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Welding machine with electronic adjustment of the welding current. Encyclopedia of radio electronics and electrical engineering

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A feature of the DC transformer welding machine presented in the article is the electronic adjustment of the welding current using a controlled trinistor rectifier. With the appropriate power of the power supply, the device is suitable for welding with coated electrodes up to 4 mm in diameter.

An apparatus for welding ferrous metal products is very useful in a home workshop. There are many such devices on the market, but they are quite expensive. Cheap ones provide only alternating welding current, which degrades the quality of welding. The welding current of such devices is regulated by moving the transformer windings or switching their sections, and this reduces the service life of the device and the efficiency of working with it. The proposed welding machine is devoid of these disadvantages.

Main Specifications

• Supply voltage, V ....... 230
• Current consumption, A ....... 24
• Limits of regulation of welding current, A ....... 40 ... 160
• Open circuit voltage, V ....... 80
• Load duration,%.......60

The scheme of the power part of the device is shown in fig. 1 . Its basis is the transformer T1, which has two secondary windings. Four winding sections III and trinistors VS1 and VS2 form a controlled full-wave rectifier. Compared to a bridge, it has a higher efficiency, requires a smaller cross-section of the secondary winding wire and contains fewer rectifier elements (trinistors).

Rice. 1. Scheme of the power part of the welding machine

The welding current is regulated and stabilized by changing the turn-on delay angle of the trinistors. The rectifier output has a choke L2, which ensures stable arc burning and facilitates its ignition [1].

A rectifier for feeding the arc is assembled on the VD1 diode bridge. Its output voltage is about 80 V. The need for it is due to the following reasons: firstly, at large angles of delay in opening the trinistors of the main rectifier, the arc burns very unstable, and secondly, to facilitate its ignition, the maximum possible voltage must be applied to the electrodes. However, according to the requirements of [2], it should not exceed 80 V. The output of the auxiliary rectifier also has a choke L1. Resistor R2 limits the current of this rectifier to approximately 7 A (with the arc burning). In the case of "sticking" of the electrode, the current increases to 12 A.

Cooling of the device - compulsory, by means of the fan M1. As practice shows, trinistors do not heat up very much even without a fan, but its use allows increasing the relative duration of operation under load (PN) and facilitating the thermal regime of the device, which favorably affects its reliability.

The control unit A1 generates control signals for the trinistors and ensures the stabilization of the welding current, the sensor of which is the current transformer T3. In fact, the block is a phase-pulse controller with feedback on the load current. Its advantages include the absence of a galvanic connection with the rectifier SCRs, as well as the fact that the pulses generated by it arrive at the control electrode of each SCR only when the voltage at its anode is positive relative to the cathode. It should be noted that the last property of the control unit is used only partially due to the presence of an additional arc feed rectifier. The control unit is powered by transformer T2.

Block diagram A1 is shown in fig. 2. On transistors A1.VT1 and A1.VT2, a synchronization unit with mains alternating voltage is made, and each of the transistors opens only in its "own" half-cycle. The pulses from the collectors of the transistors control the sawtooth voltage generator on the logic elements A1.DD2.1 and A1.DD2.2 connected in parallel to increase the load capacity. At the border of half-cycles, when the instantaneous value of the voltage in the network is close to zero, both transistors are closed, and the voltage at the outputs of the elements A1.DD2.1 and A1.DD2.2 has a low logic level. Capacitor A1 .C7 is discharged through the opened diodeA1 .VD11. With the beginning of the next half-cycle, the transistor A1 .VT1 (or A1 .VT2) opens and the charging of the capacitor A1.C7 begins with the current flowing through the resistors A1 .R12 and A1 .R13.

Rice. 2. Scheme of block A1 (click to enlarge)

The resulting sawtooth voltage is applied to the non-inverting input of the OU A1.DA1, which serves as a voltage comparator. Its inverting input receives an exemplary voltage U_arr with trimmer A1 .R15. In each half-cycle, as soon as the voltage at the non-inverting input of the op-amp A1.DA1 exceeds U_arr, a high logic level pulse appears at its output. The delay of the rising drop of this pulse relative to the beginning of the half-cycle depends on the voltage U_arr, and the falling drop is tied to the moment when the mains voltage passes through zero. By changing the exemplary voltage, it is possible to regulate the duration of the open state of the trinistors, and, consequently, the power in the load.

The feedback voltage proportional to the welding current across the resistor R1 rectifies the diode bridge A1.VD5-A1.VD8. The rectified voltage is supplied to the variable resistor R3, which serves as a regulator of this current. The trimmer resistor A1.R15 sets the minimum value of the comparator operation voltage when the slider of the variable resistor R3 is in the position corresponding to the maximum welding current.

While the welding machine is idling, the voltage across the variable resistor R3 is zero. The reference voltage at the inverting input OUA1 .DA1 is minimal, and its output is set to a high logic level. The duration of the open state of trinistors in this mode is maximum, and they work like ordinary diodes.

When the arc is ignited, the voltage at the inverting input of the OU A1.DA1 increases. At its output, high-level pulses appear, the duration of which is the shorter, the greater the welding current. This leads to a decrease in the duration of the open state of the SCRs and the average welding current. It is easy to see that when the welding current is set to the maximum (the slider of the resistor R3 is in the extreme right position according to the diagram), the feedback does not affect the operation of the regulator. In this mode, as in idle, the SCRs work like diodes, and the maximum welding current depends only on the parameters of the transformer T1.

From the output of the OU A1.DA 1, the signal is fed to the arc control unit built on the logic element A1 .DD2.3. The purpose of this node is to block the operation of the regulator when the welding electrode "sticks". For the device, this is a short circuit mode. The voltage from the divider A12.R1, A2.3.R1 is applied to terminal 18 of the A1.DD19 element, which the zener diode A1.VD14 limits to a safe value for the microcircuit (about 9 V). While the load of the device is a welding arc, the voltage at pin 12 of element A1 .DD2.3 corresponds to a high logic level, therefore the voltage level at the output of this element is inverted relative to the output of op-amp A1.DA1. When the output of the op-amp is high, a low level from the output of the element A1.DD2.3 allows the operation of a pulse generator with a frequency of about 5 kHz on the elements A1.DD1.3 and A1.DD1.4.

When the electrode sticks, the voltage at the output of the device drops sharply. At the output of element A1.DD2.3, the level becomes high, prohibiting the operation of the generator. The supply of opening pulses to the trinistors is stopped. The device will remain in this state until the short circuit is eliminated. Trimmer resistor A1.R19 set the operating voltage of the arc control unit.

This node can also be used to control the welding machine using the button [1]. To realize this possibility, it is necessary to break the output circuit 11 of the control unit at point A (see Fig. 1) and install a button with normally open contacts in the gap. Then the controlled rectifier will work only when this button is held down, and the blocking of the device in case of "sticking" of the electrode will remain.

Packets of pulses from the output of the generator, as well as pulses from the collectors of transistors A1.VT1 and A1.VT2, are fed to the logical elements OR-NOT A1.DD1.1 and A1.DD1.2. A high level appears at the output of that element, at both inputs of which the level is low. On fig. 3 shows voltage diagrams at various points of the control unit circuit, as well as at the output of the device (under load).

Rice. 3. Plots of voltage at various points of the circuit of the control unit at the output of the apparatus (under load)

The output signals of the elements A1.DD1.1 and A1.DD1.2 amplify the transistors A1.VI3 and A1.VI4, loaded by the primary windings of the isolation transformers A1.T1 and A1.T2. To protect transistors from self-induction EMF, the primary windings of transformers are shunted by diode-resistive circuits A1.R10, A1.VD10 and A1.R21, A1.VD13.

The control unit is assembled on a printed circuit board made of foil fiberglass in accordance with the drawing in fig. 4. It uses fixed resistors MLT and trimming resistors SP3-38g. Capacitors - K73-17, oxide - any type for the corresponding voltage, for example K50-35. KT315G transistors can be replaced by any low-power silicon transistors of the npn structure, and KT829A - KT972A, KT972B. Diodes 1N4007 are replaced by KD105V, KD247A - by KD226A. Instead of the MB5010 diode bridge, it is allowed to install four separate diodes for a current of at least 25 A, for example, the D132 series. SCRs T160 can be replaced by others rated for a current of 160 A or more, for example, T171-200, T123-200. When replacing, the design features of the trinistors and their cooling should be taken into account.

Rice. 4. A printed circuit board made of foil fiberglass and a drawing of the control unit

Chips of the K561 series can be replaced with their functional counterparts from the K176 or KR1561 series, and the KR544UD1A chip with any op amp with a high input impedance.

The fan motor is a three-phase AV-042-2MU3 with a power of 40 W. You can use fans with other motors.

Transformer T1 is made in accordance with the recommendations set out in [3]. Its magnetic core is assembled from U-shaped plates of electrical hot-rolled steel 0,5 mm thick, assembled in a overlap. Its dimensions, shape and arrangement of winding sections are shown in Fig. 5. Transformer windings - disk [3]. The width of the gap between the windings II and III does not matter.

Winding I consists of two sections of 100 turns of copper wire with a diameter of 3 mm. Winding II has two sections of 38 turns of PEV-2 wire with a diameter of 1,8 mm. Winding III is divided into four sections of 20 turns of a 2x9 mm copper bus. A 20 mm wide cotton tape was used as insulation.

Sections of each winding are located on different cores of the magnetic circuit (winding sections III - in pairs). Their numbers are shown in Fig. 5. All of them are frameless, wound on wooden mandrels. To prevent the coils from spreading, they are fixed with a fabric tape with a mandatory subsequent impregnation with varnish.

Rice. 5. Sections of each winding are located on different cores of the magnetic circuit

Transformer T2 is used ready-made with a voltage on the winding II of 10 ... 12 V at a load current of at least 150 mA. The current transformer T3 is wound on half of the ShL16x20 magnetic circuit, pulled together with a 0,2 mm thick sheet metal clamp. In order not to make unnecessary connections, the terminals of winding III of the transformer T1 are used as its primary windings (one turn each). The secondary winding of the T3 transformer has 300 turns of PEV-2 wire with a diameter of 0,4 mm.

Transformers T1 and T2 of block A1 are wound on B26 magnetic cores made of 2000NM ferrite without a non-magnetic gap. Winding I contains 150 turns, and winding II - 100 turns of PEV-2 wire with a diameter of 0,18 mm.

The winding of the inductor L1 is wound on a magnetic circuit from a TC-180 transformer with a non-magnetic gap of 1 mm with a PEV-2 wire with a diameter of 1,8 mm until the window is filled. The L2 inductor is wound on a ShL32x40 magnetic circuit with a non-magnetic gap of 1 mm. Its winding contains 60 turns of the same bus as winding III of transformer T1. Textolite 0,5 mm thick was used as a material for non-magnetic gaskets in the magnetic circuits of the chokes.

Resistor R1 - imported wire. You can use domestic C5-35 (PEV) or C5-37 with a power of 10 W, or connect in parallel five MLT-2 resistors with a nominal value of 110 Ohms.

Resistor R2 is made of nichrome wire 1 mm in diameter and 1,7 m long, wound on ceramic tubes from KTs109A diode posts, as shown in fig. 6. A variant of parallel connection of six PEV-30 resistors of 18 ohms was tested. When the electrodes stick, they overheat greatly, but since this is a short-term mode, such overheating can be considered acceptable. In any case, it is recommended to place resistor R2 for better cooling in the airflow from the fan.

Rice. 6. Resistor R2 is made of nichrome wire with a diameter of 1 mm and a length of 1,7 m, wound on ceramic tubes from diode pillars KTs109A

If the useless power dissipation on resistor R2 is undesirable, it can be removed from the machine by limiting the auxiliary rectifier current as recommended in [1], using a bank of capacitors connected in parallel. It is connected in series with the winding II of the transformer T1 and the diode bridge VD1. MBGP capacitors with a total capacity of 240 uF are suitable for such a battery. Variable resistor R3 - SP-I group A.

SCRs must be installed on standard coolers (heat sinks). The diode bridge MB5010 is equipped with a separate heat sink with an effective cooling surface of about 300 cm². KT829A transistors do not need heat sinks.

The body of the device can be anything. In the author's version, all the details of the device are placed on a frame made of corners bent from sheet steel 2 mm thick. The casing of the apparatus is made of steel sheet 0,8 mm thick. The front and rear walls of the casing are made of welded wire mesh with meshes of 10x10 mm. The metal case must be grounded.

To set up the device, an oscilloscope and an adjustable DC voltage source of 0 ... 12 V, as well as a multimeter are required. Adjustment should begin with a thorough check of the correct installation. After making sure that there are no errors, apply voltage to terminals 3 and 4 of block A1 from winding II of transformer T2 with transformer T1 and fan turned off. Using an oscilloscope, make sure that there are similar ones shown in Fig. 3 pulses on the collectors of transistors VT1 and VT2, as well as sawtooth voltage on the capacitor A1 .C7.

Next, set the trimmer A1.R15 slider to the top position according to the diagram, and the variable resistor R3 slider to the right position according to the diagram. In this case, the output of the op-amp A1 .DA1 should be a constant low level or short high-level pulses should be observed. Then, smoothly moving the slider of the tuning resistor A1.R15 down (according to the diagram), reduce the pauses between the pulses until they disappear completely and the high level is constantly present at the output of the op-amp.

Set the trimmer resistor A1.R19 to the top position according to the diagram. Then, apply +11 V voltage from an additional source to terminal 1 of block A8 and, by moving the slider of resistor A1.R15 down (according to the diagram), achieve a low level at the output of element A1.DD2.3. Packets of pulses at the outputs of the elements DD1.1 and DD1.2 must correspond to Fig. 3. If you need to change the pulse frequency, you should select the resistor A1 .R23. When the voltage at terminal 11 of block A1 drops below 8 V, the pulse generator should turn off. Next, check for pulses between terminals 5, 6 and between terminals 7, 8 of block A1 with the control circuits of trinistors VS1 and VS2 connected.

The next stage of adjustment is to check the operation of the feedback circuits. Move the trimmer resistor A1.R7 to the left position according to the diagram, temporarily apply a voltage of +11 V to terminal 1 of block A9, and a constant voltage of 1 ... 4 V from an additional source to capacitor A0.C10. When this voltage changes, as well as when the engine of the variable resistor R3 rotates, pulses should appear at the output of the op-amp A1.DA1 and their duty cycle should change.

Set the slider of the resistor R3 to the extreme right (according to the diagram) position. Connect a 36 V incandescent lamp with a power of at least 20 W to the output of the device. Temporarily turn off the inductor L1 and connect the primary winding of the transformer T1 to the network. In this case, the lamp should light up. Otherwise, pins 3 and 4 of block A1 should be swapped. Applying voltage to the capacitor A1.C4 from an additional source, check the operation of the current regulator. As the voltage on this capacitor increases, the brightness of the lamp should decrease.

Check if the fan rotates in the right direction. To change the direction of its rotation, it is necessary to swap any two of its three conclusions. The motor current must not exceed the maximum allowable value.

Next, turn off the additional voltage source, connect inductor L1 and pin 11 of block A1 according to the diagram. Connect welding cables to the output terminals of the device via a 200 A ammeter, set the variable resistor R3 slider to the minimum current position and turn on the device. Light the arc and use trimmer resistor A1.R7 to set the current in the welding circuit to about 40 A. Then, monitoring the current with an ammeter, calibrate the scale of variable resistor R3.

Literature

1. BogdanovA. Electronic control of welding current. - Radio, 2006, No. 4, p. 36-38.
2. GOST 95-77. Transformers single-phase single-station for manual arc welding. General specifications. - URL: files.stroyinf.ru/Data1/10/10478.
3. Volodin V. Welding transformer: calculation and production. - Radio, 2002, No. 11, p. 35, 36; No. 12, p. 38,39.

Author: E. Gerasimov

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