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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Quarter-wave electric welding. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / welding equipment Shortwave amateur radio operators and anyone who has ever been seriously interested in radio communication knows that standing waves at high power levels are an unambiguous evil. Once established in the RF power transmission path, standing waves can cause a lot of trouble. For example, disable the power amplifier, burn the cable to the antenna, burn the antenna relay, etc. I'll tell you a story. Once I needed a piece of a 75-ohm coaxial cable exactly 2 m long. I kept a coil of cable in one piece 30 m long. I cut off the desired piece, cut the ends, checked the breakage of the central core with an ohmmeter. I decided that since the piece is from the end of the bay, it can be broken off. Again he cut off the necessary piece, butchered it, checked it - again the breakage of the central core. I thought it was a used cable, lying somewhere in the control room, and it could have been trampled. The other end of the cable should be at the antenna, there is no one to stomp there. Cut off a piece from the other end of the bay. The same thing - a break in the central vein. My patience snapped, I carried the whole bay into the yard and began to cut it. Having cut the bay into 17 pieces and not having received a single good one, I decided to go to the store and buy a new cable. On the way, I thought about how you can burn the cable at the same time in many places. At direct current, the circuit usually burns out in one, the weakest place, other places after that no longer burn. Returning home with a new cable, I decided to remove the entire braid in pieces of the old cable. After that, darkened places and wire breaks of 24 mm were visible through the translucent insulation. The diameter of the central core of the RK-75-4-11 cable is 0,72 mm; in order to burn such a wire, a current of 21 A is needed. The places of burns were located with a certain frequency - a little less than 1 m. Later, I managed to find out that the damaged cable was used as part of a 54 MHz radio station. The wavelength in the cable was 3,66 m (taking into account the shortening factor of 1,52). And then I realized that the cable was "cut" into quarter-wave segments of 0,915 m. I could not find a clear explanation for this effect in the literature. And then I came up with a suitable model, which I propose below.
Initial prerequisites (symbols are shown in Fig. 1): 1) an ideal coaxial line with a uniform distribution of parameters along the length in the load break mode; 2) the insulation between the central core and the braid is ideally electrically strong and cannot be broken through by any voltage; 3) the central core has a small ohmic resistance and has the ability to increase resistance at the place of heating, a uniformly heated core has a uniformly distributed resistance along the entire length; 4) the central core can be burned with high current in a preheated place, in this place a capsule is formed, filled with vapors of the metal of the core; 5) the capsule at the site of burnout breaks through and is ionized by increased voltage, ionization persists for a long time in the capsule, and the conductivity in it increases with increasing current in the ionized gas (arc) and heat release. Repeated breakdowns occur at a much lower voltage than the primary ones. Figure 1 a, b shows the graphs of the distribution of voltages and currents along the length of the line in the extreme mismatch mode (load break or short circuit - the graphs are shifted by λ / 4). In this case, the maxima are called antinodes, and zero values are called nodes. Figure 1c shows an idealized long coaxial line in the standing wave mode (at load break), where the current and voltage antinodes are shown as symbols. They alternate with a period of λ/4 starting from the output end, since there is a complete reflection of the wave. The line is powered by a generator matched to the power transmission line. In the antinodes of the current, uniform heating of the line sections occurs. In this case, the resistance increases in this area and the core can melt and a capsule filled with metal vapors can form. In reality, due to the uneven distribution of cable parameters, the melting of the central core cannot occur in all current antinodes simultaneously.
Therefore, we introduce inhomogeneity into the line. Such heterogeneity can be a manufacturing defect (a decrease in the cross section of the core in a certain place, a dent, an inclusion). So, for example, in the antinode 3λ/4 from the open end of the line, a burn occurred (Fig. 2a) and a capsule filled with metal vapor was formed. Such a break in the line is perceived as a break in the load, the antinode of the voltage is shifted by λ/4, i.e. to the place of the first break and makes a primary breakdown (Fig. 2, b). The ionization in the capsule increases, and the resistance decreases due to the burning of the arc. The voltage antinode shifts again by λ/4, and the current antinode shifts in its place, restoring the conductivity in the gap, i.e. in this place, the plasma arc restores the conductivity of the core. But since the loading end of the line is open, the standing wave is restored in its previous form (Fig. 2, c). The temperature at the site of the section restored in this way increases, and due to heat transfer, it increases the resistance of the core in neighboring sections. In neighboring current antinodes, increased heat is released, which leads to the burning of the core to the right and left by λ / 4 from the place of the first damage, and the voltage antinode is shifted to these places Fig. 2, c. There is a primary breakdown of the gaps, their heating and strong ionization in the formed capsules. At this time, the previously ignited arc is maintained either by current or voltage (alternately as the following line damages occur), and there is increased heating in neighboring sections up to melting, and then the process develops, as shown in Fig. 2, g-g along the entire length cable. We see that the standing wave transfers energy (but not into the load) and releases it on the "loads" organized by it, arranged with a step of λ/4, in the form of melting of the central core. Moreover, at a relatively low generator power, very large values of current and voltage arise in the antinodes. The addition of these split values occurs due to the inertia of the ionized gaps (ionization in capsules is retained for a rather long time). In the case considered above with the RK-75-11 cable with 18 damages with an average gap of 3 mm, such a total gap was about 50 mm.
It is possible to use the energy of a standing wave if the places of formation of antinodes are removed from the power transmission line to its ends. Therefore, we consider the quarter-wave line separately. Figure 3a shows such a line, matched to the power source and load. This is the so-called quarter-wave line transformer, which transforms the load impedance into the line input impedance. Now let us consider extreme mismatch modes within the framework of the model proposed earlier and replace the load with a welding circuit consisting of an electrode holder and an electrode in the form of a welded part as a key with ionization of the gap between the contacts. Figure 3b shows the case of a load break when the electrodes are separated by a distance at which the arc breaks, then the voltage at the end of the electrode forms an antinode with subsequent breakdown of the gap, the discharge of the antinode and the formation of an ionized cloud. Figure 3c shows the case of load closure, in which the arc is extinguished and the electrode "sticks" on the workpiece being welded. In this case, the voltage drops to zero (theoretically), but the electrode current reaches very high values and burns out the closing bridge, and then intensively melts the electrode until the normal mode is reached. Figure 3d shows the case of the normal mode, this is the classic case of power transmission in the traveling wave mode at a matched load, and the matching conditions are also known to us. It is known that the arc burns at a voltage of about 20 V, and the current in it is determined by the cross section of the electrode used. By dividing the voltage by the current according to Ohm's law, we get the load resistance, which should be equal to the wave resistance of the line. It should be noted that for standard coaxial cables this resistance is low and special cables need to be developed. It is necessary to increase the cross section of the central core of the cable, since at currents less than 40 A, the arc burns unstable and does not create a temperature sufficient to melt the steel. Of the design facilitating moments, the following should be noted. A quarter-wave transformer creates almost ideal conditions for starting and burning an arc, equivalent to a steeply falling characteristic in conventional welding transformers, which is usually realized by transferring the operating point of the transformer to the core saturation limit, which is extremely uneconomical and creates huge interference in the lighting network (when the core of a conventional CT is saturated, the current pulses of the primary winding reach hundreds of amperes, the generated thermal power is measured in kilowatts). With quarter-wave electric welding, the arc is maintained by alternating and combining all three modes of operation of the quarter-wave line, since the welding circuit is powered from a power source, most likely, it will have to be done through a matching transformer from a generator operating at higher frequencies. With the help of such a quarter-wave transformer, it is possible to exclude the mode of closing the load of the generator, which will allow the use of transistor circuits of converters. The fact is that a short circuit in the load connected through a quarter-wave transformer is transmitted to the line input in the form of high resistance. But when the welding circuit is broken, the load for the generator is similar to a short circuit. But we have a huge voltage margin on the electrodes. This voltage must be limited at some level for safety reasons. By limiting the voltage on open welding electrodes, we simultaneously reduce the peak load on the generator and can build an optimized system with a power of only a few hundred watts, similar in efficiency to a multi-kilowatt machine in a classic implementation. Theoretically, there is the possibility of quarter-wave electric welding at a frequency of 50 Hz, but in practice it is very expensive. Therefore, the frequency should be raised to at least a few megahertz. In general, the higher the frequency, the simpler and more compact the design can be, but the skin effect begins to appear, which will reduce the depth of welding, and in the microwave it will turn into a "fireworks generator". I suggest quarter-wave electric welding only for sheet material, in which case it can replace KEMP type devices. The skin effect is useful in that it is able to clean the metal surface from oxide films. This film is usually dielectric and has a crystalline structure, and under it there is a region of increased resistance for surface currents, which will cause local heating under the film and at its boundaries, and the temperature difference will destroy the structure of the oxide film (the film will chip off the metal surface), which can be an alternative to fluxes for welding electrodes. Speaking about practical implementation, it should be noted that the physical length of a quarter-wave line in a coaxial version has a significant shortening (unlike twisted wires), and welding cables act as a tuning loop that extends the line so that the quarter-wave segment ends just at the end of the welding electrode.
In the usual inclusion of a coaxial line (Fig. 4, a), its wave impedance ρ is equal to the wave impedance of the cable Z. It is desirable to reduce the wave impedance of the cable line (use, for example, standard 50-ohm cables). If you connect the cable braid parallel to the central core, as shown in Fig. 4, b, then you can reduce the line resistance by 2 times. The cable braid usually has a significant cross section for copper, exceeding the cross section of the central core, although the currents flow through them are the same. I suggest using the cable braid as the secondary winding of the generator output transformer. You can combine the generator output transformer and a quarter-wave transformer on the line (Fig. 4, c), that is, you can simply wind the secondary winding with a coaxial cable that makes up a quarter-wave line. Since the circuit in Fig. 4c is resonant, we can expect the transfer of the energy of the magnetic field of the generator transformer to the electromagnetic field of the coaxial line. Figure 4d shows a diagram of the usual inclusion of a quarter-wave line. Here, the load of the transformer on the cable braid can be obtained by applying the load resistor R, as well as the previously considered cable design. Particularly convenient in this design is that one end of the line is plugged, but it will most likely have to be cooled. Author: Yu.P.Sarazh
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