We make a welding transformer ourselves

General issues of manufacturing welding transformers

   The 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 while being able to work almost anywhere there is a power grid. It seems that there are no problems with the choice and purchase of welding machines today. A lot of household and professional industrial welding machines are on sale. Vying with each other to offer their products and all kinds of handicraft workshops and craftsmen. But only the prices for factory apparatus "bite", as a rule, several times, exceeding the current average monthly earnings. Basically, it is this sad discrepancy between their own wealth and the price that always forces many people to take up welding with their own hands.

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  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. I offer the most important thing: how and from what at home make welding transformers... All the schemes of welding transformers described in the following have passed practical tests and are really suitable for manual electric welding. Some of the schemes have been worked out "among the people" for decades and have become a kind of "classics" of independent "transformer building".
  Like any transformer, a welding transformer consists of a primary and a secondary (possibly with taps) windings wound on a large magnetic core made of transformer iron. Welding is distinguished from a conventional transformer by the operating mode: it operates in an arc mode, i.e. in the mode of practically maximum possible power. And hence the strong vibrations, intense heating, the need to use a large cross-section wire. Such a transformer is powered from a single-phase 220-240 V network. The output voltage of the secondary winding in no-load mode (h.x.) (when no load is connected to the output) for self-made welders is, as a rule, in the range of 45-50 V, less often up to 70 V. In general, the output voltages for industrial welding machines are limited (80 V AC, 90 V DC). Therefore, large stationary units have an output of 60-80 V.
      The main power characteristic of the welding transformer it is customary to consider 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 size of the gap is 0.5 ... 1.1 d (d is the diameter of the electrode), it is manually maintained. For portable structures, operating currents are 40-200 A. The welding current is determined by the power of the transformer. The choice of the diameter of the electrodes used and the optimal thickness of the metal to be welded depend on the output current of the welding transformer.
     The most common are electrodes with 3 mm steel rods ("three"), which require currents of 90-150 A (more often 100-130 A). In skillful hands, the "three" will burn at 75 A. At currents above 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 a 3 mm electrode, a current of 20-30 A flows through the primary winding of the transformer (usually about 25 A).
     If the output current is lower than the required one, then the electrodes begin to "stick" or "stick", welding with their tips to the metal being welded: for example, the shunt transformer starts to work with a dangerous overload in the short circuit mode. At currents higher than permissible, the electrodes begin to cut the material: this way you can ruin the entire product.
     For electrodes with an iron rod of 2 mm, a current of 40-80 A (usually 50-70 A) is required. They can gently weld thin steel with a thickness of 1-2 mm. 4 mm electrodes work well at a current of 150-200 A. Higher currents are used for less common (05-6 mm) electrodes and metal cutting.
     In addition to power, an important property of a welding transformer is its dynamic characteristics. Transformer dynamic response largely determines the stability of arc burning, and hence the quality of welded joints. From the dynamic characteristics, one can distinguish steeply dipping and gently dipping. In manual welding, inevitable oscillations of the end of the electrode occur and, accordingly, a change in the arc burning length (at the time of arc ignition, when adjusting the arc length, on irregularities, from hand tremors). If the dynamic characteristic of the transformer is steeply falling, then with fluctuations in the arc length, slight changes in the operating current occur in the secondary winding of the transformer: the arc burns stably, the weld lies flat. With a shallow or rigid characteristic of the transformer: 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 welding machine manually. For manual arc welding, a steeply dipping dynamic response of the transformer 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 welding transformers, it is hardly possible. Therefore, in practice, they are divided into those that weld better and which work worse. When the transformer works well, the welders say, "It brews softly." This should be understood as the high quality of the seam, the absence of metal spatter, the arc burns steadily all the time, the metal is deposited evenly. All transformer designs described below are really suitable for manual arc welding.

Welding transformer operating mode

      The operating mode of the welding transformer can be characterized as short-term repetitive. In real conditions, after welding, as a rule, assembly, assembly and other works follow. Therefore, the transformer, after operating in the arc mode, has some time to cool down in idle mode. In arc mode, the welding transformer heats up intensively, and in idle mode it cools down, but much more slowly. The situation is worse when a transformer is used for cutting metal, which is very common. To cut thick rods, sheets, pipes, etc. with an arc, with a not too high current of a home-made transformer, you have to overheat the apparatus too much. Any apparatus of industrial manufacture is characterized by such an important parameter as the coefficient of duration of operation (LR), measured in %. For domestic factory portable vehicles weighing 40-50 kg, PR usually does not exceed 20%. This means that the welding transformer can operate in arc mode no more than 20% total time, the rest 80% must be in idle mode. For most homemade designs, the PR should be taken even less. The intensive mode of operation of the transformer will be considered as such when the time of arc burning is of the same order of magnitude as the time of breaks.
   Homemade welding transformers are performed according to different schemes: on U- and W-shaped magnetic wires or  toroidal, with various combinations of winding arrangements. The transformer manufacturing scheme and the number of turns of future windings are mainly determined by the available magnetic circuit. In the future, the article will consider real diagrams of homemade transformers and materials for them. Now we will determine what winding and insulating materials we need.

    Given the high power, a relatively thick wire is used for the transformer windings. Developing significant currents during operation, any welder 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 transformer occurs. The winding wires can be copper or aluminum. Copper allows the use of 1.5 times higher 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 a copper wire. For homemade options, 10 A / mm2 for copper can be considered a satisfactory result. With an increase in the 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 transformer will heat up to a temperature of 60 ° C after using 2 or 3 electrodes. If you think that you will have to weld a little, not quickly, and you still don’t have the best materials, then you can wind the primary winding with a wire and with a strong overload. Although this, of course, will inevitably reduce the reliability of the device.

    In addition to the cross-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). The best is insulation made of fiberglass, impregnated with 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, PEV 1.6-2.4 mm wires in simple varnish insulation. Such wires are the most common, they can be removed from the coils of chokes, transformers of old equipment. Carefully removing the old wires from the coil frames, it is necessary to monitor the condition of their coating and insulate slightly damaged areas. If the coils of wire were additionally impregnated with varnish, their coils stuck together, and when trying to disconnect, the hardened impregnation often tears off its own varnish coating of the wire, exposing the metal. In rare cases, in the absence of other options, "home-builders" wind the primary windings even with a PVC-insulated mounting wire. Its disadvantages: excess insulation and poor heat dissipation.
   The most attention should always be paid to the quality of the laying of the primary winding of the transformer. 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, when its turn-to-turn closure or insulation breakdown, for example, through ingress of moisture, the entire coil quickly "burns out". As a rule, it is impossible to restore it without disassembling the entire structure.
  The secondary winding 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. These rectangular wires (commonly referred to as bars) are used for industrial welding transformers. However, in most self-made structures, 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 a solid copper wire with a cross section of 16 mm2. 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 PVC 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 old multi-core wires in thick rubber insulation are used, which are used in powerful three-phase cables. The rubber is easy to remove, and instead of it, 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 those with which the primary winding is wound. To do this, 2-5 wires of 1.6-2.5 mm are carefully pulled together with tape and used as one stranded. This multi-wire busbar is small and flexible enough to facilitate installation. If it is difficult to get the required wire, then the secondary winding can be made of thin, most common wires PEV, PEL 0.5-0.8 mm, although this will take an hour or two. To begin with, you need to choose a flat surface where two pegs or hooks are rigidly installed with a distance between them equal to the length of the secondary winding wire of 20-30 m.Then stretch several tens of strands of a thin wire between them without deflection, you get one elongated bundle. Then disconnect one of the ends of the beam from the support and clamp it into the chuck of an electric or hand drill. At low speeds, the entire bundle in a slightly taut state is twisted 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 strip the ends of each wiring separately, and then securely solder everything together. After all, it is advisable to insulate the wire by wrapping it along its entire length with a layer, for example, of scotch tape.
   For laying the windings, fastening the wire, inter-row insulation, insulation 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 designs of welding transformers, the volume of the windows of the magnetic circuit, in which it is necessary to lay several windings with thick wires, is strongly limited. Therefore, in this "vital" space of the magnetic circuit, every millimeter is expensive. With small core sizes, insulating materials should occupy as little volume as possible, i.e. be as thin and elastic as possible. The widespread PVC insulating tape can be excluded immediately from the use on the heating sections of the transformer. Even with a slight overheating, it becomes soft and gradually spreads out or is pressed through by wires, and with significant overheating it melts and foams. For insulation and bandage, you can use fluoroplastic, glass and varnished keeper tapes, and between the rows - ordinary scotch tape. Scotch tape is one of the most convenient insulating materials. After all, having a sticky surface, small thickness, elasticity, it is sufficiently heat-resistant and strong. Moreover, nowadays scotch tape is sold almost everywhere on spools of various widths and diameters. Small diameter coils are perfect for pulling through narrow windows of compact magnetic drives. Two or three layers of scotch tape between the rows of wire practically do not increase the volume of the coils.
   Finally, the most important element of any transformer is the magnetic circuit. As a rule, for homemade products, magnetic cores of old electrical appliances are used, which before that had nothing to do with a welding transformer, 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 flux circulates. Magnetic cores with a cross-sectional area of 25-60 cm2 (usually 30-50 cm2) are suitable for the manufacture of a transformer. 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 circuit, the best performance is at an average power of 30 cm2.
  There are standard methods for calculating the parameters of the magnetic circuit and windings for industrial welder 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 transformer, and only in a single version. 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 a magnetic circuit for the same power can be very wide. There is no connection between an arbitrary section and turns in the standard formulas. For homemade welding transformers, any magnetic cores 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.
   The power of the welding transformer 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. To calculate transformers designed for welding with 3-4 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 transformers with a large magnetic circuit area (more than 50 cm2) and a relatively high efficiency, it can be recommended to increase the number of turns calculated according to the formula by 10-20%. For transformers made on cores with a small area (less than 30 cm), on the contrary, it may be necessary to reduce the number of calculated turns by 10-20%. In addition, the useful power will be determined by a number of factors: efficiency, voltage of the secondary winding, supply voltage in the network. (Practice shows that the mains voltage, depending on the location and time, can fluctuate between 190-250 V). The resistance of the power line is also important. Composing only a unit of Ohm, it practically does not affect the readings of a voltmeter, which has a large resistance, but it can strongly extinguish the power of the transformer. The influence of the line resistance in places remote from transformer substations (for example, summer cottages, garage cooperatives, in rural areas, where lines are laid with thin wires with a large number of connections) can especially affect. Therefore, initially it is hardly possible to accurately calculate the output current 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, choosing the best option for yourself, or adjust to the mains voltage. To obtain higher powers from the welding transformer, for example, to work with a 4 mm electrode at currents greater than 150 A, it is necessary to further 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 the windings. The output current can also be increased slightly by increasing the number of turns of the secondary winding, so that the open circuit output voltage rises from the assumed 50 V to higher values (70-80 V).
   Having included the primary winding in the network, it is necessary to measure the idle run, it should not have much knowledge (0.1-2 A). (When the welding transformer is connected to the network, a short but powerful current surge occurs). In general, by no-load current. it is impossible to judge the output power of the tr: it can be different even for the same types of transformers. However, by examining the curve of the dependence of the no-load current on the voltage supplying the welder, one can more confidently judge the properties of the transformer. To do this, the primary winding must be connected through the LATR, which will allow smoothly changing the voltage on it from 0 to 250 V. The current-voltage characteristics of the transformer in idle mode with different numbers of turns of the primary winding are shown in the figure, where 1 - the winding contains few turns; 2 - the transformer operates at its maximum power; 3, 4 - moderate power. At first, the current curve is hollow, almost linearly increases to a small value, then the rate of increase increases - the curve smoothly bends upward, followed by a rapid increase in current. When the current tends to infinity up to the point of the operating voltage of 240 V (curve 1), this means that the primary winding contains few turns, and it must be wound (it must be borne in mind that the welding transformer, connected to the same voltage without LATR, will consume a current of approximately 30% more). If the point of the operating voltage lies on the bend of the curve, that transformer will deliver its maximum power (curve 2, welding current of about 200 A). Curves 3 and 4 correspond to the case when the transformer has a power resource and an insignificant no-load current: most homemade products are focused on this case. In reality, no-load currents are different for different types of welding transformers: most are in the range of 100-500 mA. I do not recommend setting the no-load current to more than 2 A.

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