter than on a cardboard frame, and more turns will enter the window.

When assembling the magnetic circuit, special attention should be paid to the reliability of fastening and tight fit of the individual halves of the PU-shaped core. As already mentioned, the mating halves of the magnetic circuit must be from the same transformers and installed on the same sides as at the factory. Under the nuts of the tie-down studs, it is imperative to place large-diameter washers and a grower. On my CT, the primary contains 250 turns of 1,5 mm varnished wire, the secondary contains 65 turns of 10 mm2 stranded wire, which provides an output of 55 V at a mains voltage of 230 V. With these data, the no-load current is 450 mA; current in arc mode in the secondary circuit 60-70 A; arcing performance is good. It was assembled on the basis of ST-270 parts. The welding transformer is used to work with an electrode with a diameter of 2 mm, the "three" burns steadily, but weakly on it.

The advantages of ST of this type are ease of manufacture and the prevalence of material for it. The main disadvantage is the imperfection of the magnetic circuit, which has a compressed gap between the two halves. During factory production, for transformers of this type, the gaps of the magnetic circuit are filled with a special filler. At home, they have to be pulled "dry", which, of course, worsens the characteristics and efficiency of the transformer. It is not possible to lay thick wires in a small volume window, which greatly reduces the coefficient of MT operation. It should be noted that the primary winding of this ST is heated more strongly than, for example, the winding with the same wire of the ST on LATRs - "eared". This is affected, firstly, by a large number of turns of the windings and, probably, by the imperfection of the magnetic system of the transformer. Nevertheless, ST can be successfully used for auxiliary purposes, especially for welding thin automotive metal. It is characterized by particularly compact dimensions and a low weight of 14,5 kg.

Other types of welding transformers

ST, in addition to special production, can be obtained by converting ready-made transformers for various purposes. Powerful transformers of a suitable type are used to create networks with a voltage of 36, 40 V, usually in places with increased fire hazard, humidity and for other needs. For these purposes, different types of transformers are used: different capacities, included in 220, 380 V in a single or three-phase circuit. The most powerful of the portable types, as a rule, have a power of up to 2,5 kVA. The wire and iron of such transformers are selected by power, based on continuous operation (current density 2-4 A / mm2), so they have significant cross sections. In the arc welding mode, the transformer is able to develop power several times higher than the nominal one, and its wire fearlessly endures short-term current overloads.

If you have a powerful single-phase transformer with terminals for switching on 220/380 V and a 36 V output (possibly 12 V), then there are no problems with its connection. You may have to wind a few turns of the secondary winding to increase the output voltage. Suitable transformers with a primary wire diameter of about 2 mm, having a magnetic circuit area of ​​up to 60 cm2.

There are transformers for a voltage of 36 V, designed to be connected to a three-phase 380 V network. Transformers with a power of 2,5 kVA are well suited for conversion, and a power of 1,25 and 1,5 kVA can only be used in short-term mode, since their windings with significant overloads quickly overheat.

To use three-phase transformers from a single-phase 220 V network, their windings must be connected to each other in a different way. Then, with a good voltage in the network, the power of the received ST will be enough to work with a D4 mm electrode.

Three-phase transformers were made on a W-shaped magnetic circuit with a cross section of one arm of at least 25 cm2 (Fig. 11).

Two windings are wound on each arm - inside the primary and secondary on top of it. Thus, the transformer has six windings. First you need to disconnect the windings from the previous circuit and find the beginning and end of each. In this case, the coils of the middle arm will not be needed at all, only the windings on the extreme arms will work. Two primary windings from the extreme shoulders must be connected to each other in parallel. Due to the fact that the magnetic flux must circulate in the magnetic circuit in one direction, the coils on opposite arms must create flows in opposite directions relative to, for example, the axis of the central arm: one up, the other down. Since the coils are wound in the same way, the currents in them must flow in opposite directions. This means that you need to connect them in parallel with different ends: connect the beginning of the 1st with the end of the 2nd, the end of the 1st with the beginning of the 2nd (Fig. 12).

Secondary windings are connected in series with each other by ends or beginnings (Fig. 12). If the windings are connected correctly, then the output voltage x.x. should not exceed 50V.

Transformers of this type are often built into a convenient metal case with handles and a hinged lid. Converting them into welding machines is quite common.

Most industrial single-phase MTs are made according to the U-shaped scheme, the magnetic circuit of which is assembled from a set of rectangular plates of the appropriate length and width. The windings on the U-shaped magnetic core can be arranged in two ways: in the first (Fig. 13, a) the transformer has a high efficiency, in the second (Fig. 13, b) the ST is easier to manufacture, and then, if necessary, add or remove some the number of turns in the already assembled transformer. In this case, the transformer is easier to repair, since only one winding burns out, and the second usually remains intact. When using the circuit (Fig. 13, a), when one winding ignites, the second one is always charred.

If there are suitable plates made of transformer iron, then ST on a U-shaped magnetic circuit is easy to make on your own. The windings are wound separately on the frame, and then installed on the assembled magnetic circuit. How a U-shaped magnetic circuit is assembled is easiest to see by disassembling any small transformer of a similar design. In large transformers, the plates are installed not through one, but in packages of 3-4 pieces, this is faster.

The magnetic circuit for ST can be used, for example, from U-shaped transformers removed from old equipment, if they have sufficient window volume and cross section of the magnetic circuit. But, as a rule, most instrument transformers have limited dimensions. It makes sense to assemble one magnetic circuit from two identical transformers, thus increasing the cross-sectional area. An increase in the cross section of the magnetic circuit gives a gain in turns: now they will have to be wound much less. And the fewer turns, the smaller the window can be installed windings. The reasonable limit is 5060 cm2.

ST can be made on an W-shaped magnetic core, provided that the required number of turns of thick winding wires fit into its windows. The author made a CT from the magnetic cores of two identical W-shaped transformers with the outer dimensions of the W-shaped plate 122x182 mm and the window size 31x90 mm. The cross-sectional area of ​​the magnetic circuit folded from a set of plates from two transformers exceeded 60 cm2, which made it possible to reduce the number of turns of its windings to a minimum. The primary winding of 176 turns of wire D1,68 mm and the secondary winding into two wires D2,5 mm with an output voltage of 46 V came close to each other. At a mains voltage of 235 V, the ST developed an arc current of 160 A, although it heated up more than we would like .. .

As a rule, the cores of industrial transformers folded from plates can be easily disassembled: it is not difficult to remove old wires and wind new windings. Sometimes it makes sense to first install a secondary winding (low voltage) on the W-shaped magnetic circuit, and a primary winding (high voltage) on top of it. The characteristics of the CT do not deteriorate from this, but many problems are avoided. The number of turns of the secondary winding can be very approximate, focused on 40-60 V. The turns of the primary winding will have to be selected, adjusting the ST to the desired power. So, having calculated and laid first the low voltage winding, focusing on approximately 50 V, then you can always remove or add a certain number of turns from the upper primary winding of an already finished ST.

In units and equipment that have served their "life" you can find quite powerful and large transformers.

For stationary transformers, the limiting capabilities of either iron or winding wires are never used - everything is done with a margin. Wires often have significant cross sections, as they are designed for a current density 3-4 times less than that allowed for ST. Very often, large transformers have many secondary windings designed for different voltages and powers. The primary winding in the transformer is always one, and its wire is designed for full power. In this case, you can leave the primary winding completely or partially unwind, and remove all secondary windings by winding one thick wire in their place. If the primary winding is also unsuitable, but the magnetic circuit itself is suitable for the manufacture of ST, then all the windings will have to be wound.

In equipment, low voltages are more often used - 12; 27 V. Therefore, powerful transformers wound with thick wire can have an output of 2x12 V, 27 V and others, which are clearly insufficient for use as a ST. If there are two such transformers, then they can be combined, without redoing, into one welding one. To do this, the primary windings are connected in parallel, and the secondary windings are connected in series, and their voltages are summed up.

It may turn out that such a combined MT will have a poor, close to hard, characteristic. To correct the characteristics, it is necessary to include in the secondary winding circuit, in series with the arc, a ballast resistance - a piece of nichrome or other high-resistance wire. Possessing a resistance of the order of hundredths of an ohm, it will somewhat reduce the power of the ST, but it will allow you to work in manual mode.

Welding transformer current adjustment

An important design feature of any welding machine is the ability to adjust the operating current.

There are various ways to adjust the CT current. The easiest way to wind the windings is to make them with taps and, by switching the number of turns, change the current. However, this method can only be used to adjust the current, rather than adjust it over a wide range. Indeed, in order to reduce the current by 2-3 times, it will be necessary to increase the number of turns of the primary winding too much, which will inevitably lead to a voltage drop in the secondary circuit.

In industrial devices, different methods of current regulation are used: shunting with the help of various types of chokes; change in the magnetic flux due to the mobility of the windings or magnetic shunting, etc.; the use of stores of active ballast resistances and rheostats; the use of thyristor, triac and other electronic power control circuits. Most industrial power control schemes are too complex to be fully implemented on homemade PTs. Let's consider the simplified methods actually used in home-made performance.

Recently, thyristor and triac power control circuits have gained some distribution.

Usually a triac is included in the primary winding circuit, a thyristor can only be used at the output. Power regulation occurs by the method of periodic shutdown for a fixed period of time of the primary or secondary winding of the ST at each half-cycle of current; the average value of the current decreases. Naturally, the current and voltage after that have a non-sinusoidal shape. Such schemes allow you to adjust the power over a wide range. A person versed in radio electronics can make such a circuit on his own, although this is very difficult.

In different magazines you can find many very simple circuits with the same principle of operation, consisting of only a few parts. They are intended mainly for adjusting the incandescence of light bulbs and electric heaters. As power regulators for ST, these circuits are of little use. Most of them are unstable: their scales are not linear, and the calibration changes with the mains voltage, the current through the thyristor gradually increases during operation due to the heating of the circuit elements, in addition, the output power of the ST is usually strongly extinguished even at the maximum unlocking position of the regulator.

Do not be surprised if, when connecting the triac circuit to the primary winding, the ST starts to "knock" already at cold speed. This knock is heard in the literal sense of the word, moreover, from ST, who previously worked on x.x. practically silent. This is not surprising, because with each unlocking of the triac, an instantaneous increase in voltage occurs, causing powerful short-term pulses of self-induction EMF and surges in the consumed current. Industrial apparatus, wound with thick wire in reliable insulation, endure this power defect without any consequences. For "feeble" home-made designs, I would not recommend using a triac for the primary winding.

For homemade designs, it is better to use a triac or thyristor regulator in the secondary winding circuit. This will save the ST from unnecessary loads. Almost the same circuit, but with a more powerful device, is suitable for this, although the arc burning process is somewhat worse when using regulators of this type. After all, now with a decrease in power, the arc begins to burn with separate increasingly short-term flashes. This method of adjusting the current, due to the complexity of manufacturing and low reliability, has not become widespread for home-made STs.

The most widely used is a very simple and reliable way to adjust the current using the ballast resistance included at the output of the secondary winding. Its resistance is on the order of hundredths, tenths of an ohm, and it is selected experimentally.

For these purposes, powerful wire resistances used in cranes and trolleybuses, or segments of heating element spirals (thermal electric heater), pieces of thick high-resistance wire have long been used. It is possible to reduce the current somewhat even with the help of a stretched steel door spring. The ballast resistance can be switched on stationary (Fig. 14) or so that later it would be relatively easy to select the desired current. Most high-power wire resistors are made in the form of an open spiral, mounted on a ceramic frame up to half a meter long, as a rule, wire from heating elements is also wound into a spiral.

One end of such a resistance is connected to the output of the ST, and the end of the "ground" wire or the electrode holder is equipped with a removable clamp, which is easy to transfer along the length of the resistance spiral, choosing the desired current (Fig. 15). The industry produces special resistance boxes with switches and powerful rheostats for STs. The disadvantages of this method of adjustment include the bulkiness of the resistances, their strong heating during operation, and inconvenience when switching.

But on the other hand, ballast resistances, although often having a rough and primitive design, improve the dynamic characteristic of the ST, shifting it towards a steeply falling one. There are STs that work extremely unsatisfactorily without ballast resistance.

In industrial devices, current regulation by turning on active resistance has not found distribution due to their bulkiness and heating. But reactive shunting is very widely used - inclusion in the secondary circuit of the inductor. Inductors have a variety of designs, often combined with the CT magnetic circuit as a whole, but they are made in such a way that their inductance, which means reactance, is mainly controlled by moving parts of the magnetic circuit.

At the same time, the throttle improves the arc burning process. Due to the design complexity, chokes are not used in the secondary circuit of self-made STs.

Adjusting the current in the secondary circuit of the ST is associated with certain problems. Thus, significant currents pass through the control device, which leads to its bulkiness. In addition, for the secondary circuit it is almost impossible to select such powerful standard switches that they can withstand currents up to 200 A. Another thing is the primary winding circuit, where the currents are five times less, the switches for which are consumer goods. Active and reactive resistances can be connected in series with the primary winding. Only in this case, the resistance of the resistors and the inductance of the inductors must be significantly greater than in the secondary winding circuit.

So, a battery of several PEV-50 ... 100 resistors connected in parallel with a total resistance of 6-8 Ohms is capable of reducing the output current of 100 A by half. You can collect several batteries and install a switch. If there is no powerful switch at your disposal, then you can get by with several.

By setting the resistors according to the scheme (Fig. 16), you can achieve a combination of 0; 4; 6; 10 ohm. Instead of resistors that will get very hot during operation, you can install a reactance choke.

The inductor can be wound on a frame from a 200-300 W transformer, for example, from a TV, by making taps every 40-60 turns connected to the switch (Fig. 17). You can turn off the power by turning on the secondary winding of a transformer (200-300 W) with a secondary winding rated at about 40 V as a choke. The choke can also be made on an open - straight core.

This is convenient when there is already a ready-made coil with 200-400 turns of suitable wire. Then inside it you need to fill a package of straight plates of transformer iron. The required reactance is selected depending on the thickness of the package, guided by the welding current ST.

For example: a choke made from a coil containing presumably about 400 turns of wire with a diameter of 1,4 mm is packed with an iron package with a total cross section of 4,5 cm2, a length equal to the length of the coil, 14 cm. This made it possible to reduce the CT current to 120 A, t .e. about 2 times. A choke of this type can also be made with a continuously adjustable reactance. It is necessary to make a structure for adjusting the depth of insertion of the core rod into the cavity of the coil (Fig. 18, where 1 is the core; 2 is the latch; 3 is the coil). A coil without a core has negligible resistance, with the core fully inserted, its resistance is maximum. The inductor, wound with a suitable wire, heats up a little, but its core vibrates strongly. This must be taken into account when screeding and fixing a set of iron plates.

It should be noted that for transformers with small currents x.x. (0,1 ... 0,2 A), the above-described resistances in the primary winding circuit have practically no effect on the output voltage of the c.x. ST, and this does not affect the ignition process of the arc. ST with current x.x. 1-2 A when ballast resistance is introduced into the primary circuit, the output voltage decreases already significantly. From my own experience, I can say that active and reactive resistances added in series to the primary winding do not have any pronounced negative effects on the ignition and burning of the arc.

Although the quality of the arc is still degraded, compared with the inclusion of a quenching resistor in the secondary winding circuit.

CTs can also combine regulators or current limiters of different types. For example, you can use the switching of turns of the primary winding in combination with the connection of an additional resistor or in another way.

Reliability of the welding transformer

The reliability of the welding machine depends both on design factors and on the mode and operating conditions. Reliable, carefully manufactured transformers work for many years, without problems withstanding short overloads and flaws in operation. Lightweight portable structures, with varnished wires, and even developing exorbitant power for themselves, as a rule, do not live long. They gradually wear out in the same way as, for example, clothes or shoes wear out over time. Although, given the significant amount of work performed and the low cost of their manufacture, this fully justifies their existence.

CT's worst enemies are overheating and moisture penetration. The most effective means against overheating are reliable winding wires with a current density of not more than 5-7 A / mm2. In order for the wire to cool quickly, it must have good contact with air. To do this, slots are made in the windings (Fig. 19).

First, the first layer is wound and wooden or getinax strips 5-10 mm thick are inserted from the outer sides, then the strips are inserted every two layers of wire: so each layer has contact with air on one side. If the ST is installed without airflow, then the slots should be oriented vertically. Then air will constantly circulate through them: warm air rises, and cold air is sucked in from below. It is even better if the CT is constantly blown by a fan. In fact, forced airflow has little effect on the heating rate of the transformer, but it significantly speeds up its cooling.

Toroidal transformers heat up the fastest and cool the worst. For a very hot ST, even a powerful airflow will not solve this problem, and here it will be necessary to maintain the temperature of the windings with a moderate mode of operation. The number of turns of the windings also affects the cooling of the transformer: the fewer turns, the higher it is.

In addition to the objective and understandable reasons for the failure of welding transformers, mainly related to the imperfection of the design, based on my experience, I want to note one more, seemingly implicit, but nevertheless very common way: how to ruin the ST.

The reason in this case, oddly enough, is the voltage drop in the mains ... ST ceases to weld normally if the mains voltage drops sharply or the power line has a significant intrinsic resistance of the order of several ohms. Unfortunately, both the first and the second are widespread in our country.

If, with a drop in voltage, you can at least accurately find out the cause by taking a voltmeter and measuring the voltage, then in the second case the situation is more complicated: a high-resistance voltmeter does not feel line resistance of several ohms and shows normal voltage, but these few ohms can easily turn off the power of the ST, its own whose resistance in the arc mode is negligible. But what does the drop in power to the "combustion" of ST have to do with it? And here's the thing. When the owner of the "welding", having suffered quite a lot with the device that is not working from the 220 V network, realizes that he cannot change anything, but work oh, as it should be: earnings are lost or construction is underway, the solution is freezing, ... then in such cases very often the device is connected to a 380 V network.

The fact is that all wiring is usually done from a three-phase line: "zero" and three "phases". If you connect to "zero" and one "phase" - phase voltage, then this is the usual 220 V. If you connect to "phase" and "phase" - line voltage, then 380 V will be removed from two wires. Namely, make negligent welders with single-phase machines rated for 220 V.

At the same time, ST starts to work perfectly, however, very often for a very short time. They "fire" as weak home-made designs, as well as reliable industrial devices. And everything is very simple: if the voltage in the common electrical network drops, for example, by 50 V, and the device does not want to work from 170 V, then 330 V remains between the "phases", which is deadly for any ST ...

Often, the owners of welding machines are simply too lazy to endure their “weldings” once again: after all, the mass is considerable, and they remain on the street, get wet in the rain, they are covered with snow ... the whole structure falls apart.

But still, the main enemy of ST is overheating. Well, if you have to weld a lot and quickly, and the ST is wound with not so hot wires and heats up catastrophically quickly, ... you can offer one cardinal means of combating overheating.

You can not be afraid of overheating if the entire transformer is completely immersed in transformer oil. Having a significant thermal conductivity, the oil not only removes heat from the windings, but also acts as an additional insulator. In its simplest form, this is a bucket of oil with a ST recessed in it, from where only four wires come out, such a "miracle" can sometimes be seen in rural yards. Some transformer oil can be drained, for example, from old refrigeration units. Although people say that in case of emergency, other types are also suitable, up to sunflower ... I don’t know about sunflower, I didn’t check it myself.

Another important design element of the CT is the outer casing. When installing the CT in the case, special attention should be paid to its material and the possibility of air flow for cooling, while the top must be closed, protecting the transformer from rain. Cases or at least some of their parts are best made from non-magnetic materials (brass, duralumin, getinaks, plastics). CT creates a powerful magnetic field that attracts steel elements to it. If the case is made of tin or steel panels are screwed opposite the axis of the primary winding, then during operation this entire structure will be drawn inward and vibrate. The sound at the same time is sometimes such that it can only be compared with the work of a saw of a powerful "circular". Therefore, it is possible to install the ST either in a solidly curved rigid steel case, which is not so susceptible to vibrations, or to make panels opposite at least the primary winding from non-magnetic materials.

You can install a fan in the case or make it airtight and fill it with transformer oil.

And finally, the last recommendation. If you nevertheless made a ST, but are a beginner in welding, then it is better to invite a specialist to test it. Welding is a very difficult task, and a person without experience is unlikely to succeed immediately. Be sure to purchase or make a mask with glass number C-4 or E2. The electric arc emits powerful ultraviolet radiation, which negatively affects the skin and, first of all, the eyes. When the eyes are damaged, a yellow spot appears in the field of vision, which then gradually disappears, they say "catch a bunny."

If you manage to "catch" two such "bunnies" in a row at once, then immediately stop all experiments with an electric arc. When several "bunnies" appear before the eyes, they, as a rule, then disappear, and the person calms down, but later, after a few hours, this phenomenon is fraught with such consequences that it is better not to experience it yourself.

Author: I.Zubal

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