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 the necessary conditions for long lasting combustion 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. The upper part of 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 values welding current 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. At the same time, the electric arc produces 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 electrical circuit at constant current, it 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 of which are 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 overlap the arc column (position III), creating a short circuit in the welding circuit for a short time, after which the resulting liquid metal bridge breaks, the arc appears 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, the chemical composition and physical state of the electrode metal, the composition of the coating 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 metal pool 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 resulting magnetic field that is formed in the welding circuit. IN normal conditions The gas column of the 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 axis of the electrode in the transverse or longitudinal direction, which in appearance is similar to the displacement of an open flame under 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.
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 electric arcs low pressure 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 charcoal points. 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 English language(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 a welder begins welding a joint by instantly touching the welding electrode to the object.
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 current patterns 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), the increase in current results in a lower voltage between the 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 the reason why uncontrolled electrical arcs in the apparatus become so destructive, because after its occurrence the arc will consume more and more current from the source DC voltage until the device is destroyed.
Practical use
IN industrial scale electric arcs are used for welding, plasma cutting, mechanical processing by electric discharge, 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 because a large amount of energy is required to achieve an endothermic reaction (at temperatures of 2500 ° C).
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 Sulfur hexafluoride under high pressure is also used.
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. Voltaic arc injury is not fatal.
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.
Self-regulation of the arc during electric welding
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 fluctuations is processed automatically with rigid I-V 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 switches, 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 in this case next:
- 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 shaft (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).
- usage
1. Conditions for the occurrence and burning of an arc
Opening an electrical circuit when there is current in it is accompanied by an electrical discharge between the contacts. If in the disconnected circuit the current and voltage between the contacts are greater than critical for the given conditions, then a arc, the duration of combustion of which depends on the parameters of the circuit and the conditions of deionization of the arc gap. The formation of an arc when copper contacts are opened is possible already at a current of 0.4-0.5 A and a voltage of 15 V.
Rice. 1. Location of voltage U(a) and voltage in a stationary DC arcE(b).
In the arc there are distinguished the near-cathode space, the arc shaft and the near-anode space (Fig. 1). All stress is distributed between these areas U To, U sd, U A. The cathode voltage drop in a DC arc is 10-20 V, and the length of this section is 10-4-10-5 cm, thus, a high electric field strength is observed near the cathode (105-106 V/cm). At such high voltages, impact ionization occurs. Its essence lies in the fact that electrons torn from the cathode by electric field forces (field emission) or due to heating of the cathode (thermionic emission) are accelerated into electric field and when they hit a neutral atom, they give it their kinetic energy. If this energy is enough to remove one electron from the shell of a neutral atom, then ionization will occur. The resulting free electrons and ions make up the plasma of the arc barrel.
Rice. 2. .
Plasma conductivity approaches the conductivity of metals [ at= 2500 1/(Ohm×cm)]/ A large current passes in the arc barrel and a high temperature is created. The current density can reach 10,000 A/cm2 or more, and the temperature can range from 6,000 K at atmospheric pressure to 18,000 K or more at elevated pressures.
High temperatures in the arc barrel lead to intense thermal ionization, which maintains high plasma conductivity.
Thermal ionization is the process of formation of ions due to the collision of molecules and atoms with high kinetic energy at high speeds their movements.
The greater the current in the arc, the lower its resistance, and therefore less voltage is required to burn the arc, i.e., it is more difficult to extinguish an arc with a large current.
With AC power supply voltage u cd changes sinusoidally, the current in the circuit also changes i(Fig. 2), and the current lags behind the voltage by approximately 90°. Arc voltage u d, burning between the contacts of the switch, intermittently. At low currents, the voltage increases to a value u h (ignition voltage), then as the current in the arc increases and thermal ionization increases, the voltage drops. At the end of the half-cycle, when the current approaches zero, the arc goes out at the quenching voltage u d. In the next half-cycle, the phenomenon repeats if measures are not taken to deionize the gap.
If the arc is extinguished by one means or another, then the voltage between the switch contacts must be restored to the supply voltage - u vz (Fig. 2, point A). However, since the circuit contains inductive, active and capacitive resistances, a transient process occurs, voltage fluctuations appear (Fig. 2), the amplitude of which U in,max can significantly exceed normal voltage. For switching equipment, it is important how quickly the voltage in the AB section is restored. To summarize, the arc discharge is initiated by impact ionization and electron emission from the cathode, and after ignition, the arc is maintained by thermal ionization in the arc barrel.
In switching devices it is necessary not only to open the contacts, but also to extinguish the arc that arises between them.
In alternating current circuits, the current in the arc passes through zero every half-cycle (Fig. 2), at these moments the arc goes out spontaneously, but in the next half-cycle it can arise again. As the oscillograms show, the current in the arc becomes close to zero somewhat earlier than the natural transition through zero (Fig. 3, A). This is explained by the fact that when the current decreases, the energy supplied to the arc decreases, therefore, the arc temperature decreases and thermal ionization stops. Duration of dead time t n is small (from tens to several hundred microseconds), but plays an important role in arc extinction. If you open the contacts during a dead time and move them apart at a sufficient speed to such a distance that an electrical breakdown does not occur, the circuit will be turned off very quickly.
During the dead pause, the ionization intensity drops significantly, since thermal ionization does not occur. In switching devices, in addition, artificial measures are taken to cool the arc space and reduce the number of charged particles. These deionization processes lead to a gradual increase in the electrical strength of the gap u pr (Fig. 3, b).
A sharp increase in the electrical strength of the gap after the current passes through zero occurs mainly due to an increase in the strength of the near-cathode space (in AC circuits 150-250V). At the same time, the recovery voltage increases u V. If at any time u pr > u the gap will not be pierced, the arc will not light up again after the current passes through zero. If at some point u pr = u c, then the arc re-ignites in the gap.
Rice. 3. :
A– extinction of the arc when the current naturally passes through zero; b– increase in the electrical strength of the arc gap when the current passes through zero
Thus, the task of extinguishing the arc comes down to creating such conditions that the electrical strength of the gap between the contacts u there was more tension between them u V.
The process of voltage increase between the contacts of the switched-off device can be of a different nature depending on the parameters of the switched circuit. If a circuit with a predominance of active resistance is turned off, then the voltage is restored according to an aperiodic law; if inductive reactance predominates in the circuit, then oscillations occur, the frequencies of which depend on the ratio of capacitance and inductance of the circuit. The oscillatory process leads to significant speeds of voltage recovery, and the greater the speed du V/ dt, the more likely it is that the gap will break down and the arc will re-ignite. To facilitate the conditions for extinguishing the arc, active resistances are introduced into the disconnected current circuit, then the nature of the voltage recovery will be aperiodic (Fig. 3, b).
3. Methods for extinguishing arcs in switching devices up to 1000IN
In switching devices up to 1 kV, the following arc extinguishing methods are widely used:
Lengthening the arc with rapid divergence of contacts.
The longer the arc, the greater the voltage required for its existence. If the power source voltage is lower, the arc goes out.
Dividing a long arc into a number of short ones (Fig. 4, A).
As shown in Fig. 1, the arc voltage is the sum of the cathode voltage U k and anode U and voltage drops and arc shaft voltage U sd:
U d= U k+ U a+ U sd= U e+ U sd.
If a long arc that occurs when the contacts open is pulled into an arc-extinguishing grid made of metal plates, then it will split into N short arcs. Each short arc will have its own cathode and anode voltage drops U e. The arc goes out if:
U n U uh,
Where U- mains voltage; U e - the sum of the cathode and anode voltage drops (20-25 V in a DC arc).
The AC arc can also be divided into N short arcs. At the moment the current passes through zero, the near-cathode space instantly acquires an electrical strength of 150-250 V.
The arc goes out if
Arc extinction in narrow slots.
If an arc burns in a narrow gap formed by an arc-resistant material, then due to contact with cold surfaces, intense cooling and diffusion of charged particles occurs in environment. This leads to rapid deionization and arc extinction.
Rice. 4.
A– dividing a long arc into short ones; b– drawing the arc into a narrow slot in the arc-extinguishing chamber; V– rotation of the arc in a magnetic field; G– arc extinction in oil: 1 – fixed contact; 2 – arc trunk; 3 – hydrogen shell; 4 – gas zone; 5 – oil vapor zone; 6 – moving contact
Movement of an arc in a magnetic field.
An electric arc can be considered as a conductor carrying current. If the arc is in a magnetic field, then it is acted upon by a force determined by the left-hand rule. If you create a magnetic field directed perpendicular to the axis of the arc, then it will receive translational motion and will be pulled inside the slot of the arc-extinguishing chamber (Fig. 4, b).
In a radial magnetic field, the arc will receive rotational movement(Fig. 4, V). A magnetic field can be created permanent magnets, special coils or the circuit of live parts itself. Rapid rotation and movement of the arc contributes to its cooling and deionization.
The last two methods of extinguishing the arc (in narrow slots and in a magnetic field) are also used in disconnecting devices with voltages above 1 kV.
4. The main methods of extinguishing the arc in devices above 1kV.
In switching devices over 1 kV, methods 2 and 3 described in paragraphs are used. 1.3. and the following arc extinguishing methods are also widely used:
1. Arc extinction in oil .
If the contacts of the disconnecting device are placed in oil, then the arc that occurs during opening leads to intense gas formation and evaporation of the oil (Fig. 4, G). A gas bubble is formed around the arc, consisting mainly of hydrogen (70-80%); rapid decomposition of the oil leads to an increase in pressure in the bubble, which contributes to its better cooling and deionization. Hydrogen has high arc-quenching properties. Contacting directly with the arc shaft, it contributes to its deionization. Inside the gas bubble there is a continuous movement of gas and oil vapor. Arc quenching in oil is widely used in circuit breakers.
2. Gas-air blowing .
Arc cooling is improved if a directed movement of gases is created - blasting. Blowing along or across the arc (Fig. 5) promotes the penetration of gas particles into its barrel, intense diffusion and cooling of the arc. Gas is created during the decomposition of oil by an arc (oil switches) or solid gas-generating materials (autogas blast). It is more effective to blow with cold, non-ionized air coming from special compressed air cylinders (air switches).
3. Multiple current circuit break .
Switching off large currents at high voltages is difficult. This is explained by the fact that when large values With the added energy and recovery voltage, deionization of the arc gap becomes more complicated. Therefore, in high-voltage circuit breakers, multiple arc breaks are used in each phase (Fig. 6). Such switches have several extinguishing devices designed for part of the rated value. 
yarn. The number of breaks per phase depends on the type of switch and its voltage. In 500-750 kV circuit breakers there can be 12 breaks or more. To facilitate arc extinction, the recovery voltage must be evenly distributed between the breaks. In Fig. Figure 6 schematically shows an oil switch with two breaks per phase.
When a single-phase short circuit is disconnected, the recovering voltage will be distributed between the breaks as follows:
U 1/U 2 = (C 1+C 2)/C 1
Where U 1 ,U 2 - stresses applied to the first and second breaks; WITH 1 – capacitance between the contacts of these gaps; C 2 – capacity of the contact system relative to the ground.
Rice. 6. Voltage distribution over breaks in the switch: a – voltage distribution over breaks in the oil switch; b – capacitive voltage dividers; c – active voltage dividers.
Because WITH 2 is much more C 1, then the voltage U 1 > U 2 and, therefore, extinguishing devices will operate under different conditions. To equalize the voltage, capacitances or active resistances are connected parallel to the main contacts of the circuit breaker (MC) (Fig. 16, b, V). The values of capacitances and active shunt resistances are selected so that the voltage at the breaks is distributed evenly. In switches with shunt resistances, after extinguishing the arc between the main circuits, the accompanying current, limited in value by the resistances, is broken by the auxiliary contacts (AC).
Shunt resistances reduce the rate of rise of the recovery voltage, which makes it easier to extinguish the arc.
4. Arc extinction in vacuum .
Highly rarefied gas (10-6-10-8 N/cm2) has an electrical strength tens of times greater than gas at atmospheric pressure. If the contacts open in a vacuum, then immediately after the first passage of the current in the arc through zero, the strength of the gap is restored and the arc does not light up again.
5. Arc extinction in gases high pressure .
Air at a pressure of 2 MPa or more has high electrical strength. This makes it possible to create fairly compact devices for extinguishing an arc in a compressed air atmosphere. The use of high-strength gases, such as sulfur hexafluoride SF6 (SF6 gas), is even more effective. SF6 gas not only has greater electrical strength than air and hydrogen, but also better arc-extinguishing properties even at atmospheric pressure.
The principle of electric arc welding is based on the use of the temperature of the electrical discharge that occurs between the welding electrode and the metal workpiece.
An arc discharge is formed due to electrical breakdown of the air gap. When this phenomenon occurs, gas molecules are ionized, its temperature and electrical conductivity increase, and it transitions to the plasma state.
The burning of the welding arc is accompanied by the release large quantity light and especially thermal energy, as a result of which the temperature rises sharply and local melting of the workpiece metal occurs. This is welding.
During operation, in order to initiate an arc discharge, the workpiece is briefly touched by the electrode, that is, the creation short circuit followed by breaking the metal contact and establishing the required air gap. In this way, the optimal length of the welding arc is selected.
With a very short discharge, the electrode may stick to the workpiece, melting occurs too intensely, which can lead to the formation of sagging. A long arc is characterized by instability of combustion and insufficiently high temperature in the welding zone.
Instability and visible bending of the welding arc shape can often be observed during the operation of industrial welding units with fairly massive parts. This phenomenon is called magnetic blowing.
Its essence lies in the fact that the welding arc current creates a certain magnetic field that interacts with magnetic field, created by current flowing through a massive workpiece.
That is, the deflection of the arc is caused by magnetic forces. The process is called blowing because the arc is deflected, as if under the influence of wind.
There are no radical ways to combat this phenomenon. To reduce the influence of magnetic blast, welding with a shortened arc is used, and the electrode is also placed at a certain angle.
Combustion medium
There are several different welding technologies that use electric arc discharges, differing in properties and parameters. The electric welding arc has the following types:
- open. The discharge occurs directly in the atmosphere;
- closed. The high temperature generated during combustion causes abundant release of gases from the burning flux. Flux is contained in the coating of welding electrodes;
- in a protective gas environment. In this option, gas is supplied to the welding zone, most often helium, argon or carbon dioxide.
Protection of the welding zone is necessary to prevent active oxidation of the melting metal under the influence of atmospheric oxygen.
The oxide layer prevents the formation of a continuous weld, the metal at the junction becomes porous, resulting in a decrease in the strength and tightness of the joint.
To some extent, the arc itself is capable of creating a microclimate in the combustion zone due to the formation of an area high blood pressure, preventing the flow of atmospheric air.
The use of flux allows for more active squeezing of air from the welding zone. The use of protective gases supplied under pressure solves this problem almost completely.
Discharge duration
In addition to the protection criteria, the arc discharge is classified by duration. There are processes in which arc combustion occurs in a pulsed mode.
In such devices, welding is carried out in short bursts. During the outbreak, the temperature manages to increase to a value sufficient for local melting small area, in which a point connection is formed.
Most of the welding technologies used use a relatively long arc burning time. During the welding process, the electrode constantly moves along the edges being joined.
Region elevated temperature, creating, moves after the electrode. After moving welding electrode Consequently, the arc discharge, the temperature of the area passed through decreases, crystallization of the weld pool occurs and the formation of a strong weld.
Arc discharge structure
The arc discharge area is conventionally divided into three sections. The areas immediately adjacent to the poles (anode and cathode) are called anode and cathode, respectively.
The central part of the arc discharge, located between the anode and cathode regions, is called the arc column. The temperature in the welding arc zone can reach several thousand degrees (up to 7000 °C).
Although the heat is not completely transferred to the metal, it is quite enough to melt. Thus, the melting point of steel, for comparison, is 1300-1500 °C.
To ensure stable combustion of an arc discharge, it is necessary following conditions: the presence of a current of the order of 10 Amperes (this is the minimum value, the maximum can reach 1000 Amperes), while maintaining the arc voltage from 15 to 40 Volts.
This voltage drop occurs in an arc discharge. The voltage distribution across the arc zones is uneven. Most of the applied voltage drop occurs in the anodic and cathodic zones.
It has been experimentally established that at , the greatest voltage drop is observed in the cathode zone. The highest temperature gradient is observed in this part of the arc.
Therefore, when choosing the polarity of the welding process, the cathode is connected to the electrode when they want to achieve its greatest melting, increasing its temperature. On the contrary, for deeper penetration of the workpiece, the cathode is attached to it. The smallest part of the voltage drops in the arc column.
When welding with a non-consumable electrode, the cathode voltage drop is less than the anode, that is, the high temperature zone is shifted towards the anode.
Therefore, with this technology, the workpiece is connected to the anode, which ensures good heating and protection of the non-consumable electrode from excessive temperature.
Temperature zones
It should be noted that with any type of welding, both with consumable and non-consumable electrodes, the arc column (its center) has the most high temperature- about 5000-7000 °C, and sometimes higher.
The lowest temperature zones are located in one of the active areas, cathode or anodic. In these zones, 60-70% of the arc heat can be released.
In addition to intensely increasing the temperature of the workpiece and welding electrode, the discharge emits infrared and ultraviolet waves that can have a harmful effect on the welder’s body. This necessitates the use of protective measures.
As for AC welding, the concept of polarity does not exist there, since the position of the anode and cathode changes at an industrial frequency of 50 vibrations per second.
The arc in this process is less stable compared to direct current, its temperature fluctuates. The advantages of welding processes using alternating current include simpler and cheaper equipment, and even the almost complete absence of such a phenomenon as magnetic blast, which is mentioned above.
Volt-ampere characteristics
The graph shows the dependence of the power source voltage on the welding current, called the current-voltage characteristics of the welding process.
The red curves display the change in voltage between the electrode and the workpiece in the phases of excitation of the welding arc and its stable combustion. The starting points of the curves correspond to the voltage idle move power supply.
At the moment the welder initiates an arc discharge, the voltage drops sharply until the period when the arc parameters stabilize and the value of the welding current is established, depending on the diameter of the electrode used, the power of the power source and the set arc length.
With the onset of this period, the arc voltage and temperature stabilize, and the whole process becomes stable.