Braces correct your bite by gradually moving your teeth into the correct position. This is achieved through the interaction of elements of the orthodontic system that perform certain functions.
With all the variety of types of modern braces: lingual and vestibular, self-ligating and classic, made from various materials, their main elements are a metal arch that creates pressure, and the braces themselves, which fix the orthodontic arch and provide the desired direction of tooth movement.
What is an electric arc?
This mysterious phenomenon was first described by the Russian scientist V. Petrov. He created an electric arc using a battery consisting of thousands of copper and zinc plates. Studying the process of igniting an arc with direct current, the scientist came to the conclusion that the air gap between the electrodes under certain conditions becomes electrically conductive.
One of the conditions for the occurrence of electrical breakdown is a sufficiently high potential difference at the ends of the electrodes. The higher the voltage, the larger the gas gap the discharge can bridge. In this case, an electrically conductive gas column is formed, which becomes very hot during the burning of the arc.
Rice. 2. Electric arc
A reasonable question arises : “Why does air, which is an excellent insulator in its normal state, suddenly become a conductor?”
There can be only one explanation - charge carriers are formed in the arc barrel, capable of moving under the influence of an electric field. Since there are no free electrons in air, unlike metals, only one conclusion suggests itself - ionization of gases (see Fig. 3). That is, starting the process of gas saturation with ions that are carriers of electric charge.
Rice. 3. Physics of the electric arc
Air ionization occurs under the influence of various types of radiation, including X-ray and cosmic irradiation. Therefore, there are always small amounts of ions in the air. But since the ions almost immediately recombine (turn into neutral atoms and molecules), the concentration of charged particles is always tiny. It is impossible to produce an arc flash at such a concentration.
For an arc discharge to occur, an avalanche-like ionization process is required. It can be caused by the strong heating of the gas that occurs during ignition.
When the contacts open, electrons are emitted and accumulate in a very small space. Under the influence of electric field strength, negative charges rush towards the electrode with a positive sign.
When the breakdown voltage is reached, a spark discharge occurs between the electrodes, heating the area between the electrodes. If the current is large enough, then the amount of heat will be sufficient to trigger an avalanche-like process of air ionization.
In an area called the arc gap, a barrel is formed, called an arc column, consisting of hot conductive plasma. A current flows through this barrel, maintaining the heating of the plasma. This is how the arc discharge ignites.
Saturation of the plasma barrel with ions of different signs leads to a significant increase in the current density, as well as to the recombination of some of the ions. Warming up the plasma also leads to an increase in pressure in the barrel. Therefore, some of the ions escape into the surrounding space.
If the formation of new charges is not maintained, the arc will be extinguished. As we have already found out, stable combustion is accompanied by 2 factors: the presence of voltage between the electrodes and maintaining a high plasma temperature. Excluding one of them will lead to arc extinguishing.
Thus, we can formulate the definition of an electric arc . Namely, an electric arc is a type of spark discharge, accompanied by a high current density, burning duration, and a small voltage drop across the barrel, characterized by increased gas pressure in which a high temperature is maintained.
An electric arc differs from a conventional discharge in that it burns longer.
Primary requirements
The power source for welding work of any type and class must satisfy the following key characteristics:
- ensure ease of arc ignition;
- maintain stable combustion;
- control the upper threshold of short circuit current;
- have good dynamics;
- comply with electrical safety requirements.
In this case, dynamics is understood as the speed of voltage recovery from the moment the electrode contacts the ground (the occurrence of a short circuit) until the arc flashes, that is, the formation of an electrical breakdown of the air.
The arc flashes at a voltage of about 20 V. The time from the moment of a short circuit to the arc flash with a good power source should be no more than 0.05 seconds. The smaller it is, the higher the dynamics.
In addition, it is very important that the source maintains a stable arc, that is, automatically regulates the voltage change from no-load mode (60-90 V) to operating voltage (18-20 V).
These requirements apply to all devices without exception. Even a homemade welding machine assembled for manual arc welding from a computer power supply must comply with them.
By the way, using the latter to assemble a device for home use is not so difficult. The switching power supply is precisely designed to reduce the mains voltage. But only thin metal can be welded.
Welding arc power sources are classified into many gradations. Among them:
- by purpose - for manual welding, submerged arc welding or shielding gas welding (for example, argon arc);
- by the number of welding stations that can be connected at a time;
- according to the ability to move - mobile and stationary;
- according to the method of energy production - converters or producers;
- by type of output current;
- according to the current-voltage characteristic (volt-ampere characteristic).
The main parameters of a welding machine for a welder are the purpose of this particular unit and the welding current that it produces. In many cases, the key requirement is the selection of the desired current-voltage characteristic (volt-ampere characteristic).
For example, for welding in a shielding gas environment, devices with a rigid characteristic that cook with direct current are required. For manual and semi-automatic submerged arc welding, alternating and direct current devices with a decreasing characteristic are used.
We invite you to familiarize yourself with Edible mushrooms: name, photo and description
Some modern welding arc power sources are universal: they have many operating modes, including the ability to change the type of welding current and change its current-voltage characteristic.
The main difference between welding arc power sources, which determines their technical characteristics, weight, dimensions and scope of application, is the difference in the principle of electric current conversion.
The following types of sources exist:
- transformers;
- rectifiers;
- converters;
- inverters.
Generators, the so-called units, stand apart. These machines are not secondary, but primary sources of energy; they do not convert power from a city or industrial network in one way or another, but generate it themselves.
As a rule, units are built on the basis of an internal combustion engine - gasoline or diesel. The former are cheaper, the latter have greater power and service life.
Transformer
This is the simplest type of welding machine. It is based on a choke - a reactive inductor.
A simple step-down transformer lowers the network voltage to the no-load value - 60...80 V. Subsequently, during operation, the welding voltage is maintained at 20 V.
The transformer operates only with alternating current. Its advantage is its simplicity of design (you can make it yourself by calculating the number of turns of both windings).
It has high efficiency, relatively low energy consumption, and is distinguished by reliability combined with maintainability. The transformer arc power source operates silently and costs relatively little.
But using alternating current for welding also has certain disadvantages. This welding arc power source has large dimensions and a very large mass.
The arc burns unstably and is highly dependent on surges in the supply voltage. There is a need to use special coated electrodes. The list of metals and alloys that can be welded with alternating current (mainly low-carbon steels) is limited.
Rectifier
As the name suggests, this is a device that rectifies alternating current, that is, converts it into direct current. For this purpose, semiconductor elements based on selenium or silicon are used.
Rectifiers can be single-phase and three-phase, stationary or mobile, and have any current-voltage characteristic - either rigidly specified by the manufacturer, or changed by the user according to his needs.
Straighteners have many advantages. This is silent operation, high efficiency (higher than that of transformers), a wide range of uses (you can weld any metals and alloys). Such a power source has low no-load losses, relatively small dimensions and weight, and low energy consumption.
They have few disadvantages, but, unfortunately, they are quite significant. Rectifiers, as power sources for the welding arc, get very hot during the working process, so they need a good cooling system, which must be carefully monitored.
In addition, they are very sensitive to power surges, do not like dust, which can damage the cooling system, and are quite expensive.
Converters
A converter is a device that mechanically converts alternating current into direct current. Essentially, it is an electric motor that rotates the shaft of a DC generator. Once upon a time, these were the first devices capable of welding with direct current.
Generators powered by a gasoline or diesel engine operate on a similar principle.
Despite the seemingly illogical design, converters also have their pros and cons. Their main advantage is that these devices are insensitive to voltage drops - the output current always has a stable characteristic.
In addition, they can produce a very high current - 300, 500, some models 1000 A. In some types of work, for example, when welding thick metal plates, this is important.
Their disadvantages are their large mass (up to 500 kg), as well as the need for regular maintenance due to the presence of parts rotating at high speed. The efficiency of the converters is low due to the energy spent on spinning up the motor shaft.
Inverters
Inverters are a special class of welding arc power sources. These are welding machines that are optimally suited for domestic needs.
- Due to their small size and ease of handling, they are actively used where mobility is needed, and there are also restrictions on the power that can be taken from the network.
- Most inverter power sources for the welding arc can be plugged into a regular outlet without fear of network overload.
The principle of operation of these devices is inversion - a mirror transformation of one state of energy into another. The inverter machine welds with high-frequency alternating current, which it receives from direct current, and this, in turn, from industrial alternating current.
Inversion allows you to increase the current frequency by 1000 times - up to 50 kHz. Due to this, it was possible to achieve a significant reduction in the size and weight of the device.
Thanks to some inverter power sources for the welding arc, it is possible to weld with both direct and alternating current, depending on the mode.
Their advantages, in addition to their dimensions, include low energy consumption, a high level of safety, smooth adjustment of the output current and low melt spatter during welding.
The list of shortcomings is short. The device requires careful care and protection from dust, does not like frost, and is not very cheap to repair. The inverter can be called the optimal device for manual welding.
Structure
The electric arc consists of three main zones:
- cathode;
- anodic;
- plasma column.
In welding arcs, the dimensions of the cathode and anode zones are small compared to the length of the column. The thickness of these zones is thousandths of a millimeter. In the area of the cathode voltage drop (at the end of the negative electrode), the presence of cathode spots is observed, which are formed as a result of strong heating.
Figure 4 shows a diagram of the structure of the arc created by the welding machine.
Rice. 4. Welding arc structure
Please note: for the purpose of clarity, the electrode zones are greatly exaggerated in the picture. In reality, their thickness is measured in microns.
Combustion conditions
Under standard conditions, the temperature in the welding arc column reaches 7000 degrees, at its maximum value. Using the cathode, it is necessary to achieve a constant temperature at which the arc will occur and burn. In this case, factors such as diameter, size and ambient temperature are also taken into account.
Classification of the welding arc.
It is important to ensure that the value does not fluctuate, so you can weld absolutely any material. A working power source is the key to a constant temperature indicator of the element; this is what influences the operating properties of the element.
The main areas of the welding arc are the work of ionized gas, as well as the use of an alkaline or alkaline earth group in the form of potassium or calcium to promote a reliable and good burning of the welding arc. The question in what environment a welding arc can burn is quite relevant.
It is necessary to take into account many physical and chemical factors, be able to calculate how much energy is expended to remove an electron from an atom, depending on the nature of the new gas formation, etc.
Useful Application
Oddly enough, physicists found application for this electrical phenomenon even at the stage of development of the science of electricity. An example of this is the Yablochkov light bulb . It consisted of two carbon electrodes, between which an electric arc was ignited.
This lamp had two drawbacks. The electrodes quickly wore out (burned out), and the light spectrum shifted to the ultraviolet zone, which negatively affected vision. For these reasons, arc lamps did not find widespread use and were quickly replaced by incandescent lamps, which exist to this day.
The exception is arc-discharge lamps, as well as high-power spotlights used primarily for military purposes. The arc discharge has been widely used in practice since the invention of the welding machine. Arc welding is used for welding metals. (see Fig. 5)
Rice. 5. Arc welding
Using plasma conductivity, including special welding electrodes in the welding circuit, a high temperature is achieved in a concentrated spot. By adjusting the welding current, the welder has the opportunity to adjust the machine to the desired temperature of the arc discharge. To protect the barrel from heat loss, the metal electrodes are coated with a special mixture that ensures combustion stability.
An electric arc is used in blast furnaces for melting metals. Arc melting is convenient in that you can regulate its temperature by changing the current parameters.
Along with useful applications, in electrical engineering we often have to deal with arc discharges. An uncontrolled arc discharge can cause significant damage on power lines, industrial and domestic networks.
Rice. 6. Arc discharge on power lines
Causes
Based on the definition, we can name the conditions for the occurrence of an electric arc:
- the presence of multipolar electrodes with high currents;
- creation of a spark discharge;
- maintaining voltage on the electrodes;
- providing conditions for maintaining the temperature of the barrel.
A spark discharge occurs in two cases: during short-term contact of the electrodes or when approaching breakdown parameters. A powerful electrical breakdown always ignites the barrel.
While maintaining the optimal arc length, the plasma temperature is maintained independently. However, as the gap between the electrodes increases, intense heat exchange occurs between the barrel and the surrounding air. Eventually, in the barrel, due to a drop in temperature, the formation of ions will stop like an avalanche, resulting in the extinguishing of the flame.
Breakdowns often occur on high-voltage power lines. They can lead to the destruction of insulators and other negative consequences. A long electric arc goes out quite quickly, but even in a short burning time its destructive power is enormous.
An arc tends to form when contacts open. In this case, the switch contacts quickly burn out, the electrical circuit remains closed until the barrel disappears. This is dangerous not only for networks, but also for humans.
Extinguishing methods
It should be noted that arc extinction occurs for various reasons. For example, as a result of cooling of the column, a voltage drop, or when the air between the electrodes is displaced by external vapors that prevent ionization.
In order to prevent the formation of arcs on high-voltage power line wires, they are spaced over a long distance, which eliminates the possibility of breakdown. If a breakdown between the wires does occur, the long barrel will quickly cool and extinguishment will occur.
To cool the barrel, it is sometimes divided into several components. This principle is often used in the designs of air circuit breakers designed for voltages up to 1 kV.
Some circuit breaker models contain multiple arc chutes to promote rapid cooling.
Rapid ionization can be achieved by evaporating certain materials surrounding the space of the moving knives. High-pressure evaporation blows away the plasma of the barrel, resulting in extinguishing.
There are other methods: placing contacts in oil, auto-blowing, using electromagnetic damping, etc.
Exposure to humans and electrical equipment
An electric arc poses a danger to humans due to its thermal effects, as well as the ultraviolet effect of the emitting light. High AC voltage poses a huge danger. If an unprotected person is at a critically close distance from live parts of devices, an electrical breakdown may occur with the formation of an arc. Then, in addition to the effect of current, the body will be affected by a thermal component.
The spread of an arc discharge through structural parts of equipment threatens to burn out electronic elements, boards and connections.
AC Welding
Everything described above concerned the procedure for welding with direct current. However, alternating current can also be used for these purposes. As for the negative aspects, there is a noticeable deterioration in stability, as well as frequent jumps in the combustion temperature of the welding arc. One of the advantages is that you can use simpler, and therefore cheaper equipment. In addition, in the presence of a variable component, such an effect as magnetic blast practically disappears. The last difference is that there is no need to select polarity, since with alternating current the change occurs automatically with a frequency of about 50 times per second.
It can be added that when using manual equipment, in addition to the high temperature of the welding arc with the manual arc method, infrared and ultraviolet waves will be emitted. In this case, they are emitted by a discharge. This requires maximum protection for the worker.
