High frequency induction. Induction high frequency heating. Inductive discharge without external magnetic field

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

Introduction

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

Inductive RF discharge has been known for more than a hundred years. 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 an eddy 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 the inductive HF discharge is supported not by vortex electric fields, but by potential fields that appear due to the presence of a potential difference between the turns of the inductor. In 1929, K. McKinton experimentally demonstrated the possibility of the existence of two discharge combustion modes. At low HF voltage amplitudes, the discharge actually occurred under the influence of the electric field between the turns of the coil and had the character of a weak longitudinal glow along the entire gas-discharge tube. As the amplitude of the RF voltage increased, the glow became brighter and finally a bright ring discharge appeared. The glow caused by the longitudinal electric field disappeared. Subsequently, these two forms of discharge were called E-H - discharge, respectively.

The areas of existence of an inductive discharge can be divided into two large areas: this high pressure(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. Plasma RF and microwave discharges. Types of high frequency discharges

To initiate and maintain a DC glow discharge, it is necessary that two conductive (metal) electrodes be 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. Firstly, when carrying out processes of plasma deposition of dielectric coatings, a non-conducting film can also form on the electrodes. This will lead to increased instability of the discharge and ultimately to its attenuation. Secondly, in reactors with internal electrodes there is always the problem of contamination of the target process with materials removed from the electrode surface during physical spraying or chemical reactions with plasma particles. To avoid these problems, including completely abandoning 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 typical cases:

Low frequencies. At external field frequencies up to 10 2 - 10 3 Hz the situation is close to that realized in constant electric field. However, if the characteristic frequency of charge destruction v d is less than the field frequency w(v d ? w), the charges, after changing the sign of the field, manage to disappear before the field strength reaches a value sufficient to maintain the discharge. Then the discharge will be extinguished and ignited twice during the period of field change. The discharge re-ignition voltage should depend on the frequency. 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 re-ignition potential. At low frequencies after breakdown, the relationship between current and 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 an increase in frequency, when the characteristic frequencies of plasma processes are comparable and slightly less than the field frequency (v d ? w), the discharge state does not have time to “follow” the change in the supply voltage. Hysteresis appears in the dynamic current-voltage characteristic of the discharge (Fig. 1, curve 2).

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

Rice. 1. Current-voltage characteristics of periodic discharges: 1 - static current-voltage characteristic, 2 - current-voltage characteristic in the transition frequency region, 3 - steady-state dynamic current-voltage characteristic

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

For HF discharges, the following excitation methods exist: 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 generators of these ranges. With the capacitive excitation method, 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 it. An induction discharge is easy to observe if an evacuated vessel is placed inside a solenoid through which a sufficiently strong high-frequency current flows. Under the influence 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. Maintaining the discharge (ionization) requires the Joule heat of ring induction currents flowing in the ionized gas along the vortex electric field lines (magnetic field lines inside a long solenoid are parallel to the axis; Fig. 3).

Fig. 3 Field diagram in the solenoid

Among the old works on electrodeless discharge, the most thorough research belongs to J. Thomson 2, who, in particular, experimentally proved the inductive nature of the discharge and derived theoretical ignition conditions: the dependence of the threshold magnetic field for breakdown on 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 a 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.

Burning 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 question in the physics of low-pressure inductive discharge is the question of the mechanisms and efficiency of RF power absorption by plasma. It is known that with purely inductive excitation of an HF discharge, its equivalent circuit can be represented in the form shown in Fig. 1 year 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 represents the active resistance and inductance of the antenna, equivalent resistance and inductance of the plasma connected in series (Fig. 4d), so that the power of the RF generator P gen is connected with the power Pan t released in the antenna, and the power P p1 released in the plasma, 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 plasma resistance.

From formulas (1) and (2) it is clear that when the load is matched with the generator, the active RF power Pgen supplied 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. Previously, the overwhelming majority of works 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, which is completely absorbed by the plasma. In the mid-1990s, V. Godyak and his colleagues convincingly showed that in low-pressure discharges, relation (3) can be violated. Obviously, provided

Rpi? Rant (4)

the behavior of the inductive RF discharge changes radically.

Rice. 4. Circuits of (a, b) inductive plasma sources and (c) 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 emergence of new effects associated with self-consistent redistribution of power 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 regimes corresponding to inequality (4), as well as to optimizing the operation of plasma devices, lies in the patterns of changes in the equivalent plasma resistance when changing plasma parameters and conditions for maintaining the discharge.

Design of HF induction discharge

The foundations for modern research and applications of electrodeless discharges were laid by the work of G.I. Babat, which was carried out just before the war at the Leningrad Electric Lamp Plant? Svetlana?. These works were published in 1942 3 and became widely known abroad after publication in England in 1947. 4. Babat created high-frequency tube 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 of that time, up to several tens of kilowatts, was introduced into the high-pressure discharge (however, such values are high for modern installations). ?Punch? air or other gas at atmospheric pressure, of course, was not possible even with the highest currents in the inductor, so special measures had to be taken to ignite the discharge. The easiest way was to excite the discharge at low pressure, when the breakdown fields are small, and then gradually increase the pressure, bringing it to atmospheric pressure. Babat noted that when gas flows through the discharge, the latter can be extinguished if the blast is too intense. At high pressures, the effect of contraction, i.e. separation of the discharge from the walls of the discharge chamber, was discovered. In the 50s, several papers appeared on electrodeless discharge 5~7. Cabanne 5 studied 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 ignition curves, measured the power introduced into the discharge using a calorimetric method, and measured electron concentrations using probes. Ignition curves for many gases were also obtained in Ref. 7. In Ref. 6 an attempt was made to use the discharge for ultraviolet spectroscopy. The electrodeless plasma torch, to which current installations are very close, was designed by Reed in 1960. 8. A diagram and photograph of it are shown in Fig. 2. A quartz tube with a diameter of 2.6 cm was covered 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. A rod pushed into the inductor heats up in a high-frequency field and emits electrons. The surrounding gas heats up and expands, causing breakdown. After ignition, the rod is removed and the discharge continues to burn. The most significant point in this installation was the use of tangential gas supply. Reed pointed out that the resulting plasma should spread quickly enough against the flow of gas tending to carry it away. Otherwise, the discharge will go out, as happens with unstabilized flames. At low flow rates, plasma can be maintained by ordinary thermal conductivity. (The role of thermal conductivity in high-pressure discharges was also noted by Cabanne5). However, at high gas supply rates it is necessary to take measures to recirculate part of the plasma. A satisfactory solution to this problem was the vortex stabilization used by Reed, in which gas is fed into the tube tangentially and flows through it, performing a helical motion. Due to the centrifugal expansion of the gas, a column of low 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 speed, the higher the luminous plasma penetrates against the flow. In addition, with this method of supply, gas flows along the tube mainly at its walls, presses the discharge away from the walls and isolates the latter from the destructive effects 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 completely accurately reflect the essence of the matter. We will return to the issue of plasma maintenance, 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 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 (average gas velocity over the cross-section of the tube was 30-40 cm/sec) when a power of 1.5-3 kW was introduced into the discharge, which was approximately half the power consumed by the generator. Reed determined the energy balance in the plasmatron and measured the spatial distribution of temperature in the plasma using an optical method.

He published several more articles: on powerful induction discharges at low pressures9, on measurements of heat transfer to probes introduced into various points of a plasma torch10, on growing crystals of refractory materials using an induction torch, etc.

An induction plasma torch, similar in design to Reed's, was described somewhat later in the works of Rebu4 5 "4 6. Rebu used it for growing crystals and producing 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 distributions of electron concentrations are measured. Here, as a rule, well-known optical, spectral and probe methods are used, usually used in the study of arc discharge plasma. The powers put into the discharge are measured at different voltages on the inductor, different gas flow rates, different dependences of the parameters for different gases, frequencies, etc. It is difficult to establish any uniform dependences, say, of the plasma temperature on the power put into the discharge, so how everything depends on specific conditions: tube diameter, inductor geometry, gas supply speed, etc. The general result of many works is the conclusion that with a power of the order of several or tens of kilowatts, the temperature of argon plasma reaches approximately 9000-10,000 ° K .

The temperature distribution mainly has a plateau character. in the middle of the tube and drops sharply near the walls, however? a plateau? not quite level, in the central part there is a small dip, 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. In air, temperatures are lower than in argon at the same power, and, conversely, to achieve the same temperatures, several times higher powers are required 31. The temperature increases slightly with increasing power and weakly depends on gas flow. 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 increasing gas supply speed and gas flow rate (with tangential supply), the discharge is increasingly pressed away from the walls and the discharge radius changes from approximately 0.8 to 0.4 of the tube radius. As the gas flow rate increases, the power put into the discharge also decreases somewhat, which is associated with a decrease in the discharge radius, i.e., the plasma flow or consumption. During discharges in closed vessels, without gas flow, the luminous area of the discharge usually comes very close to the side walls of the vessel. Measurements of electron concentrations showed that the state of the plasma at atmospheric pressure is close to thermodynamic equilibrium. The measured concentrations and temperatures fit the Saha equation with satisfactory accuracy.

Induction HF discharge

Currently, low-pressure plasma sources are known, the operating principle 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 an induction corresponding to the conditions of electron cyclotron resonance (ECR) and conditions excitation of helicons and Trivelpiece-Gold (TG) waves (hereinafter called helicon sources).

It is known that in the plasma of an inductive discharge, HF electric fields are skinned, i.e. Electrons are heated in a narrow wall layer. When an inductive HF discharge of an external magnetic field is applied to the plasma, regions of transparency appear in which HF fields penetrate deep into the plasma and electrons are heated throughout its entire volume. This effect is used in plasma sources, the operating principle of which is based on ECR. Such 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 a non-uniform magnetic field is formed using magnets. The magnetic field is characterized by the presence of one or several resonant zones in which the ECR conditions are met and RF power is introduced into the plasma. In the radio frequency range, ECR is used in so-called neutral loop plasma sources. A significant role in the generation of plasma and the formation of the discharge structure is played by the neutral circuit, which is a continuous sequence of points with a zero magnetic field. 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 current in the middle coil flows in the opposite direction. An RF induction discharge with a neutral circuit is characterized by a high plasma density (10 11 - 10 12 cm~ 3) and low electron temperature (1 -4 eV).

Inductive discharge without external magnetic field

The independent variable on the abscissa axis is the power P pi absorbed by the plasma. It is natural to assume that the plasma density n e is proportional to P pi , but it should be noted that for different plasma sources the proportionality coefficients between P pi and n e will differ. As can be seen, 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 predominates, i.e. in the region of the anomalous skin effect, the dependence R pl (n e) is close to that obtained for media with strong spatial dispersion. In general, the non-monotonic dependence of the equivalent resistance on the plasma density is explained by the competition of two factors: on the one hand, RF power absorption increases with increasing electron concentration, on the other hand, the skin layer depth, which determines the width of the RF power absorption region, decreases with increasing p 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 skin depth 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 experimental conditions, the depth of the skin layer is obviously less than the length of the plasma sources, so it is not surprising that the equivalent plasma resistance of sources equipped with an 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 length of the source, accompanied by a simultaneous increase in the length of the antenna, leads to an increase in the area in which RF power is absorbed, i.e. to the elongation of the skin layer, therefore, in the case of a side antenna, the equivalent resistance increases with increasing source length.

Experiments and calculations have shown that at low pressures the absolute values of the equivalent plasma resistance are small. An increase in working gas pressure leads to a significant increase in equivalent resistance. This effect has been noted many times in both 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 absorption of RF power. As can be seen from Fig. 5, at the minimum pressure considered, p -- 0.1 mTorr, the Cherenkov dissipation mechanism is predominant. Electron-atom collisions have virtually no effect on the value of the equivalent resistance, and electron-ion collisions lead to only a slight increase in the equivalent resistance at n e > 3 x 10 11 cm-- 3. Increase in pressure, i.e. frequency of electron-atom collisions leads to an increase in equivalent resistance due to the increased 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 the collisional and collisionless absorption mechanisms to the equivalent resistance calculated taking into account only collisions.

Rice.5 . Dependence of the ratio of the equivalent resistance Rpi, calculated taking into account 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

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

At low working gas pressures (p < 5 mTorr), the equivalent resistance of the plasma without a magnetic field is significantly smaller in magnitude than in the “helicon” region. The values of R pl obtained for the ECR region occupy an intermediate position, and here the equivalent resistance monotonically increases 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 side helical antenna. The position and 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.

Increasing the input power, i.e. electron concentration 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 higher magnetic fields, and in some cases to the appearance of additional local maxima. A similar effect is observed with increasing length of the plasma source.

The pressure increase is in the range of 2-5 mTorr, as can be seen from Fig. 4b, does not lead to significant changes in the nature of the dependence ^ pl (B), however, at pressures exceeding 10 mTorr, the non-monotonicity of the dependence of the equivalent resistance on the magnetic field disappears, the absolute values of the equivalent resistance fall and become less than the values obtained without a magnetic field.

The analysis of the physical mechanisms of absorption of RF power by an inductive discharge plasma under ECR conditions and conditions of excitation of helicons and TG waves was carried out in many theoretical works. Analytical consideration of the problem of excitation of helicons and TG waves in the general case is associated with significant difficulties, since it is necessary to describe two interconnected waves. Let us recall that the helicon is a fast transverse wave, and the TG wave is a slow longitudinal wave. 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 magnetized plasma oscillations. In the case of a limited cylindrical plasma source, the problem can only be solved numerically. However, the main features of the physical mechanism of RF power absorption at B > 1 mT can be illustrated using the helicon approximation developed, which describes the process of excitation of waves in plasma provided that the inequalities are satisfied

Application area

high frequency combustion magnetic plasma

Plasma reactors and ion sources, the operating principle of which is based on low-pressure inductive RF discharge, have been a critical component of modern terrestrial and space technologies for several decades. The wide spread 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 of the plasma with 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, allows one to avoid damage to the surface of the samples when they are treated in a discharge with high-energy ions.

Typical examples of plasma sources operating on an inductive RF discharge without a magnetic field are plasma reactors designed for etching substrates, ion sources intended for the implementation of terrestrial ion beam technologies and operation in space as spacecraft orbit correction engines, 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. Using an antenna connected to a high-frequency generator, RF power is introduced into the volume of the GDC and an electrodeless discharge is ignited. Currents flowing through the antenna induce an eddy 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 neutral gas in plasma reactors varies from 1 to 30 mTorr, in ion sources it is 0.1 mTorr, in light sources it is 0.1-10 torr.

Plasma reactors and ion sources, the operating principle of which is based on low-pressure inductive RF discharge, have been a critical component of modern terrestrial and space technologies for several decades. The wide spread 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 plasma contact with 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, allows one to avoid damage to the surface of the samples when they are treated in a discharge with high-energy ions.

The results obtained in recent years, both experimental and theoretical, show that the plasma parameters of an inductive RF discharge depend on power losses in the external circuit and the amount of power entering the discharge through the inductive and capacitive channels. The parameters of the plasma, 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 powers entering different channels and, ultimately, the power absorbed by the plasma. This determines the self-consistent nature of the discharge. Self-consistency is most clearly manifested in the strong non-monotonicity of the dependence of plasma parameters on the magnetic field and discharge disruptions. Significant power losses in the external circuit and the non-monotonic dependence of the ability of the plasma to absorb RF power on the plasma density lead to saturation of the plasma density with increasing power of the RF generator and the appearance of hysteresis in the dependence of plasma parameters on the power of the RF generator and the external magnetic field.

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

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

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Vortex induction heaters

An induction coil is capable of heating any metal, heaters are assembled using transistors 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 non-contact heating has no losses in adjusting the resonant coincidence of the operating parameters of the installation with the parameters of the output oscillatory circuit. Vortex-type heaters assembled on transistors are able to perfectly analyze and adjust the output frequency in automatic mode.

Metal induction heaters

Heaters for induction heating of metal have a non-contact 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 in 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 Every day it acquires new ways of application. The heater operates on alternating electric current. Most often, induction electric heaters are used to bring metals to the required temperatures during the following operations: forging, soldering, welding, bending, hardening, etc. Electric induction heaters operate at a high frequency of 30-100 kHz and are used to heat various types of media and coolants.

Electric heater used in many areas:

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

Medium Frequency Induction Heaters

When deeper heating is required, medium-frequency type induction heaters are used, operating at medium frequencies from 1 to 20 kHz. Compact inductors for all types of heaters come in a variety of shapes, which are selected so as to ensure uniform heating of samples of the most varied shapes, while it is also possible to carry out specified local heating. The mid-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%, induction mid-frequency heaters are used for a wide range of technologies in metallurgy (also for melting various metals), mechanical engineering, instrument making and other fields.

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 the chemical properties of the metal.

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

Ultra High Frequency Induction Heaters

Microwave induction heaters operate at superfrequencies (100-1.5 MHz) and penetrate to a heating depth (up to 1 mm). The ultra-high-frequency type is indispensable for processing thin, small, small-diameter parts. The use of such heaters allows one to avoid unwanted deformations associated with heating.

Ultra-high-frequency induction heaters based on JGBT modules and MOSFET transistors have power limits 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 requiring special precision and filigree.

Forge induction heaters

The main purpose of forging-type induction heaters (IH) is heating of parts or parts thereof, prior to subsequent forging. Blanks can be of different types, alloys and shapes. Induction forging heaters allow you to process cylindrical workpieces of any diameter in automatic mode:

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

Induction shaft heaters

Induction heaters for hardening shafts work together with the hardening complex. The workpiece is in a vertical position and rotates inside a stationary inductor. The heater allows the use of all types of shafts for consistent local heating; the hardening depth can be fractions of millimeters in depth.

As a result of induction heating of the shaft along its entire length with instant cooling, its strength and durability increases many times over.

Induction pipe heaters

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

Air Cooled Tube Induction Heating produced using a flexible inductor and a thermal blanket. Heating temperature up to temperature 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 used to improve the quality of any steel pipe.

Pyrometer for heating control

One of the most important operating parameters of induction heaters is temperature. For more careful monitoring of it, in addition to built-in sensors, infrared pyrometers are often used. These optical devices allow you to quickly and easily determine the temperature of hard-to-reach (due to high heat, the possibility of exposure to electricity, etc.) surfaces.

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

Operating principle of induction heaters

During operation, a magnetic field is formed in the inductor, into which the part is placed. Depending on the task (depth of heating) 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 states: when a conductor is in an alternating electromagnetic field, an EMF (electromotive force) is formed in it. The amplitude graph shows that it moves proportionally to the change in the speed of the magnetic flux. 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, current leads to heating of a conductor that has resistance.

The operating principle of all types of induction heaters is similar to a transformer. The conductive workpiece, which is located in the inductor, is similar to a transformer (without a magnetic core). The primary winding is an inductor, the secondary inductance of the part, and the load is the metal resistance. During high-frequency heating, a “skin effect” is formed; eddy currents that form inside the workpiece displace the main current onto the surface of the conductor, because the heating of the metal on the surface is stronger than inside.

Advantages of induction heaters

An induction heater has undoubted advantages and is a leader among all types of devices. This advantage is as follows:

It consumes less electricity and does not pollute the surrounding space.
Easy to use, it provides high quality work and allows you to control the process.
Heating through the walls of the chamber ensures special purity and the ability to obtain ultra-pure alloys, while melting can be carried out in different atmospheres, including inert gases and vacuum.
With its help, it is possible to uniformly heat parts of any shape or selective heating
Finally, induction heaters are universal, which allows them to be used everywhere, displacing outdated energy-consuming and inefficient installations.

Repair of induction heaters is carried out 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 do not allow excessive operating conditions - first of all, monitor the temperature and proper water cooling.

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

Video of induction mid-frequency heaters working

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

Induction heating is carried out in an alternating magnetic field. Conductors placed in a field are heated by eddy currents induced into them according to the laws of electromagnetic induction.

Intense heating can only be achieved in magnetic fields of high intensity and frequency, which are created by special devices - inductors (induction heaters) powered from the network or individual high-frequency current generators (Fig. 3.1). The inductor is like the primary winding of an air transformer, the secondary winding of which is the heated body.

Depending on the frequencies used, induction heating installations are divided as follows:

a) low (industrial) frequency (50 Hz);

b) medium (high) frequency (up to 10 kHz);

c) high frequency (over 10 kHz).

The division of induction heating into frequency ranges is dictated by technical and technological considerations. The physical essence and general quantitative patterns for all frequencies are the same and are based on ideas about the absorption of electromagnetic field energy by a conducting medium.

Frequency has a significant impact on the intensity and nature of heating. At a frequency of 50 Hz and a magnetic field strength of 3000-5000 A/m, the specific heating power does not exceed 10 W/cm 2 , and with high-frequency (HF) heating the power reaches hundreds and thousands of W/cm 2 . At the same time, temperatures develop that are sufficient to melt the most refractory metals.

At the same time, the higher the frequency, the shallower the depth of penetration of currents into the metal and, consequently, the thinner the heated layer, and vice versa. Surface heating is carried out at high frequencies. By reducing the frequency and thereby increasing the depth of current penetration, it is possible to achieve deep or even through heating, uniform over the entire cross-section of the body. Thus, by choosing the frequency, it is possible to obtain the heating character and its intensity required by the technological conditions. The ability to heat products to almost any thickness is one of the main advantages of induction heating, which is widely used for hardening surfaces of parts and tools.

Surface hardening after induction heating significantly increases the wear resistance of products compared to heat treatment in furnaces. Induction heating is also successfully used for melting, heat treatment, metal deformation and other processes.

An inductor is a working part of an induction heating installation. The closer the type of electromagnetic wave emitted by the inductor is to the shape of the heated surface, the higher the heating efficiency. The type of wave (flat, cylindrical, etc.) is determined by the shape of the inductor.

The design of inductors depends on the shape of the heated bodies, purposes and heating conditions. The simplest inductor is an insulated conductor placed inside a metal pipe, elongated or coiled. When an industrial frequency current is passed through a conductor, eddy currents are induced in the pipe and heat it. In agriculture, attempts have been made to use this principle to heat the soil in closed ground, poultry perches, etc.

In induction water heaters and milk pasteurizers (work on them has not yet gone beyond the scope of experimental samples), the inductors are made like the stators of three-phase electric motors. A cylindrical metal vessel is placed inside the inductor. The rotating (or pulsating in single-phase version) magnetic field created by the inductor induces eddy currents in the walls of the vessel and heats them. Heat is transferred from the walls to the liquid in the vessel.

When induction drying wood, a stack of boards is laid out with metal mesh and placed (rolled on a special trolley) inside a cylindrical inductor made of large cross-section conductors wound on a frame made of insulating material. The boards are heated by metal meshes in which eddy currents are induced.

The given examples explain the principle of indirect induction heating installations. The disadvantages of such installations include low energy levels and low heating intensity. Low-frequency induction heating is quite effective when directly heating massive metal workpieces and a certain ratio between their sizes and the depth of current penetration (see below).

Inductors of high-frequency installations are made non-insulated; they consist of two main parts - an inducting wire, with the help of which an alternating magnetic field is created, and current leads for connecting the inducting wire to a source of electrical energy.

The design of the inductor can be very diverse. To heat flat surfaces, flat inductors are used, cylindrical workpieces - cylindrical (solenoid) inductors, etc. (Fig. 3.1). Inductors can have a complex shape (Fig. 3.2), due to the need to concentrate electromagnetic energy in the desired direction, supply cooling and quenching water, etc.

To create high-intensity fields, large currents, amounting to hundreds and thousands of amperes, are passed through the inductors. In order to reduce losses, inductors are made with the lowest active resistance possible. Despite this, they still heat up intensively both by their own current and due to heat transfer from the workpieces, so they are equipped with forced cooling. Inductors are usually made of copper tubes of round or rectangular cross-section, inside which running water is passed for cooling.

Specific surface power. The electromagnetic wave emitted by the inductor falls on a metal body and, being absorbed in it, causes heating. The power of the energy flow flowing through a unit surface of the body is determined by formula (11)

taking into account the expression

In practical calculations, the dimension D is used R in W/cm2, then

Substituting the resulting value H 0 into formula (207), we get

(3.7)

Thus, the power released in the product is proportional to the square of the ampere-turns of the inductor and the power absorption coefficient. At a constant magnetic field strength, the heating intensity is greater, the greater the resistivity r, the magnetic permeability of the material m and the frequency of the current f.

Formula (208) is valid for a plane electromagnetic wave (see § 2 of Chapter I). When cylindrical bodies are heated in solenoid inductors, the picture of wave propagation becomes more complicated. The smaller the ratio, the greater the deviations from the relations for a plane wave. r/z a, Where r- cylinder radius, z a- current penetration depth.

In practical calculations, they still use the simple dependence (208), introducing correction factors into it - Birch functions, depending on the ratio r/z a(Fig. 43). Then

Formula (212) is valid for a solid inductor without gaps between the turns. If there are gaps, losses in the inductor increase. As the frequency of the function increases F a (r a, z a) And F and (r and, z a) tend to unity (Fig. 43), and the power ratio tends to the limit

From expression (3.13) it follows that efficiency decreases with increasing air gap and resistivity of the inductor material. Therefore, inductors are made of massive copper tubes or busbars. As follows from expression (214) and Figure 43, the efficiency value approaches its limit already at r/z a>5÷10. This allows us to find a frequency that provides a sufficiently high efficiency. Using the above inequality and formula (15) for the penetration depth z a , we get

(3.14)

It should be noted that simple and visual dependencies (3.13) and (3.14) are valid only for a limited number of relatively simple cases of induction heating.

Inductor power factor. The power factor of a heating inductor is determined by the ratio of the active and inductive resistance of the inductor-product system. At high frequencies, the active and internal inductive reactances of the product are equal, since the phase angle between the vectors and is 45° and |D R| = |D Q|. Therefore, the maximum power factor value

Where A - air gap between the inductor and the product, m.

Thus, the power factor depends on the electrical properties of the product material, air gap and frequency. As the air gap increases, the leakage inductance increases and the power factor decreases.

The power factor is inversely proportional to the square root of the frequency, therefore an unreasonable increase in frequency reduces the energy performance of installations. You should always strive to reduce the air gap, but there is a limit due to the breakdown voltage of the air. During the heating process, the power factor does not remain constant, since r and m (for ferromagnets) change with temperature. In real conditions, the power factor of induction heating installations rarely exceeds 0.3, decreasing to 0.1-0.01. To unload the networks and generator from reactive currents and increase the sof, compensating capacitors are usually connected in parallel with the inductor.

The main parameters characterizing induction heating modes are current frequency and efficiency. Depending on the frequencies used, two induction heating modes are conventionally distinguished: deep heating and surface heating.

Deep heating (“low frequencies”) is carried out at this frequency f when penetration depth z a approximately equal to the thickness of the heated (hardened) layer x k(Fig. 3.4, a). Heating occurs immediately to the entire depth of the layer x k the heating rate is chosen such that the transfer of heat by thermal conductivity deep into the body is insignificant.

Since in this mode the penetration depth of currents z a relatively large ( z a » x k), then, according to the formula:

Surface heating (“high frequencies”) is carried out at relatively high frequencies. In this case, the penetration depth of currents z a significantly less than the thickness of the heated layer x k(Fig. 3.4,6). Heating throughout the entire thickness x k occurs due to the thermal conductivity of the metal. When heating in this mode, less generator power is required (in Figure 3.4, the useful power is proportional to the double-hatched areas), but the heating time and specific energy consumption increase. The latter is associated with heating due to thermal conductivity of the deep layers of the metal. Efficiency heating, proportional to the ratio of the double-hatched areas to the entire area bounded by the curve t and coordinate axes, in the second case lower. At the same time, it should be noted that heating to a certain temperature a layer of metal with a thickness b lying behind the hardening layer and called the transition layer is absolutely necessary for reliable connection of the hardened layer with the base metal. With surface heating, this layer is thicker and the connection is more reliable.

With a significant decrease in frequency, heating becomes completely impossible, since the penetration depth will be very large and the energy absorption in the product will be insignificant.

The induction method can be used to carry out both deep and surface heating. With external heat sources (plasma heating, resistance electric furnaces), deep heating is impossible.

Based on the principle of operation, there are two types of induction heating: simultaneous and continuous-sequential.

During simultaneous heating, the area of the inductive wire facing the heated surface of the product is approximately equal to the area of this surface, which allows simultaneous heating of all its areas. During continuous-sequential heating, the product moves relative to the induction wire, and heating of its individual sections occurs as it passes through the working area of the inductor.

Frequency selection. Sufficiently high efficiency can be obtained only with a certain ratio between the size of the body and the frequency of the current. The choice of the optimal current frequency was mentioned above. In the practice of induction heating, the frequency is selected according to empirical dependencies.

When heating parts for surface hardening to depth x k(mm) the optimal frequency (Hz) is found from the following dependencies: for parts of simple shape (flat surfaces, bodies of rotation)

When through-heating steel cylindrical workpieces with a diameter d(mm) the required frequency is determined by the formula

During heating, the resistivity of metals r increases. For ferromagnets (iron, nickel, cobalt, etc.), the value of magnetic permeability m decreases with increasing temperature. When the Curie point is reached, the magnetic permeability of ferromagnets drops to 1, that is, they lose their magnetic properties. The usual heating temperature for hardening is 800-1000° C, for pressure treatment 1000 - 1200° C, that is, above the Curie point. A change in the physical properties of metals with a change in temperature leads to a change in the power absorption coefficient and specific surface power (3.8) entering the product during the heating process (Fig. 3.5). Initially, due to an increase in r, the specific power D R increases and reaches the maximum value D P max= (1.2÷1.5) D R start, and then, due to the loss of magnetic properties by steel, drops to a minimum D Р min. To maintain heating in an optimal mode (with a sufficiently high efficiency), the installations are equipped with devices for matching the parameters of the generator and the load, that is, the ability to regulate the heating mode.

If we compare the through heating of workpieces for plastic deformation using the induction method and the electric contact method (both refer to direct heating), then we can say that in terms of energy consumption, electric contact heating is appropriate for long workpieces of a relatively small cross-section, and induction heating for short workpieces of relatively large diameters.

A rigorous calculation of inductors is quite cumbersome and requires the use of additional semi-empirical data. We will consider a simplified calculation of cylindrical inductors for surface hardening, based on the dependencies obtained above.

Thermal calculation. From consideration of induction heating modes it follows that the same thickness of the hardened layer x k can be obtained at different values of specific power D R and heating duration t. The optimal mode is determined not only by the layer thickness x k, but also by the size of the transition zone b, connecting the hardened layer with the deep layers of the metal.

In the absence of generator power control devices, the nature of the change in the specific power consumed by the steel product is shown in the graph shown in Figure 3.5. During the heating process, the rc value changes and towards the end of heating, after passing through the Curie point, it sharply decreases. The steel product seems to switch off automatically, which ensures high quality hardening without burnouts. If there are control devices, power D R may be equal to or even less than D Р min(Fig. 3.5), which allows, by lengthening the heating process, to reduce the specific power required for a given thickness of the hardened layer x k.

Graphs of heating modes for surface hardening for carbon and low-alloy steels with a transition zone thickness of 0.3-0.5 of the hardened layer are shown in Figures 3.6 and 3.7.

By choosing the value D R, it is not difficult to find the power supplied to the inductor,

where h tr- efficiency of high-frequency (quenching) transformer.

Power consumed from the network

determined by specific energy consumption A(kWh/t) and productivity G(t/h):

for surface heating

(3.26)

where D i- increment in the heat content of the workpiece as a result of heating, kJ/kg;

D- density of the workpiece material, kg/m 3 ;

M 3 - workpiece mass, kg;

S 3- surface of the hardened layer, m2;

b- metal waste (with induction heating 0.5-1.5%);

h tp- efficiency of heat transfer due to thermal conductivity inside the workpiece (with surface hardening h tp = 0,50).

The remaining notations are explained above.

Approximate values of specific energy consumption for induction heating: tempering - 120, hardening - 250, carburization - 300, through heating for mechanical processing - 400 kWh/t.

Electrical calculation. The electrical calculation is based on dependence (3.7). Let us consider the case when the penetration depth z a significantly smaller than the dimensions of the inductor and the part, and the distance A between the inductor and the product is small compared to the width of the inducting conductor b(Fig. 3.1). For this case the inductance L with inductor-product systems can be expressed by the formula

Substituting the current value into formula (3.7) and keeping in mind that

Formula (3.30) gives the relationship between specific power, electrical parameters and geometric dimensions of the inductor, and the physical characteristics of the heated metal. Taking the dimensions of the inductor as a function, we obtain

for heated state

Inductor power factor

where P is the active power of the inductor, W;

U and- voltage across the inductor, V;

f- frequency Hz.

When connecting capacitors to the primary circuit of a high-frequency transformer, the capacitance of the capacitors must be increased to compensate for the reactivity of the transformer and connecting conductors.

Example. Calculate the inductor and select a high-frequency installation for surface hardening of cylindrical carbon steel workpieces with a diameter d a= 30 mm and height h a= 90 mm. Depth of hardened layer x k = 1mm, inductor voltage U and = 100 V. Find the recommended frequency using formula (218):

We stop at the closest of the applied frequencies f=67 kHz.

From the graph (Fig. 3.7) we take D R= 400 W/cm2.

Using formula (3.33) we find al for cold condition:

cm 2.

We accept A= 0.5 cm, then the diameter of the inductor

cm.

Inductive conductor length

Number of inductor turns

Inductor height

Power supplied to the inductor, according to

where 0.66 is the efficiency of the inductor (Fig. 3.8).

Generator oscillatory power

kW.

We choose a high-frequency installation LPZ-2-67M, which has an oscillating power of 63 kW and an operating frequency of 67 kHz.

The induction heating technique uses currents of low (industrial) frequency 50 Hz, medium frequency 150-10000 Hz and high frequency from 60 kHz to 100 MHz.

Medium frequency currents are obtained using machine generators or static frequency converters. In the range of 150-500 Hz, generators of the usual synchronous type are used, and above (up to 10 kHz) machine generators of the inductor type are used.

Recently, machine generators have been replaced by more reliable static frequency converters based on transformers and thyristors.

High frequency currents from 60 kHz and above are obtained exclusively using tube generators. Installations with lamp generators are used to perform various operations of heat treatment, surface hardening, metal smelting, etc.

Without touching on the theory of the issue, presented in other courses, we will consider only some of the features of heating generators.

Heating generators are usually self-excited (autogenerators). Compared to independent excitation generators, they are simpler in design and have better energy and economic performance.

The circuits of tube generators for heating are not fundamentally different from radio engineering ones, but they have some features. These circuits are not required to have strict frequency stability, which greatly simplifies them. A schematic diagram of a simple generator for induction heating is shown in Figure 3.10.

The main element of the circuit is the generator lamp. Heating generators most often use three-electrode lamps, which are simpler than tetrodes and pentodes and provide sufficient reliability and stability of generation. The load of the generator lamp is an anode oscillatory circuit, the parameters of which are inductance L and capacity WITH are selected from the operating conditions of the circuit in resonance at the operating frequency:

Where R- reduced loop loss resistance.

Contour Options R, L, C are determined taking into account changes introduced by the electrophysical properties of heated bodies.

The anode circuits of generator lamps are powered by direct current from rectifiers assembled on thyratrons or gastrons (Fig. 3.10). For economic reasons, AC power is used only for low powers (up to 5 kW). The secondary voltage of the power (anode) transformer feeding the rectifier is 8 - 10 kV, the rectified voltage is 10 - 13 kV.

Undamped oscillations in a self-oscillator occur when there is sufficient positive feedback from the grid to the circuit and certain conditions are met that connect the parameters of the lamp and the circuit.

Grid Feedback Coefficient

Where U with , U to , U a- voltage, respectively, on the grid, oscillatory circuit and anode of the generator lamp;

D- lamp permeability;

s d- dynamic slope of the anode-grid characteristics of the lamp.

Grid feedback in generators for induction heating is most often performed using a three-point circuit, when the grid voltage is taken from part of the inductance of the anode or heating circuit. In Figure 3.10, voltage is supplied to the grid from part of the turns of the coupling coil L2, which is an inductive element of the heating circuit.

Heating generators, unlike radio generators, are most often double-circuit (Fig. 3.10) or even single-circuit. Double-circuit generators are easier to tune into resonance and more stable in operation.

Oscillations of the second kind are excited in generators. The anode current flows through the lamp in pulses, only for part (1/2-1/3) of the period. Due to this, the DC component of the anode current is reduced, the heating of the anode is reduced and the efficiency of the generator is increased. The grid current also has a pulse shape. The cutoff of the anode current (within the cutoff angle q = 70-90°) is carried out by applying a constant negative bias to the grid, which is created by the voltage drop across the gridlick resistance R g when a constant component of the grid current flows.

Heating generators have a load that changes during the heating process, caused by changes in the electrical properties of the heated materials. To ensure the generator operates in optimal mode, characterized by the highest values of output power and efficiency, the installations are equipped with load matching devices. The optimal mode is achieved by selecting the appropriate value of the mesh feedback coefficient k s and fulfillment of the condition

Where E a - power supply voltage;

E s - constant offset on the grid;

I a1-the first harmonic of the anode current.

To match the load, the circuits provide the ability to adjust the resonant resistance of the circuit R a and change the grid voltage U s. Changing these values is achieved by introducing additional capacitances or inductances into the circuit and switching the anode, cathode and grid clamps (probes) connecting the circuit to the lamp.

Induction heating installations are very common at repair plants and Agricultural Equipment enterprises.

In the repair industry, medium and high frequency currents are used for through and surface heating of cast iron and steel parts for hardening, before hot deformation (forging, stamping), when restoring parts using surfacing and high-frequency metallization methods, when brazing, etc.

Surface hardening of parts occupies a special place. The ability to concentrate power in a given location of a part makes it possible to obtain a combination of an outer hardened layer with the plasticity of deep layers, which significantly increases wear resistance and resistance to alternating and impact loads.

The advantages of surface hardening using induction heating are as follows:

1) the ability to harden parts and tools to any required thickness, if necessary, processing only the working surfaces;

2) significant acceleration of the hardening process, which ensures high productivity of installations and reduces the cost of heat treatment;

3) usually lower specific energy consumption compared to other heating methods due to the selectivity of heating (only to a given depth) and the rapidity of the process;

4) high quality of hardening and reduction of defects;

5) the possibility of organizing production flow and process automation;

6) high production standards, improvement of sanitary and hygienic working conditions.

Induction heating installations are selected according to the following main parameters: purpose, rated oscillatory power, operating frequency. Industrially produced units have a standard power scale with the following steps: 0.16; 0.25; 0.40; 0.63; 1.0 kW and further by multiplying these numbers by 10, 100 and 1000.

Installations for induction heating have powers from 1.0 to 1000 kW, including lamp generators up to 250 kW, and higher - with machine generators. The operating frequency, determined by calculation, is specified according to the frequency scale permitted for use in electrothermal applications.

High-frequency installations for induction heating have a single indexing: HF (high-frequency induction).

After the letters, a dash indicates the oscillatory power (kW) in the numerator, and the frequency (MHz) in the denominator. After the numbers are written letters indicating the technological purpose. For example: VCHI-40/0.44-ZP - high-frequency induction heating unit, oscillating power 40 kW, frequency 440 kHz; letters ZP - for hardening surfaces (NS - for through heating, ST - pipe welding, etc.).

1. Explain the principle of induction heating. Scope of its application.

2. List the main elements of an induction heating installation and indicate their purpose.

3. How is the heater winding done?

4. What are the advantages of the heater?

5. What is the phenomenon of surface effect?

6. Where can the induction air heater be applied?

7. What determines the depth of current penetration into the heated material?

8. What determines the efficiency of a ring inductor?

9. Why is it necessary to use ferromagnetic tubes to make induction heaters at industrial frequency?

10. What most significantly affects the cos of an inductor?

11. How does the heating rate change with increasing temperature of the heated material?

12. What parameters of steel are affected by temperature measurement?

And in devices, heat in the heated device is released by currents arising in the alternating electromagnetic field inside the unit. They are called induction. As a result of their action, the temperature increases. Induction heating of metals is based on two main physical laws:

Faraday-Maxwell;
Joule-Lenz.

In metal bodies, when they are placed in an alternating field, vortex electric fields begin to arise.

Induction heating device

Everything happens as follows. Under the influence of a variable, the electromotive force (EMF) of induction changes.

EMF acts in such a way that eddy currents flow inside bodies, which release heat in full accordance with the Joule-Lenz law. EMF also generates alternating current in the metal. In this case, thermal energy is released, 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 process

To provide induction heating, it is necessary 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. You can use individual power sources for this - converters and generators.

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

The use of induction heating at low frequencies is quite rare. Metal processing at medium and high frequencies is more common.

Such devices are distinguished by the fact 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 must be close in shape.

Where are they used?

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

melting of metals, their soldering using a non-contact method;
obtaining new metal alloys;
mechanical engineering;
jewelry making;
manufacturing small parts that may be damaged when using other methods;
(and the parts 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:

With its help, you can quickly heat and melt any current-conducting material.
Allows heating in any environment: vacuum, atmosphere, non-conducting liquid.
Due to the fact that only the conductive material is heated, the walls, which weakly absorb waves, remain cold.
In specialized areas of metallurgy, production of ultra-pure alloys. This is an interesting process, because the metals are mixed in a shell of protective gas.

Compared to other types, induction does not pollute the environment. If in the case of gas burners contamination is present, just as in arc heating, then induction eliminates 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 promote uniform heat distribution.
Indispensable if it is necessary to heat only a certain area of the surface.
It is not difficult to configure such equipment to the desired mode and regulate it.

Flaws

The system has the following disadvantages:

It is quite difficult to independently install and adjust the type of heating (induction) and its equipment. It's better to contact specialists.
The need to accurately match the inductor and the workpiece, otherwise induction heating will be insufficient, its power can reach low values.

Heating with induction equipment

To arrange individual heating, you can consider an option such as induction heating.

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

How does it work

The operating principle 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 pipes are connected to it: for the cold water that comes in, and at the outlet of warm water - the second pipe. Due to pressure, water constantly circulates, which eliminates 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 advantage is that the device operates silently. It can be installed in any room.

Making equipment yourself

Installing induction heating is not very difficult. Even someone who has no experience will cope with the task after careful study. Before you start, you need to stock up on the following necessary items:

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

The wire should be cut into pieces no larger than five centimeters. This is necessary for efficient heating in a high-frequency field.
The body must be no smaller 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 quantity that it fills the entire internal space.
The structure is closed and the adapter is installed.
Then a coil is constructed from this pipe. To do this, wrap it with already prepared 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 water supply pump is installed.
A temperature regulator is installed.

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 leading to the inductor, which can be much greater than the inductor itself.

About hobs

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

This surface looks like a regular transformer. Its coil is hidden under the surface of the panel, which can be glass or ceramic. Current passes through it. This is the first part of the coil. But the second is the dishes in which the food will be cooked. Eddy currents are created at the bottom of the cookware. They heat the dishes first, and then the food in them.

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

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

For such stoves you need special dishes. Most ferromagnetic metals can interact with the induction field: aluminum, stainless and enameled steel, cast iron. The only ones not suitable for such surfaces are: copper, ceramic, glass and utensils made from non-ferromagnetic metals.

Naturally, it will turn on only when suitable dishes are installed on it.

Modern stoves are equipped with an electronic control unit, which allows you to recognize empty and unsuitable cookware. The main advantages of cookers are: safety, ease of cleaning, speed, efficiency, and cost-effectiveness. You should never get burned on the surface of the panel.

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