What can be cooked from squid: quick and tasty

INDUCTION HEATER Is electric heater operating when changing the flux of magnetic induction in a closed conducting loop. This phenomenon is called electromagnetic induction. Want to know how an induction heater works? ZAVODRR is a trade information portal where you will find information about heaters.

Vortex induction heaters

An induction coil is capable of heating any metal, transistor heaters are assembled and have a high efficiency of more than 95%, they have long replaced lamp induction heaters, whose efficiency did not exceed 60%.

A vortex induction heater for contactless heating has no losses for tuning the resonant coincidence of the operating parameters of the installation with the parameters of the output oscillatory circuit. Vortex heaters assembled on transistors are able to perfectly analyze and adjust the output frequency in automatic mode.

Induction metal heaters

Heaters for induction heating of metal have a contactless method due to the action of a vortex field. Different types of heaters penetrate the metal to a certain depth from 0.1 to 10 cm, depending on the selected frequency:

high frequency;
average frequency;
ultra high frequency.

Metal induction heaters allow you to process parts not only on open areas, but also to place heated objects in isolated chambers, in which you can create any environment, as well as a vacuum.

Electric induction heater

High Frequency Electric Induction Heater gets new ways of application every day. The heater operates on a variable electric current... Most often, induction electric heaters are used to bring metals to the required temperatures during the following operations: forging, brazing, welding, bending, hardening, etc. Electric induction heaters, operate at a high frequency of 30-100 kHz and are used for heating different types environments and coolants.

Electric heater applied in many areas:

metallurgical (HFC heaters, induction furnaces);
instrumentation (soldering of elements);
medical (production and disinfection of instruments);
jewelry (jewelry making);
housing and communal (induction heating boilers);
food (induction steam boilers).

Medium Frequency Induction Heaters

When deeper heating is required, medium-frequency induction heaters are used, operating at medium frequencies from 1 to 20 kHz. A compact inductor for all types of heaters can be of a very different shape, which is selected so as to ensure uniform heating of samples of the most diverse shapes, while a given local heating can be carried out. Medium frequency type will process materials for forging and hardening, as well as through heating for stamping.

Easy to operate, with an efficiency of up to 100%, medium-frequency induction heaters are used for large circle technologies in metallurgy (also for melting various metals), mechanical engineering, instrument making and other areas.

High frequency induction heaters

The widest range of applications is for high-frequency induction heaters. The heaters are characterized by a high frequency of 30-100 kHz and a wide power range of 15-160 kW. The high-frequency type provides shallow heating, but this is enough to improve Chemical properties metal.

High frequency induction heaters are easy to operate and economical, while their efficiency can reach 95%. All types operate continuously for a long time, and the two-unit version (when the high-frequency transformer is placed in a separate unit) allows round-the-clock operation. The heater has 28 types of protection, each of which is responsible for its own function. Example: monitoring the water pressure in the cooling system.

Ultra high frequency induction heaters

Microwave induction heaters operate at superfrequency (100-1.5 MHz) and penetrate to a heating depth (up to 1 mm). The ultrahigh-frequency type is indispensable for processing thin, small parts with a small diameter. The use of such heaters makes it possible to avoid unwanted deformations accompanying heating.

Ultra-high-frequency induction heaters based on JGBT-modules and MOSFET-transistors have a power range of 3.5-500 kW. They are used in electronics, in the production of high-precision instruments, watches, jewelry, for the production of wire and for other purposes that require special precision and delicacy.

Forging induction heaters

The main purpose of forging-type induction heaters (TSC) is to heat parts or their parts prior to subsequent forging. Blanks can be different types, alloy and shape. Induction forging heaters allow automatic processing of cylindrical workpieces of any diameter:

economical, as they spend only a few seconds on heating and have a high efficiency up to 95%;
easy to use, allow for: full control of the process, semi-automatic loading and unloading. There are options with full automation;
reliable and can work continuously for a long time.

Shaft induction heaters

Shaft hardening induction heaters work in conjunction with a hardening complex. The workpiece is in a vertical position and rotates inside a stationary inductor. The heater allows the use of all types of rolls for sequential local heating; the hardening depth can be fractions of a millimeter in depth.

As a result of induction heating of the shaft along the entire length with instant cooling, its strength and durability are greatly increased.

Induction tube heaters

All types of pipes can be treated with induction heaters. The pipe heater can be air-cooled or water-cooled, with a capacity of 10-250 kW, with the following parameters:

Air cooled induction tube heating produced with a flexible inductor and thermal blanket. Heating temperature up to temperature of 400 ° C, and use pipes with a diameter of 20 - 1250 mm with any wall thickness.
Induction Heating Water Cooled Pipe has a heating temperature of 1600 ° C and is used for bending pipes with a diameter of 20 - 1250 mm.

Each heat treatment option is applied to improve the quality of any steel pipe.

Heating control pyrometer

One of the most important parameters of the operation of induction heaters is temperature. For a more thorough control over it, in addition to built-in sensors, infrared pyrometers are often used. These optical instruments allow you to quickly and easily determine the temperature of hard-to-reach (due to high heat, the likelihood of exposure to electricity, etc.) surfaces.

If you connect a pyrometer to an induction heater, you can not only monitor temperature regime but also automatically maintain the heating temperature for a preset time.

The principle of operation of induction heaters

During operation, a magnetic field is generated in the inductor, into which the part is placed. Depending on the task (heating depth) and the part (composition), the frequency is selected, it can be from 0.5 to 700 kHz.

The principle of operation of the heater according to the laws of physics says: when a conductor is in an alternating electromagnetic field, an EMF (electromotive force) is formed in it. The amplitude graph shows that it moves in proportion to the change in magnetic flux velocity. Due to this, eddy currents are formed in the circuit, the magnitude of which depends on the resistance (material) of the conductor. According to the Joule-Lenz law, the current leads to heating of the conductor, which has a resistance.

The principle of operation of all types of induction heaters is similar to that of a transformer. The conductive workpiece, which is located in the inductor, is similar to a transformer (without a magnetic circuit). The primary winding is the inductor, the secondary inductance of the part, and the load is the resistance of the metal. During HFC heating, a "skin effect" is formed, eddy currents that are formed inside the workpiece displace the main current to the surface of the conductor, because the heating of the metal on the surface is stronger than inside.

Benefits of induction heaters

The induction heater has undeniable advantages and is the leader among all types of devices. This advantage is summed up in the following:

It uses less energy and does not pollute the surrounding area.
Easy to operate, it provides high quality work and allows you to control the process.
Heating through the walls of the chamber provides special purity and the ability to obtain ultrapure alloys, while melting can be performed in different atmospheres, including inert gases and in vacuum.
With its help, uniform heating of parts of any shape or selective heating is possible.
Finally, induction heaters are versatile, allowing them to be used universally, replacing obsolete energy-intensive and inefficient installations.

Induction heaters are repaired using spare parts from our warehouse. At the moment we can repair all types of heaters. Induction heaters are quite reliable if you strictly follow the operating instructions and avoid extreme operating modes - first of all, monitor the temperature and proper water cooling.

The subtleties of the operation of all types of induction heaters are often not fully published in the manufacturer's documentation; their repair should be carried out by qualified specialists who are well familiar with the detailed principle of operation of such equipment.

Video of operation of induction medium-frequency heaters

You can watch a video of the operation of a medium frequency induction heater .. The medium frequency is used for deep penetration into all types of metal products. Medium frequency heater is a reliable and modern equipment that works around the clock for the benefit of your company.

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HF - induction discharge: combustion conditions, design and field of application

Introduction

One of critical issues organizing plasma technological processes is the development of plasma sources with properties that are optimal for this technology, for example: high homogeneity, given plasma density, energy of charged particles, concentration of chemically active radicals. Analysis shows that high-frequency (HF) plasma sources are the most promising for use in industrial technologies, since, firstly, they can be used to process both conductive and dielectric materials, and Secondly, as working gases, you can use not only inert, but also chemically active gases. Plasma sources based on capacitive and inductive RF discharges are known today. A feature of the capacitive RF discharge, which is most often used in plasma technologies, is the existence of a space charge at the electrode layers, in which a time-average potential drop is formed, which accelerates ions in the direction of the electrode. This makes it possible to process samples of materials located on the electrodes of an HF capacitive discharge using accelerated ions. The disadvantage of capacitive RF discharge sources is the relatively low concentration of electrons in the main volume of the plasma. A significantly higher concentration of electrons at the same RF powers is characteristic of inductive RF discharges.

Inductive RF discharge has been known for over a century. This is a discharge excited by a current flowing through an inductor located on the side or end surface of a usually cylindrical plasma source. Back in 1891, J. Thomson suggested that an inductive discharge is caused and maintained by a vortex electric field, which is created by a magnetic field, in turn, induced by a current flowing through the antenna. In 1928-1929, arguing with J. Thomson, D. Townsend and R. Donaldson expressed the idea that an inductive HF discharge is supported not by vortex electric fields, but by potential ones that appear due to the presence of a potential difference between the turns of the inductor. In 1929, K. McKinton experimentally showed the possibility of the existence of two modes of discharge combustion. At low RF voltage amplitudes, the discharge actually appeared under the action of an electric field between the turns of the coil and had the character of a weak longitudinal glow along the entire gas-discharge tube. With an increase in the HF voltage amplitude, the glow became brighter, and finally a bright ring discharge appeared. In this case, the glow caused by the longitudinal electric field disappeared. Subsequently, these two forms of discharge were called E - H - discharge, respectively.

The regions of existence of an inductive discharge can be conditionally divided into two large regions: these are high pressures(of the order of atmospheric pressure) at which the generated plasma is close to equilibrium, and low pressures at which the generated plasma is nonequilibrium.

Periodic discharges. HF and microwave plasma. Types of high-frequency discharges

To excite and maintain a direct current glow discharge, it is necessary that two conducting (metal) electrodes are in direct contact with the plasma zone. From a technological point of view, such a design of a plasma-chemical reactor is not always convenient. First, when carrying out the processes of plasma deposition of dielectric coatings, a non-conductive film can also form on the electrodes. This will lead to an increase in the instability of the discharge and, ultimately, to its attenuation. Secondly, in reactors with internal electrodes, there is always a problem of contamination of the target process with materials removed from the electrode surface during physical sputtering or chemical reactions with plasma particles. To avoid these problems, including completely, to abandon the use of internal electrodes, allows the use of periodic discharges, excited not by a constant, but by an alternating electric field.

The main effects that occur in periodic discharges are determined by the relationships between the characteristic frequencies of plasma processes and the frequency of the applied field. It is advisable to consider three characteristic cases:

Low frequencies. At frequencies of the external field up to 10 2 - 10 3 Hz, the situation is close to that realized in a constant electric field. However, if the characteristic frequency of the loss of charges v d is less than the frequency of the field w (v d? W), the charges after the change in the sign of the field have time to disappear before the value of the field reaches a value sufficient to maintain the discharge. Then the discharge will be extinguished twice and ignited during the period of the field change. The reignition voltage should be frequency dependent. The higher the frequency, the smaller the fraction of electrons will have time to disappear during the existence of a field insufficient to maintain the discharge, the lower the potential for reignition. At low frequencies after breakdown, the ratio between the current and the combustion voltage corresponds to the static current-voltage characteristic of the discharge (Fig. 1, curve 1). The discharge parameters "track" voltage changes.

Intermediate frequencies. With increasing frequency, when the characteristic frequencies of plasma processes are comparable and somewhat lower than the field frequency (vd? W), the state of the discharge does not have time to “follow” the change in the supply voltage. A hysteresis appears in the dynamic I – V characteristic of the discharge (Fig. 1, curve 2).

High frequencies. If the condition is met< v d <

Rice. 1. Volt-ampere characteristics of periodic discharges: 1 - static I – V characteristic, 2 –– I – V characteristic in the transition frequency range, 3 - steady-state dynamic I – V characteristic

There are many types of electric discharges in a gas, depending on the nature of the applied field (constant electric field, alternating, pulsed, (HF), ultra-high frequency (UHF)), on the gas pressure, the shape and location of the electrodes, etc.

For HF discharges, there are the following excitation methods: 1) capacitive at frequencies less than 10 kHz, 2) inductive at frequencies in the range 100 kHz - 100 MHz. These excitation methods involve the use of range data generators. With the capacitive method of excitation, the electrodes can be installed inside the working chamber or outside, if the chamber is made of dielectric (Fig. 2 a, b). For the induction method, special coils are used, the number of turns of which depends on the frequency used (Fig. 2 c).

HF induction discharge

High-frequency induction (electrodeless) discharge in gases has been known since the end of the last century. However, it was not immediately possible to fully understand him. An inductive discharge can be easily observed if an evacuated vessel is placed inside a solenoid, through which a sufficiently strong high-frequency current flows. Under the action of a vortex electric field, which is induced by an alternating magnetic flux, a breakdown occurs in the residual gas and a discharge is ignited. The Joule heat of the ring induction currents flowing in the ionized gas along the lines of force of the vortex electric field is spent to maintain the discharge (ionization) (the magnetic lines of force inside the long solenoid are parallel to the axis; Fig. 3).

Fig. 3 Field diagram in the solenoid

Among the old works on electrodeless discharges, the most comprehensive studies belong to J. Thomson, 2 who, in particular, experimentally proved the inductive nature of the discharge and derived the theoretical conditions for ignition: the dependence of the threshold magnetic field for breakdown on the gas pressure (and frequency). Like the Paschen curves for the breakdown of the discharge gap in a constant electric field, the ignition curves have a minimum. For the practical frequency range (from tenths to tens of megahertz), the minima lie in the low pressure region; therefore, the discharge was usually observed only in highly rarefied gases.

Combustion conditions of HF - induction discharge

An inductive RF discharge is a discharge excited by a current flowing through an inductor located on the side or end surface of a usually cylindrical plasma source (Fig. 4a, b). The central issue in the physics of low-pressure inductive discharge is the question of the mechanisms and efficiency of absorption of RF power by plasma. It is known that with a purely inductive excitation of an RF discharge, its equivalent circuit can be represented in the form shown in Fig. 1d. The RF generator is loaded onto a transformer, the primary winding of which consists of an antenna through which the current generated by the generator flows, and the secondary winding is the current induced in the plasma. The primary and secondary windings of the transformer are connected by the coefficient of mutual induction M. The transformer circuit can be easily reduced to a circuit that is a series-connected active resistance and antenna inductance, equivalent resistances and plasma inductance (Fig.4e), so that the power of the RF generator P gen turns out to be connected with the power P an t emitted in the antenna, and the power P p1 emitted in the plasma, by the expressions

where I is the current flowing through the antenna, P ant is the active resistance of the antenna, R p 1 is the equivalent resistance of the plasma.

From formulas (1) and (2) it can be seen that when the load is matched with the generator, the active RF power Pgen, given by the generator to the external circuit, is distributed between two channels, namely: one part of the power goes to heating the antenna, and the other part is absorbed plasma. Earlier, in the overwhelming majority of works, it was assumed a priori that under experimental conditions

R pl> R antvv (3)

and the properties of the plasma are determined by the power of the RF generator completely absorbed by the plasma. In the mid-1990s, V. Godyak and his coworkers convincingly showed that in low-pressure discharges, relation (3) can be violated. It is obvious that under the condition

R pi? R ant (4)

the behavior of an HF inductive discharge changes dramatically.

Rice. 4... Schemes (a, b) of inductive plasma sources and (c) an inductive plasma source with a capacitive component, (d, e) equivalent circuits of a purely inductive discharge.

Now, the plasma parameters depend not only on the power of the RF generator, but also on the equivalent plasma resistance, which, in turn, depends on the plasma parameters and the conditions for its maintenance. This leads to the appearance of new effects associated with self-consistent power redistribution in the external discharge circuit. The latter can significantly affect the efficiency of plasma sources. Obviously, the key to understanding the behavior of the discharge in modes corresponding to inequality (4), as well as to optimizing the operation of plasma devices, lies in the regularities of the change in the equivalent plasma resistance when changing the plasma parameters and the conditions for maintaining the discharge.

RF design - induction discharge

The foundations for modern research and applications of electrodeless discharges were laid by the works of G.I.Babat, which were carried out before the war itself at the Leningrad electric lamp plant "Svetlana". These works were published in 1942 3 and became widely known abroad after their publication in England in 1947. 4. Babat created high-frequency lamp generators with powers of the order of hundreds of kilowatts, which allowed him to obtain powerful electrodeless discharges in air at pressures up to atmospheric ... Babat worked in the frequency range 3--62 MHz, the inductors consisted of several turns with a diameter of about 10 cm. A huge power for that time, up to several tens of kilowatts, was introduced into the high-pressure discharge (however, such values are also high for modern installations). ? Punch? air or other gas at atmospheric pressure, of course, was not possible even at the highest currents in the inductor, so special measures had to be taken to ignite the discharge. The easiest way was to initiate a discharge at low pressure, when the breakdown fields are small, and then gradually increase the pressure, bringing it to atmospheric. Babat noted that when gas flows through the discharge, the latter can be extinguished if the blowing is too intense. At high pressures, the effect of contraction was discovered, i.e., the detachment of the discharge from the walls of the discharge chamber. In the 1950s, several articles appeared on the electrodeless discharge 5 ~ 7. Cabann 5 investigated discharges in inert gases at low pressures from 0.05 to 100 mm Hg. Art. and low powers up to 1 kW at frequencies of 1--3 MHz, determined the ignition curves, measured the power introduced into the discharge using the calorimetric method, and measured electron concentrations using probes. Ignition curves for many gases were also obtained in Ref. 7, an attempt was made to use the discharge for ultraviolet spectroscopy. An electrodeless plasma torch, to which current installations are very close, was designed by Reed in 1960. 8. Its schematic and photograph are shown in Fig. 2. A quartz tube 2.6 cm in diameter was enclosed by a five-turn inductor made of a copper tube with a distance between turns of 0.78 cm. The power source was an industrial high-frequency generator with a maximum output power of 10 kW; operating frequency 4 MHz. A movable graphite rod was used to ignite the discharge. The rod, inserted into the inductor, heats up in the high-frequency field and emits electrons. The surrounding gas heats up and expands, and breakdown occurs in it. After ignition, the rod is removed, and the discharge continues to burn. The most significant aspect of this setup was the use of a tangential gas feed. Reed pointed out that the resulting plasma should propagate quickly enough against the flow of gas that tends to carry it away. Otherwise, the discharge will go out, as happens with unstabilized flames. At low flow rates, maintaining the plasma can provide normal thermal conductivity. (The role of thermal conductivity in high-pressure discharges was also noted by Cabann 5). However, at high gas flow rates, it is necessary to take measures to recirculate a part of the plasma. A satisfactory solution to this problem was the vortex stabilization used by Reed, in which gas is fed tangentially into the tube and flows through it in a helical motion. Due to the centrifugal runaway of the gas, a column of reduced pressure is formed in the axial part of the tube. There is almost no axial flow here, and part of the plasma is sucked upstream. The higher the feed rate, the higher the luminous plasma penetrates against the flow. In addition, with this method of supply, the gas flows along the tube mainly at its walls, squeezes the discharge from the walls and insulates the latter from the destructive effect of high temperatures, which makes it possible to work at increased powers. These qualitative considerations, briefly expressed by Reed, are very important for understanding the phenomena, although they may not quite accurately reflect the essence of the matter. We will return to the question of maintaining plasma, which seems to be the most serious when considering a stationary stabilized discharge in a gas flow, below, in Chap. IV.

Reed worked with argon and with mixtures of argon with helium, hydrogen, oxygen, and air. He noted that it is easiest to maintain a discharge in pure argon. The argon flow rate was 10--20 L / min (the average gas velocity over the tube cross section is 30--40 cm / s) when a power of 1.5--3 kW was introduced into the discharge, which is approximately half the power consumed by the generator. Read determined the energy balance in the plasmatron and measured the spatial temperature distribution in the plasma by the optical method.

He published several more articles: on powerful induction discharges at low pressures, 9 on measurements of heat transfer to probes introduced at various points of the plasma torch, 10 on growing crystals of refractory materials using an induction torch, etc.

An induction plasma torch, similar in design to the Reed one, was described somewhat later in the works of Rebu.45'46 Rebu used it for growing crystals and making spherical particles of refractory materials.

Since about 1963, many works have appeared in our and foreign press devoted to the experimental study of high-pressure induction discharges both in closed vessels and in a gas flow1 2-3 3 ЃE 4 0-4 4-5 3 ЃE 8 0.

The spatial distributions of temperature in the discharge region and in the plasma plume, and the distribution of electron concentrations are measured. Here, as a rule, well-known optical, spectral and probe methods are used, which are usually used in the study of plasma of arc discharges. The powers put into the discharge are measured at different voltages on the inductor, different gas flow rates, different dependences of parameters for different gases, frequencies, etc. how it all depends on the specific conditions: the diameter of the tube, the geometry of the inductor, the gas flow rate, etc. The general result of many works is the conclusion that at a power of the order of several or ten kilowatts, the temperature of argon plasma reaches about 9000-10000 ° K ...

The temperature distribution generally has the character of a plateau. in the middle of the tube and falls off sharply near the walls; however, the? plateau? not quite even, in the central part a small dip is obtained, usually several hundred degrees in size. In other gases, temperatures are also of the order of 10,000 °, depending on the type of gas and other conditions. The temperatures in air are lower than in argon at the same power, and, conversely, several times higher powers are required to reach the same temperatures 31. The temperature rises slightly with increasing power and weakly depends on the gas flow rate. In fig. 3 and 4 are given to illustrate the temperature distribution along the radius, the temperature field (isotherms), and the distribution of electron concentrations. Experiments27 have shown that with an increase in the feed rate and gas flow rate (with a tangential feed), the discharge is more and more squeezed out of the walls and the discharge radius changes from about 0.8 to 0.4 of the tube radius. With an increase in the gas flow rate, the power deposited into the discharge also slightly decreases, which is associated with a decrease in the discharge radius, ie, the plasma flow or flow rate. During discharges in closed vessels, without a gas flow, the luminous region of the discharge usually comes very close to the side walls of the vessel. Measurements of electron concentrations have shown that the state of the plasma at atmospheric pressure is close to thermodynamic equilibrium. The measured concentrations and temperatures fit within the Saha equation with satisfactory accuracy.

Induction HF - discharge

Currently, low-pressure plasma sources are known, the principle of operation of which is based on an inductive HF discharge in the absence of a magnetic field, as well as on an inductive HF discharge placed in an external magnetic field with induction corresponding to the conditions of electron cyclotron resonance (ECR) and conditions excitation of helicons and Trivelpeace - Gold (TG) waves (hereinafter referred to as helicon sources).

It is known that in the plasma of an inductive discharge, HF electric fields are skinned, i.e. heating of electrons is carried out in a narrow wall layer. When an external magnetic field is applied to the plasma of an inductive high-frequency discharge, regions of transparency appear, in which high-frequency fields penetrate deep into the plasma and electrons are heated throughout its volume. This effect is used in plasma sources, the operating principle of which is based on ECR. These sources operate primarily in the microwave range (2.45 GHz). Microwave radiation is introduced, as a rule, through a quartz window into a cylindrical gas-discharge chamber, in which an inhomogeneous magnetic field is formed with the help of magnets. The magnetic field is characterized by the presence of one or several resonance zones, in which the ECR conditions are fulfilled and the RF power is injected into the plasma. In the radio frequency range, ECR is used in so-called neutral-loop plasma sources. A neutral circuit, which is a continuous sequence of points with zero magnetic field, plays an essential role in plasma generation and the formation of the discharge structure. A closed magnetic circuit is formed using three electromagnets. The currents in the windings of the upper and lower coils have the same direction. The middle coil current flows in the opposite direction. HF induction discharge with a neutral circuit is characterized by a high plasma density (10 11 - 10 12 cm ~ 3) and a low electron temperature (1 -4 eV).

Inductive discharge without external magnetic field

The power P pi absorbed by the plasma is plotted as an independent variable along the abscissa. It is natural to assume that the plasma density ne is proportional to P pi, but it should be noted that for different plasma sources the proportionality coefficients between P pi and ne e will differ. As you can see, the general tendency of the behavior of the equivalent resistance R pi is its increase in the region of relatively small values of the input power, and then its saturation.

On the contrary, in the region of high electron concentrations, where collisionless absorption prevails, i.e. in the region of the anomalous skin effect, the R pl (ne) dependence is close to that obtained for media with strong spatial dispersion. In general, the nonmonotonicity of the dependence of the equivalent resistance on the plasma density is explained by the competition of two factors: on the one hand, the absorption of rf power increases with an increase in the electron concentration; on the other hand, the depth of the skin layer, which determines the width of the region of absorption of rf power, decreases with increasing n e.

The theoretical model of a plasma source excited by a spiral antenna located on its upper end surface predicts that the equivalent plasma resistance does not depend on the length of the plasma source, provided that the depth of the skin layer is less than the length of the plasma source. Physically, this result is obvious, since the absorption of RF power occurs within the skin layer. Under the experimental conditions, the depth of the skin layer is obviously less than the length of the plasma sources; therefore, it is not surprising that the equivalent plasma resistance of the sources equipped with the upper end antenna does not depend on their length. On the contrary, if the antenna is located on the side surface of the sources, an increase in the source length, accompanied by a simultaneous increase in the antenna length, leads to an increase in the region in which the RF power is absorbed, i.e. to the elongation of the skin layer; therefore, in the case of a side antenna, the equivalent impedance increases with increasing source length.

Experiments and calculations have shown that at low pressures, the absolute values of the equivalent plasma resistance are low. An increase in the pressure of the working gas leads to a significant increase in the equivalent resistance. This effect has been repeatedly noted both in theoretical and experimental works. The physical reason for the increase in the ability of plasma to absorb RF power with increasing pressure lies in the mechanism of RF power absorption. As can be seen from Fig. 5, at the minimum of the considered pressures, p - 0.1 mTorr, the Cherenkov dissipation mechanism is predominant. Electron-atom collisions have practically no effect on the value of the equivalent resistance, and electron-ion collisions lead only to an insignificant increase in the equivalent resistance at n> 3 x 10 11 cm - 3. Increase in pressure, i.e. the frequency of electron-atom collisions, leads to an increase in the equivalent resistance due to an increase in the role of the collisional mechanism of RF power absorption. This can be seen from Fig. 5, which shows the ratio of the equivalent resistance calculated taking into account collisional and collisionless absorption mechanisms to the equivalent resistance calculated only with collisions taken into account.

Rice.5 . Dependence of the ratio of the equivalent resistance Rpi, calculated taking into account the collisional and collisionless absorption mechanisms, to the equivalent resistance Rpi, calculated only taking into account collisions, on the plasma density. The calculation was performed for flat disk-shaped sources with a radius of 10 cm at a neutral gas pressure of 0.3 mtorr (1), 1 mtorr (2), 10 mtorr (3), 100 mtorr (7), 300 mtorr (5).

Inductive discharge with external magnetic field

Plasma sources equipped with spiral antennas located on the side and end surfaces of the sources, as well as Nagoya III antennas, were used in the experiments. For an operating frequency of 13.56 MHz, the region of magnetic fields B «0.4-1 mT corresponds to the ECR conditions, and the region B> 1 mT corresponds to the conditions for the excitation of helicons and Trivelpeace-Gold waves.

At low pressures of the working gas (p> 5 mTorr), the equivalent resistance of the plasma without a magnetic field is substantially less in magnitude than in the "helicon" region. The Rpl values obtained for the ECR region occupy an intermediate position, and here the equivalent resistance increases monotonically with increasing magnetic field. The “helicon” region is characterized by a nonmonotonic dependence of the equivalent resistance on the magnetic field, and the nonmonotonicity of R pl (B) in the case of the end helical antenna and the Nagoya III antenna is much more pronounced than in the case of the lateral helical antenna. The position and the number of local maxima of the ^ pi (B) curve depend on the input RF power, the length and radius of the plasma source, the type of gas and its pressure.

An increase in the input power, i.e. the concentration of electrons n e, leads to an increase in the equivalent resistance and a shift of the main maximum of the function ^ pi (B) to the region of high magnetic fields, and in some cases to the appearance of additional local maxima. A similar effect is observed with an increase in the length of the plasma source.

An increase in pressure in the range of 2-5 mtorr, as seen from Fig. 4b, does not lead to significant changes in the character of the ^ pl (B) dependence; however, at pressures exceeding 10 mTorr, the nonmonotonicity of the dependence of the equivalent resistance on the magnetic field disappears, the absolute values of the equivalent resistance decrease and become less than the values obtained without a magnetic field.

The analysis of the physical mechanisms of absorption of HF power by inductive discharge plasma under ECR conditions and conditions for the excitation of helicons and TG waves was carried out in many theoretical works. Analytical consideration of the problem of the excitation of helicons and TG waves in the general case is associated with significant difficulties, since it is necessary to describe two waves associated with each other. Recall that the helicon is a fast shear wave, and the TG wave is a slow longitudinal one. Helicons and TG waves turn out to be independent only in the case of a spatially unlimited plasma, in which they represent the eigenmodes of oscillations of a magnetized plasma. In the case of a limited cylindrical plasma source, the problem can be solved only numerically. However, the main features of the physical mechanism of HF power absorption at B> 1 mT can be illustrated with the help of the developed in the helicon approximation, which describes the process of wave excitation in plasma provided that the inequalities are satisfied

Application area

high frequency burning magnetic plasma

Plasma reactors and ion sources, the principle of which is based on a low-pressure inductive high-frequency discharge, have been an essential component of modern terrestrial and space technologies for several decades. The widespread use of technical applications of inductive RF discharge is facilitated by its main advantages: the possibility of obtaining a high concentration of electrons at a relatively low level of RF power, the absence of contact between the plasma and metal electrodes, the low temperature of the electrons, and, consequently, the low potential of the plasma relative to the walls limiting the discharge. The latter, in addition to minimizing power losses on the walls of the plasma source, makes it possible to avoid damage to the surface of the samples during their processing in the discharge with high-energy ions.

Typical examples of plasma sources operating on an inductive RF discharge without a magnetic field are plasma reactors intended for etching substrates, ion sources intended for the implementation of terrestrial ion-beam technologies and work in space as engines for correcting the orbit of spacecraft, and light sources. A common design feature of the listed devices is the presence of a gas-discharge chamber (GDC), on the outer surface of which or inside it there is an inductor or antenna. With the help of an antenna connected to a high-frequency generator, high-frequency power is introduced into the volume of the GDK and an electrodeless discharge is ignited. The currents flowing through the antenna induce a vortex electric field in the plasma, which heats the electrons to the energies required for effective ionization of the working gas. Typical plasma densities in plasma reactors are 10 11 - 3 x 10 12 cm ~ 3, and in ion sources - 3 x 10 10 - 3 x 10 11 cm ~ 3. The characteristic pressure of a neutral gas in plasma reactors varies from 1 to 30 mtorr, in ion sources it is 0.1 mtorr, in light sources - 0.1-10 torr.

Plasma reactors and ion sources, the principle of which is based on a low-pressure inductive high-frequency discharge, have been an essential component of modern terrestrial and space technologies for several decades. The widespread use of technical applications of an inductive RF discharge is facilitated by its main advantages - the possibility of obtaining a high concentration of electrons at a relatively low level of RF power, the absence of plasma contact with metal electrodes, a low electron temperature, and, consequently, a low plasma potential relative to the walls limiting the discharge. The latter, in addition to minimizing power losses on the walls of the plasma source, makes it possible to avoid damage to the surface of the samples during their processing in the discharge with high-energy ions.

The results obtained in recent years, both experimental and theoretical, show that the parameters of the inductive high-frequency discharge plasma depend on the power losses in the external circuit and the values of the power supplied to the discharge through the inductive and capacitive channels. The plasma parameters, on the one hand, are determined by the values of the absorbed power, and on the other hand, they themselves determine both the ratio of the powers supplied to different channels and, ultimately, the power absorbed by the plasma. This is responsible for the self-consistent nature of the discharge. Self-consistency is most clearly manifested in the strong nonmonotonicity of the dependence of the plasma parameters on the magnetic field and in the disruption of the discharge. Significant power losses in the external circuit and the nonmonotonic dependence of the ability of the plasma to absorb RF power on the plasma density lead to saturation of the plasma density with an increase in the power of the RF generator and the appearance of a hysteresis in the dependence of plasma parameters on the power of the RF generator and the external magnetic field.

The presence of the capacitive component of the discharge causes a change in the fraction of the power introduced into the plasma through the inductive channel. This causes a shift in the position of the transition of the discharge from a low to a high mode to the region of lower powers of the RF generator. When passing from a low discharge mode to a high one, the presence of a capacitive component manifests itself in a smoother change in the plasma density with an increase in the generator power and in the disappearance of hysteresis. An increase due to the contribution of power through the capacitive channel of the concentration of electrons to values exceeding the value at which the equivalent resistance reaches a maximum, leads to a decrease in the contribution of RF power through the inductive channel. Physically, it is not justified to compare the modes of an inductive rf discharge with low and high electron concentrations with the capacitive and inductive modes, since the presence of one channel for inputting power into the plasma leads to a change in the fraction of the power entering the plasma through the other channel.

Refinement of the picture of physical processes in a low-pressure HF inductive discharge makes it possible to optimize the parameters of plasma devices operating on its basis.

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Induction heating device

Everything happens in the following way. Under the influence of the variable, the electromotive force (EMF) of induction changes.

The EMF acts in such a way that eddy currents flow inside the bodies, which generate heat in full accordance with the Joule-Lenz law. Also EMF generates alternating current in the metal. In this case, the release of thermal energy occurs, which leads to an increase in the temperature of the metal.

This type of heating is the simplest, as it is non-contact. It allows you to reach very high temperatures at which you can handle

To provide induction heating, it is required to create a certain voltage and frequency in electromagnetic fields. This can be done in a special device - an inductor. It is powered from an industrial network at 50 Hz. For this you can use individual power sources - converters and generators.

The simplest device of a low-frequency inductor is a spiral (insulated conductor), which can be placed inside a metal pipe or wound around it. The passing currents heat the pipe, which in turn transfers heat to the environment.

The use of induction heating at low frequencies is quite rare. Medium and high frequency metal processing is more common.

Such devices differ in that the magnetic wave hits the surface, where it is attenuated. The body converts the energy of this wave into heat. To achieve maximum effect, both components should be close in shape.

Where are used

The use of induction heating in the modern world is widespread. Scope of use:

smelting metals, soldering them in a contactless way;
obtaining new metal alloys;
mechanical engineering;
jewelry making;
making small parts that can be damaged by other methods;
(moreover, the details can be of the most complex configuration);
heat treatment (processing of machine parts, hardened surfaces);
medicine (disinfection of devices and instruments).

Induction heating: positive characteristics

This method has many advantages:

It can quickly heat and melt any conductive material.
Allows heating in any environment: vacuum, atmosphere, non-conductive liquid.
Due to the fact that only the conductive material is heated, the walls, weakly absorbing waves, remain cold.
In specialized areas of metallurgy, obtaining ultrapure alloys. This is an entertaining process as the metals are mixed in a shielding gas sheath.

Compared to other types, induction does not pollute the environment. If, in the case of gas burners, contamination is present, as well as in arc heating, then induction excludes this, due to "pure" electromagnetic radiation.
Small dimensions of the inductor device.
The ability to manufacture an inductor of any shape, this will not lead to local heating, but will contribute to an even distribution of heat.
Indispensable if it is necessary to heat only a certain area of the surface.
It is not difficult to set up such equipment to the desired mode and regulate it.

disadvantages

The system has the following disadvantages:

It is rather difficult to independently install and adjust the type of heating (induction) and its equipment. It is better to consult a specialist.
The need to accurately match the inductor and the workpiece, otherwise induction heating will be insufficient, its power can reach small values.

Heating by induction equipment

For the arrangement of individual heating, you can consider such an option as induction heating.

As a unit, a transformer will be used, consisting of two types of windings: primary and secondary (which, in turn, is short-circuited).

How does it work

The principle of operation of a conventional inductor: vortex flows pass inside and direct the electric field to the second body.

In order for water to pass through such a boiler, two nozzles are brought to it: for cold, which comes in, and at the outlet of warm water - a second nozzle. Due to the pressure, the water is constantly circulating, which excludes the possibility of heating the inductor element. The presence of scale is excluded here, since constant vibrations occur in the inductor.

Such an element will be inexpensive to maintain. The main plus is that the device works silently. It can be installed in any room.

Making equipment yourself

Installation of induction heating will not be very difficult. Even someone who has no experience, after careful study, will cope with the task. Before starting work, you need to stock up on the following necessary elements:

Inverter. It can be used from a welding machine, it is inexpensive and is of the required high frequency. You can make it yourself. But this is a time consuming task.
Heater body (a piece of plastic pipe is suitable for this, induction heating of the pipe in this case will be most effective).
Material (wire no more than seven millimeters in diameter will do).
Devices for connecting the inductor to the heating network.
Mesh for holding the wire inside the inductor.
An induction coil can be created from (it must be enameled).
Pump (so that water is supplied to the inductor).

Rules for making equipment yourself

In order for the induction heating installation to work correctly, the current for such a product must correspond to the power (it must be at least 15 amperes, if required, then more).

The wire should be cut into pieces no more than five centimeters long. This is necessary for efficient heating in a high-frequency field.
The body should be no less in diameter than the prepared wire and have thick walls.
For attachment to the heating network, a special adapter is attached to one side of the structure.
A mesh should be placed at the bottom of the pipe to prevent the wire from falling out.
The latter is needed in such an amount that it fills the entire internal space.
The structure is closed, an adapter is installed.
Then a coil is constructed from this pipe. To do this, they wrap it with already harvested wire. The number of turns must be observed: minimum 80, maximum 90.
After connecting to the heating system, water is poured into the device. The coil is connected to the prepared inverter.
A pump is installed to supply water.
A temperature controller is mounted.

Thus, the calculation of induction heating will depend on the following parameters: length, diameter, temperature and processing time. Pay attention to the inductance of the buses supplying the inductor, which can be much higher than the indicators of the inductor itself.

About hobs

Another use in household use, in addition to the heating system, found this type of heating in hobs.

This surface looks like a conventional transformer. Its coil is hidden under the surface of the panel, which can be glass or ceramic. A current flows through it. This is the first part of the coil. But the second is the dishes in which the cooking will take place. Eddy currents are generated at the bottom of the cookware. They first heat the dishes, and then the food in it.

Heat will be generated only when the dishes are placed on the surface of the panel.

If it is absent, no action takes place. The induction heating zone will correspond to the diameter of the cookware placed on it.

For such stoves, special dishes are needed. Most ferromagnetic metals can interact with an induction field: aluminum, stainless and enamelled steel, cast iron. Not suitable for such surfaces only: copper, ceramic, glass and cookware made of non-ferromagnetic metals.

Naturally, it will only turn on when a suitable cookware is installed on it.

Modern cookers are equipped with an electronic control unit, which makes it possible to recognize empty and unusable dishes. The main advantages of cooking boilers are: safety, ease of cleaning, speed, efficiency, economy. Never burn yourself on the surface of the panel.

So, we found out where this type of heating (induction) is used.

Induction Heating is a method of non-contact heating by high frequency currents (RFH - radio-frequency heating) of electrically conductive materials.

Description of the method.

Induction heating is the heating of materials by electric currents that are induced by an alternating magnetic field. Consequently, this is the heating of products made of conductive materials (conductors) by the magnetic field of inductors (sources of an alternating magnetic field). Induction heating is carried out as follows. An electrically conductive (metal, graphite) workpiece is placed in a so-called inductor, which is one or more turns of wire (most often copper). Powerful currents of various frequencies (from ten Hz to several MHz) are induced in the inductor with the help of a special generator, as a result of which an electromagnetic field arises around the inductor. The electromagnetic field induces eddy currents in the workpiece. Eddy currents heat the workpiece under the influence of Joule heat (see Joule-Lenz law).

The workpiece inductor system is a coreless transformer in which the inductor is the primary winding. The workpiece is a short-circuited secondary winding. The magnetic flux between the windings is closed in the air.

At a high frequency, eddy currents are displaced by the magnetic field formed by them into the thin surface layers of the workpiece Δ (Surface-effect), as a result of which their density increases sharply, and the workpiece heats up. The underlying metal layers are heated due to thermal conductivity. It is not the current that is important, but the high current density. In the skin layer Δ, the current density decreases by a factor of e relative to the current density on the surface of the workpiece, while 86.4% of heat is released in the skin layer (of the total heat release. The depth of the skin layer depends on the radiation frequency: the higher the frequency, the thinner skin layer It also depends on the relative magnetic permeability μ of the workpiece material.

For iron, cobalt, nickel and magnetic alloys at temperatures below the Curie point μ has a value from several hundred to tens of thousands. For other materials (melts, non-ferrous metals, liquid low-melting eutectics, graphite, electrolytes, electrically conductive ceramics, etc.) μ is approximately equal to unity.

For example, at a frequency of 2 MHz, the depth of the skin layer for copper is about 0.25 mm, for iron ≈ 0.001 mm.

The inductor gets very hot during operation, as it absorbs its own radiation. In addition, it absorbs heat radiation from a hot workpiece. Inductors are made from copper tubes cooled by water. Water is supplied by suction - this ensures safety in case of burn-through or other depressurization of the inductor.

Application:
Ultrapure non-contact metal melting, brazing and welding.
Obtaining prototypes of alloys.
Bending and heat treatment of machine parts.
Jewelry making.
Processing small parts that can be damaged by flame or arc heating.
Surface hardening.
Quenching and heat treatment of complex-shaped parts.
Disinfection of medical instruments.

Advantages.

High speed heating or melting of any electrically conductive material.

Heating is possible in a protective gas atmosphere, in an oxidizing (or reducing) environment, in a non-conductive liquid, in a vacuum.

Heating through the walls of a protective chamber made of glass, cement, plastics, wood - these materials absorb electromagnetic radiation very weakly and remain cold during the operation of the installation. Only electrically conductive material is heated - metal (including molten), carbon, conductive ceramics, electrolytes, liquid metals, etc.

Due to the arising MHD forces, the liquid metal is intensively mixed, up to keeping it suspended in air or shielding gas - this is how ultrapure alloys are obtained in small quantities (levitation melting, melting in an electromagnetic crucible).

Since the heating is carried out by means of electromagnetic radiation, there is no contamination of the workpiece by the products of torch combustion in the case of gas-flame heating, or by the electrode material in the case of arc heating. Placing the samples in an inert gas atmosphere and a high heating rate will eliminate scale formation.

Ease of use due to the small size of the inductor.

The inductor can be made of a special shape - this will allow evenly heating parts of a complex configuration over the entire surface, without leading to their warping or local non-heating.

Local and selective heating is easy.

Since the most intense heating occurs in the thin upper layers of the workpiece, and the underlying layers are heated more gently due to thermal conductivity, the method is ideal for surface hardening of parts (the core remains viscous).

Easy automation of equipment - heating and cooling cycles, temperature control and retention, supply and removal of workpieces.

Induction heating installations:

In installations with an operating frequency of up to 300 kHz, inverters are used on IGBT assemblies or MOSFET transistors. Such installations are designed for heating large parts. To heat small parts, high frequencies are used (up to 5 MHz, the range of medium and short waves), high-frequency installations are built on electronic tubes.

Also, for heating small parts, installations of increased frequency on MOSFET transistors are being built for operating frequencies up to 1.7 MHz. Controlling transistors and protecting them at higher frequencies presents certain difficulties, therefore, higher frequency settings are still quite expensive.

The inductor for heating small parts has a small size and low inductance, which leads to a decrease in the quality factor of the operating oscillating circuit at low frequencies and a decrease in efficiency, and also poses a danger to the master oscillator (the quality factor of the oscillating circuit is proportional to L / C, an oscillating circuit with a low quality factor is too good "Pumped" with energy, forms a short circuit in the inductor and disables the master oscillator). To increase the quality factor of the oscillatory circuit, two ways are used:
- an increase in the operating frequency, which leads to the complication and rise in the cost of the installation;
- the use of ferromagnetic inserts in the inductor; gluing the inductor with panels made of ferromagnetic material.

Since the inductor works most efficiently at high frequencies, induction heating received industrial application after the development and start of production of powerful generator lamps. Before World War I, induction heating was of limited use. At that time, machine generators of increased frequency (the work of V.P. Vologdin) or spark discharge installations were used as generators.

The generator circuit can be, in principle, any (multivibrator, RC generator, generator with independent excitation, various relaxation generators), operating on a load in the form of a coil-inductor and having sufficient power. It is also necessary that the vibration frequency be high enough.

For example, in order to "cut" a steel wire with a diameter of 4 mm in a few seconds, an oscillatory power of at least 2 kW at a frequency of at least 300 kHz is required.

The scheme is chosen according to the following criteria: reliability; stability of fluctuations; stability of the power released in the workpiece; ease of manufacture; ease of customization; the minimum number of parts to reduce cost; the use of parts that together give a reduction in weight and dimensions, etc.

For many decades, an inductive three-point was used as a generator of high-frequency oscillations (Hartley generator, generator with autotransformer feedback, circuit on an inductive loop voltage divider). This is a self-excited circuit of parallel power supply of the anode and a frequency-selective circuit made on an oscillatory circuit. It has been successfully used and continues to be used in laboratories, jewelry workshops, industrial enterprises, as well as in amateur practice. For example, during the Second World War, surface hardening of the rollers of the T-34 tank was carried out on such installations.

Disadvantages of the three points:

Low efficiency (less than 40% when using a lamp).

A strong frequency deviation at the time of heating the workpieces made of magnetic materials above the Curie point (≈700С) (μ changes), which changes the depth of the skin layer and unpredictably changes the heat treatment mode. When heat-treating critical parts, this may be unacceptable. Also, powerful TV-sets should operate in a narrow range of frequencies allowed by Rossvyazokhrankultura, since with poor shielding they are actually radio transmitters and can interfere with television and radio broadcasting, coastal and rescue services.

When changing workpieces (for example, a smaller one for a larger one), the inductance of the inductor-workpiece system changes, which also leads to a change in the frequency and depth of the skin layer.

When changing from single-turn inductors to multi-turn inductors, to larger or smaller ones, the frequency also changes.

Under the leadership of Babat, Lozinsky and other scientists, two- and three-circuit generator circuits were developed that have a higher efficiency (up to 70%), as well as better maintain the operating frequency. Their principle of operation is as follows. Due to the use of coupled circuits and weakening the connection between them, a change in the inductance of the working circuit does not entail a strong change in the frequency of the frequency setting circuit. Radio transmitters are designed according to the same principle.

Modern TVF generators are inverters based on IGBT-assemblies or powerful MOSFET-transistors, usually made in a bridge or half-bridge scheme. Operate at frequencies up to 500 kHz. The gates of the transistors are opened using a microcontroller control system. The control system, depending on the task at hand, allows you to automatically hold

A) constant frequency
b) constant power released in the workpiece
c) the highest possible efficiency.

For example, when a magnetic material is heated above the Curie point, the thickness of the skin layer increases sharply, the current density drops, and the workpiece begins to heat worse. Also, the magnetic properties of the material disappear and the process of magnetization reversal stops - the workpiece starts to heat up worse, the load resistance abruptly decreases - this can lead to the "separation" of the generator and its failure. The control system monitors the transition through the Curie point and automatically increases the frequency when the load is abruptly reduced (or decreases the power).

Remarks.

The inductor should be positioned as close to the workpiece as possible. This not only increases the density of the electromagnetic field near the workpiece (proportional to the square of the distance), but also increases the power factor Cos (φ).

Increasing the frequency dramatically decreases the power factor (proportional to the cube of the frequency).

When magnetic materials are heated, additional heat is also released due to magnetization reversal; their heating to the Curie point is much more efficient.

When calculating the inductor, it is necessary to take into account the inductance of the buses supplying the inductor, which can be much higher than the inductance of the inductor itself (if the inductor is made in the form of one turn of a small diameter or even part of a turn - an arc).

There are two cases of resonance in oscillatory circuits: voltage resonance and current resonance.
Parallel oscillatory circuit - current resonance.
In this case, the voltage on the coil and on the capacitor is the same as that of the generator. At resonance, the loop resistance between the branch points becomes maximum, and the current (I total) through the load resistance Rн will be minimal (the current inside the loop I-1L and I-2c is greater than the generator current).

Ideally, the loop impedance is infinity - the circuit does not draw any current from the source. When the frequency of the generator changes in either direction from the resonant frequency, the total resistance of the circuit decreases and the line current (I total) increases.

Serial oscillatory circuit - voltage resonance.

The main feature of a series resonant circuit is that its impedance is minimal at resonance. (ZL + ZC - minimum). When the frequency is tuned above or below the resonant frequency, the impedance increases.
Output:
In a parallel circuit at resonance, the current through the circuit terminals is 0, and the voltage is maximum.
In a series circuit, on the contrary, the voltage tends to zero, and the current is maximum.

The article was taken from the site http://dic.academic.ru/ and reworked into a text that is more understandable for the reader by the company Prominductor LLC.