Consists of an electric arc. Electric arc and its use for welding

• Electric arc (voltaic arc, arc discharge) - physical phenomenon, one of the types of electrical discharge in gas.

  It was first described in 1802 by the Russian scientist V. Petrov in the book “News of Galvanic-Volta experiments using a huge battery, sometimes consisting of 4200 copper and zinc circles” (St. Petersburg, 1803). An electric arc is a special case of the fourth form of state of matter - plasma - and consists of an ionized, electrically quasi-neutral gas. The presence of free electric charges ensures the conductivity of the electric arc.

  An electric arc between two electrodes in air at atmospheric pressure is formed as follows:

  When the voltage between two electrodes increases to a certain level, an electrical breakdown occurs in the air between the electrodes. The electrical breakdown voltage depends on the distance between the electrodes and other factors. The ionization potential of the first electron of metal atoms is approximately 4.5 - 5 V, and the arcing voltage is twice as high (9 - 10 V). It is necessary to expend energy to release an electron from the metal atom of one electrode and to ionize the atom of the second electrode. The process leads to the formation of plasma between the electrodes and the burning of an arc (for comparison: the minimum voltage for the formation of a spark discharge is slightly higher than the electron output potential - up to 6 V).

  To initiate breakdown at the existing voltage, the electrodes are brought closer to each other. During a breakdown, a spark discharge usually occurs between the electrodes, pulse-closing electrical circuit.

  Electrons in spark discharges ionize molecules in the air gap between the electrodes. With sufficient power of the voltage source in the air gap, a sufficient amount of plasma is formed for a significant drop in the breakdown voltage or resistance of the air gap. In this case, spark discharges turn into an arc discharge - a plasma cord between the electrodes, which is a plasma tunnel. The resulting arc is, in fact, a conductor and closes the electrical circuit between the electrodes. As a result, the average current increases even more, heating the arc to 5000-50000 K. In this case, it is considered that the ignition of the arc is completed. After ignition stable combustion The arc is provided by thermionic emission from the cathode, heated by current and ion bombardment.

  The interaction of electrodes with arc plasma leads to their heating, partial melting, evaporation, oxidation and other types of corrosion.

  After ignition, the arc can remain stable when the electrical contacts are separated to a certain distance.

  When operating high-voltage electrical installations, in which the appearance of an electric arc is inevitable, it is combated using electromagnetic coils combined with arc extinguishing chambers. Among other methods, the use of vacuum, air, SF6 and oil circuit breakers is known, as well as methods of diverting current to a temporary load that independently breaks the electrical circuit.

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

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

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

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

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

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

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

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

Figure 3.4 – Arc burning diagram

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

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

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

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

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

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

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

Q a > Q k; U a< U к.

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

In modern industry, welding has great value, it has a very wide range of applications in all industries. To carry out the welding process, a welding arc is required.

What is a welding arc, its definition

A welding arc is considered to be a very large electrical discharge in terms of power and duration that exists between the electrodes to which voltage is applied in a mixture of gases. Its properties are characterized by high temperature and current density, thanks to which it is capable of melting metals with a melting point above 3000 degrees. In general, we can say that an electric arc is a conductor made of gas that transforms electrical energy to thermal. Electric charge is the passage of electric current through a gaseous medium.

There are several types of electrical discharge:

• Glow discharge. Occurs at low pressure, used in fluorescent lamps and plasma screens;
• Spark discharge. Occurs when the pressure is equal to atmospheric pressure and has an intermittent shape. Lightning corresponds to a spark discharge; it is also used to ignite internal combustion engines;
• Arc discharge. Used for welding and lighting. It is characterized by a continuous form and occurs at atmospheric pressure;
• Crown. It occurs when the body of the electrode is rough and inhomogeneous, the second electrode may be missing, that is, a jet appears. Used for purifying gases from dust;

Nature and structure

The nature of the welding arc is not as complicated as it might seem at first glance. The electric current, passing through the cathode, then penetrates the ionized gas, a discharge occurs with a bright glow and a very high temperature, so the temperature of the electric arc can reach 7000 - 10000 degrees. After this, the current flows to the material being welded. Since the temperature is so high, the arc emits harmful human body ultraviolet and infrared radiation, it can damage the eyes or cause light burns on the skin, so proper protection is necessary when carrying out the welding process.

The structure of the welding arc consists of three main areas: anodic, cathodic and arc column. During arc burning, active spots are formed on the cathode and anode - areas in which the temperature reaches the highest values; it is through these areas that all electric current passes; the anode and cathode areas represent larger voltage drops. And the pillar itself is located between these areas, the voltage drop in the pillar is very small. Thus, the length of the welding arc is the sum of the above areas, usually the length is several millimeters, when the anodic and cathode areas are, respectively, 10-4 and 10-5 cm. The most favorable length is approximately 4-6 mm, with this length a constant and favorable temperature.

Varieties

Types of welding arc differ in the welding current supply circuit and the environment in which they occur; the most common options are:

• Direct action. With this method, the welding machine is located parallel to the metal structure being welded and the arc occurs at an angle of ninety degrees relative to the electrode and the metal;
• Indirect welding arc. Occurs when two electrodes are used, which are located at an angle of 40-60 degrees to the surface of the part being welded, an arc occurs between the electrodes and welds the metal;

There is also a classification depending on the atmosphere in which they occur:

• Open type. Arc of this type burns in air and a gas phase is formed around it, containing vapors of the material being welded, electrodes and their coatings;
• Closed type. The combustion of such an arc occurs under a layer of flux; vapors of metal, electrode and flux enter the gas phase formed around the arc;
• Arc with gas supply. Compressed gases - helium, argon, carbon dioxide, hydrogen and other various mixtures of gases, they are supplied so that the welded metal does not oxidize; their supply contributes to a reducing or neutral environment. The gas phase around the arc includes the supplied gas, metal and electrode vapors;

They are also distinguished by the duration of action - stationary (for long-term use) and pulsed (for one-time use), by the material of the electrode used - carbon, tungsten - non-consumable electrodes and metal - consumable. The most common consumable electrode is steel. Today, welding with a non-consumable electrode is most often used. Thus, the types of welding arcs are varied.

Combustion conditions

At standard conditions, that is, at a temperature of 25 degrees and a pressure of 1 atmosphere, gases are not capable of conducting electric current. In order for an arc to form, it is necessary that the gases between the electrodes be ionized, that is, they contain various charged particles - electrons or ions (cations or anions). The process of formation of an ionized gas will be called ionization, and the work that must be spent on removing an electron from an atomic particle to form an electron and an ion will be called the ionization work, which is measured in electron volts and is called the ionization potential. Exactly what energy needs to be expended to remove an electron from an atom depends on the nature of the gas phase; values ​​can be from 3.5 to 25 eV. The metals of the alkali and alkaline earth groups – potassium, calcium and, accordingly, their chemical compounds – have the lowest ionization potential. Electrodes are coated with such compounds so that they contribute to the stable existence and burning of the welding arc.

Also, for the arc to occur and burn, a constant temperature is required at the cathode, which depends on the nature of the cathode, its diameter, size and ambient temperature. The temperature of the electric arc must therefore be constant and not fluctuate; thanks to the enormous current values, the temperature can reach 7 thousand degrees, thus absolutely all materials can be connected by welding. A constant temperature is ensured by a properly functioning power supply, so its choice when designing welding machine very important, it affects the properties of the arc.

Emergence

It occurs during a rapid short circuit, that is, when the electrode comes into contact with the surface of the material being welded, due to the colossal temperature, the surface of the material melts, and a small strip of molten material forms between the electrode and the surface. By the time the electrode and the material being welded diverge, a neck of material is formed, which instantly breaks and evaporates due to high values current density. The gas becomes ionized and an electric arc occurs. You can excite her by touching or scratching.

Peculiarities

She has following features compared to other electric charges:

• High current density, which reaches several thousand amperes per square centimeter, due to which very high temperatures are achieved;
• Uneven distribution electric field in the space between the electrodes. Near the electrodes the voltage drop is very high, when in the column it is the opposite;
• Huge temperatures that reach the most large values in the column due to the high current density. As the length of the column increases, the temperature decreases, and when it narrows, on the contrary, it increases;
• Using welding arcs, you can obtain a wide variety of current-voltage characteristics - the dependence of the voltage drop on the current density at a constant length, that is, steady combustion. On at the moment There are three current-voltage characteristics.

The first is falling, when with an increase in strength and, accordingly, current density, the voltage drops. The second is hard, when a change in current does not affect the voltage value in any way, and the third is increasing, when as the current increases, the voltage also increases.

Thus, the welding arc can be called the best and most reliable way of fastening metal structures. The welding process has great influence on today's industry, because only the high temperature of the welding arc is capable of holding most metals together. To obtain high-quality and reliable seams, it is necessary to correctly and correctly take into account all the characteristics of the arc, monitor all values, thanks to this the procedure will be quick and most effective. It is also necessary to take into account the properties of the arc: current density, temperature and voltage.

Electrical arcing can be extremely destructive to equipment and, more importantly, dangerous to people. An alarming number of accidents caused by it occur each year, often resulting in serious burns or death. Fortunately, significant progress has been made in the electrical industry in terms of creating means and methods of protection against arc exposure.

Causes and places of occurrence

Electrical arcing is one of the deadliest and least understood electrical hazards and is prevalent in most industries. It is widely accepted that the higher the voltage electrical system, the greater the risk to people working on or near live wires and equipment.

The thermal energy from an arc flash, however, can actually be greater and occur more frequently at lower voltages with the same destructive consequences.

An electric arc usually occurs when there is accidental contact between a live conductor, such as a trolleybus or tram line contact wire with another conductor, or a grounded surface.

When this happens, the resulting short circuit current melts the wires, ionizes the air and creates a fiery channel of conducting plasma with a characteristic arc-shaped shape (hence the name), and the temperature of the electric arc at its core can reach over 20,000 ° C.

What is an electric arc?

In fact, this is the common name for an arc discharge, well known in physics and electrical engineering - a type of independent electric discharge in a gas. What are physical properties electric arc? It burns in a wide range of gas pressure, at constant or alternating (up to 1000 Hz) voltage between the electrodes in the range from several volts (welding arc) to tens of kilovolts. Maximum Density arc current is observed at the cathode (10 2 -10 8 A/cm 2), where it is contracted into a cathode spot, very bright and small in size. It moves randomly and continuously over the entire area of ​​the electrode. Its temperature is such that the cathode material boils in it. Therefore, there are ideal conditions for thermionic emission of electrons into the cathode space. A small layer is formed above it, charged positively and providing acceleration of emitted electrons to speeds at which they impact ionize atoms and molecules of the medium in the interelectrode gap.

The same spot, but somewhat larger and less mobile, forms on the anode. The temperature in it is close to the cathode spot.

If the arc current is of the order of several tens of amperes, then from both electrodes flows high speed Plasma jets or torches are normal to their surfaces (see photo below).

At high currents (100-300 A), additional plasma jets appear, and the arc becomes similar to a bundle of plasma filaments (see photo below).

How does an arc manifest itself in electrical equipment?

As mentioned above, the catalyst for its occurrence is strong heat generation in the cathode spot. The temperature of the electric arc, as already mentioned, can reach 20,000 ° C, about four times higher than on the surface of the sun. This heat can quickly melt or even vaporize the copper of the conductors, which has a melting point of about 1084 ° C, much lower than in an arc. Therefore, copper vapors and splashes of molten metal often form in it. When copper changes from solid to vapor, it expands to several tens of thousands of times its original volume. This is equivalent to a one cubic centimeter piece of copper changing to a size of 0.1 cubic meters in a fraction of a second. This will create high-intensity pressure and sound waves propagating around at high speed (which can be over 1100 km per hour).

Exposure to electric arc

If it occurs, serious injuries, and even death, can occur not only to persons working on electrical equipment, but also to people nearby. Arc injuries can include external skin burns, internal burns from inhaling hot gases and vaporized metal, hearing damage, vision damage such as blindness from ultraviolet flash light, and many other devastating injuries.

A particularly powerful arc may also cause it to explode, creating a pressure of more than 100 kilopascals (kPa) and releasing shrapnel-like debris at speeds of up to 300 meters per second.

Persons who have been exposed to electrical arc current may require serious treatment and rehabilitation, and the cost of their injuries can be extreme - physically, emotionally and financially. Although legislation requires enterprises to conduct risk assessments for all types labor activity, however, the risk of electrical arc hazards is often overlooked because most people do not know how to assess and effectively manage this hazard. Protection against the effects of an electric arc involves the use of a whole range of means, including the use when working with live electrical equipment, special electrical protective equipment, special clothing, as well as the equipment itself, especially high-low voltage switching electrical devices designed using arc extinguishing means.

Arc in electrical apparatus

In this class of electrical devices ( circuit breakers, contactors, magnetic starters) the fight against this phenomenon is of particular importance. When the contacts of a switch that is not equipped with special devices to prevent an arc are opened, it is sure to ignite between them.

At the moment when the contacts begin to separate, the area of ​​​​the latter decreases rapidly, which leads to an increase in current density and, consequently, to an increase in temperature. The heat generated in the gap between the contacts (the usual medium is oil or air) is sufficient to ionize the air or evaporate and ionize the oil. The ionized air or steam acts as a conductor for the arc current between the contacts. The potential difference between them is very small, but it is enough to maintain the arc. Consequently, the current in the circuit remains continuous until the arc is eliminated. Not only does it delay the interruption process, but it also generates a huge amount of heat that can damage the breaker itself. Thus, main problem in a switch (primarily a high-voltage one) - this is extinguishing the electric arc in as soon as possible so that the heat generated in it cannot reach a dangerous value.

Factors for maintaining an arc between switch contacts

These include:

2. Ionized particles between them.

Accepting this, we note additionally:

• When there is a small gap between the contacts, even a small potential difference is enough to maintain the arc. One way to extinguish it is to separate the contacts at such a distance that the potential difference becomes insufficient to maintain the arc. However, this method is not practical in high voltage applications where separation over many meters may be required.
• Ionized particles between the contacts tend to support the arc. If its path is deionized, then the quenching process will be facilitated. This can be achieved by cooling the arc or removing ionized particles from the space between the contacts.
• There are two ways by which arc protection is provided in circuit breakers:

High resistance method;

Zero current method.

Extinguishing the arc by increasing its resistance

In this method, the resistance along the arc path increases over time so that the current decreases to a value insufficient to support it. Consequently, it is interrupted and the electric arc goes out. The main disadvantage of this method is that the extinction time is quite long, and enormous energy has time to dissipate in the arc.

Arc resistance can be increased by:

• Arc elongation - the resistance of the arc is directly proportional to its length. The arc length can be increased by changing the gap between the contacts.
• Cooling the arc, or more precisely the medium between the contacts. Effective fan cooling must be directed along the arc.
• By placing the contacts in a difficult-to-ionize gas environment (gas switches) or in a vacuum chamber (vacuum switches).
• Decrease cross section arc by passing it through a narrow hole, or by reducing the contact area.
• By dividing the arc - its resistance can be increased by dividing it into a number of small arcs connected in series. Each of them experiences the action of elongation and cooling. The arc can be divided by introducing some conductive plates between the contacts.

Arc extinction using zero current method

This method is only used in circuits AC. It keeps the arc resistance low until the current drops to zero, where it goes out naturally. Its re-ignition is prevented despite the increased voltage at the contacts. All modern high-alternating current circuit breakers use this arc extinguishing method.

In an alternating current system, the latter drops to zero after each half cycle. At each such zeroing, the arc goes out by short time. In this case, the medium between the contacts contains ions and electrons, so its dielectric strength is low and can be easily destroyed by increasing voltage at the contacts.

If this happens, the electric arc will burn for the next half cycle of the current. If immediately after it is reset to zero, the dielectric strength of the medium between the contacts increases faster than the voltage across them, then the arc will not ignite and the current will be interrupted. A rapid increase in the dielectric strength of the medium near zero current can be achieved by:

• recombination of ionized particles in the space between contacts into neutral molecules;
• by removing ionized particles away and replacing them with neutral particles.

Thus, the real problem in interrupting AC arc current is the rapid deionization of the medium between the contacts as soon as the current becomes zero.

Methods for deionization of the medium between contacts

1. Gap lengthening: The dielectric strength of the medium is proportional to the length of the gap between the contacts. Thus, by quickly opening the contacts, a higher dielectric strength of the medium can be achieved.

2. High blood pressure. If it increases in the immediate vicinity of the arc, the density of particles making up the arc discharge channel also increases. Increased particle density leads to high level their deionization and, consequently, the dielectric strength of the medium between the contacts increases.

3. Cooling. The natural recombination of ionized particles occurs faster as they cool. Thus, the dielectric strength of the medium between the contacts can be increased by cooling the arc.

4. Explosion effect. If the ionized particles between the contacts are swept away and replaced by non-ionized ones, the dielectric strength of the medium can be increased. This can be achieved using gas explosion, directed to the discharge zone, or by injecting oil into the intercontact space.

These switches use sulfur hexafluoride (SF6) gas as the arc extinguishing medium. It has a strong tendency to absorb free electrons. Switch contacts open in the flow high pressure SF6) between them (see figure below).

The gas captures free electrons in the arc and forms an excess of low-mobility negative ions. The number of electrons in the arc quickly decreases and it goes out.

An electric arc is a type of discharge characterized by a high current density, high temperature, high gas pressure and a small voltage drop across the arc gap. In this case, intense heating of the electrodes (contacts) takes place, on which so-called cathode and anode spots are formed. The cathode glow is concentrated in a small bright spot, the hot part of the opposite electrode forms an anode spot.

In the arc, three regions can be noted, very different in the nature of the processes occurring in them. Directly adjacent to the negative electrode (cathode) of the arc is the area of ​​the cathode voltage drop. Next comes the plasma arc barrel. Directly adjacent to the positive electrode (anode) is the region of the anode voltage drop. These areas are shown schematically in Fig. 1.

Rice. 1. The structure of the electric arc

The sizes of the cathode and anode voltage drop regions in the figure are greatly exaggerated. In reality, their extent is very small. For example, the extent of the cathode voltage drop is of the order of the path of free electron movement (less than 1 μ). The length of the region of the anode voltage drop is usually somewhat greater than this value.

Under normal conditions, air is a good insulator. Thus, the voltage required to break down an air gap of 1 cm is 30 kV. In order for an air gap to become a conductor, it is necessary to create a certain concentration of charged particles (electrons and ions) in it.

How does an electric arc occur?

An electric arc, which is a flow of charged particles, at the initial moment of contact divergence arises as a result of the presence of free electrons of the arc gap gas and electrons emitted from the cathode surface. Free electrons located in the gap between the contacts move at high speed in the direction from the cathode to the anode under the influence of electric field forces.

The field strength at the beginning of contact divergence can reach several thousand kilovolts per centimeter. Under the influence of the forces of this field, electrons are ejected from the surface of the cathode and move to the anode, knocking out electrons from it, which form an electron cloud. The initial flow of electrons created in this way subsequently forms intense ionization of the arc gap.

Along with ionization processes, deionization processes occur in parallel and continuously in the arc. Deionization processes consist in the fact that when two ions of different signs or a positive ion and an electron come together, they are attracted and, colliding, are neutralized; in addition, charged particles move from the combustion area of ​​souls with a higher concentration of charges to environment with a lower charge concentration. All these factors lead to a decrease in the temperature of the arc, to its cooling and extinction.

Rice. 2. Electric arc

Arc after ignition

In a steady-state combustion mode, the ionization and deionization processes in it are in equilibrium. The arc barrel with an equal number of free positive and negative charges is characterized by a high degree of gas ionization.

A substance whose degree of ionization is close to unity, i.e. in which there are no neutral atoms and molecules is called plasma.

The electric arc is characterized by the following features:

1. A clearly defined boundary between the arc shaft and the environment.

2. High temperature inside the barrel there is an arc reaching 6000 - 25000K.

3. High Density current and arc barrel (100 - 1000 A/mm 2).

4. Small values ​​of the anode and cathode voltage drop and practically does not depend on current (10 - 20 V).

Current-voltage characteristics of an electric arc

The main characteristic of a DC arc is the dependence of the arc voltage on the current, which is called current-voltage characteristic (VAC).

An arc occurs between the contacts at a certain voltage (Fig. 3), called the ignition voltage Uз and depending on the distance between the contacts, on the temperature and pressure of the medium and on the speed of divergence of the contacts. The arc extinction voltage Ug is always less than the voltage Uz.

Rice. 3. Volt-ampere characteristic of a DC arc (a) and its equivalent circuit (b)

Curve 1 represents the static characteristic of the arc, i.e. obtained by slowly changing the current. The characteristic has a falling character. As the current increases, the arc voltage decreases. This means that the resistance of the arc gap decreases faster as the current increases.

If, at one speed or another, the current in the arc is reduced from I1 to zero and at the same time the voltage drop across the arc is recorded, then curves 2 and 3 will be obtained. These curves are called dynamic characteristics.

The faster the current is reduced, the lower the dynamic current-voltage characteristics will lie. This is explained by the fact that when the current decreases, arc parameters such as the barrel cross-section and temperature do not have time to quickly change and acquire values ​​corresponding to a lower current value at steady state.

Voltage drop across the arc gap:

Ud = U з + EdId,

Where U z = U k + U a - near-electrode voltage drop, Ed - longitudinal voltage gradient in the arc, Id - arc length.

It follows from the formula that as the arc length increases, the voltage drop across the arc will increase, and the current-voltage characteristic will be located higher.

Electric arcs are dealt with when designing electrical switching devices. The properties of the electric arc are used in and in.