Electric arc structure. Electric arc (voltaic arc, arc discharge)

Electric arc- this is a powerful, long-lasting electrical discharge between energized electrodes in a highly ionized mixture of gases and vapors. Characterized by high gas temperatures and high current in the discharge zone.

The electrodes are connected to AC sources ( welding transformer) or direct current (welding generator or rectifier) ​​with direct and reverse polarity.

When welding with direct current, the electrode connected to the positive pole is called the anode, and to the negative pole is called the cathode. The space between the electrodes is called the arc gap region or arc gap (Figure 3.4). The arc gap is usually divided into 3 characteristic areas:

1. anode region adjacent to the anode;
2. cathode region;
3. arc pillar.

Any arc ignition begins with short circuit, i.e. from the connection of the electrode with the product. In this case, U d = 0, and current I max = I short circuit. A cathode spot appears at the location of the short circuit, which is an indispensable (necessary) condition for the existence of an arc discharge. When the electrode is removed, the resulting liquid metal is stretched, overheated and the temperature reaches boiling point - an arc is excited (ignited).

The arc can be ignited without contact of the electrodes due to ionization, i.e. breakdown of the dielectric air (gas) gap by increasing the voltage by oscillators (argon arc welding).

The arc gap is a dielectric medium that must be ionized.

For the existence of an arc discharge, U d = 16÷60 V is sufficient. Passage electric current through the air (arc) gap is possible only if there are electrons (elementary negative particles) and ions in it: positive (+) ions - all molecules and atoms of elements (me metals form more easily); negative (–) ions – more easily form F, Cr, N 2, O 2 and other elements with an affinity for electrons e.

Figure 3.4 – Arc burning diagram

The cathode region of the arc is a source of electrons that ionize the gases in the arc gap. Electrons released from the cathode are accelerated by the electric field and move away from the cathode. At the same time, under the influence of this field, + ions are directed to the cathode:

U d = U k + U c + U a;

The anode region has a significantly larger volume U a< U к.

Arc column - the main part of the arc gap is a mixture of electrons, + and – ions and neutral atoms (molecules). The arc column is neutral:

∑charge.neg. = ∑charges of positive particles.

The energy to maintain a stationary arc comes from the IP power supply.

Different temperatures, sizes of the anodic and cathodic zones and different amounts of heat released determine the existence of direct and reverse polarity when welding with direct current:

Q a > Q k; Ua< U к.

• when a large amount of heat is required to heat the edges of large metal thicknesses, direct polarity is used (for example, when surfacing);
• for thin-walled metals being welded that do not allow overheating, reverse polarity (+ on the electrode).

Arc welding, whether manual or mechanized, is carried out thanks to an electric arc, which, in essence, is an electric discharge. A welding electric arc is characterized by the release of large amounts of heat and light. Note that the arc temperature can reach up to 6,000 degrees Celsius.

It is worth paying attention to the fact that the light and heat generated by the arc can be harmful to human health. Therefore, all welding work using the arc welding method is carried out exclusively in special clothing and a mask or goggles that protect the welder’s eyes.

The welding electric arc is not always the same; there are several types of it, which depend on the environment where welding is carried out, on the metal product and other factors.

Types of welding electric arc.

If we talk about the dependence of the medium and the arc, then we can distinguish the following types of electrical discharge:

• Open electric arc. Welding of metal products is carried out in the open air, without using special gases for protection. The arc burns in a medium formed by the surrounding air and vapors that appear during welding of metal products, melting of an electrode or wire, or their coatings.
• Closed electric arc. This type of arc is formed during submerged arc welding. The arc is protected during welding by a gas mixture that is formed as a result of mixing vapors from the metal product being welded, the consumable electrode and, in fact, the flux.
• Arc in a protective gas environment. In this case we are talking about welding in an environment of so-called shielding gases: inert or active (both pure gases and their mixtures are used). As a result of welding, a gaseous environment is formed consisting of shielding gas, metal vapor and electrode.

Power supply for the welding arc.

A welding arc is formed when electric current is applied. Note that the arc can be powered from both alternating current and direct current sources. Various sources food is given different types arc.

When using direct current, two types of arc can be obtained: welders use both an arc of direct polarity and reverse polarity. The difference between these two types is the power connection. So, with direct polarity, minus is applied directly to the electrode, and plus is applied to the metal product that will be welded. With reverse polarity, the connection occurs in reverse: the plus is supplied to the electrode, while the minus is supplied to the metal product being welded.

We also note that the metal product being welded is sometimes not included in the electrical circuit. In such cases, they say that an indirect arc is used, that is, the current is supplied only to the electrode. If both an electrode and a metal product are connected to the power source, then in this case they speak of a direct arc. It is worth noting that this electric arc is most often used; welders use an indirect arc extremely rarely.

Current density values ​​for a welding arc.

When welding metal products with an electric arc, the current density indicator also plays an important role. In the mode of conventional manual arc welding, the current density is standard, namely 10-20 A/mm 2. Welders set the same value when welding in certain gases. High current density, namely 80-120 A/mm 2 and also higher, is used for semi-automatic or other types of welding carried out under the protection of gases or flux.

Current density affects arc voltage. This dependence is usually called the static characteristic of the arc (it is depicted graphically). Note that if the current density is small, then this characteristic can be decreasing: that is, a voltage drop occurs when the current, on the contrary, increases. This phenomenon is due to the fact that as the current value increases, the conductivity of electricity increases, as well as the cross-sectional area of ​​the arc column, while the current density decreases.

When the current density usual for manual welding is used, the voltage loses its dependence on the current value. In this case, the area of ​​the column increases in proportion to the current. Note also that the electrical conductivity remains virtually unchanged, and the current density in the column also remains constant.

How does a welding arc occur?

A welding arc occurs only when the gas column located between the metal product and the electrode is sufficiently ionized (that is, it has required quantity electrons and ions). To achieve a normal level of ionization, electricity is transferred to gas molecules. As a result of this process, electrons begin to be released. In fact, the arc medium is a gas conductor of current; it has a round-cylindrical shape.

Note that the electric arc itself consists of 3 components:

• anode part;
• electric arc column;
• cathode part.

The stability of the electric arc during the welding process is influenced by many factors, including no-load voltage, the type of electric current, its magnitude, polarity, etc. During the welding process, all these indicators must be carefully monitored and the welding mode must be correctly set at in different ways and for various metal products.

An electric arc is an arc discharge that occurs between two electrodes or an electrode and a workpiece and which allows the connection of two or more parts by welding.

The welding arc, depending on the environment in which it occurs, is divided into several groups. It can be open, closed, or in a protective gas environment.

An open arc flows in the open air through the ionization of particles in the combustion area, as well as due to the metal vapors of the parts being welded and the electrode material. The closed arc, in turn, burns under a layer of flux. This allows you to change the composition of the gas environment in the combustion area and protect the metal workpieces from oxidation. In this case, the electric arc flows through metal vapor and flux additive ions. The arc, which burns in a protective gas environment, flows through the ions of this gas and metal vapor. This also allows you to prevent oxidation of parts, and, consequently, increase the reliability of the formed connection.

An electric arc differs in the type of current supplied - alternating or direct - and in the duration of combustion - pulsed or stationary. In addition, the arc can have direct or reverse polarity.

Based on the type of electrode used, non-melting and melting are distinguished. The use of a particular electrode directly depends on the characteristics that it has welding machine. The arc that occurs when using a non-consumable electrode, as the name implies, does not deform it. When welding with a consumable electrode, the arc current melts the material and it is fused onto the original workpiece.

The arc gap can be conditionally divided into three characteristic sections: near-cathode, near-anode, and also the arc shaft. In this case, the last section, i.e. The arc shaft has the greatest length, however, the characteristics of the arc, as well as the possibility of its occurrence, are determined precisely by the near-electrode areas.

In general, the characteristics that an electric arc has can be combined into the following list:

1. Arc length. This refers to the total distance of the cathode and anode regions, as well as the arc shaft.

2. Arc voltage. Consists of the sum on each of the areas: barrel, near-cathode and near-anode. In this case, the change in voltage in the near-electrode regions is significantly greater than in the remaining region.

3. Temperature. An electric arc, depending on the composition of the gaseous medium and the material of the electrodes, can develop a temperature of up to 12 thousand degrees Kelvin. However, such peaks are not located over the entire plane of the electrode end. Because even with the most better processing the material of the conductive part has various irregularities and tubercles, due to which many discharges occur, which are perceived as one. Of course, the arc temperature largely depends on the environment in which it burns, as well as on the parameters of the supplied current. For example, if you increase the current value, then, accordingly, the temperature value will increase.

And finally, the current-voltage characteristic or I-V characteristic. It represents the dependence of voltage on length and current magnitude.

Electric arc and its properties

Electric arc welding is most widely used in mechanical engineering. Let's take a closer look at the features of electric arc welding.

An electric arc is a continuous discharge of electric current between two electrodes, occurring in a gaseous environment. The electric arc used to weld metals is called a welding arc. In most cases, such an arc burns between the electrode and the product, i.e. is an arc of direct action.

A direct direct current arc burning between a metal electrode (cathode) and the metal being welded (anode) has several clearly distinguishable areas (Fig. 2.3). The electrically conductive gas channel connecting the electrodes has the shape of a truncated cone or cylinder. Its properties are different distances from the electrodes are not the same. Thin layers of gas adjacent to the electrodes have relatively low temperature. Depending on the polarity of the electrode to which they are adjacent, these layers are called cathode 2 and anode 4 arc areas.

Length of cathode region l k is determined by the free path of neutral atoms and is

̃about 10 -5 cm. Length of the anode region l a is determined by the free path of the electron and is approximately 10 -3 cm. Between the near-electrode regions there is the longest, high-temperature region of the discharge - the arc column l c 3.

Spots are formed on the surface of the cathode and anode, called, respectively, cathode 1 and anode 5 spots, which are the bases of the arc column through which all welding current passes. Electrode spots are distinguished by the brightness of their glow at their relatively low temperature (2600... 3200 K). The temperature in the arc column reaches 6000...8000 K.

Total arc length l d equal to the sum of the lengths of all three of its regions (l d ​​=l a +l k) and for real conditions it is 2...6 mm.

The total voltage of the welding arc, accordingly, is composed of the sum of the voltage drops in certain areas arcs and ranges from 20 to 40 V. The dependence of the voltage in the welding arc on its length is described by the equation , Where A - sum of voltage drops in the cathode and anode regions, V; l d- arc column length, mm; b- specific voltage drop in the arc, i.e. referred to 1 mm of arc column length, V/mm.

One of the main characteristics of an electric arc discharge is the static current-voltage characteristic - the dependence of the arc voltage at a constant length on the current strength in it (Fig. 2.4).

As the arc length increases, the voltage increases and the curve of the static current-voltage characteristic of the arc rises higher, while approximately maintaining its shape (curves a, b, c). It distinguishes three regions: decreasing I, rigid (almost horizontal) II and increasing III. Depending on the arc burning conditions, one of the characteristic sections corresponds to it. In manual arc welding with coated electrodes, gas-shielded welding with a non-consumable electrode and submerged arc welding, the high densities current, the arc characteristic will initially be falling, and with an increase in current it will completely turn into hard. In this case, with an increase in welding current, the cross-section of the arc column and the cross-sectional area of ​​the anode and cathode spots increase proportionally. The current density and arc voltage remain constant.

When welding under submerged arc and in shielding gases with a thin electrode wire at high current densities, the arc characteristic becomes increasing. This is explained by the fact that the diameters of the cathode and anode spots become equal to the diameter of the electrode and cannot increase any further. In the arc gap, complete ionization of gas molecules occurs and a further increase in the welding current can only occur by increasing the speed of movement of electrons and ions, i.e. by increasing the voltage electric field. Therefore, to further increase the welding current, an increase in arc voltage is required.

The welding arc is a powerful concentrated source of heat. Almost all electrical energy, consumed by the arc, turns into heat. Full thermal power arcs Q=I St U d(J/s) depends on the strength of the welding current I St.(A) and arc voltage U d(IN).

It should be noted that not all the heat of the arc is spent on heating and melting the metal. Part of it is uselessly spent on heating the surrounding air or shielding gas, radiation, etc. In this regard, the effective thermal power of the arc qeff(J/s) (that part of the heat of the welding arc that is introduced directly into the product) is determined by the following relationship: where η is the coefficient of performance (efficiency) of the process of heating a product with a welding arc, determined experimentally.

The coefficient η depends on the welding method, electrode material, coating or flux composition and a number of other factors. For example, when welding with an open arc with a carbon or tungsten electrode, it averages 0.6; when welding with coated (high-quality) electrodes - about 0.75; when welding under submerged arcs - 0.8 or more.

During operation, electrical circuits are constantly closed and opened. It has long been noted that at the moment of opening an electric arc is formed between the contacts. For its appearance, a voltage of more than 10 volts and a current of over 0.1 ampere are sufficient. With more high values current and voltage, the internal temperature of the arc often reaches 3-15 thousand degrees. This becomes the main cause of melted contacts and live parts.

If the voltage is 110 kilovolts or higher, in this case the length of the arc can reach a length of more than one meter. Such an arc poses a serious danger to persons working with powerful power plants, therefore its maximum limitation and rapid extinguishing are required in any circuits, regardless of the voltage.

What is an electric arc

The most typical example is the electric welding arc, which manifests itself in the form of a prolonged electrical discharge in the plasma. In turn, plasma is ionized gases mixed with each other and pairs of components of the protective atmosphere, base and filler metal.

Thus, an electric arc is the burning of an electrical discharge between two electrodes located in a horizontal plane. Under the influence of heated gases tending to the top, this discharge bends and becomes visible as an arc or arch.

These properties made it possible to use the arc in practice as a gas conductor, with the help of which electrical energy is converted into thermal energy, creating a high heating intensity. This process can be relatively easily controlled by changing electrical parameters.

Under normal conditions, gases do not conduct current. However, if there are favorable conditions, they can be ionized. Their atoms or molecules become positive or negative ions. Under the influence high temperature and an external electric field with high intensity, the gases change and transform into a plasma state, which has all the properties of a conductor.

How a welding arc is formed

• Initially, contact appears between the end of the electrode and the workpiece, affecting both surfaces.
• Under the influence of current high density, surface particles quickly melt, forming a layer of liquid metal. It constantly increases in the direction of the electrode, after which it ruptures.
• At this moment, the metal evaporates very quickly and ions and electrons begin to fill the discharge gap. The applied voltage causes them to move towards the anode and cathode, resulting in the initiation of a welding arc.
• The process of thermal ionization begins, during which positive ions and free electrons continue to concentrate, the gas in the arc gap becomes even more ionized, and the arc itself becomes stable.
• Under its influence, the metals of the workpiece and the electrode melt and, being in liquid state, mix with each other.
• After cooling, a weld seam is formed in this place.

Extinguishing the electric arc in switching equipment

Disabling elements electrical circuit must be done very carefully, without damaging the switching equipment. Opening the contacts alone will not be enough; it is necessary to properly extinguish the arc that occurs between them.

The processes of burning and extinguishing the arc differ significantly depending on the use in the network. If there are no special problems with direct current, then if there is AC There are a number of factors to consider. First of all, the arc current passes the zero mark at each half-cycle. At this moment, the release of energy stops, as a result, the arc spontaneously goes out and lights up again. In practice, the current approaches zero even before crossing the zero mark. This is due to a decrease in current and a decrease in energy supplied to the arc.

Accordingly, its temperature decreases, which causes the cessation of thermal ionization. Intense deionization occurs in the arc gap itself. If at this moment you quickly open and route the contacts, then a breakdown may not occur, the circuit will turn off without the appearance of an arc.

In practice, create similar ideal conditions very difficult. In this regard, special measures have been developed to accelerate arc extinction. Various technical solutions allow you to quickly cool the arc gap and reduce the number of charged particles. As a result, there is a gradual increase in the electrical strength of this gap and a simultaneous increase in the restoring voltage on it.

Both quantities are interdependent and affect the ignition of the arc in the next half-cycle. If the electrical strength exceeds the restoring voltage, the arc will no longer ignite. Otherwise, it will burn steadily.

Basic methods of arc extinguishing

Quite often the method of arc extension is used, when in the process of divergence of contacts when the circuit is disconnected, it stretches (Fig. 1). By increasing the surface, cooling conditions are significantly improved, and to support combustion it is required higher value voltage.

1.

In another case, the total electric arc is divided into separate short arcs (Fig. 2). A special metal grid can be used for this. An electromagnetic field is induced in its plates, which draws in the arc for separation. This method widely used in switching equipment with voltages less than 1 kV. A typical example are air circuit breakers.

2.

Extinguishing in small volumes, that is, inside the arc chambers, is considered quite effective. These devices have longitudinal slots that coincide along the axes with the direction of the arc shaft. As a result of contact with cold surfaces, the arc begins to cool intensively, actively releasing charged particles into the environment.

Usage high pressure. In this case, the temperature remains unchanged, the pressure increases, and ionization decreases. Under such conditions, the arc is intensively cooled. Tightly closing chambers are used to create high pressure. The method is especially effective for fuses and other equipment.

Arc extinguishing can occur with the help of oil where the contacts are placed. When they open, an arc appears, under the influence of which the oil begins to actively evaporate. It turns out to be covered with a gas bubble or shell, consisting of 70-80% hydrogen and oil vapor. Under the influence of the released gases entering directly into the barrel area, the cold and hot gas inside the bubble mixes, intensively cooling the arc gap.

Other quenching methods

Extinguishing an electric arc can be accomplished by increasing its resistance. It gradually increases, and the current decreases to a value insufficient to maintain combustion. The main disadvantage of this method is the long extinguishing time, during which the arc dissipates large number energy.

Increasing arc resistance is achieved in different ways:

• Lengthening the arc because its resistance is in a straight line proportional dependence with length. To do this, you need to change the gap between the contacts upward.
• Cooling of the medium between the contacts where the arc is located. Most often, blowing is used, directed along the arc.
• The contacts are placed in a gas environment with a low degree of ionization or in a vacuum chamber. This method used in gas and vacuum circuit breakers.
• Cross section The arc can be reduced by passing it through a narrow hole or reducing the contact area.

In AC voltage circuits, the zero current method is used to extinguish the arc. In this case, the resistance remains at a low level until the current value drops to zero. As a result, quenching occurs naturally, and ignition does not recur, although the voltage at the contacts may increase. The drop to zero occurs at the end of each half-cycle and the arc goes out at short time. If you increase the dielectric strength of the gap between the contacts, the arc will remain extinguished.

Consequences of an electric arc

The destructive effects of an arc pose a serious danger not only to equipment, but also to working people. Under unfavorable circumstances, you can get serious burns. Sometimes arc damage is fatal.

As a rule, an electric arc occurs at the moment of accidental contact with live parts or conductors. Under the influence of a short circuit current, the wires melt, the air is ionized, and other favorable conditions are created for the formation of a plasma channel.

Currently, in the field of electrical engineering, significant positive results have been achieved with the help of modern protective equipment, designed against electric arc.