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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Manufacturing of a welding transformer. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / welding equipment As a rule, when making a welding machine yourself, the first question that arises is where to get the magnetic core for the transformer. This issue becomes especially acute for rural radio amateurs. The author of the article below talks about his experience in manufacturing a welding transformer. The requirements for the transformer of an electric welding machine are very specific and it is very difficult to satisfy them in a real design, especially if the capabilities of the radio amateur are limited. I managed to find a design version of the magnetic core, which, on the one hand, is quite easy to manufacture and the source materials are readily available, and on the other hand, the technical characteristics of the assembled unit turned out to be quite satisfactory. The magnetic core is made up of six sets of magnetic cores from network transformers TSA-270-1 of old tube color TVs ULPTsT-61, ULPTsTI-61-ll and the like. The magnetic core of the TSA-270-1 transformer - PL25x45 - consists of two U-shaped parts. When assembled, the composite magnetic circuit is a six-petalled daisy (Fig. 1). The top view shows the bottom half of the magnetic circuit; The coil has been conditionally removed. It is easy to see from the drawing that to assemble the magnetic circuit, it is necessary to cut two ribs of the internal elbows of all U-shaped parts 2 and 3 along the entire length at an angle of 30 degrees.
The manufacture of the magnetic core begins with the fact that when disassembling the transformers, all U-shaped parts are marked in such a way that, during the final assembly of the unit, each “own” pair is joined in the same relative position as it was in the transformer initially. This will minimize the air gap between the upper half of the composite magnetic circuit and its lower half. Then they begin to process the inner elbows of the U-shaped parts. To do this, the outer elbow of each of them is tightly wrapped with any non-stretch tape, as shown in Fig. 2. Black adhesive fabric insulating tape is suitable, as well as adhesive mylar tape, known as “scotch tape”.
The inner elbow is clamped into a small homemade simple clamp, bent from a piece of hard sheet 3...3,5 mm thick with a threaded hole MB for a clamping screw. The clamp is installed so that one of the ribs of the inner elbow remains free for processing. These measures - wrapping with tape and clamping with a clamp - are necessary to prevent the U-shaped package from delaminating into separate plates during processing. There are several ways to cut a corner on a metal part, but two of them are the most accessible. The first is to saw off with a rapidly rotating thin corundum disk ("grinder") and the second - the simplest - is to grind off on a regular sharpening machine. The abrasive disc of a sharpening machine must have a medium grain size and a fairly high rotation speed - at least 2800...3000 min-1. You must work extremely carefully, with very little pressure on the disk, avoiding excessive heating of the part. It is very important to observe what is shown in Fig. 2 direction of rotation of the disk relative to the part. Under no circumstances should you work on a sharpening machine that does not have a support tool and a disk guard. Be sure to wear safety glasses while sharpening. After processing one edge of the part, the clamp is moved to this side and the second edge is turned. In order to maintain stitching angles with acceptable accuracy, it is advisable to make several homemade goniometer templates of different shapes. Using the described method, the inner elbow of all twelve U-shaped parts of the future composite magnetic circuit is processed one by one. All treated surfaces should be coated with epoxy varnish. Do not allow varnish to come into contact with the joining surfaces of the U-shaped parts. As soon as the varnish has dried, a test assembly of the composite magnetic circuit is performed and a dodecagonal coupling holder 1 (see Fig. 1), consisting of three identical parts, is made locally for it. It is cut from non-magnetic sheet metal 1 mm thick. Compression of the lower and upper halves of the composite magnetic core in the axial direction is provided by two plates 5 and 7 made of sheet metal with a thickness of 10...12 mm, tightened by six threaded rods 4. On both sides of the magnetic core, six rectangular gaskets 6 made of rigid sheet sponge are installed under the plates rubber 5 mm thick. In each slab, six holes with a diameter of 20 mm are drilled, located around the circumference, and one in the center - 50 mm. These holes are necessary to ensure the flow of convection air currents. In general, special attention should be paid to cooling the transformer, since it will operate in severe thermal conditions. In addition to the ventilation holes, six holes with a diameter of 5,3 mm were drilled in the plates for tie rods (M5) 4. The network winding of a 220 V transformer contains 183 turns of copper wire PEV-2 1,43 (with a current density of about 9 A/mm2). The winding has two taps - from the 150th turn and from the 162nd turn. The secondary winding for a voltage of 40 V consists of 34 turns of wire with a cross-section of 17,64 mm2 with taps from the 22nd, 26th and 30th turns. Additional winding III contains 36 turns of MGShV-0,35 wire. The secondary winding is placed closer to the central rod. It is wound on a wooden template with a 4,2x4,2 mm square wire in cotton insulation. The finished winding is removed from the template and wrapped on top with two or three layers of lacquer tape. To better fix the turns, the coil should be pre-impregnated with epoxy varnish. The winding terminals should be made from copper tape of the appropriate cross-section and carefully soldered as shown in Fig. 3.
To wind the primary winding, you can cut out an individual template, but it is better to use a ready-made secondary winding as it, placing a gasket made of two layers of pressed line on it - this will make it easier to remove the finished winding from the template. If there is no copper wire of the specified cross-section, you can wind it with aluminum enameled wire removed from the coils of disassembled TSA-270-1 transformers. The winding is wound into three wires, laying them turn to turn. It is advisable to impregnate the layers of wire with epoxy varnish as they are laid. Manufacturing winding III usually does not cause problems. It is best to combine it with the secondary one. When all the parts of the transformer are ready, we begin to assemble it. The lower U-shaped parts 1 of the magnetic circuit are installed in the coupling clip 1 (see Fig. 2), then the windings are placed in the annular groove. They must be mutually placed so that air gaps are formed between them and the edges of the magnetic circuit. To fix these gaps, getinaks or wooden spacers of square cross-section, lubricated with epoxy varnish, are inserted into them every 40...50 mm. The lower U-shaped parts 2 are covered with the upper 3, in exact accordance with the markings on them. The screws 8 of the clamping clip uniformly compress all the elements of the magnetic circuit into a single whole in the radial direction, but the compression force has not yet been brought to the maximum. Then the magnetic circuit is placed on the bottom plate 7, not forgetting the rubber gaskets 6, and it is precisely oriented relative to all the ventilation holes. Install rubber gaskets 6 on top and cover with top plate 5. Six threaded rods 4 with nuts are inserted into the corresponding holes of the plates and the elements of the magnetic circuit are evenly tightened in the axial direction. Now all that remains is to finally tighten the screws of the holder 1. Do not compress the holder excessively, otherwise the shape of the U-shaped parts of the magnetic circuit may be disrupted with the appearance of an air gap between them. In conclusion, it is advisable to fill all the gaps between the parts of the central rod with epoxy varnish through the central holes in the plates - this will reduce the hum of the transformer during operation. The construction of a welding transformer was based on the recommendations and calculated ratios set out in the article by V. Baranov “Low-power welding machine” in “Radio”, 1996, No. 7, p. 52 - 54. The ability to regulate the steepness of the drop in the I-V characteristic of the transformer is implemented in the same way as indicated in this article. Author: S. Evdokimov, Novosibirsk
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