Adjust the required current for what. Power supply with current and voltage regulation. Welding with direct and alternating current

An important design feature of any welding machine is the ability to adjust the operating current. The following methods are known for adjusting the current in welding transformers: shunting using chokes of various types, changing the magnetic flux due to the mobility of the windings or magnetic shunting, using stores of active ballast resistances and rheostats. All these methods have both their advantages and disadvantages. For example, the disadvantage of the latter method is the complexity of the design, the bulkiness of the resistances, their strong heating during operation, and inconvenience when switching.

The most optimal method is to adjust the current stepwise by changing the number of turns, for example, by connecting to taps made when winding the secondary winding of the transformer. However, this method does not allow the current to be adjusted over a wide range, so it is usually used to adjust the current. Among other things, adjusting the current in the secondary circuit of a welding transformer is associated with certain problems. In this case, significant currents pass through the control device, which causes an increase in its dimensions. For the secondary circuit, it is practically impossible to select powerful standard switches that could withstand currents of up to 260 A.

If we compare the currents in the primary and secondary windings, it turns out that the current in the primary winding circuit is five times less than in the secondary winding. This suggests the idea of placing a welding current regulator in the primary winding of the transformer, using thyristors for this purpose. In Fig. Figure 20 shows a diagram of the welding current regulator using thyristors. With extreme simplicity and accessibility of the element base, this regulator is easy to operate and does not require configuration.

Rice. 1 Schematic diagram of the current regulator of the welding transformer:
VT1, VT2 -P416

VS1, VS2 - E122-25-3

C1, C2 - 0.1 µF 400 V

R5, R6 - 1 kOhm

Power regulation occurs when the primary winding of the welding transformer is periodically turned off for a fixed period of time at each half-cycle of the current. The average current value decreases. The main elements of the regulator (thyristors) are connected opposite and parallel to each other. They are alternately opened by current pulses generated by transistors VT1, VT2.

When the regulator is connected to the network, both thyristors are closed, capacitors C1 and C2 begin to charge through the variable resistor R7. As soon as the voltage on one of the capacitors reaches the avalanche breakdown voltage of the transistor, the latter opens and the discharge current of the capacitor connected to it flows through it. Following the transistor, the corresponding thyristor opens, which connects the load to the network.

By changing the resistance of resistor R7, you can regulate the moment the thyristors are turned on from the beginning to the end of the half-cycle, which in turn leads to a change in the total current in the primary winding of the welding transformer T1. To increase or decrease the adjustment range, you can change the resistance of the variable resistor R7 up or down, respectively.

Transistors VT1, VT2 operating in avalanche mode, and resistors R5, R6 included in their base circuits can be replaced with dinistors (Fig. 2)

Rice. 2 Schematic diagram of replacing a transistor with a resistor with a dinistor, in the current regulator circuit of a welding transformer.
The anodes of the dinistors should be connected to the extreme terminals of resistor R7, and the cathodes should be connected to resistors R3 and R4. If the regulator is assembled using dinistors, then it is better to use devices of the KN102A type.

Old-style transistors such as P416, GT308 have proven themselves well as VT1, VT2, but these transistors, if desired, can be replaced with modern low-power high-frequency transistors that have similar parameters. The variable resistor is SP-2 type, and the fixed resistors are MLT type. Capacitors type MBM or K73-17 for an operating voltage of at least 400 V.

All parts of the device are assembled using hinged mounting on a textolite plate 1...1.5 mm thick. The device has a galvanic connection to the network, so all elements, including thyristor heat sinks, must be isolated from the housing.

A correctly assembled welding current regulator does not require any special adjustment; you just need to make sure that the transistors are stable in avalanche mode or, when using dinistors, that they are switched on stable.

There are various ways to regulate the welding current, but we can say that the most widely used method among people is a very simple and reliable method of adjusting the current - using a ballast resistance switched on at the output of the secondary winding. The method is not only simple and reliable, but also useful, as it improves the external characteristic of the transformer, increasing the steepness of its drop. In some cases, ballast resistances are used purely to correct the rigid characteristics of the welding machine.

The value of ballast resistance for the welding current regulator is on the order of hundredths to tenths of an Ohm and is, as a rule, selected experimentally. Powerful resistance wires, used in cranes, trolleybuses, or sections of spirals of heating elements (thermal electric heaters), or pieces of thick high-resistance wire have long been used as ballast resistance. You can even reduce the current somewhat by using a stretched steel door spring. The ballast resistance can be switched on either permanently.

Or so that the welding current can then be relatively easily adjusted. One end of such a resistance is connected to the output of the transformer, and the end of the welding wire is equipped with a removable clamp, which is easily thrown along the length of the resistance spiral, selecting the desired current.

Nichrome wire as ballast resistance (4 mm in diameter and 8 m in length). The wire can be of a smaller diameter, and a shorter length will be needed, but it will heat up more.

Most high-power wirewound resistors are made in the form of an open spiral mounted on a ceramic frame up to half a meter long; as a rule, the wire from the heating elements is also wound into the spiral. If the resistor is made of magnetic alloys, then in the case of its spiral arrangement, and even more so with any steel structural elements inside the spiral, when large currents pass, the spiral begins to vibrate strongly. After all, a spiral is the same solenoid, and huge welding currents generate powerful magnetic fields. You can reduce the influence of vibrations by stretching the spiral and fixing it on a rigid base. In addition to the spiral, the wire can also be bent into a snake, which also reduces the size of the finished resistor. The cross-section of the conductive material of the resistor should be selected larger, because it gets very hot during operation. A wire or tape that is too thin will become red-hot, although even this, in principle, does not exclude the effectiveness of using it as a current regulator for a welding machine. It is difficult to judge the real value of the resistance of ballast wire resistors, since in a heated state the properties of the materials change greatly.

In industrial welding machines, the method of adjusting the current by turning on active resistances, due to their bulkiness and heating, has not become widespread. But reactance is very widely used - inclusion in the secondary circuit of a choke. Chokes have a variety of designs, often combined with the magnetic core of the transformer into one whole, but they are made in such a way that their inductance, and therefore reactance, is regulated mainly by the movement of parts of the magnetic circuit. At the same time, the choke improves the arc burning process.

Adjusting the current in the secondary circuit of a welding transformer is associated with certain problems. Significant currents pass through the control device, which makes it bulky. Another inconvenience is switching. For the secondary circuit, it is almost impossible to select standard switches so powerful that they can withstand currents up to 200A. Another thing is the primary winding circuit, where the currents are about five times less, the switches for which are consumer goods. In series with the primary winding, just as in the previous case, ballast resistors can be connected. Only in this case, the resistance of the resistors should be an order of magnitude greater than in the secondary winding circuit. Thus, a battery of several parallel-connected resistors PEV-50...100 with a total resistance of 6-8 Ohms can reduce the output current by half or even three times, depending on the design of the transformer. You can collect several batteries and install a switch. If you don’t have a powerful switch at your disposal, then you can get by with several switches. By installing resistors according to the diagram shown below, you can, for example, make a welding current regulator with the combination: 0; 4; 6; 10 ohms.

True, when turning on the ballast resistance in the primary circuit, the benefit that the resistance gives in the secondary circuit is lost - improving the falling characteristic of the transformer. But on the other hand, resistors switched on at high voltage do not lead to any negative consequences in arc combustion: if the transformer welded well without them, then it will cook with additional resistance in the primary winding.

In no-load mode, the transformer consumes a small current, which means its winding has significant resistance. Therefore, an additional few ohms have virtually no effect on the output no-load voltage.

Instead of resistors, which will get very hot during operation, you can install a reactance - a choke - in the primary winding circuit.

This measure should be considered rather as a way out if no other means of reducing power are available. The inclusion of reactance in the high voltage circuit can greatly reduce the open circuit output voltage of the transformer. A drop in output voltage is observed in transformers with a relatively large no-load current - 2-3A. With low current consumption - about 0.1A - the drop in output voltage is almost imperceptible. In addition, a choke connected to the primary winding of a transformer may lead to some deterioration in the welding characteristics of the transformer, although not so much that it cannot be used. In the latter case, it still strongly depends on the properties of the particular transformer. For some welding machines, the inclusion of a choke in the primary circuit of the transformer does not in any way affect, at least according to subjective sensations, the quality of the arc.

As a welding machine choke, to regulate the current, you can use a ready-made secondary winding of some transformer, designed with an output of about 40V and a power of 200-300 W, then you won’t have to redo anything. Although it is still better to make a homemade choke by winding a wire on a separate frame from the same transformer - 200-300 W, for example from a TV, making taps every 30-60 turns connected to the switch.

A homemade choke can also be made using an open-ended - straight core. This is convenient when you already have a ready-made coil with several hundred turns of suitable wire. Then you need to stuff a package of straight transformer iron plates inside it. The required reactance is set by selecting the thickness of the package, guided by the welding current of the transformer. For example: a choke made from a coil containing supposedly about 400 turns of wire with a diameter of 1.4 mm was stuffed with a package of iron with a total cross-section of 4.5 cm 2, a length equal to the length of the coil, 14 cm. This made it possible to reduce the welding current of the 120A transformer approximately twice. A choke of this type can also be made with adjustable reactance. To do this, you can change the depth of insertion of the core rod into the cavity of the coil. A coil without a core has low resistance; when the core is fully inserted, its resistance is maximum. A choke wound with a suitable wire does not heat up much, but its core vibrates strongly. This must be taken into account when screeding and fixing a set of iron plates.

For homemade welding machines, the easiest way is to make them with taps while winding the windings and, by switching the number of turns, change the current. However, this method can only be used to adjust the current rather than to regulate it over a wide range. After all, in order to reduce the current by 2-3 times, you will have to increase the number of turns of the primary winding too much, which will inevitably lead to a voltage drop in the secondary circuit. Otherwise, you will have to increase the turns of all coils, which will lead to excessive wire consumption and an increase in the dimensions and weight of the transformer.

For finer adjustment of the welding current to a lesser extent, you can use the inductance of the welding cable, laying it in rings. But don't overdo it, because... the cable will become hot.

Recently, thyristor and triac circuits for adjusting the welding current have become somewhat widespread. When a voltage of a certain magnitude is applied to the control terminal of a thyristor or triac, the regulator opens and begins to freely pass current through itself. In current control circuits operating from alternating voltage, control pulses usually arrive at every half-cycle. The regulator opens at strictly defined (set) moments of time, thus cutting off the beginning of each half-cycle of the current sinusoid, which reduces the total power of the passing electrical signal.

Naturally, the current and voltage after this do not have a sinusoidal shape. Such circuits allow you to regulate power over a wide range. A person who understands radio electronics will be able to make such a circuit on his own, although, it must be said, devices of this kind cannot be considered perfect. When using regulators of this type, the arc burning process is somewhat deteriorated. Indeed, now, as the power decreases, the arc begins to burn in separate, increasingly short-term flashes. For most of the thyristor regulator circuits, the scales are not linear, and the calibration changes with changes in the network voltage; the current through the thyristor gradually increases during operation due to heating of the circuit elements. In addition, the output power is usually noticeably damped even at the maximum unlocking position of the regulator, to which welding transformers are very sensitive. This method of adjusting the welding current, due to the complexity of manufacturing and low reliability, has not become widespread among homemade welding current regulators.

Welding current measurement

To measure large currents, in this case up to 200A, devices are required that have their own specific characteristics and are not widely used in everyday life. One of the simplest solutions is to use a clamp meter.

The specificity of measurement with this device is that it does not require connection to an electrical circuit for measurement. The current strength is measured at a distance from the wire without touching it. The device has a special dividing circuit, which is why the name is “pincers”, which covers the current-carrying wire. The electromagnetic field of the current flowing in the covered wire induces a current in a closed circuit, which is measured. On the body of the “clamps” there is a current measurement limit switch, the maximum values of which usually reach from 100A to 500A for different models of devices. Clamp meters can be quickly used in almost any situation without having any effect on the electrical circuit. They can only measure alternating current, which creates an alternating electromagnetic field; for direct current, this instrument is useless. The accuracy class in this case is very low, so one can judge, rather, only approximate values.

Another way to measure welding current is to install an ammeter designed for high current values into the electrical circuit of the welding machine being manufactured or an industrial machine being modified, or even simply connect it temporarily to an open circuit of the welding wires.

The inclusion of an ammeter in the welding circuit is also marked by some specifics. The fact is that it is not the device itself (arrow pointer) that is connected in series to the circuit, but its shunt (resistor), while the pointer indicator is connected to the shunt in parallel.

The shunt has its own resistance: presumably hundredths of an Ohm (since it cannot be measured with a regular ohmmeter). In appearance, it is a piece of metal several centimeters long with a rectangular cross-section with powerful contact pads on both sides. The accuracy of the device readings also depends on the accuracy of the shunt resistance. Each ammeter model has a specific shunt resistance and should be sold together.

And what you should never do is try to connect a pointer device to a circuit without a shunt at all. If you have a pointer device lying around somewhere, the scale of which shows hundreds of amperes, this does not mean at all that it measures them itself. Check it: and the device itself turns out to be just a micro- or milliammeter. Sometimes you come across pointer instruments in which the shunt is mounted inside the housing and nothing else needs to be connected to it. As a rule, these are distinguished by their huge sizes and low accuracy class.

Of considerable importance is the ability of the pointer pointer of the measuring device to be set to the current value, overcoming oscillatory transient processes when the current changes, otherwise the pointer will frantically dance along the scale even with minor changes in current, which are inevitable when the welding arc burns.

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This is a fairly common question that has several solutions. There is one of the most popular ways to solve the problem; adjustment occurs through an active ballast connection at the output of the winding (secondary).

On the territory of the Russian Federation, welding for alternating current consists of a frequency of 50 Hz. A 220V network is used as a power source. And all transformers for welding have a primary and secondary winding.

In units used in an industrial area, current regulation is carried out differently. For example, using the movable functions of the windings, as well as magnetic shunting, inductive shunting of various types. Ballast resistance stores (active) and a rheostat are also used.

This choice of welding current cannot be called a convenient method, due to the complex design, overheating and discomfort when switching.

A more convenient way to regulate the welding current is to wind the secondary (secondary winding) by making taps, which will allow you to change the voltage when switching the number of turns.

But in this case, it will not be possible to control the voltage over a wide range. They also note certain disadvantages when adjusting from the secondary circuit.

Thus, the welding current regulator, at the initial speed, passes through itself a high-frequency current (HFC), which entails a cumbersome design. And standard secondary circuit switches do not require a load of 200 A. But in the primary winding circuit, the indicators are 5 times less.

As a result, an optimal and convenient tool was found in which adjusting the welding current does not seem so confusing - this is a thyristor. Experts always note its simplicity, ease of use and high reliability. The strength of the welding current depends on turning off the primary winding for specific periods of time, at each half-cycle of the voltage. At the same time, the average voltage readings will decrease.

The principle of operation of a thyristor

The regulator parts are connected both in parallel and counter to each other. They are gradually opened by current pulses, which are formed by transistors vt2 and vt1. When the device starts, both thyristors are closed, C1 and C2 are capacitors, they will be charged through resistor r7.

At the moment the voltage of any of the capacitors reaches the avalanche breakdown voltage of the transistor, it opens, and the discharge current of the joint capacitor flows through it. After the transistor opens, the corresponding thyristor opens and connects the load to the network. Then the opposite half-cycle of the alternating voltage begins, which implies the closing of the thyristor, then a new cycle of recharging the capacitor follows, this time in the opposite polarity. Then the next transistor opens, but again connects the load to the network.

Welding with direct and alternating current

In the modern world, DC welding is used to a greater extent. This is due to the possibility of reducing the amount of filler material of the electrodes in the weld. But when welding with alternating voltage, you can achieve very high-quality welding results. Welding power sources operating with alternating voltage can be divided into several types:

Instruments for argon arc welding. Special electrodes are used here that do not melt, making argon welding as comfortable as possible;
Apparatus for the production of RDS by alternating electric current;
Equipment for semi-automatic welding.

Alternating welding methods are divided into two types:

use of non-consumable electrodes;
piece electrodes.

There are two types of DC welding, reverse and direct polarity. In the second option, the welding current moves from negative to positive, and the heat is concentrated on the workpiece. And the reverse concentrates attention on the end of the electrode.

A DC welding generator consists of a motor and a current generator itself. They are used for manual welding during installation work and in the field.

Manufacturing of the regulator

To make a control device for welding current, you will need the following components:

Resistors;
Wire (nichrome);
Coil;
design or diagram of the device;
Switch;
Spring made of steel;
Cable.

Operation of the ballast connection

The ballast resistance of the control apparatus is at the level of 0.001 Ohm. It is selected through experiment. Directly to obtain resistance, resistance wires of high power are mainly used; they are used in trolleybuses or on lifts.

You can even reduce the high frequency welding voltage by using a steel door spring.

Such resistance is turned on permanently or in another way, so that in the future it will be possible to easily adjust the indicators. One edge of this resistance is connected to the output of the transformer structure, the other is provided with a special clamping tool that can be thrown along the entire length of the spiral, which will allow you to select the desired voltage force.

The main part of resistors using high-power wire is produced in the form of an open spiral. It is mounted on a structure half a meter long. Thus, the spiral is also made from heating element wire. When resistors made of a magnetic alloy are combined with a spiral or any part made of steel, in the process of passing high current, it will begin to tremble noticeably. The spiral has this dependence only until it stretches.

How to make a throttle yourself?

It is quite possible to make your own throttle at home. This occurs when there is a straight coil with a sufficient number of turns of the desired cord. Inside the coil are straight metal plates from the transformer. By choosing the thickness of these plates, it is possible to select the starting reactance.

Let's look at a specific example. A choke with a coil with 400 turns and a cord with a diameter of 1.5 mm is filled with plates with a cross-section of 4.5 square centimeters. The length of the coil and wire should be the same. As a result, the 120 A transformer current will be reduced by half. Such a choke is made with a resistance that can be changed. To carry out such an operation, it is necessary to measure the deepening of the passage of the core rod into the coil. Without this tool, the coil will have little resistance, but if the rod is inserted into it, the resistance will increase to the maximum.

A choke that is wound with the correct cord will not overheat, but the core may experience strong vibration. This is taken into account when screeding and fastening iron plates.

Quite a large number of industrial electric drives and technological processes use direct current for their power supply. Moreover, in such cases it is quite often necessary to change the value of this voltage. Such types of transport as the subway, trolleybuses, electric cars and other types of transport receive power from DC networks with constant voltage. But many of them need to change the voltage value supplied to the armature of the electric motor. The classic means of obtaining the required values are resistive control, or the Leonardo system. But these systems are outdated, and they can be found quite rarely (especially the generator-motor system). More modern and actively being implemented now are the thyristor converter-motor and pulse converter-motor systems. Let's look at each system in more detail.

Resistor regulation

To regulate the starting current and voltage supplied to the electric motor, resistors are connected to the armature circuit in series with the armature (or the armature and the field winding in the case of a series-excited motor):

In this way, the current supplied to the electrical machine is regulated. Contactors K1, K2, K3 bypass resistors if it is necessary to change any parameter or coordinate of the electric drive. This method is still quite widespread, especially in traction electric drives, although it is accompanied by large losses in resistors and, as a consequence, rather low efficiency.

Generator-engine system

In such a system, the required voltage level is formed by changing the excitation flow of the generator:

The presence of three electric machines in such a system, large weight and dimensions and a long repair time in case of breakdowns, as well as expensive maintenance and the large inertia of such an installation made the efficiency of such a machine very low. Nowadays there are practically no generator-engine systems left; they are all being actively replaced by systems, which have a number of advantages.

Thyristor converter – motor

It received its massive development in the 60s, when thyristors began to appear. It was on their basis that the first static low-power thyristor converters were created. Such devices were connected directly to AC networks:

Voltage regulation occurs by changing. Regulation through a thyristor converter has a number of advantages over the generator-motor installation, such as high speed and efficiency, smooth regulation of DC voltage and many others.

Converter with intermediate voltage link

This is where things get a little more complicated. To obtain a constant voltage of the required value, additional auxiliary devices are used, namely an inverter, transformer, rectifier:

Here, direct current is converted into alternating current using a current inverter, then lowered or increased using a transformer (depending on the need), and then rectified again. The presence of a transformer and inverter significantly increases the cost of installation and enlarges the system, which reduces efficiency. But there is also a plus - galvanic isolation between the network and the load due to the presence of a transformer. In practice, such devices are extremely rare.

Switching DC-DC converters

These are perhaps the most modern control devices in DC circuits. It can be compared to a transformer, since the behavior of a pulse converter is similar to a transformer with a smoothly varying number of turns:

Such systems actively replace electric drives with resistive regulation, by connecting them to the machine armature in series, instead of a resistive-contactor group. I use them quite often in electric cars, and they have also gained quite a lot of popularity in underground transport (subway). Such converters emit minimal heat, which does not heat up the tunnels and can implement regenerative braking mode, which is a big plus for electric drives with frequent starting and braking.

The big advantage of such devices is that they can recuperate energy into the network, smoothly regulate the rate of current rise, and have high efficiency and speed.