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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
Welding machine with voltage boost and smooth current adjustment. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Electrician's Handbook Readers are offered a description of a welding machine that is easy to manufacture and reliable in operation. It allows you to weld both direct and alternating current, and in both cases it is possible not only to step, but also to smoothly adjust. To facilitate the ignition of the arc, a voltage boost is provided. There are a wide variety of welding machines on the market today. Portable welding machines (so-called inverters) operate only on direct current. Their cheap models, designed for non-professional use, are relatively small in power and not reliable enough. Welding machines on high-power low-frequency transformers are produced mainly for industrial use. They have, as a rule, high power, significant weight and dimensions, and are relatively expensive. In addition, they allow the possibility of long continuous operation. The welding current in such devices is regulated smoothly or in steps by changing the inductance of the additional choke or the leakage inductance of the welding transformer itself. The large mass and high price make the purchase of such a device for personal (non-professional) use impractical. There are also cheap low-power welding machines on low-frequency transformers on sale. But in the formation of the desired load characteristics, the active resistance of the windings takes part in them. Therefore, such welding machines get very hot during operation. Many people make welding transformers on their own. All that is needed for this is a suitable magnetic circuit and a winding wire. But to perform high-quality welding, a home-made device must provide the ability to select the type of current (direct or alternating) and regulate the welding current. In addition, to facilitate arc ignition at low voltage, it is desirable to have a voltage boost in the apparatus. The following is a description of a simple and reliable welding machine with a transformer based on the stator of an asynchronous three-phase electric motor and ensuring the fulfillment of the above requirements. It has a number of significant features that significantly improve its performance and reduce the complexity of manufacturing compared to those previously described in amateur radio literature and on the Internet. The scheme of the apparatus is shown in fig. 1. Mains voltage through a stepped rheostat, consisting of wire resistors R1-R4 and switch SA1, is supplied to winding I of the welding transformer T2. The node, consisting of a current transformer T1, a rectifier on diodes VD1, VD2 and a measuring head PA1, measures the current consumed from the network. The voltage from the winding II of the transformer T2 through the switch SA2 and a full-wave rectifier on the diodes vD5, VD7 and trinistors VS1, VS2 is fed into the welding circuit.
The rectifier is combined with the welding current regulator. With the extreme right position of the sliders of variable resistors R5 and R6 according to the scheme, the trinistors VS1 and VS2 open at an instantaneous voltage value slightly different from zero on the winding II of the transformer T2. In this case, the current cutoff angle is close to 180 degrees. and the welding current is maximum. When moving the sliders of these resistors to the left, the opening voltage of the trinistors VS1 and VS2 increases, and the current cutoff angle decreases to 90 degrees. As a result, the welding current is reduced by approximately two times compared to the maximum. With a further increase in the resistance of the control resistors, the rectifier trinistors cease to open, so the output voltage and current become equal to zero. Transistor VT1 serves as a control current amplifier. It can be excluded from the circuit, but then the resistance of the resistors R5 and R6 will have to be reduced by about 30 times. At the same time, several watts of power will be dissipated on resistors R5 and R6 in some modes. It is difficult to find variable resistors with a sufficiently large allowable dissipation power, so it was decided to use high-resistance resistors with a transistor current amplifier in the controller. Two variable resistors connected in series made it possible to ensure smooth adjustment of the current over a wide range of its change. In some welding machines, trinistor current controllers are used, which provide a smooth change in the cutoff angle in the range from 0 to 180 degrees, which corresponds to a change in current from zero to maximum. The trinistors in such regulators are controlled, as a rule, with the help of short pulses. But these regulators are more complicated and do not work stably enough for a load with a low differential resistance (a welding arc or a charging battery). Instability manifests itself in the fact that with a constant position of the regulator knob, the output current randomly changes relative to a given average value. Regulators in which SCRs are controlled by direct current work more stably under these conditions. In addition, the welding current regulator must regulate the welding current, but not the amplitude of the output voltage of the welding machine. And when you change the cutoff angle from 90 to 0 degrees. the amplitude of the voltage pulses at the output of the rectifier decreases, which is undesirable, since the conditions for ignition of the arc worsen. To expand the limits of current regulation without complicating the trinistor regulator, the device has a powerful stepped rheostat on resistors R1-R4. Such rheostats are often included in the secondary winding circuit of the welding transformer. But putting it in series with the primary provides several advantages. In particular, the transformer in this case operates at a lower voltage, so it heats up less. In addition, in this case, it is easier to choose a high-resistance wire for the manufacture of rheostat resistors, and as a SA1 switch, you can use a typical package switch for current up to 30 A. The voltage boost circuit is a half-wave rectifier on a VD3 diode, in series with which an EL1 incandescent lamp is connected as a current limiter. In idle mode (when the welding arc is not lit), capacitor C1 is charged through diode VD3 to a voltage of about 76 V at any position of switch SA2. Since the resistance of the cold filament of the lamp is minimal, the capacitor C1 charges quickly. After the arc is ignited, the voltage across the capacitor C1 becomes smaller. In this mode, the current flowing through the VD3 diode is limited by the resistance of the EL1 lamp, which increases as the filament heats up, so the current remains within the allowable range for the diode and only slightly increases the welding current. The booster is a very useful device. In its absence and low open-circuit voltage at the output of the welding machine, the arc is ignited with difficulty, which reduces the productivity of the welder and makes him very tired. Increasing the open circuit voltage without the use of a voltage boost dramatically reduces the efficiency of the welding machine and increases the load on the electrical network. But in many cases, voltage boost units are too complex, and in some cases not effective enough. For example, in [1] this node is designed so that when the arc burns, a rather large current can flow through the voltage boost circuit, limited only by the active resistance of the inductor. To keep this current within acceptable limits, the boost voltage is chosen to be small (10...12 V), which reduces its efficiency. It is desirable that the voltage boost increase the open circuit voltage to 80 ... 90 V. In addition, in the device described in [1], the output current at the moment of arc ignition is limited by the inductive resistance of the inductor, which further complicates its formation. Practice shows that the arc is best ignited when a capacitor is installed at the output of the welding rectifier. The result is slightly worse when the rectifier does not have any smoothing filter at all. But the hardest arc is ignited if the smoothing filter consists only of a choke or ends with a choke. The capacitance of the capacitor C1 must be such as to ensure a quick transition of the spark discharge into a low-power arc. Practice shows that its capacity of 3000 microfarads is enough for this. Such a capacitor cannot smooth out the variable component of the welding current, and there is no need for this. When the welding arc burns, the voltage across the capacitor C1 pulsates from zero to the peak value. Therefore, the capacitor C1 must withstand the voltage ripple with this amplitude. In this case, it must be borne in mind that the permissible amplitude of voltage ripples on oxide capacitors usually does not exceed 10 ... 20% of their rated operating voltage. The question of which smoothing filter is better to use in the rectifier of the welding machine is debatable. Many authors of articles published in magazines and especially on the Internet believe that it is better to use a choke in the rectifier filter of a welding machine. For example, there is an opinion that its presence prevents the electrode from sticking to the workpiece being welded. But the reason for sticking is usually the insufficient power of the welding power source (or the inability to weld). At the same time, a low-power arc melts the electrode and the part a little, and in order to create a powerful arc, the source does not have enough power. As a result, if the electrode accidentally touches the workpiece to be welded, the molten metal of the electrode crystallizes upon contact with the colder workpiece and the electrode is welded to the workpiece. The throttle cannot facilitate the ignition of the arc either, because in idle mode it does not store energy in itself. At the moment the electrode touches the part, the current begins to increase from zero, the inductor begins to store energy. At this time, the energy of the source is not used to create an arc discharge, but is accumulated in the magnetic field of the inductor. In the descriptions of welding machines whose transformers are made on the basis of asynchronous electric motors, it is usually recommended to remove the shroud strips located on the outside of the stator plate package and the protrusions on the inside of these plates. At the same time, the finished transformer is mounted in the body of the welding machine like low-power transformers with toroidal magnetic circuits. But the welding transformer has a large mass, and during operation it can get very hot. The weight of the transformer with this mounting puts pressure on the insulation of the winding wires, which can lead to damage and interturn short circuits. This problem is especially pronounced when the insulation of the wires is not sufficiently heat-resistant. Removal of shroud strips and protrusions of stator plates is a very laborious and not only useless, but even harmful operation. However, it is considered that the shroud strips should be removed so that they do not short-circuit the stator plates. The removal of protrusions is not justified at all. Maybe they do this to increase the area of the magnetic circuit window or slightly reduce the wire consumption. But the fact is that the size of the magnetic circuit window, as a rule, is quite sufficient, and the wire savings are very small. The protrusions of the plates and the bandage are usually removed with a chisel and a hammer. After such a removal, many points of electrical contact are formed between the plates, which can create paths for eddy currents in the magnetic circuit. The magnetic flux in the annular part of the magnetic circuit of the electric motor and transformer flows parallel to the shroud strips, without crossing them, and cannot create eddy currents in them. The only difference is that in the motor stator the flow is divided into two halves, flowing in diametrically opposite sections of the annular magnetic circuit in one direction, and in the transformer a single flow flows through the ring. Therefore, the effective cross section of the same magnetic circuit in the transformer turns out to be approximately two times less than in the engine, and the average length of the power line is greater. As a result, the required number of turns of the transformer winding is greater than the motor winding for the same voltage. It is better to determine it experimentally. The design of the magnetic circuit of the transformer of the proposed welding machine is shown in fig. 2. Banding strips and protrusions of the stator plates are left in place. In order for the turns of the windings not to fall between the projections of the stator plates, two annular plates 5 are attached to the ends of their package 3. Between the projections of the stator plates there are four studs 4 isolated from the stator plates (gaskets are used that were used in the electric motor to isolate the windings). The studs are screwed into posts 2 with internal thread, fixed on a wooden base 1. Therefore, the load from the weight of the transformer is transferred to the base 1 only through the posts 2, and not through the insulation of the wires. This allows you to increase the maximum allowable operating temperature of the transformer without the risk of deformation of the wire insulation and short circuits.
In the upper part of the magnetic circuit on two of the four studs 4 tightening the package, brackets 6 with a handle 7 made of non-magnetic material (for example, aluminum) are fixed. It is desirable to make both brackets 6 and racks 2 from the same material, but there is no great need for this. To leave more space for the winding, you can use only three studs, placing them (in the top view) at the vertices of an equilateral triangle, but then you have to change the design of the handle. The stator of an asynchronous motor with a power of 7,5 kW was used as the actual magnetic circuit. Winding I consists of 305 turns of aluminum wire with a cross section of 4 mm2 in refractory plastic insulation. Winding II is wound with two APV-10 aluminum wires folded together with a cross section of 10 mm2 every. It contains 77 turns. The taps are made from the 48th, 58th and 69th turns. To determine the required number of turns, a test winding was wound on the magnetic circuit and its inductance was measured. The number of winding turns I was then calculated to obtain an inductive reactance of 220 ohms at a frequency of 50 Hz. As a result, the no-load current of the transformer turned out to be about 1 A. Then, based on the required transformation ratio, the number of turns of the winding II was calculated. The current transformer T1 is made on the magnetic circuit from the vertical scan output transformer TVK-110. Its primary winding is one turn of a mounting wire with a cross section of 2,5 mm2. The secondary winding contains 100 turns of PEV-2 wire with a diameter of 0,5 mm. If a pointer avometer with a measurement limit of 1 A is used as a measuring head PA0,5, then its pointer will completely deviate at a current of 100 A through the I winding. changes. As a result, the pointer of a device with a low total deflection current often hits against the stops, which leads to a rapid failure of the measuring mechanism. The current measurement unit can be easily transferred to the winding circuit II of the transformer T2. But there is no great need for this. The transformation ratio is known, and knowing the current in the winding I, the value of the welding current can always be calculated. Resistors R1-R4 of the rheostat are made of three nichrome wires put together from a 2 kW electric heating coil. These resistors can get very hot during the operation of the welding machine, so they are installed on a heat-resistant base made of refractory lightweight bricks with holes through which nichrome wires are passed. To make the rheostat more compact, you can cut the brick into two pieces and use only one half. Instead of a rheostat, you can use a choke with several taps from the winding. But the mass and dimensions of the inductor are much larger than those of a rheostat made of brick and nichrome wire. The expediency of regulating the welding current with a choke depends on several circumstances. For example, when performing a large amount of welding work, the choke will reduce the consumption of electricity and, consequently, its cost, since the active power dissipated by it is negligible. If it is necessary to weld with alternating current, then the welding circuit should be included in the wire break at point A (see fig. 1). In this case, the terminals of the capacitor C1 must be closed with a jumper capable of withstanding the welding current without noticeable heating. In this case, the current regulator works as usual, but there is no voltage boost. Before performing welding work, it is recommended to set the operating mode of the welding machine in the following order. First, depending on the required power of the welding arc, switch SA2 to set the required output voltage, and move the sliders of variable resistors R5 and R6 to the right (according to the diagram) position. Then you should put the switch SA1 in the desired position and, without turning on the device, connect the terminals of the capacitor C1 with a jumper. Having connected the device to the network, using variable resistors R5 and R6, set the short-circuit current to 30 ... 50% more than the required welding current. The short circuit mode should be short-term, no more than 2 ... 3 s, after which you should disconnect the device from the network and remove the jumper from the terminals of the capacitor C1. Now you can turn on the machine again and start welding. In the future, variable resistors R5 and R6, if necessary, you can adjust the current. Typical welding modes for various parts are given in special literature. The trinistor controller used in the described welding machine is similar in terms of output current stability to that described, for example, in [2], but the circuit is much simpler. This is due to the fact that it does not have an additional rectifier to power the control electrode circuit of the trinistor. But it can be introduced by building a welding machine according to the scheme shown in Fig. 3. Additional winding III of transformer T2 must contain 10 turns of mounting wire with a cross section of 1,5 mm2 (for mechanical strength). In this case, the rectified voltage across the resistor R5, smoothed by the capacitor C1, will be about 10 V. The current of the control electrodes of the trinistors will not become pulsating, but constant, depending on the position of the variable resistor R5 engine.
Literature
Author: A. Sergeev
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