Electric welding arc is a long-term electrical discharge in plasma, which is a mixture of ionized gases and vapors of components of the protective atmosphere, filler and base metal.
The arc got its name from characteristic shape, which it takes when burning between two horizontally located electrodes; heated gases tend to rise upward and this electrical discharge bends, taking the shape of an arch or arc.
From a practical point of view, the arc can be considered as a gas conductor that transforms electrical energy to thermal. It provides high heating intensity and is easily controlled through electrical parameters.
A general characteristic of gases is that they are normal conditions are not conductors electric current. However, under favorable conditions (high temperature and the presence of external electric field high voltage) gases can be ionized, i.e. their atoms or molecules can release or, for electronegative elements, on the contrary, capture electrons, turning into positive or negative ions, respectively. Thanks to these changes, gases move into the fourth state of matter called plasma, which is electrically conductive.
Excitation of the welding arc occurs in several stages. For example, when welding MIG/MAG, when the end of the electrode and the part being welded come into contact, contact occurs between the micro protrusions of their surfaces. The high current density contributes to the rapid melting of these protrusions and the formation of a layer of liquid metal, which constantly increases towards the electrode, and eventually ruptures.
At the moment the jumper breaks, rapid evaporation of the metal occurs, and the discharge gap is filled with ions and electrons that arise in this case. Due to the fact that voltage is applied to the electrode and the product, electrons and ions begin to move: electrons and negatively charged ions to the anode, and positively charged ions to the cathode, and thus a welding arc is excited. After the arc is excited, the concentration of free electrons and positive ions in the arc gap continues to increase, since electrons collide with atoms and molecules on their way and “knock out” even more electrons from them (at the same time, atoms that have lost one or more electrons become positively charged ions ). Intense ionization of the gas in the arc gap occurs and the arc acquires the character of a stable arc discharge.
A few fractions of a second after the arc is excited, a weld pool begins to form on the base metal, and a drop of metal begins to form at the end of the electrode. And after about another 50 - 100 milliseconds, a stable transfer of metal from the end of the electrode wire into the weld pool is established. It can be carried out either by drops that freely fly over the arc gap, or by drops that first form a short circuit and then flow into the weld pool.
The electrical properties of the arc are determined by the processes occurring in its three characteristic zones - the column, as well as in the near-electrode regions of the arc (cathode and anode), which are located between the arc column on one side and the electrode and the product on the other.
To maintain the arc plasma when welding with a consumable electrode, it is enough to provide a current of 10 to 1000 amperes and apply an electric voltage of about 15 to 40 volts between the electrode and the product. In this case, the voltage drop across the arc column itself will not exceed several volts. The remaining voltage drops at the cathode and anode regions of the arc. The length of the arc column on average reaches 10 mm, which corresponds to approximately 99% of the arc length. Thus, the electric field strength in the arc column lies in the range from 0.1 to 1.0 V/mm. The cathode and anode regions, on the contrary, are characterized by a very short length (about 0.0001 mm for the cathode region, which corresponds to the mean free path of the ion, and 0.001 mm for the anodic region, which corresponds to the mean free path of the electron). Accordingly, these regions have a very high electric field strength (up to 104 V/mm for the cathode region and up to 103 V/mm for the anodic region).
It has been experimentally established that for the case of welding with a consumable electrode, the voltage drop in the cathode region exceeds the voltage drop in the anode region: 12 - 20 V and 2 - 8 V, respectively. Considering that heat generation at objects electrical circuit depends on current and voltage, it becomes clear that when welding with a consumable electrode, more heat is released in the area where more voltage drops, i.e. in the cathode. Therefore, when welding with a consumable electrode, mainly the reverse polarity of the welding current is used, when the product serves as the cathode to ensure deep penetration of the base metal (in this case, the positive pole of the power source is connected to the electrode). Direct polarity is sometimes used when performing surfacing (when the penetration of the base metal, on the contrary, is desirable to be minimal).
Under TIG welding conditions (non-consumable electrode welding), the cathode voltage drop, on the contrary, is significantly lower than the anode voltage drop and, accordingly, under these conditions more heat is generated at the anode. Therefore, when welding with a non-consumable electrode, to ensure deep penetration of the base metal, the product is connected to the positive terminal of the power source (and it becomes the anode), and the electrode is connected to the negative terminal (thus, also protecting the electrode from overheating).
In this case, regardless of the type of electrode (consumable or non-consumable), heat is generated mainly in the active regions of the arc (cathode and anode), and not in the arc column. This property of the arc is used to melt only those areas of the base metal to which the arc is directed.
Those parts of the electrodes through which the arc current passes are called active spots (on the positive electrode - anode spot, and on the negative electrode - cathode spot). The cathode spot is a source of free electrons, which contribute to the ionization of the arc gap. At the same time, streams of positive ions rush towards the cathode, bombarding it and transferring their kinetic energy to it. The temperature on the cathode surface in the area of the active spot during welding with a consumable electrode reaches 2500 ... 3000 °C.
Lk - cathode region; La - anode region (La = Lk = 10 -5 -10 -3 cm); Lst - arc column; Ld - arc length; Ld = Lk + La + Lst
Streams of electrons and negatively charged ions rush to the anode spot, which transfer their kinetic energy to it. The temperature on the anode surface in the area of the active spot during welding with a consumable electrode reaches 2500 ... 4000°C. The temperature of the arc column when welding with a consumable electrode ranges from 7,000 to 18,000 ° C (for comparison: the melting point of steel is approximately 1500 ° C).
Influence on the arc of magnetic fields
When welding with direct current, a phenomenon such as magnetic is often observed. It is characterized by the following features:
The welding arc column sharply deviates from its normal position;
- the arc burns unsteadily and often breaks off;
- the sound of the arc burning changes - popping sounds appear.
Magnetic blast disrupts the formation of the seam and can contribute to the appearance of such defects in the seam as lack of penetration and lack of fusion. The cause of magnetic blast is the interaction magnetic field welding arc with other nearby magnetic fields or ferromagnetic masses.
The welding arc column can be considered as part of the welding circuit in the form of a flexible conductor around which there is a magnetic field.
As a result of the interaction of the magnetic field of the arc and the magnetic field that arises in the part being welded during the passage of current, the welding arc is deflected in the direction opposite to the place where the current conductor is connected.
The influence of ferromagnetic masses on arc deflection is due to the fact that, due to the large difference in resistance to the passage of magnetic field lines of the arc through air and through ferromagnetic materials (iron and its alloys), the magnetic field turns out to be more concentrated on the side opposite to the location of the mass, so the arc column shifts to the side ferromagnetic body.
The magnetic field of the welding arc increases with increasing welding current. Therefore, the effect of magnetic blast is more often manifested when welding at high conditions.
You can reduce the influence of magnetic blast on the welding process:
Performing short arc welding;
- tilting the electrode so that its end is directed towards the action of the magnetic blast;
- bringing the current supply closer to the arc.
The effect of magnetic blast can also be reduced by replacing direct welding current with alternating current, in which the magnetic blast appears much less. However, it must be remembered that the arc AC less stable, since due to a change in polarity it goes out and lights up again 100 times per second. In order for the alternating current arc to burn stably, it is necessary to use arc stabilizers (easily ionized elements), which are introduced, for example, into the electrode coating or into the flux.
Material from Wikipedia - the free encyclopedia
Electric arc (voltaic arc, arc discharge) - a physical phenomenon, one of the types of electrical discharge in a gas.
Arc structure
The electric arc consists of cathode and anode regions, arc column, and transition regions. The thickness of the anode region is 0.001 mm, the cathode region is about 0.0001 mm.
The temperature in the anodic region when welding with a consumable electrode is about 2500 ... 4000 ° C, the temperature in the arc column is from 7,000 to 18,000 ° C, in the cathode region - 9,000 - 12,000 ° C.
The arc column is electrically neutral. In any of its sections there are the same number of charged particles of opposite signs. The voltage drop in the arc column is proportional to its length.
Welding arcs are classified according to:
- Electrode materials - with consumable and non-consumable electrode;
- Degrees of column compression - free and compressed arc;
- According to the current used - DC arc and AC arc;
- According to the polarity of direct electric current - direct polarity ("-" on the electrode, "+" - on the product) and reverse polarity;
- When using alternating current - single-phase and three-phase arcs.
Arc self-regulation
When external compensation occurs - changes in network voltage, wire feed speed, etc., a disturbance occurs in the established equilibrium between the feed speed and the melting rate. As the length of the arc in the circuit increases, the welding current and the melting speed of the electrode wire decrease, and the feed speed, while remaining constant, becomes greater than the melting speed, which leads to the restoration of the arc length. As the arc length decreases, the wire melting speed becomes greater than the feed speed, this leads to the restoration of the normal arc length.
The efficiency of the arc self-regulation process is significantly influenced by the shape of the current-voltage characteristic of the power source. The high speed of arc length oscillations is processed automatically with rigid current-voltage characteristics of the circuit.
Fighting an electric arc
In a number of devices, the phenomenon of an electric arc is harmful. These are primarily contact switching devices used in power supply and electric drives: high-voltage circuit breakers, circuit breakers, contactors, sectional insulators on the contact network of electrified railways and urban electric transport. When the loads are disconnected by the above devices, an arc occurs between the opening contacts.
The mechanism of arc occurrence in this case is as follows:
- Reducing contact pressure - the number of contact points decreases, the resistance in the contact unit increases;
- The beginning of contact divergence - the formation of “bridges” from the molten metal of the contacts (at the last contact points);
- Rupture and evaporation of “bridges” from molten metal;
- Formation of an electric arc in metal vapor (which contributes to greater ionization of the contact gap and difficulty in extinguishing the arc);
- Stable arc burning with fast burnout of contacts.
To minimize damage to the contacts, it is necessary to extinguish the arc in a minimum time, making every effort to prevent the arc from remaining in one place (as the arc moves, the heat released in it will be evenly distributed over the contact body).
To meet the above requirements, the following arc control methods are used:
- arc cooling by a flow of cooling medium - liquid (oil switch); gas - (air circuit breaker, autogas circuit breaker, oil circuit breaker, SF6 gas circuit breaker), and the flow of the cooling medium can pass both along the arc barrel (longitudinal quenching) and across (transverse quenching); sometimes longitudinal-transverse damping is used;
- use of the arc-extinguishing ability of vacuum - it is known that when the pressure of the gases surrounding the switched contacts is reduced to a certain value, a vacuum circuit breaker leads to effective extinguishing of the arc (due to the absence of carriers for arc formation).
- use of more arc-resistant contact material;
- use of contact material with a higher ionization potential;
- use of arc extinguishing grids (circuit breaker, electromagnetic switch). The principle of using arc extinguishing on gratings is based on the use of the effect of near-cathode drop in the arc (most of the voltage drop in the arc is the voltage drop at the cathode; the arc extinguishing grating is actually a series of serial contacts for the arc that gets there).
- use of arc extinguishing chambers - entering a chamber made of an arc-resistant material, such as mica plastic, with narrow, sometimes zigzag channels, the arc stretches, contracts and is intensively cooled from contact with the walls of the chamber.
- the use of “magnetic blast” - since the arc is highly ionized, it can be considered as a first approximation as a flexible conductor with current; By creating a magnetic field with special electromagnets (connected in series with the arc), it is possible to create arc movement to uniformly distribute heat across the contact, and to drive it into the arc-extinguishing chamber or grid. Some switch designs create a radial magnetic field that imparts torque to the arc.
- bypassing of contacts at the moment of opening by a power semiconductor switch with a thyristor or triac connected in parallel with the contacts; after opening the contacts, the semiconductor switch is turned off at the moment the voltage passes through zero (hybrid contactor, thyricon).
See also
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Literature
- Electric arc- article from.
- Spark discharge- article from the Great Soviet Encyclopedia.
- Raiser Yu. P. Physics of gas discharge. - 2nd ed. - M.: Nauka, 1992. - 536 p. - ISBN 5-02014615-3.
- Rodshtein L. A. Electrical devices, L 1981
- Clerici, Matteo; Hu, Yi; Lassonde, Philippe; Milian, Carles; Couairon, Arnaud; Christodoulides, Demetrios N.; Chen, Zhigang; Razzari, Luca; Vidal, François (2015-06-01). "Laser-assisted guiding of electric discharges around objects". Science Advances 1(5):e1400111. Bibcode:2015SciA....1E0111C. doi:10.1126/sciadv.1400111. ISSN 2375-2548.
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An excerpt characterizing the electric arc
– On fera du chemin cette fois ci. Oh! quand il s"en mele lui meme ca chauffe... Nom de Dieu... Le voila!.. Vive l"Empereur! Les voila donc les Steppes de l"Asie! Vilain pays tout de meme. Au revoir, Beauche; je te reserve le plus beau palais de Moscow. Au revoir! Bonne chance... L"as tu vu, l"Empereur? Vive l" Empereur!.. preur! Si on me fait gouverneur aux Indes, Gerard, je te fais ministre du Cachemire, c"est arrete. Vive l"Empereur! Vive! vive! vive! Les gredins de Cosaques, comme ils filent. Vive l"Empereur! Le voila! Le vois tu? Je l"ai vu deux fois comme jete vois. Le petit caporal... Je l"ai vu donner la croix a l"un des vieux... Vive l"Empereur!.. [Now let's go! Oh! as soon as he takes charge, things will boil. By God... Here he is... Hurray, Emperor! So here they are, the Asian steppes... However, good-bye, Bose. I will leave you the best palace in Moscow. Goodbye. Have you seen the emperor? Here he is! I saw him twice like you. Little corporal... I saw how he hung a cross on one of the old men... Hurray, emperor!] - said the voices of old and young people, of the most diverse characters and positions in society. All the faces of these people had one common expression of joy at the beginning of the long-awaited campaign and delight and devotion to the man in a gray frock coat standing on the mountain.On June 13, Napoleon was given a small purebred Arabian horse, and he sat down and galloped to one of the bridges over the Neman, constantly deafened by enthusiastic cries, which he obviously endured only because it was impossible to forbid them to express their love for him with these cries; but these screams, accompanying him everywhere, weighed on him and distracted him from the military worries that had gripped him since the time he joined the army. He drove across one of the bridges swinging on boats to the other side, turned sharply to the left and galloped towards Kovno, preceded by enthusiastic Guards horse rangers who were transfixed with happiness, clearing the way for the troops galloping ahead of him. Arriving at the wide Viliya River, he stopped next to a Polish Uhlan regiment stationed on the bank.
- Vivat! – the Poles also shouted enthusiastically, disrupting the front and pushing each other in order to see him. Napoleon examined the river, got off his horse and sat down on a log lying on the bank. At a wordless sign, a pipe was handed to him, he placed it on the back of a happy page who ran up and began to look at the other side. Then he went deep into examining a sheet of map laid out between the logs. Without raising his head, he said something, and two of his adjutants galloped towards the Polish lancers.
- What? What did he say? - was heard in the ranks of the Polish lancers when one adjutant galloped up to them.
It was ordered to find a ford and cross to the other side. The Polish Lancer colonel, a handsome old man, flushed and confused in his words with excitement, asked the adjutant if he would be allowed to swim across the river with his Lancers without looking for a ford. He, with obvious fear of refusal, like a boy who asks permission to mount a horse, asked to be allowed to swim across the river in the eyes of the emperor. The adjutant said that the emperor would probably not be dissatisfied with this excessive zeal.
As soon as the adjutant said this, an old mustachioed officer with a happy face and sparkling eyes, raising his saber, shouted: “Vivat! - and, commanding the lancers to follow him, he gave spurs to his horse and galloped up to the river. He angrily pushed the horse that had hesitated beneath him and fell into the water, heading deeper into the rapids of the current. Hundreds of lancers galloped after him. It was cold and terrible in the middle and at the rapids of the current. The lancers clung to each other, fell off their horses, some horses drowned, people drowned too, the rest tried to swim, some on the saddle, some holding the mane. They tried to swim forward to the other side and, despite the fact that there was a crossing half a mile away, they were proud that they were swimming and drowning in this river under the gaze of a man sitting on a log and not even looking at what they were doing. When the returning adjutant, having chosen a convenient moment, allowed himself to draw the emperor’s attention to the devotion of the Poles to his person, a small man in a gray frock coat stood up and, calling Berthier to him, began to walk with him back and forth along the shore, giving him orders and occasionally looking displeasedly at the drowning lancers who entertained his attention.
It was not new for him to believe that his presence at all ends of the world, from Africa to the steppes of Muscovy, equally amazes and plunges people into the madness of self-forgetfulness. He ordered a horse to be brought to him and rode to his camp.
About forty lancers drowned in the river, despite the boats sent to help. Most washed back to this shore. The colonel and several people swam across the river and with difficulty climbed out to the other bank. But as soon as they got out with their wet dress flopping around them and dripping in streams, they shouted: “Vivat!”, looking enthusiastically at the place where Napoleon stood, but where he was no longer there, and at that moment they considered themselves happy.
In the evening, Napoleon, between two orders - one about delivering the prepared counterfeit Russian banknotes for import into Russia as soon as possible, and the other about shooting the Saxon, in whose intercepted letter information about orders for the French army was found - made a third order - about the inclusion of the Polish colonel, who unnecessarily threw himself into the river, into the cohort of honor (Legion d'honneur), of which Napoleon was the head.
Qnos vult perdere – dementat. [Whoever he wants to destroy, he will deprive him of his mind (lat.)]
Meanwhile, the Russian emperor had already lived in Vilna for more than a month, making reviews and maneuvers. Nothing was ready for the war that everyone expected and for which the emperor came from St. Petersburg to prepare. There was no general plan of action. Hesitation about which plan, out of all those that were proposed, should be adopted, only intensified even more after the emperor's month-long stay in the main apartment. The three armies each had a separate commander-in-chief, but there was no common commander over all the armies, and the emperor did not assume this title.
How lived longer The emperor in Vilna prepared less and less for war, tired of waiting for it. All the aspirations of the people surrounding the sovereign seemed to be aimed only at making the sovereign, while having a pleasant time, forget about the upcoming war.
After many balls and celebrations among the Polish magnates, among the courtiers and the sovereign himself, in June one of the Polish general adjutants of the sovereign came up with the idea of giving a dinner and ball to the sovereign on behalf of his general adjutants. This idea was joyfully accepted by everyone. The Emperor agreed. The general's adjutants collected money by subscription. The person who could be most pleasing to the sovereign was invited to be the hostess of the ball. Count Bennigsen, a landowner of the Vilna province, offered his country house for this holiday, and on June 13 a dinner, a ball, boating and fireworks were scheduled in Zakret, country house Count Bennigsen.
On the very day on which Napoleon gave the order to cross the Neman and his advanced troops, pushing back the Cossacks, crossed the Russian border, Alexander spent the evening at Bennigsen’s dacha - at a ball given by the general’s adjutants.
It was a cheerful, brilliant holiday; experts in the business said that rarely so many beauties gathered in one place. Countess Bezukhova, along with other Russian ladies who came for the sovereign from St. Petersburg to Vilna, was at this ball, darkening the sophisticated Polish ladies with her heavy, so-called Russian beauty. She was noticed, and the sovereign honored her with a dance.
Boris Drubetskoy, en garcon (a bachelor), as he said, having left his wife in Moscow, was also at this ball and, although not an adjutant general, was a participant at a large amount in the subscription for the ball. Boris was now a rich man, far gone in honors, no longer looking for patronage, but straight leg standing with the highest of his peers.
At twelve o'clock at night they were still dancing. Helen, who did not have a worthy gentleman, herself offered the mazurka to Boris. They sat in the third pair. Boris, coolly looking at Helen's shiny bare shoulders protruding from her dark gauze and gold dress, talked about old acquaintances and at the same time, unnoticed by himself and others, never for a second stopped watching the sovereign, who was in the same hall. The Emperor did not dance; he stood in the doorway and stopped first one or the other with those gentle words that he alone knew how to speak.
At the beginning of the mazurka, Boris saw that General Adjutant Balashev, one of the closest persons to the sovereign, approached him and stood un-courtly close to the sovereign, who was speaking with a Polish lady. After talking with the lady, the sovereign looked questioningly and, apparently realizing that Balashev acted this way only because there were important reasons, nodded slightly to the lady and turned to Balashev. As soon as Balashev began to speak, surprise was expressed on the sovereign’s face. He took Balashev by the arm and walked with him through the hall, unconsciously clearing three fathoms of wide road on both sides of those who were shunning in front of him. Boris noticed Arakcheev's excited face while the sovereign walked with Balashev. Arakcheev, looking from under his brows at the sovereign and snoring his red nose, moved out of the crowd, as if expecting that the sovereign would turn to him. (Boris realized that Arakcheev was jealous of Balashev and was dissatisfied that some obviously important news was not conveyed to the sovereign through him.)
But the sovereign and Balashev walked, without noticing Arakcheev, through the exit door into the illuminated garden. Arakcheev, holding his sword and looking around angrily, walked about twenty paces behind them.
When it comes to the characteristics of a voltaic arc, it is worth mentioning that it has a lower voltage than a glow discharge and relies on thermionic radiation of electrons from the electrodes that support the arc. In English-speaking countries, the term is considered archaic and outdated.
Arc suppression techniques can be used to reduce the duration or likelihood of arc formation.
In the late 1800s, the voltaic arc was widely used for public lighting. Some low pressure electric arcs are used in many applications. For example, for lighting they use fluorescent lamps, mercury, sodium and metal halide lamps. Xenon arc lamps used for film projectors.
Opening a voltaic arc
The phenomenon is believed to have been first described by Sir Humphry Davy in an 1801 article published in William Nicholson's Journal of Natural Philosophy, Chemistry and Arts. However, the phenomenon described by Davy was not an electric arc, but only a spark. Later researchers wrote: “This is obviously a description not of an arc, but of a spark. The essence of the first is that it must be continuous, and its poles must not touch after it has arisen. The spark produced by Sir Humphry Davy was clearly not continuous, and although it remained charged for some time after contact with the carbon atoms, there was probably no arc connection required for its classification as voltaic.”
That same year, Davy publicly demonstrated the effect before the Royal Society by passing an electric current through two touching carbon rods and then pulling them a short distance apart. The demonstration showed a "weak" arc, barely distinguishable from a sustained spark, between the points charcoal. The scientific community has provided him with more powerful battery of 1000 plates, and in 1808 he demonstrated the occurrence of a voltaic arc on a large scale. He is also credited with naming it in English (electric arc). He called it an arc because it takes the shape of an ascending bow when the distance between the electrodes becomes close. This is due to the conductive properties of hot gas.
How did the voltaic arc appear? The first continuous arc was independently observed in 1802 and described in 1803 as a "special liquid with electrical properties" by Russian scientist Vasily Petrov, experimenting with a copper-zinc battery consisting of 4,200 disks.
Further Study
In the late nineteenth century, the voltaic arc was widely used for public lighting. The tendency of electrical arcs to flicker and hiss was a serious problem. In 1895, Hertha Marx Ayrton wrote a series of articles on electricity, explaining that the voltaic arc was the result of oxygen coming into contact with the carbon rods used to create the arc.
In 1899, she was the first woman ever to read her own paper before the Institution of Electrical Engineers (IEE). Her report was entitled "The Mechanism of the Electric Arc." Shortly afterwards, Ayrton was elected as the first female member of the Institution of Electrical Engineers. The next woman was admitted to the institute in 1958. Ayrton applied to read a paper before the Royal Society, but she was not allowed to do so because of her gender, and The Mechanism of the Electric Arc was read in her place by John Perry in 1901.
Description
An electric arc is the type with the highest current density. The maximum amount of current carried by the arc is limited only by the external environment and not by the arc itself.
An arc between two electrodes can be initiated by ionization and glow discharge when the current through the electrodes increases. The electrode gap breakdown voltage is a combined function of pressure, the distance between the electrodes, and the type of gas surrounding the electrodes. When an arc begins, its terminal voltage is much lower than that of a glow discharge, and the current is higher. An arc in gases near atmospheric pressure is characterized by visible light, high density current and high temperature. It differs from a glow discharge in approximately the same effective temperatures of both electrons and positive ions, and in a glow discharge the ions have a much lower thermal energy than electrons.
When welding
An extended arc can be initiated by two electrodes initially in contact and separated during the experiment. This action can initiate an arc without a high voltage glow discharge. This is the way in which the welder begins welding a joint by instantly touching welding electrode to the subject.
Another example is the separation of electrical contacts on switches, relays or circuit breakers. High energy circuits may require arc suppression to prevent contact damage.
Voltaic arc: characteristics
Electrical resistance along a continuous arc creates heat that ionizes more gas molecules (where the degree of ionization is determined by temperature), and according to this sequence the gas gradually turns into thermal plasma, which is in thermal equilibrium, since the temperature is distributed relatively uniformly across all atoms, molecules, ions and electrons. The energy transferred by electrons is quickly dispersed with heavier particles due to elastic collisions due to their high mobility and large numbers.
The current in the arc is maintained by thermionic and field emission of electrons at the cathode. The current can be concentrated into a very small hot spot on the cathode - on the order of a million amperes per square centimeter. Unlike a glow discharge, the arc has a subtle structure, since the positive column is quite bright and extends almost to the electrodes at both ends. The cathode drop and the anode drop of several volts occur within a fraction of a millimeter of each electrode. The positive column has a lower voltage gradient and may be absent in very short arcs.
Low frequency arc
A low frequency (less than 100 Hz) AC arc resembles a DC arc. At each cycle, the arc is initiated by breakdown and the electrodes switch roles as the current changes direction. As the frequency of the current increases, there is not enough time to ionize at the divergence of each half cycle, and breakdown is no longer needed to maintain the arc - the voltage and current characteristics become more ohmic.
Place among other physical phenomena
Various shapes Electric arcs are emergent properties of nonlinear models of current and electric field. The arc occurs in the gas-filled space between two conductive electrodes (often tungsten or carbon), resulting in very high temperatures capable of melting or vaporizing most materials. An electric arc is a continuous discharge, while a similar electric spark discharge is instantaneous. A voltaic arc can occur either in direct current circuits or in alternating current circuits. In the latter case, it can strike again every half-cycle of current generation. An electric arc differs from a glow discharge in that the current density is quite high and the voltage drop inside the arc is low. At the cathode, the current density can reach one megaampere per square centimeter.
Destructive potential
An electric arc has a nonlinear relationship between current and voltage. Once the arc has been created (either by progression from the glow discharge or by momentarily touching the electrodes and then separating them), increasing the current results in more low voltage between arc terminals. This negative resistance effect requires that some positive form of impedance (like electrical ballast) be placed in the circuit to maintain a stable arc. This property is why uncontrolled electrical arcs in a device become so destructive, because once the arc occurs, it will draw more and more current from the DC voltage source until the device is destroyed.
Practical Application
IN industrial scale Electric arcs are used for welding, plasma cutting, electric discharge machining, as an arc lamp in film projectors and in lighting. Electric arc furnaces are used to produce steel and other substances. Calcium carbide is obtained in this way, since to achieve an endothermic reaction (at temperatures of 2500 ° C) it is required large number energy.
Carbon arc lights were the first electric lights. They were used for street lamps in the 19th century and for specialized devices such as floodlights until World War II. Today, low pressure electric arcs are used in many areas. For example, fluorescent lamps, mercury vapor lamps, sodium vapor lamps and metal halide lamps are used for lighting, while xenon arc lamps are used for film projectors.
The formation of an intense electrical arc, similar to a small-scale arc flash, is the basis of explosive detonators. When scientists learned what a voltaic arc is and how it can be used, the variety of world weapons was replenished with effective explosives.
The main remaining application is high voltage switchgear for transmission networks. Modern devices also use sulfur hexafluoride under high pressure.
Conclusion
Despite the frequency of voltaic arc burns, it is considered a very useful physical phenomenon, still widely used in industry, production and the creation of decorative objects. She has her own aesthetic, and her image often appears in science fiction films. Voltage arc injury is not fatal.
TO category:
Assembly of metal structures
Electric arc and its properties
An electric arc is a long-term electrical discharge occurring in the gas gap between two conductors - the electrode and the metal being welded at a significant current. The ionization of the air layer, which continuously arises under the influence of a rapid flow of positive and negative ions and electrons in the arc, creates necessary conditions for long, stable burning of the welding arc.
Rice. 1. Electric arc between a metal electrode and the metal being welded: a - diagram of the arc, b - graph of arc voltages 4 mm long; 1 - electrode, 2 - flame halo, 3 - arc column, 4 - metal being welded, 5 - anode spot, 6 - molten pool, 7 - crater, 8 - cathode spot; h - depth of penetration in the arc, A - moment of arc ignition, B - moment of stable combustion
The arc consists of a column, the base of which is located in a depression (crater) formed on the surface of the molten pool. The arc is surrounded by a halo of flame formed by vapors and gases coming from the arc column. The column has the shape of a cone and is the main part of the arc, since the main amount of energy is concentrated in it, corresponding to the highest density of the electric current passing through the arc. Upper part The column located on electrode 1 (cathode) has a small diameter and forms a cathode spot 8. The largest number of electrodes emit through the cathode spot. The base of the arc column cone is located on the metal being welded (anode) and forms the anode spot. Anode spot diameter at average welding current values larger diameter cathode spot approximately 1.5 ... 2 times.
Direct and alternating current are used for welding. When using direct current, the minus of the current source is connected to the electrode (straight polarity) or to the workpiece being welded “” (reverse polarity). Reverse polarity is used in cases where it is necessary to reduce the release of heat on the product being welded: when welding thin or low-melting metal, alloy, stainless and high-carbon steels that are sensitive to overheating, as well as when using certain types of electrodes.
Producing a large amount of heat and having a high temperature. electric arc At the same time, it gives very concentrated heating of the metal. Therefore, during welding, the metal remains relatively slightly heated even at a distance of several centimeters from the welding arc.
The action of the arc melts the metal to a certain depth h, called the depth of penetration or penetration.
The arc is excited when the electrode approaches the metal being welded and short-circuits the welding circuit. Due to the high resistance at the point of contact of the electrode with the metal, the end of the electrode quickly heats up and begins to emit a stream of electrons. When the end of the electrode is quickly moved away from the metal to a distance of 2...4 mm, an electric arc occurs.
The voltage in the arc, i.e. the voltage between the electrode and the base metal, depends mainly on its length. At the same current, the voltage in a short arc is lower than in a long arc. This is due to the fact that with a long arc the resistance of its gas gap is greater. The increase in resistance in the electrical circuit when constant force current requires an increase in voltage in the circuit. The higher the resistance, the higher the voltage must be in order to ensure the same current passes through the circuit.
The arc between the metal electrode and the metal burns at a voltage of 18 ... 28 V. To initiate the arc, a higher voltage is required than that necessary to maintain its normal combustion. This is explained by the fact that at the initial moment the air gap is not yet sufficiently heated and it is necessary to give the electrons a high speed to decouple the molecules and atoms of the air. This can only be achieved with a higher voltage at the moment of arc ignition.
The graph of changes in current I in the arc during its ignition and stable burning (Fig. 1, b) is called the static characteristic of the arc and corresponds to steady arc burning. Point A characterizes the moment of arc ignition. The arc voltage V quickly drops along the AB curve to a normal value corresponding to a stable arc at point B. A further increase in current (to the right of point B) increases the heating of the electrode and the rate of its melting, but does not affect the stability of the arc.
A stable arc is one that burns evenly, without arbitrary breaks requiring re-ignition. If the arc burns unevenly, often breaks and goes out, then such an arc is called unstable. The stability of the arc depends on many reasons, the main ones being the type of current, the composition of the electrode coating, the type of electrode, the polarity and length of the arc.
With alternating current, the arc burns less steadily than with direct current. This is explained by the fact that at the moment when the current n, reaches zero, the ionization of the arc gap decreases and the arc can go out. To increase the stability of the alternating current arc, it is necessary to apply coatings to the metal electrode. Pairs of elements included in the coating increase the ionization of the arc gap and thereby contribute to the stable burning of the arc at alternating current.
The arc length is determined by the distance between the end of the electrode and the surface of the molten metal of the work being welded. Typically, the normal arc length should not exceed 3...4 mm for a steel electrode. Such an arc is called short. A short arc burns steadily and ensures the normal flow of the welding process. An arc longer than 6 mm is called long. With it, the process of melting the metal of the electrode proceeds unevenly. In this case, the drops of metal flowing from the end of the electrode can be oxidized to a greater extent by oxygen and enriched with air nitrogen. The deposited metal turns out to be porous, the seam has an uneven surface, and the arc burns unsteadily. With a long arc, welding productivity decreases, metal spatter increases and the number of places of lack of penetration or incomplete fusion of the deposited metal with the base metal increases.
The transfer of electrode metal to the product during consumable-electrode arc welding is a complex process. After ignition of the arc (position /), a layer of molten metal is formed on the surface of the end of the electrode, which, under the influence of gravity and surface tension, collects into a drop (position //). Drops can reach large sizes and block the arc column (position III), creating a short circuit in the welding circuit for a short time, after which the resulting bridge of liquid metal breaks, the arc occurs again, and the droplet formation process is repeated.
The size and number of drops passing through the arc per unit time depend on the polarity and strength of the current, chemical composition and the physical state of the electrode metal, coating composition and a number of other conditions. Large drops, reaching 3...4 mm, are usually formed when welding with uncoated electrodes, small drops (up to 0.1 mm) - when welding with coated electrodes and high current. The fine-droplet process ensures stable arc combustion and favors the conditions for transfer of molten electrode metal in the arc.
Rice. 2. Scheme of metal transfer from the electrode to the metal being welded
Rice. 3. Deflection of the electric arc by magnetic fields (a-g)
Gravity can promote or hinder the transfer of droplets in the arc. In ceiling and partially vertical welding, the gravity of the drop counteracts its transfer to the product. But thanks to the force of surface tension, the liquid pool of metal is kept from flowing out when welding in the ceiling and vertical positions.
The passage of electric current through the elements of the welding circuit, including the product being welded, creates a magnetic field, the strength of which depends on the strength of the welding current. The gas column of an electric arc is a flexible conductor of electric current, so it is subject to the resultant magnetic field that is formed in the welding circuit. Under normal conditions, the gas column of an arc burning openly in the atmosphere is located symmetrically to the axis of the electrode. Under the influence of electromagnetic forces, the arc deflects from the electrode axis in the transverse or longitudinal direction, which external signs similar to the displacement of an open flame at strong air currents. This phenomenon is called magnetic blast.
Accession welding wire in close proximity to the arc, it sharply reduces its deflection, since the current’s own circular magnetic field has a uniform effect on the arc column. The supply of current to the product at a distance from the Arc will lead to its deflection due to the condensation of the power lines of the circular magnetic field from the side of the current conductor.
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 something was wrong with my right hand (with which I was holding a screwdriver), 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 buzzing.
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.