Introduction

Methods for extinguishing an electric arc... The topic is relevant and interesting. So let's begin. We ask ourselves the questions: What is an electric arc? How to control it? What processes occur during its formation? What does it consist of? And what it looks like.

What is an electric arc?

Electric arc(Volta arc, Arc discharge) is a physical phenomenon, one of the types of electrical discharge in a gas. It was first described in 1802 by the Russian scientist V.V. Petrov.

Electric arc is a special case of the fourth form of state of matter - plasma - and consists of an ionized, electrically quasi-neutral gas. Presence of free electric charges ensures the conductivity of the electric arc.

Arc formation and properties

When the voltage between two electrodes increases to a certain level, an electrical breakdown occurs in the air between the electrodes. The electrical breakdown voltage depends on the distance between the electrodes, etc. Often, to initiate breakdown at the existing voltage, the electrodes are brought closer to each other. During a breakdown, a spark discharge usually occurs between the electrodes, pulse-closing the electrical circuit.

Electrons in spark discharges ionize molecules in the air gap between the electrodes. With sufficient power of the voltage source, a sufficient amount of plasma is formed in the air gap so that the breakdown voltage (or air gap resistance) in this place drops significantly. In this case, spark discharges turn into an arc discharge - a plasma cord between the electrodes, which is a plasma tunnel. This arc is essentially a conductor, and closes the electrical circuit between the electrodes, the average current increases even more, heating the arc to 5000-50000 K. In this case, it is considered that the ignition of the arc is completed.

The interaction of electrodes with arc plasma leads to their heating, partial melting, evaporation, oxidation and other types of corrosion. An electric welding arc is a powerful electrical discharge flowing in a gaseous environment. An arc discharge is characterized by two main features: the release of a significant amount of heat and a strong light effect. Normal temperature welding arc about 6000°C.

Arc light is dazzlingly bright and is used in a variety of lighting applications. The arc radiates large number visible and invisible thermal (infrared) and chemical (ultraviolet) rays. Invisible rays cause inflammation of the eyes and burn human skin, so welders use special shields and special clothing to protect against them.

Using an arc

Depending on the environment in which the arc discharge occurs, the following welding arcs are distinguished:

1. Open arc. Burns in the air. The composition of the gas environment of the arc zone is air mixed with vapors of the metal being welded, the material of the electrodes and electrode coatings.

2. Closed arc. Burns under a layer of flux. The composition of the gas environment of the arc zone - vapor of the base metal, electrode material and protective flux.

3. Arc with supply of protective gases. Various gases are fed into the arc under pressure - helium, argon, carbon dioxide, hydrogen, illuminating gas and various mixtures of gases. The composition of the gas environment in the arc zone is an atmosphere of protective gas, vapor of the electrode material and the base metal.

The arc can be powered from DC or AC sources. In the case of DC power, a distinction is made between an arc of direct polarity (minus the power source on the electrode, plus on the base metal) and reverse polarity (minus on the base metal, plus on the electrode). Depending on the material of the electrodes, arcs are distinguished with fusible (metal) and non-fusible (carbon, tungsten, ceramic, etc.) electrodes.

When welding, the arc may be direct action(the base metal is involved in electrical circuit arc) and indirect action (the base metal does not participate in the electrical circuit of the arc). The arc of indirect action is used relatively little.

The current density in the welding arc can be different. Arcs with normal current density are used - 10--20 a/mm2 (regular manual welding, welding in some shielding gases) and with high current density - 80--120 a/mm2 and more (automatic, semi-automatic welding submerged, in protective gases).

The occurrence of an arc discharge is possible only in the case when the gas column between the electrode and the base metal is ionized, that is, it contains ions and electrons. This is achieved by imparting the appropriate energy to the gas molecule or atom, called ionization energy, as a result of which electrons are released from the atoms and molecules. The arc discharge medium can be represented as a gas conductor electric current having a round-cylindrical shape. The arc consists of three regions - the cathode region, the arc column, and the anode region.

During arc burning, active spots are observed on the electrode and base metal, which are heated areas on the surface of the electrode and base metal; The entire arc current passes through these spots. On the cathode, the spot is called cathode, on the anode - anodic. The cross section of the middle part of the arc column is several more sizes cathode and anode spots. Its size accordingly depends on the size of active spots.

The arc voltage varies depending on the current density. This dependence, depicted graphically, is called the static characteristic of the arc. At low values ​​of current density, the static characteristic has a decreasing character, i.e., the arc voltage decreases as the current increases. This is due to the fact that with increasing current, the cross-sectional area of ​​the arc column and electrical conductivity increase, and the current density and potential gradient in the arc column decrease. The magnitude of the cathode and anode arc voltage drops does not change with the current value and depends only on the electrode material, base metal, gas environment and gas pressure in the arc zone.

At current densities of the welding arc of conventional modes used for manual welding, the arc voltage does not depend on the current value, since the cross-sectional area of ​​the arc column increases in proportion to the current, and the electrical conductivity changes very little, and the current density in the arc column practically remains constant. In this case, the magnitude of the cathode and anode voltage drops remains unchanged. In an arc of high current density, with increasing current strength, the cathode spot and the cross-section of the arc column cannot increase, although the current density increases in proportion to the current strength. In this case, the temperature and electrical conductivity of the arc column increase slightly.

Voltage electric field and the potential gradient of the arc column will increase with increasing current. Cathode voltage drop increases, as a result of which the static characteristic will have an increasing character, i.e., the arc voltage will increase with increasing arc current. Increasing static characteristic is a feature of the arc high density current in various gas environments. Static characteristics refer to the steady stationary state of the arc with its length unchanged.

A stable arc burning process during welding can occur if certain conditions. The stability of the arc burning process is influenced by a number of factors; voltage idle speed arc power source, type of current, current magnitude, polarity, presence of inductance in the arc circuit, presence of capacitance, current frequency, etc.

Contribute to improving arc stability by increasing the current, open-circuit voltage of the arc power source, including inductance in the arc circuit, increasing the frequency of the current (when powered by alternating current) and a number of other conditions. Stability can also be significantly improved through the use of special electrode coatings, fluxes, shielding gases and a number of other technological factors.

extinguishing electric arc welding

LECTURE 5

ELECTRIC ARC

Occurrence and physical processes in an electric arc. Opening an electrical circuit at significant currents and voltages is accompanied by an electrical discharge between diverging contacts. The air gap between the contacts ionizes and becomes conductive, and an arc burns in it. The shutdown process consists of deionizing the air gap between the contacts, i.e., stopping the electrical discharge and restoring dielectric properties. At special conditions: low currents and voltages, a break in the alternating current circuit at the moment the current passes through zero can occur without an electrical discharge. This shutdown is called a non-sparking break.

The dependence of the voltage drop across the discharge gap on the electric discharge current in gases is shown in Fig. 1.

An electric arc is accompanied by high temperatures. Therefore, an arc is not only an electrical phenomenon, but also a thermal one. Under normal conditions, air is a good insulator. To break down a 1cm air gap, a voltage of 30kV is required. In order for the air gap to become a conductor, it is necessary to create a certain concentration of charged particles in it: free electrons and positive ions. The process of separation of electrons from a neutral particle and the formation of free electrons and positively charged ions is called ionization. Gas ionization occurs under the influence of high temperature and electric field. For arc processes in electrical devices highest value have processes at the electrodes (thermionic and field emission) and processes in the arc gap (thermal and impact ionization).

Thermionic emission called the emission of electrons from a heated surface. When the contacts diverge, the contact resistance and current density in the contact area increase sharply. The area heats up, melts, and a contact isthmus of molten metal is formed. With further divergence of the contacts, the isthmus breaks and evaporation of the metal of the contacts occurs. A hot area (cathode spot) is formed on the negative electrode, which serves as the base of the arc and the source of electron radiation. Thermionic emission causes an electric arc to occur when the contacts open. Thermionic emission current density depends on the temperature and electrode material.

Autoelectronic emissions is the phenomenon of electron emission from the cathode under the influence of a strong electric field. When the contacts are open, mains voltage is applied to them. When the contacts are closed, as the moving contact approaches the stationary one, the electric field strength between the contacts increases. At a critical distance between the contacts, the field strength reaches 1000 kV/mm. This electric field strength is sufficient to rip electrons out of the cold cathode. The field emission current is small and only serves as the beginning of an arc discharge.

Thus, the occurrence of an arc discharge at diverging contacts is explained by the presence of thermionic and field electron emissions. The occurrence of an electric arc when the contacts are closed is due to field electronic emission.

Impact ionization called the creation of free electrons and positive ions when electrons collide with a neutral particle. A free electron breaks up a neutral particle. The result will be a new free electron and a positive ion. The new electron, in turn, ionizes the next particle. In order for an electron to ionize a gas particle, it must move at a certain speed. The speed of the electron depends on the potential difference across the mean free path. Therefore, it is usually not the speed of movement of the electron that is indicated, but the minimum potential difference along the length of the free path so that the electron acquires the required speed. This potential difference is called ionization potential. The ionization potential of a gas mixture is determined by the lowest ionization potential of the components included in the gas mixture and depends little on the concentration of the components. The ionization potential for gases is 13÷16V (nitrogen, oxygen, hydrogen), for metal vapors it is approximately two times lower: 7.7V for copper vapors.

Thermal ionization occurs under the influence of high temperature. The temperature of the arc barrel reaches 4000÷7000 K, and sometimes 15000 K. At this temperature, the number and speed of moving gas particles increases sharply. When they collide, atoms and molecules are destroyed, forming charged particles. The main characteristic of thermal ionization is the degree of ionization, which is the ratio of the number of ionized atoms to the total number of atoms in the arc gap. Maintaining the resulting arc discharge with a sufficient number of free charges is ensured by thermal ionization.

Simultaneously with the ionization processes in the arc, reverse processes occur deionization– reunification of charged particles and formation of neutral molecules. When an arc occurs, ionization processes predominate; in a steadily burning arc, ionization and deionization processes are equally intense; when deionization processes predominate, the arc goes out.

Deionization occurs mainly through recombination and diffusion. Recombination is a process in which differently charged particles come into contact to form neutral particles. Diffusion charged particles is the process of removal of charged particles from the arc gap into the surrounding space, which reduces the conductivity of the arc. Diffusion is caused by both electrical and thermal factors. The density of charges in the arc shaft increases from the periphery to the center. In view of this, it is created electric field, causing ions to move from the center to the periphery and leave the arc region. The temperature difference between the arc shaft and the surrounding space also acts in the same direction. In a stabilized and freely burning arc, diffusion plays a negligible role. In an arc blown with compressed air, as well as in a rapidly moving open arc, deionization due to diffusion can be close in value to recombination. In an arc burning in a narrow gap or closed chamber, deionization occurs due to recombination.

VOLTAGE DROP ACROSS ELECTRIC ARC

The voltage drop along a stationary arc is distributed unevenly. Pattern of voltage drop change U d and longitudinal voltage gradient (voltage drop per unit arc length) E d along the arc is shown in Fig. 2.

Progress of characteristics U d And E d in the near-electrode regions differs sharply from the course of characteristics in the rest of the arc. At the electrodes, in the near-cathode and near-anode regions, over a gap of about 10 -3 mm, there is a sharp voltage drop, called near-cathode U To and anode U A . IN cathode region, a deficiency of electrons is formed due to their high mobility. A positive volumetric charge is formed in this area, which causes a potential difference U To, about 10÷20V. The field strength in the cathode region reaches 10 5 V/cm and ensures the release of electrons from the cathode due to field emission. In addition, the voltage at the cathode ensures the release of the necessary energy to heat the cathode and ensure thermionic emission.

Rice. 2. Voltage distribution across stationary DC arc

IN anode area, a negative space charge is formed, causing a potential difference U A. The electrons heading towards the anode are accelerated and knock out secondary electrons from the anode, which exist near the anode.

The total value of the near-anode and near-cathode voltage drops is called the near-electrode voltage drop:
and is 20-30V.

In the rest of the arc, called the arc shaft, the voltage drop U d directly proportional to the length of the arc:

,

Where E ST– longitudinal stress gradient in the arc shaft, l ST– arc barrel length.

The gradient here is constant along the trunk. It depends on many factors and can vary widely, reaching 100÷200 V/cm.

Thus, the voltage drop across the arc gap is:

DC ELECTRIC ARC STABILITY

To extinguish a direct current electric arc, it is necessary to create conditions under which deionization processes in the arc gap would exceed ionization processes at all current values.

For a circuit (Fig. 3) containing resistance R, inductance L, arc gap with voltage drop U d, DC voltage source U, in transition mode ( ) the Kirchhoff equation is valid: , (1) Where – voltage drop across the inductance when the current changes. With a steadily burning arc (stationary state ) expression (1) takes the form: . (2) To extinguish the arc, it is necessary that the current in it decreases all the time. This means that :

Hello to all visitors to my blog. The topic of today's article is electric arc and protection against electric arc. The topic is not random, I am writing from the Sklifosovsky Hospital. Can you guess why?

What is an electric arc

This is one of the types of electrical discharge in gas (physical phenomenon). It is also called – Arc discharge or Voltaic arc. Consists of ionized, electrically quasi-neutral gas (plasma).

It can occur between two electrodes when the voltage between them increases or approaches each other.

Briefly about properties: electric arc temperature, from 2500 to 7000 °C. Not a low temperature, however. The interaction of metals with plasma leads to heating, oxidation, melting, evaporation and other types of corrosion. Accompanied by light radiation, explosive and shock waves, ultra-high temperature, fire, release of ozone and carbon dioxide.

There is a lot of information on the Internet about what an electric arc is, what its properties are, if you are interested in more details, take a look. For example, in ru.wikipedia.org.

Now about my accident. It's hard to believe, but 2 days ago I directly encountered this phenomenon, and unsuccessfully. It happened like this: on November 21, at work, I was tasked with wiring lamps in a junction box and then connecting them to the network. There were no problems with the wiring, but when I climbed into the shield, some difficulties arose. It’s a pity I forgot my android at home, I didn’t take a photo of the electrical panel, otherwise it would have been more clear. Maybe I'll do more when I get back to work. So, the shield was very old - 3 phases, a zero bus (also known as grounding), 6 circuit breakers and a batch switch (it seemed simple), the condition initially did not inspire confidence. I struggled with it for a long time zero bus, since all the bolts were rusty, after which I easily installed the phase on the machine. Everything is fine, I checked the lamps, they work.

Afterwards, I returned to the switchboard to carefully lay the wires and close it. I would like to note that the electrical panel was located at a height of ~2 meters, in a narrow passage, and to get to it, I used a stepladder (ladder). While laying out the wires, I discovered sparks on the contacts of other machines, which caused the lamps to blink. Accordingly, I pulled out all the contacts and continued inspecting the remaining wires (to do it once and not return to this again). Having discovered that one contact on the packet has high temperature, decided to extend it too. I took a screwdriver, leaned it against the screw, turned it, bang! There was an explosion, a flash, I was thrown back, hitting the wall, I fell to the floor, nothing was visible (blinded), the shield did not stop exploding and buzzing. I don't know why the protection didn't work. Feeling the falling sparks on me, I realized that I had to get out. I got out by touch, crawling. Having got out of this narrow passage, he began to call his partner. Already at that moment I felt that with my right hand(I held the screwdriver to her) something was wrong, I felt terrible pain.

Together with my partner, we decided that we needed to run to the first aid station. I don’t think it’s worth telling what happened next, I just got injected and went to the hospital. I will never forget this terrible sound of a long short circuit - itching with a buzzing sound.

Now I’m in the hospital, I have an abrasion on my knee, the doctors think that I was electrocuted, this is the way out, so they are monitoring my heart. I believe that I was not shocked, but the burn on my hand was caused by an electric arc that occurred during a short circuit.

I don’t yet know what happened there, why the short circuit occurred, I think that when the screw was turned, the contact itself moved and a phase-to-phase short circuit occurred, or there was a bare wire behind the packet switch and when the screw approached, a electric arc. I'll find out later if they figure it out.

Damn, I went to get a bandage, they wrapped my hand so much that I’m writing with my left hand now)))

I didn’t take a photo without bandages; it was a very unpleasant sight. I don’t want to scare novice electricians….

What are the protection measures against electric arcs that could protect me? After analyzing the Internet, I saw that the most popular means of protecting people in electrical installations from electric arcs is a heat-resistant suit. In North America, special machines from Siemens are very popular, which protect against both electric arc and maximum current. In Russia, on at the moment, such machines are used only at high-voltage substations. In my case it would be enough for me dielectric glove, but think for yourself how to connect lamps to them? This is very inconvenient. I also recommend using safety glasses to protect your eyes.

In electrical installations, the fight against an electric arc is carried out using vacuum and oil switches, as well as using electromagnetic coils together with arc extinguishing chambers.

This is all? No! The most reliable way to protect yourself from an electric arc, in my opinion, is stress relief work . I don’t know about you, but I won’t work under voltage anymore...

That's it for my article electric arc And arc protection ends. Do you have anything to add? Leave a comment.

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 a 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 spot, which are the bases of the arc column through which the entire welding current. 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 length welding arc 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. During manual arc welding with coated electrodes, gas-shielded welding with a non-consumable electrode and submerged arc welding at relatively low current densities, the arc characteristic will initially decrease, 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 due to an increase in the speed of movement of electrons and ions, i.e., due to an increase in the electric field strength. 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 useful action(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.

2.1. NATURE OF WELDING ARC

An electric arc is one of the types of electrical discharges in gases, in which the passage of an electric current through a gas gap under the influence of an electric field is observed. The electric arc used to weld metals is called a welding arc. The arc is part of the electrical welding circuit and experiences a voltage drop across it. When welding with direct current, the electrode connected to the positive pole of the arc power source is called the anode, and to the negative pole is called the cathode. If welding is carried out on alternating current, each of the electrodes is alternately an anode and a cathode.

The space between the electrodes is called the arc area or arc gap. The length of the arc gap is called the arc length. IN normal conditions at low temperatures gases consist of neutral atoms and molecules and do not have electrical conductivity. The passage of electric current through a gas is possible only if it contains charged particles - electrons and ions. The process of formation of charged gas particles is called ionization, and the gas itself is called ionized. The appearance of charged particles in the arc gap is caused by the emission (emission) of electrons from the surface of the negative electrode (cathode) and the ionization of gases and vapors located in the gap. The arc burning between the electrode and the welding object is a direct arc. Such an arc is usually called a free arc, in contrast to a compressed arc, the cross-section of which is forcibly reduced due to the burner nozzle, gas flow, and electromagnetic field. The arc is excited as follows. When there is a short circuit, the electrode and the parts where they touch the surfaces heat up. When the electrodes are opened from the heated surface of the cathode, electrons are emitted - electron emission. The yield of electrons is primarily associated with the thermal effect (thermionic emission) and the presence of a high-intensity electric field at the cathode (field emission). The presence of electron emission from the cathode surface is an indispensable condition for the existence of an arc discharge.

Along the length of the arc gap, the arc is divided into three regions (Fig. 2.1): cathode, anode and the arc column located between them.

The cathode region includes the heated surface of the cathode, called the cathode spot, and the part of the arc gap adjacent to it. The length of the cathode region is small, but it is characterized by increased tension and the processes of obtaining electrons occurring in it, which are a necessary condition for the existence of an arc discharge. The cathode spot temperature for steel electrodes reaches 2400-2700 °C. It releases up to 38% of the total arc heat. The main physical process in this area is electron emission and acceleration of electrons. The voltage drop in the cathode region of the IR is about 12-17 V.

The anode region consists of an anode spot on the surface of the anode and part of the arc gap adjacent to it. The current in the anode region is determined by the flow of electrons coming from the arc column. The anode spot is the site of entry and neutralization of free electrons in the anode material. It has approximately the same temperature as the cathode spot, but as a result of electron bombardment, more heat is released on it than on the cathode. The anode region is also characterized by increased tension. The voltage drop Ua in it is of the order of 2-11 V. The extent of this region is also small.

The arc column occupies the greatest extent of the arc gap, located between the cathode and anode regions. The main process of formation of charged particles here is gas ionization. This process occurs as a result of the collision of charged (primarily electrons) and neutral gas particles. With sufficient collision energy, electrons are knocked out of gas particles and positive ions are formed. This ionization is called collision ionization. A collision can occur without ionization, then the collision energy is released in the form of heat and goes to increase the temperature of the arc column. The charged particles formed in the arc column move to the electrodes: electrons to the anode, ions to the cathode. Some of the positive ions reach the cathode spot, while the other part does not, and by adding negatively charged electrons to themselves, the ions become neutral atoms.

This process of particle neutralization is called recombination. In the arc column, under all combustion conditions, a stable equilibrium is observed between the processes of ionization and recombination. In general, the arc column has no charge. It is neutral, since in each section there are simultaneously equal quantities oppositely charged particles. The temperature of the arc column reaches 6000-8000 °C or more. The voltage drop in it (Uc) varies almost linearly along the length, increasing with increasing length of the column. The voltage drop depends on the composition of the gaseous medium and decreases with the introduction of easily ionized components into it. Such components are alkaline and alkaline earth elements (Ca, Na, K, etc.). The total voltage drop in the arc is Ud=Uk+Ua+Uc. Taking the voltage drop in the arc column in the form of a linear dependence, it can be represented by the formula Uc=Elc, where E is the tension along the length, lc is the length of the column. The values ​​of IR, Ua, E practically depend only on the material of the electrodes and the composition of the arc gap medium and, if they remain unchanged, remain constant at different conditions welding Due to the small extent of the cathode and anode regions, it can be considered practically 1s = 1d. Then we get the expression

II)( = a + Н)(, (2.1)

showing that the arc voltage directly depends on its length, where a = ik + ia; b=E. An indispensable condition for obtaining quality welded joint is the stable burning of the arc (its stability). By this we mean such a mode of its existence in which the arc long time burns at specified values ​​of current and voltage, without interruption or transition to other types of discharges. With a stable burning of the welding arc, its main parameters - current and voltage - are in a certain interdependence. Therefore, one of the main characteristics of an arc discharge is the dependence of its voltage on the current strength at a constant arc length. Graphic image This dependence when operating in static mode (in a state of stable arc burning) is called the static current-voltage characteristic of the arc (Fig. 2.2).

As the length of the arc increases, its voltage increases and the curve of the static current-voltage characteristic rises; higher, with a decrease in the length of the arc, it drops lower, while qualitatively maintaining its shape. The static characteristic curve can be divided into three regions: falling, hard and rising. In the first region, an increase in current leads to a sharp drop in arc voltage. This is due to the fact that with increasing current strength, the cross-sectional area of ​​the arc column and its electrical conductivity increase. Arc burning in regimes in this region is characterized by low stability. In the second region, the increase in current strength is not associated with a change in arc voltage. This is explained by the fact that the cross-sectional area of ​​the arc column and active spots changes in proportion to the current strength, and therefore the current density and voltage drop in the arc remain constant. Arc welding with a rigid static characteristic finds wide application in welding technology, especially in manual welding. In the third region, as the current increases, the voltage increases. This is due to the fact that the diameter of the cathode spot becomes equal to the diameter of the electrode and cannot increase further, while the current density in the arc increases and the voltage drops. An arc with increasing static characteristics is widely used in automatic and mechanized submerged arc and shielding gas welding using thin welding wire.

Rice. 2.3. Statistical current-voltage characteristic of the arc at different speeds electrode wire feed: a - low speed; b — average speed, in — high speed

When mechanized welding with a consumable electrode, a static current-voltage characteristic of the arc is sometimes used, taken not at a constant length, but at a constant feed speed of the electrode wire (Fig. 2.3).

As can be seen from the figure, each electrode wire feed speed corresponds to a narrow range of currents with stable arc burning. Too little welding current can lead to a short circuit between the electrode and the workpiece, and too much can lead to a sharp increase in voltage and breakage.