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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Repair of imported power transformers. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Power Supplies If the transformer is not working, inspect it carefully. In many cases, the appearance of the transformer and especially its windings allows you to determine the nature of the fault. If the frame is badly melted, or traces of soot or small balls of molten copper are visible, serious repairs cannot be avoided. If the appearance of the transformer is not changed, try to get to the primary winding, if the design of the transformer allows. The fact is that by the color of the wires of the primary winding, especially in the middle of the frame, you can determine how much the transformer was overloaded. If the transformer has been working in heavy duty for a long time, then its primary winding changes its color. In this case, there is a violation of the insulation of the winding wire, a breakdown between the turns, melting and breakage. If the color of the winding has not changed over the entire width, and the winding is melted, then this is the result of a violation of the wire insulation due to natural causes or after switching on at 110 V. In this case, the coil does not melt. If the transformer fails due to a short circuit (short circuit) in the secondary circuits, then, as a rule, the secondary winding also changes. The above signs will allow you to avoid repeated failure of the transformer, if they are taken into account during repairs. Before disassembling the transformer, check if it has a thermal fuse. Figure 1 shows the design of the two most common options for the location of the transformer windings on the frames.
Figure 1a shows a design that makes it easy to check the presence and condition of the thermal fuse. To do this, without disassembling the transformer, remove the protective insulation of the primary winding. If the design of the transformer is as shown in Fig. 1b, i.e. the terminals of the thermal fuse go to the pins of the transformer, it can be checked with a tester for an open circuit. This fuse is usually a one-time action and cannot be restored. If you managed to restore the integrity of the primary circuit, try connecting the transformer to the network through a thin wire, which will serve as a fuse in the event of a short circuit. If the transformer hums a lot and immediately heats up, it will have to be disassembled, and if not, measure the no-load current of the primary circuit. For good transformers of 200-300 W, it is 20-40 mA, and for "squeezed out" low-power ones up to 80-90 mA. For powerful transformers of 200-300 W, this current can be 300-350 mA.
If disassembly of the transformer is necessary, free it from all fittings, and if the plates are not welded, as shown in Fig. 2, try to carefully knock the plates out of the frame. If they are stuck together, do not bend or break the plates - this will not help, especially if they are thin. In this case, the core is sufficiently well heated and easy to disassemble. If the transformer is small, it can be heated by putting it on a hot soldering iron for 15-20 minutes, and if it is not possible to heat the core to 130-150 ° C on a soldering iron, heat it on an electric stove or, in extreme cases, on an inverted iron. At the same time, check the hardness of the compound with which the transformer is glued and do not be afraid for the frame: it can easily withstand such temperatures. It should be noted that in this way it is easy to disassemble glued ferrite cores, including armored ones of the SB type, although in the latter, especially domestic production, the frame can be deformed. If the transformer is welded, as shown in Fig. 2, do not worry. Your task in most cases is simplified or at least solvable: with a hacksaw, as shown in Fig. 3, make cuts along the electric welding seam. Moreover, for the core in Fig. 3, a, it is enough to make one cut and bend the jumper, and for the core in Fig. 3, b, it is better to make cuts on both sides. It will take you no more time than heating the transformer. If, after sawing the welds, the coil from the core is not removed, do not knock on it hard, it is better to warm it up, as mentioned above.
Well, now you have finally disassembled the transformer without damaging the plates and keeping them all to a single This is especially important for miniature cores. If your transformer has a coil design, as shown in Fig. 1, a, proceed to unwind the primary winding. I cannot recommend cutting off the winding with subsequent winding "until the frame is filled". This is the simplest way, but quite often it leads to unpleasant consequences, especially if it is serious equipment. And if the design of the coil is as in Fig. 1, b, and the frame cannot be moved apart due to melting, do not apply great efforts. They break and still can't be taken apart. Wind the secondary winding, count the turns, and then around the perimeter with a hacksaw blade for metal in the places shown in Fig. 1, b, saw through the frame of the secondary winding and remove it, leaving the cheeks of the frame in place, bearing in mind that there is no need to restore the frame: it is sufficient to lay good insulation between the primary and secondary windings. In addition, you will have a margin "over the window" due to the thickness of the removed frame. It is far from always possible to unwind a sintered winding into one wire. Yes, there is no need for this. Even if you unwind 1-2-5, 10-100 turns and write down their number, so that you can sum it up later at 2500-3500 turns, you will be mistaken by no more than 100 turns, which will not radically affect the operation of the transformer. But if you have any doubts, it is better to increase the number of turns of the primary winding by 100 than to decrease by 50. However, when disassembling a low-power transformer, it may turn out that the primary winding was wound in two wires (usually of different enamel colors). In this case, it is necessary to count the turns by their actual number, and when winding in two wires, do not forget to set the counter to half the value. Such transformers are used in very cheap equipment, and this method of winding reduces the cost of it even more, which cannot be said about reliability. However, attempts to wind such a transformer in one wire are most often not justified. Moreover, when winding the coil into two wires, they need to be laid as close as possible to each other, for which I use a piece of polyvinyl chloride tube of the smallest possible diameter, as shown in Fig. 4. In this case, the interturn capacitance of the winding increases, which leads to some increase in reactance, i.e. to a decrease in the no-load current, which in such transformers is already close to critical.
In such transformers, you need to be very careful when choosing the diameter of the winding wire: with a smaller diameter, the transformer does not produce the initial power, so it overloads and overheats, and with a larger diameter, the resistance of the primary circuit decreases, which leads to an increase in the no-load current, and the transformer also overheats and goes out. out of service. I would like to note that in all cases of transformer repair, strive to use domestic winding wires. Their enamel coating is much stronger and more reliable than imported ones. Now you have wound all the windings, and they fit in the frame. It remains only to collect the core. The easiest way is to assemble a welded core that you did not deform when sawing. Deburr it and bond it with epoxy. To avoid magnetic gaps, clamp the core in a vise until the compound polymerizes. If you are assembling the transformer "overlapping", then before the end of the assembly, when the plates are still not very tight, insert two plates in one direction, as shown in Fig. 5. Then at the end of the assembly, when the plates are already difficult to enter along the frame, you can relatively easily insert them between one-sided plates without risking damage to the frame and winding. And only after that install jumpers.
From a low-power transformer, you should not have a single "extra" plate or jumper left. This leads to a sharp increase in the no-load current. Check the transformer without load for compliance with the voltages and especially the no-load current, and only then assemble the fittings, keeping the relative position of the fasteners and transformer leads. It is tempting to repair power transformers of microwave ovens, which, apart from magnetic shunts, practically do not differ from ordinary ones, but their repair is possible only in the manufacture of rigid and precise collapsible frames for winding coils, which is hardly available to a wide range of radio amateurs, and besides, the manufacture of such a frame for repair of one transformer (considering their wide range) is hardly economically justified. Finally, about the "pitfalls" when working with thin winding wires. Who is not aware of the phenomena of a broken wire during winding of a coil or a shortage of wire. Don't be upset. It's OK. If this is a winding of a power transformer wound in bulk, then connect the wire by soldering and continue winding. Pre-insulate the junction, as shown in Fig. 6, with insulating material, preferably with capacitor paper, in thickness corresponding to the diameter of the wire. If necessary, paper can be laid in several layers. In extreme cases, use ordinary high-quality writing paper, preferably coated. Make sure that the connection point on the coil is not from the side of the core window, but from the end: this will save you from the unnecessary problem of placing the winding in the window. If this is a high-voltage coil (for example, an ignition coil or a TVS winding, TDKS), then it is also necessary to get rid of the sharp protrusions of the soldering points.
If you tried, you probably met with another unpleasant phenomenon: a thin wire is not so easy to neatly solder. The place of soldering turns out to be fragile and most often dotted. But even in this case, there is a simple solution. Given that transformers usually contain several thousand turns, you can afford to overspend a few tens of centimeters of wire. Fold the two ends of the broken wire together and cast on a few loops, as shown in Fig. 7, a. Such a beam is already felt in the hands and can be twisted (Fig. 7, b). Now scroll the formed bundle between your fingers, holding the winding wire, stepping down a little in Fig. 7, b, and you will get a beautiful uniform twist of the winding wire, which is easy to tin. Then cut off the excess twist and form the junction, as shown in Fig. 7, c. But keep in mind that when connecting a conductor with a thickness of 0,2 mm or more in this way, make sure that the twists of the twist are connected by soldering, otherwise, with outwardly beautiful soldering, the wires may remain isolated from each other: the wire is not deep in the curls.
And the last. If you wound the winding in two wires, apply the principle: "measure seven times, ...". You must be absolutely sure that the beginning of one conductor is connected to the end of the other, only after that the transformer can be tested. Author: V.M. Paley
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