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HF - induction discharge: combustion conditions, design and scope
Introduction
One of the most important issues of the organization of plasma technological processes is the development of plasma sources with properties optimal for this technology, for example: high homogeneity defined by plasma density, energy of charged particles, and a concentration of chemically active radicals. The analysis shows that high-frequency (HF) plasma sources are most promising for use in industrial technologies, since, firstly, with their help, both conductive and dielectric materials can be processed, and secondly, not only inert, but also chemically active gases can be used as working gases. Today, plasma sources based on capacitive and inductive RF discharge are known. The feature of the capacitive RF discharge, which is most frequently used in plasma technologies, is the existence of an electrode layer of volumetric charge, in which the average in time is formed by the drop in the potential accelerating ions in the direction of the electrode. This allows you to process the samples of materials using accelerated ions located on the HF capacitive discharge electrodes. The disadvantage of the sources of the capacitive RF discharge is the relatively low concentration of electrons in the main volume of plasma. A significantly higher concentration of electrons with the same RF capacities is characteristic of inductive RF discharges.
Inductive HF-discharge is known for over a hundred years. This is a discharge excited by a current current to the inductor located on the side or end surface, as a rule, a cylindrical plasma source. Back in 1891, J. Thomson suggested that the inductive rank is called and supported by Vortex electric fieldwhich is created by the magnetic field, in turn, induced by the current current by the antenna. In 1928-1929, Paulumizing with J. Thosend, D. Townsend and R. Donaldson expressed the idea that inductive HF rank is supported by non-vortex electric fields, but by potential, which appear due to the presence of potential difference between the inductor turns. In 1929, K. Mac-Cinton experimentally showed the possibility of the existence of two modes of burning discharge. With small amplitudes of the RF voltage, the discharge really occurred under the action of the electric field between the coils coils and wore the character of a weak longitudinal glow along the entire gas-discharge tube. With an increase in the amplitude of the RF voltage, the glow became brighter and finally the bright ring discharge occurred. The glow caused by the longitudinal electric field disappeared. Subsequently, these two forms of the discharge were named electrically discharge, respectively.
The field of existence of an inductive discharge can be divided into two large areas: it high pressure (order atmospheric pressure), in which the generated plasma is close to the equilibrium, and low pressureAt which the generated plasma is nonequilibrium.
Periodic discharges. Plasma HF and microwave discharges. Types of high-frequency discharges
To excite and maintain a glow discharge direct current It is necessary that two conductive (metal) electrodes be in direct contact with the plasma area. From a technological point of view, such a design of the plasma chemical reactor is not always comfortable. First, when the processes of plasma application of dielectric coatings, the non-conductive film can also be formed on the electrodes. This will lead to an increase in the unstability of the discharge and ultimately for its attenuation. Secondly, in reactors with internal electrodes there is always a problem of the impressions of the target process with materials removed from the electrode surface during physical spraying or chemical reactions with plasma particles. Avoid these problems, including completely, to abandon the use of internal electrodes, allows the use of periodic discharges excited by not permanent, and by an alternating electric field.
The main effects that occur in periodic discharge are determined by relations between the characteristic frequencies of plasma processes and the frequency of the attached field. It is advisable to consider three characteristic cases:
Low frequencies. With the frequencies of the external field up to 10 2 - 10 3 Hz, the situation is close to the implemented in constant electric field. However, if the characteristic frequency of the destruction of charges V D is less than the frequency of the field W (v d? W), charges after changing the field of the field, you have time to disappear earlier than the field value will reach the value sufficient to maintain the discharge. Then the discharge will twice go out and approach during the period of change in the field. The discharge re-ignition voltage should depend on the frequency. The higher the frequency, the smaller the proportion of electrons will have time to disappear during the existence of the field insufficient to maintain the discharge, the lower the re-ignition potential. At low frequencies after a breakdown, the ratio between the current and the combustion voltage corresponds to the static voltampear 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 commensurate and somewhat less than the frequency of the field (V D? W), the discharge state does not have time to "follow" by changing the supply voltage. In the dynamic battery, hysteresis appears (Fig. 1, curve 2).
High frequencies. When performing the condition< v d < Fig. 1. Voltample characteristics of periodic discharges: 1-static WAY, 2 - WAs in the transition region, 3 - established dynamic flush There are many types of electrical discharges in a gas depending on the nature of the applied field (constant electric field, variable, pulsed, (HF), over high frequency (microwave)), on the pressure of the gas, shape and location of the electrodes, etc. For RF devices, there are the following excitation methods: 1) Capacitive at frequencies less than 10 kHz, 2) induction at frequencies in the range of 100 kHz - 100 MHz. These excitation methods imply the use of data generators of the ranges. With a capacitive excitation method, the electrodes can be installed inside the working chamber or outside if the camera 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 B). HF Induction discharge High-frequency induction (electrodeless) discharge in gases is known since the end of the last century. However, it was not completely understood. The induction discharge is easy to observe if inside the solenoid, according to which a rather strong high frequency current flows, put a dumped vessel. Under the action of a vortex electric field, which is induced by a variable magnetic flux, a breakdown occurs in the residual gas and the discharge is lit. On the maintenance of the discharge (ionization), Jowle is spent by the heat of ring induction currents flowing in ionized gas along the power lines of the vortex electric field (magnetic power lines inside the long solenoid parallel to the axis; Fig. 3). Fig.3 Field Scheme in Solenoid Among the old works on the electrodeless discharge, the most disconstruct studies belong to J.Tomson 2, which, in particular, experimentally proved the induction nature of the discharge and brought theoretical conditions of ignition: the dependence of the threshold for the breakdown of the magnetic field on the gas pressure (and frequency). Like curves of Pashen for breakdown of the discharge gap in a constant electric field, the ignition curves have a minimum. For the practical range of frequencies (from the tenths of up to dozen Meghertz), the minima lie in the field of low pressures; Therefore, the discharge was usually observed only in highly sparse gases. Conditions of combustion of HF - induction discharge Inductive HF-discharge is a discharge excited by a current flowing by the inductor located on the side or end surface, as a rule, a cylindrical plasma source (Fig. 4a, b). The central issue of the physics inductive discharge of low pressure is the question of the mechanisms and efficiency of the absorption of HF power plasma. It is known that with a purely inductive excitation of the HF discharge, its equivalent circuit can be represented as shown in Fig. 1g. The RF generator is loaded to the transformer, the primary winding of which consists of an antenna, which flows the current created by the generator, and the secondary winding is the current induced in plasma. The primary and secondary transformer winding is associated with the mutual induction coefficient M. The transformer scheme can be easily reduced to the diagram of a sequentially connected active impact and inductance of antenna, equivalent resistance and plasma inductance (Fig. 4d), so that the power of the RF generator P Gen turns on the associated With a power P an T released in the antenna, and the power of P1, highlighted in plasma, expressions where I is the current flowing through the antenna, P Ant - the active resistance of the antenna, R p 1 is the equivalent plasma resistance. From formulas (1) and (2) it can be seen that when the load is coordinated with the generator, the active RF power PGEN, given by the generator into the outer chain, is distributed between two channels, namely: one part of the power goes to heating antenna, and the other part is absorbed Plasma. Earlier in the overwhelming number of works A Priori was supposed to be in experimental conditions R pl\u003e R antvv (3) and plasma properties are determined by the power of the RF generator fully absorbed by the plasma. In the mid-1990s, V. Annak with employees convincingly showed that in the discharges of low pressures, the ratio (3) may violate. Obviously, provided R PI? R Ant (4) the behavior of the inductive HF discharge is changing radically. Fig. four. Schemes (A, b) inductive plasma sources and (c) inductive plasma source with a capacitive component, (g, e) equivalent schemes of pure 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 its maintenance conditions. This leads to the emergence of new effects associated with self-consistent redistribution of power in the external circuit of the discharge. 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 optimize the operation of plasma devices lies in the patterns of changes in the equivalent plasma resistance when the plasma parameters change and the conditions for maintaining the discharge. Design of HF - induction discharge The foundations for modern research and applications of non-electrode discharges were laid by the works of G. I. Babat, who were held in the war in the Leningrad Electric Power 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 lamp generators with booms of order hundreds of kilowatt, which allowed him to receive powerful electrodeless discharges in the air at pressures up to atmospheric . Babat worked in a range of 3--62 MHz, inductors consisted of several turns with a diameter of about 10 cm. In the category of high pressure, a huge power was introduced to several tens of kilowatt (however, such values \u200b\u200bare high for modern installations). ? Break? The air or other gas at atmospheric pressure, of course, was not possible even with the largest currents in the inductor, so to ignite the discharge had to take special measures. The easiest way was to excite the discharge at low pressure, when the punching fields are small, and then gradually increase the pressure, bringing it to atmospheric. Babat noted that when gas flowing through a category, the latter can be repaid if the blowing is too intense. At high pressures, the effect of contraglation was discovered, F E. separation of the discharge from the walls of the discharge chamber. In the 1950s, several articles appeared on the electrodeless discharge 5 ~ 7. Kabanne 5 examined the discharges in the inert gases at low pressures from 0.05 to 100 mm Hg. Art. and small capacities up to 1 ket at frequencies of 1--3 MHz, the ignition curves determined, the calorimetric method was measured the power introduced into the category, electron concentrations measured using probes. Ignition curves in many gases were also obtained in 7. In the work 6, an attempt was made to use a discharge for ultraviolet spectroscopy. The electrodeless plasma burner, which is very close to the current installations, was designed by Reed in 1960. 8. The scheme and photograph of it are shown in Fig. 2. Quartz tube with a diameter of 2.6 cm covered a five-way inductor made from a copper tube with a distance between the turns of 0.78 cm. The power supply was served by an industrial high-frequency generator with a maximum output power of 10 ket; Operating frequency 4 MHz. A moving graphite rod was used to ignite the discharge. The rod, dodged into the inductor, is heated in a high-frequency field and emitters electrons. It is heated and the surrounding gas is expanding, and there is a breakdown in it. After ignition, the rod is removed, and the discharge continues to burn. The most significant moment in this installation was the use of tangential gas supply. Reed indicated that the resulting plasma should quickly spread against the gas flow, aspiring it to demolish. Otherwise, the discharge will go out, as it happens with unstabilized flames. At low stream speeds, plasma maintenance can provide common thermal conductivity. (The role of thermal conductivity in high-pressure discharges also noted Kabann 5).) However, at high gas supply rates, it is necessary to take measures to recycle part of the plasma. A satisfactory solution of this problem was the vortex stabilization used by Reed, at which the gas is fed to the tube on the tangent and flows through it, making a screw movement. Due to the centrifugal gas runoff in the recovery part of the tube, a low pressure post is formed. There is almost no axial flow here, and part of the plasma is sucking upstream. The greater the feed rate, the higher the luminous plasma penetrates against the stream. In addition, with this method of supplying gas flows along the tube mainly at its walls, presses the discharge from the walls and isolates the last of the devastating action of high temperatures, which makes it possible to work at elevated facilities. These qualitative considerations, briefly expressed by Reed, are very important for understanding the phenomena, although they can, and not quite accurately reflect the creature of the case. On the issue of maintaining plasma, which seems to be the most serious when considering a stationary stabilized discharge in the gas flow, we will return lower, in ch. IV. Reed worked with argon and with argon mixtures with helium, hydrogen, oxygen, air. He noted that it is easiest to maintain a discharge in pure argon. Argon expenses were 10--20 l / min (the average section of the gas velocity tube 30--40 cm / s) when the capacity of 1,5 to 3 kets, which make up an approximately half of the power consumed by the generator in the discharge. Reed determined the balance of energy in plasma torch and the optical method measured the spatial distribution of plasma temperature. He published a few more articles: On powerful induction discharges at low pressures 9, on heat transfer measurements to probes made to different points of plasma torch10, on the cultivation of crystals of refractory materials using an induction burner and. The induction plasma burner, similar to the ridden, was somewhat later described in the works of the RB4 5 "4 6. The paper used it for the cultivation of crystals and the manufacture of spherical particles of refractory materials. Starting from around 1963, in our and foreign printing there are many works devoted to the experimental study of the induction discharges of high pressure in both closed vessels and gas flow1 2-3 3 ѓe 4 0-4 4-5 3 ѓE 8 0. The spatial distribution of temperature in the area of \u200b\u200bdischarge and in the plasma torch, the distribution of electronic concentrations is measured. Here, as a rule, known optical, spectral and probe methods are used, usually used in the study of plasma arc discharges. The capacities invested in the category are measured at different stresses on the inductor, different gas consumption, various parameter dependences for different gases, frequencies, etc. It is difficult to establish some uniform dependencies, say, plasma temperatures from power invested in a category, so How it all depends on the specific conditions: the diameter of the tube, the geometry of the inductor, the gas supply speed, etc., the total result of many works is to conclusion that with the power of the order of several or ten kilowatt, the argon plasma temperature reaches approximately 9000--10,000 ° K . The temperature distribution is mainly characterized by a plateau? In the middle of the tube and sharply falls near the walls, however? Plateau? Not quite even, in the central part it turns out a small failure of the amount is usually a few hundred degrees. In other gas gases, there are also about 10,000 °, depending on the type of gas and other conditions. In the air, temperatures are lower than in the argon at the same power, and, on the contrary, to achieve the same temperatures are required several times large power 31. The temperature grows slightly with increasing power and weakly depends on the gas flow rate. In fig. 3 and 4 are shown to illustrate the temperature distribution along the radius, temperature field (isotherm), the distribution of electronic concentrations. Experiments27 showed that with an increase in the supply rate and gas flow rate (with a tangential supply), the discharge is increasingly pressed from the walls and the discharge radius varies from about 0.8 to 0.4 of the tube radius. With increasing gas flow rate, the power is slightly decreased and the invested power is reduced, which is associated with a decrease in the discharge radius, i.e. flow or plasma consumption. When discharged in closed vessels, without gas duct, the luminous discharge area is usually 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. Thermodynamically equilibrium. The measured concentrations and temperatures with satisfactory accuracy are stacked into the SAH equation. Induction HF - discharge Low pressure plasma sources are currently known, the principle of action of which is based on an inductive RF discharge, in the absence of a magnetic field, as well as on an inductive RF discharge, placed in an external magnetic field with induction corresponding to the conditions of electronic cyclotron resonance (ECR) and conditions The excitation of Helikon and Waves of Trivelpis - Gold (TG) (hereinafter referred to as Helicon Sources). It is known that in the plasma of the inductive discharge of HF electric fields, they are skins, i.e. Electron heating is carried out in a narrow onset layer. Under the application to the plasma of the inductive RF discharge of the external magnetic field, the transparency areas appear in which the plasma income is penetrated and the heating of the electrons is carried out throughout its volume. This effect is used in plasma sources, the principle of operation is based on ECR. Such sources work mainly in the microwave (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 using magnets. The magnetic field is characterized by the presence of one or more resonant zones, in which the ECR conditions are performed and the RF power in the plasma occurs. In the radio frequency range, the ECR is used in the so-called plasma sources with a neutral contour. A significant role in the generation of plasma and the formation of the discharge structure plays a neutral contour, which is a continuous sequence of points with a zero magnetic field. A closed magnetic circuit is formed using three electromagnets. Currents in the windings of the upper and lower coils have the same direction. The current of the middle coil flows in the opposite direction. The RF induction discharge with the 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 an external magnetic field As an independent variable along the abscissa axis, the P PI power is postponed, absorbed by the plasma. It is natural to assume that the plasma density P e is proportional to P PI, but it should be noted that for various plasma sources, the proportionality coefficients between P PI and P E will differ. As can be seen, the general tendency of the behavior of the equivalent resistance R PI is its increase in the area of \u200b\u200brelatively small values \u200b\u200bof nested power, and then its saturation. In contrast, in the region of high concentrations of electrons, where invaluing absorption prevails, i.e. In the abnormal skin effect, the dependence R Pl (N e) is close to the medium-obtained dispersion. In general, the non-monotonicity of the dependence of the equivalent resistance from 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 concentration of electrons, on the other hand, the depth of the skin-layer, which determines the width of the absorption area of \u200b\u200bthe RF power, decreases with increasing e. The theoretical model of the plasma source excited by a spiral antenna located on its upper end surface predicts the dependence of the equivalent plasma resistance from the plasma source length, 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 HF power occurs within the skin-layer. Under the experiments, the depth of the skin-layer is obviously less than the length of the plasma sources, therefore it is not surprising that the equivalent resistance of the plasma of sources equipped with the upper end antenna does not depend on their length. In contrast, in the case of an antenna location on the side surface of the sources, an increase in the length of the source, accompanied by simultaneously increasing the length of the antenna, leads to an increase in the area in which the absorption of RF power occurs, i.e. To lengthen the skin-layer, so in the case of a side antenna, the equivalent resistance increases with an increase in the length of the source. Experiments and calculations showed that at low pressures, the absolute values \u200b\u200bof the equivalent resistance of the plasma is small. An increase in the pressure of the working gas leads to a significant increase in equivalent resistance. This effect was repeatedly noted both in theoretical and experimental work. The physical reason for increasing the plasma ability to absorb RF power with an increase in pressure lies in the absorption mechanism of the RF power. As can be seen from fig. 5, with minimal pressure considered, p - 0.1 morterr, the predominant is the Chenkovsky dissipation mechanism. The electron-atomic clashes practically do not affect the value of equivalent resistance, and the electron-ion collisions lead only to a minor increase in equivalent resistance at P E\u003e 3 x 10 11 cm-- 3. Increased pressure, i.e. The frequencies of electro-atomic collisions leads to an increase in the equivalent resistance due to increasing the role of the collision mechanism of the absorption of RF power. This is seen from fig. 5, which shows the ratio of equivalent resistance, calculated taking into account the collisional and clarification mechanisms of absorption, to the equivalent resistance, calculated only with the collision. Fig.5
.
The dependence of the ratio of the equivalent resistance of RPI, calculated taking into account the collisional and invaluing mechanisms of absorption, to the equivalent resistance of the RPI, calculated only with the collisions, from the plasma density. The calculation is made for flat disco-shaped sources with a radius of 10 cm at a pressure of a neutral gas 0.3 morterr (1), 1 morter (2), 10 morterr (3), 100 morter (7), 300 billboard (5). Inductive discharge with an external magnetic field The experiments used plasma sources equipped with spiral antennas located on the side and end surfaces of sources, as well as Nagoya III antennas. For the operating frequency of 13.56 MHz, the magnetic fields region in "0.4--1 MTL corresponds to the conditions of the ECR, and the region B\u003e 1 \u200b\u200bMTL - the conditions of excitation of Helikon and the Waves of Trevelpis Gold. At low operating gas pressures (p ^ 5 of the morter), the equivalent plasma resistance without a magnetic field is significantly smaller in magnitude than in the "helicon" region. The values \u200b\u200bof R Pl, obtained for the ECR region occupy an intermediate position, and here the equivalent resistance is monotonically increased with an increase in the magnetic field. For the "helical" region, the non-monotonic dependence of the equivalent resistance from the magnetic field is characterized, and the non-monotonicity of R Pl (b) in the case of an end spiral antenna and Nagoya III antenna is much stronger than in the case of a lateral spiral antenna. The position and number of local maxima curve ^ pi (b) depends on the nested RF power, the length and radius of the plasma source, the genus of the gas and its pressure. Increased power inboard, i.e. The concentrations of electrons P E, leads to an increase in the equivalent resistance and displacement of the main maximum of the function ^ Pi (b) to the region of large magnetic fields, and in some cases, the appearance of additional local maxima. A similar effect is observed and with an increase in the length of the plasma source. Increasing pressure in the range of 2-5 morterr, as can be seen from fig. 4B, does not lead to significant changes in the character of the dependence ^ Pl (b), but at pressures exceeding 10 morter, the non-monotonicity of the dependence of the equivalent resistance from the magnetic field disappears, the absolute values \u200b\u200bof the equivalent resistance are falling and becoming less than the values \u200b\u200bobtained without a magnetic field. Analysis of the physical mechanisms of absorption of the pumps of the inductive discharge plasma under the conditions of ECR \u200b\u200band the conditions of excitation of helicals and TG-waves were carried out in many theoretical work. Analytical consideration of the problem of excitation of helicals and TG-waves in the general case is associated with significant difficulties, since it is necessary to describe the two associated waves. Recall that Helicon is a fast transverse wave, and the TG-wave is slow longitudinal. Helikon and TG-waves are independent only in the case of a spatial unlimited plasma in which they constitute their own modes of vibrations of the magnetized plasma. In the case of a limited cylindrical plasma source, the task is to solve only numerically. However, the main features of the physical absorption mechanism of the RF power at B\u003e 1 \u200b\u200bMTL can be illustrated by means of developed in the Helicon approximation, which describes the process of excitation of plasma waves, subject to inequality Application area high-frequency burning magnetic plasma Plasma reactors and sources of ions, the principle of operation of which is based on the inductive RF discharge of low pressure, for several decades are the most important component of modern earthly and space technologies. The main advantages of the inductive RF discharge are promoted by the possibility of obtaining a high concentration of electrons with a relatively low level of RF power, the absence of plasma contact with metal electrodes, a small temperature of electrons, and consequently, the low plasma potential relative to the walls limiting the discharge. The latter in addition to minimizing the power loss on the walls of the plasma source makes it possible to avoid damage to the surface of the samples when they are processed in the discharge of high-energy ions. Typical examples of plasma sources working on an inductive RF discharge without a magnetic field are plasma reactors intended for the etching of substrates, sources of ions intended for the implementation of earth ion-beam technologies and work in space as the engine of the orbit of spacecraft, light sources. The overall design feature of the listed devices is the presence of a gas-discharge chamber (GRK), on the outer surface of which or inside it is located inductor or antenna. With the help of an antenna connected to a high-frequency generator, RF power is introduced into the volume of GDK and an electrode-on-time discharge is ignited. The currents current by the antenna are induced in the plasma a vortex electric field that heats the electrons to the energies necessary for efficient ionization of the working gas. Typical plasma density in plasma reactors make up a value of 10 11 - 3 x 10 12 cm ~ 3, and in the sources of ions - 3 x 10 10 - 3 x 10 11 cm ~ 3. The characteristic pressure of neutral gas in plasma reactors varies from 1 to 30 mTRs, in the sources of ions, is 0.1 mtorer, in light sources - 0.1-10 Torr. Plasma reactors and sources of ions, the principle of operation of which is based on the inductive RF discharge of low pressure, for several decades are the most important component of modern earthly and space technologies. Its main advantages are widespread of technical applications - the possibility of obtaining a high concentration of electrons with a relatively low level of RF power, the absence of plasma contact with metal electrodes, a small temperature of electrons, and, consequently, the low plasma potential relative to the walls limiting the discharge. The latter in addition to minimizing the power loss on the walls of the plasma source makes it possible to avoid damage to the surface of the samples when they are processed in the discharge of high-energy ions. The results obtained in recent years, both experimental and theoretical, show that the parameters of the plasma of the inductive RF discharge depend on the loss of power in the outer chain and the power values \u200b\u200bentering the discharge through the inductive and capacitive channels. Plasma parameters, on the one hand, are determined by the values \u200b\u200bof the absorbed power, and on the other hand, they themselves are determined as the ratio of the capacities entering different channels and ultimately the power absorbed by the plasma. This determines the self-consistent nature of the discharge. The most vividly self-consistentness is manifested in a strong non-monotonicity of the dependence of plasma parameters from the magnetic field and discharge breakdowns. Significant power loss in the outer chain and the non-monotonic dependence of the plasma ability to absorb the RF power from the plasma density lead to the saturation of the plasma density with an increase in the power of the RF generator and the appearance of the hysteresis of the dependence of the plasma parameters from the value of the power of the RF generator and the external magnetic field. The presence of a capacitive component of the discharge determines the change in the proportion of the power introduced into the plasma through the inductive channel. This causes offset the position of the transition of a low-mode discharge to high to the lower power supply area of \u200b\u200bthe HF generator. When moving from a low discharge fashion to the high presence of a capacitive component manifests itself in a smoother change in plasma density with an increase in the generator's power and in the disappearance of the hysteresis. An increase by the contribution of power through the capacitive channel of the electron concentration to values \u200b\u200bexceeding the value at which the equivalent resistance reaches the maximum leads to a decrease in the contribution of HF power through the inductive channel. The comparison of the inductive HF discharge modes with low and high concentrations of electrons with capacitive and inductive modes is physically not justified, since the presence of one plasma power input channel leads to a change in the share of the power flowing into the plasma through another channel. Clarification of the pattern of physical processes in the inductive RF discharge of low pressure allows you to optimize the parameters of plasma devices that work on it. Posted on Allbest.ru. An ion gas-discharge electric package device designed to stabilize the voltage. The principle of the action of the strabitron of the glowing discharge. Basic physical laws. Voltage stabilization area. The operation of the parametric stabilizer. examination, added 10/28/2011 Parameters of partial discharges and determining their dependencies. Basics of development of partial discharges, diagnostics of cable lines. 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Therefore, it is the heating of products from conductive materials (conductors) by a magnetic field of inducers (sources of an alternating magnetic field). Induction heating is carried out as follows. Electrically conductive (metallic, graphite) Billet is placed in the so-called inductor, which is one or more turns of the wire (most often copper). In the inductor, with the help of a special generator, powerful currents of different frequencies are subject to (from a dozen Hz to several MHz), as a result of which an electromagnetic field occurs around the inductor. The electromagnetic field suggests vortex currents in the workpiece. The vortex currents warm up the workpiece under the action of Joulehe heat (see the Joule-Lenza law). The "inductor-blank" system is a non-dedicated transformer in which the inductor is a primary winding. The workpiece is a secondary winding, closed spice. The magnetic flow between the windings is closed through the air. At high frequency, the vortex currents are displaced by the magnetic field formed by the same magnetic field into thin surface layers of the blank δ (surface effect), as a result of which their density increases sharply, and the workpiece is heated. The following metal layers are heated due to thermal conductivity. It is not current current, but a large current density. In the skin-layer Δ The current density decreases in e times relative to the current density on the surface of the workpiece, while 86.4% of heat is released into the skin-layer. The depth of the skin layer depends on the frequency of the radiation: the higher the frequency, the thinner Skin layer. It also depends on the relative magnetic permeability of μ material of the workpiece. For iron, cobalt, nickel and magnetic alloys at temperatures below the point of Curie 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 one. 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 is strongly heated during operation, as it absorbs itself its own radiation. In addition, it absorbs thermal radiation from a split billet. Make inductors from copper tubes cooled by water. Water is satisfied with suction - this ensures safety in the event of a burner or other depressurization of the inductor. Application: Benefits. High-speed heating or melting of any electrically conductive material. It is possible to heat in an atmosphere of protective gas, in an oxidative (or reducing) medium, in a non-conductive fluid, in vacuo. Heating through the walls of the protective chamber made of glass, cement, plastics, wood - these materials are very poorly absorb electromagnetic radiation and remain cold when installing installation. Only the electrically conductive material is heated - metal (including melted), carbon, conductive ceramics, electrolytes, liquid metals, etc. Due to the arising MHD, the efforts occurs intensive mixing of the liquid metal, up to hold of it in a suspended state in air or protective gas - so far be obtained alloys in small quantities (levitational smelting, smelting in an electromagnetic crucible). Since the heating is carried out by means of electromagnetic radiation, there is no contamination of the preparation of the combustion of the torch in the case of gas-flame heating, or the electrode material in the case of arc heating. The placement of samples into the atmosphere of inert gas and high heating rate will eliminate the scale. Ease of operation due to a small inductor size. The inductor can be made of a special form - this will allow it evenly to warm over the entire surface of the detail of a complex configuration, without leading to their warping or local non-rest. Easy to carry out local and selective heating. Since the most intense warm-up is in the thin upper layers of the workpiece, and the underlying layers warmer more gently due to thermal conductivity, the method is ideal for carrying out surface hardening of parts (the core remains viscous). Easy equipment automation - heating and cooling cycles, adjustment and deterring temperature, feed and eat blanks. Induction heating installations: At installations with a working frequency of up to 300 kHz, inverters are used on IGBT assemblies or MOSFET transistors. Such installations are designed for heating large parts. High frequencies are used to warm up small parts (up to 5 MHz, medium and short wave range), high frequency settings are built on electronic lamps. Also, for warming up small parts, the installation of increased frequency on the MOSFET transistors on operating frequencies to 1.7 MHz is being built. Transistor management and their protection at elevated frequencies represent certain difficulties, so the installation of an increased frequency is still quite expensive. The inductor for heating small parts has small sizes and a small inductance, which leads to a decrease in the quality of the working oscillatory circuit at low frequencies and a reduction in efficiency, and is also dangerous for the specifying generator (the voltage of the oscillating circuit is proportional to L / C, the oscillatory outline with low qualityness too good "Pumping out" with energy, forms a short circuit in the inductor and displays the specifying generator). To increase the voluntaryness of the oscillatory circuit, use two ways: Since the most efficient inductor works at high frequencies, industrial use of induction heating received after developing and started production of powerful generator lamps. Prior to World War I, induction heating had limited use. As generators, then used machine generators of an increased frequency (work V. P. Vologdin) or spark discharge settings. The generator scheme can be in principle any (multivibrator, RC generator, an independent excitation generator, various relaxation generators) operating on the load in the form of an inductor coil and with sufficient power. It is also necessary that the frequency of oscillations is high enough. For example, to "cut" in a few seconds steel wire with a diameter of 4 mm, a oscillatory capacity is needed at least 2 kW at a frequency of at least 300 kHz. Choose a scheme for the following criteria: Reliability; stability of oscillations; stability of power secreted in the billet; simplicity; convenience of setting; minimum number of parts to reduce cost; Application of parts, in the amount of mass reduction and dimensions, etc. For many decades, an inductive trimmer was used as a high-frequency oscillation generator (Hartley Generator, an autotransformed feedback generator, a circuit on an inductive contour voltage divider). This is a self-exciting scheme of parallel power of an anode and a frequency-selective chain, made on the oscillatory circuit. It was 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 on such installations, a surface hardening of the rollers of T-34 tank was performed. Disadvantages of three points: Low efficiency (less than 40% when using a lamp). The strong deviation of the frequency at the time of heating the billets from magnetic materials above the Curie point (≈700C) (changes μ), which changes the depth of the skin-layer and unpredictably changes the heat treatment mode. When thermal processing of responsible parts, it may be unacceptable. Also, powerful TDHs should operate in a narrow range of frequencies permitted Rossvyazokhrankulture, since with poor shielding are actually radio transmitters and can interfere with television and radio broadcasting, coastal and rescue services. When changing billets (for example, smaller to larger) changes the inductance of the inductor-blank system, which also leads to a change in the frequency and depth of the skin layer. When changing single inductors on multi-ski, on larger or more small-sized frequencies also changes. Under the leadership of Babat, Lozinsky and other scientists, two- and three-constructive schemes of generators with a higher efficiency (up to 70%) were developed, as well as better retaining operating frequency. The principle of their action is as follows. Due to the use of related contours and loosening the connection between them, the change in the inductance of the working circuit does not entail a strong change in frequency frequency circuit. By the same principle, radio transmitters are designed. Modern TVH-generators are inverters on IGBT assemblies or powerful MOSFET transistors, usually made according to the bridge or semi-sequence. Work at frequencies up to 500 kHz. Transistor shutters are open using a microcontroller control system. The control system, depending on the task, allows you to automatically hold A) constant frequency For example, when the magnetic material is heated above the Curie point, the skin-layer thickness increases sharply, the current density drops, and the billet begins to warm worse. The magnetic properties of the material also disappear and the magnetization process is stopped - the billet begins to heat worse, the load resistance is jumpingly decreased - this can lead to a "separation" of the generator and the failure of it. The control system monitors the transition through the Curie point and automatically increases the frequency with a hopping load reduction (or reduces power). Comments. Inductor if possible, it is necessary to have as close as possible to the workpiece. This not only increases the density of the electromagnetic field near the workpiece (in proportion to the square of the distance), but also increases the power coefficient COS (φ). Increasing the frequency sharply reduces the power coefficient (proportional to the frequency cube). When the magnetic materials are heated, additional heat is also highlighted due to reclamation, their heating to the point Curie is much more efficient. When calculating the inductor, it is necessary to take into account the inductance of the entrance to the inductor of the tire, which can be much more inductance of the inductor itself (if the inductor is made in the form of a single turn of a small diameter or even parts of the turn - arcs). There are two cases of resonance in oscillatory circuits: stress resonance and current resonance. In the ideal case, the total contour resistance is equal to infinity - the diagram does not consume the current from the source. When the generator frequency changes, to any side of the resonance frequency, the total contour resistance decreases and the linear current (I Society) increases. Sequential oscillating circuit - stress resonance. The main feature of the serial resonance contour is that its full resistance is minimally with resonance. (Zl + zc - minimum). When adjusting the frequency by a value exceeding or the resonant frequency below, the imperative increases. The article is taken from the site http://dic.academic.ru/ and recycled to a more understandable text for the reader, the company LLC "Prominductor". Induction heating is carried out in a variable magnetic field. The conductors placed in the field are heated by vortex currents are injected into them according to the laws of electromagnetic induction. Intensive heating can be obtained only in high voltage and frequency magnetic fields, which are created by special devices - inductors (induction heaters) powered by a network or individual high-frequency current generators (Fig. 3.1). The inductor is as if the primary winding of the air transformer, the secondary winding of which is the heated body. Depending on the frequencies of the installation of induction heating are divided as follows: a) low (industrial) frequency (50 Hz); b) a medium (elevated) frequency (up to 10 kHz); c) high frequency (over 10 kHz). The division of induction heating on frequency bands is dictated by technical and technological considerations. The physical entity and total quantitative patterns for all frequencies are the same and based on the views, the absorption of the conductive energy of the electromagnetic field. The frequency has a significant effect on the intensity and character of heating. At a frequency of 50 Hz and magnetic field tension 3000-5000 a / m, the temperature of heating does not exceed 10 W / cm 2, and with high-frequency (HF) heating, power reaches hundreds and a thousand W / cm 2. At the same time, temperatures are developing sufficient to melting the most refractory metals. At the same time, the higher the frequency, the less the depth of the penetration of currents into the metal and, therefore, the thinner of the heated layer, and vice versa. At high frequencies, surface heating is carried out. Reducing the frequency and thus increasing the depth of the current penetration, you can make a depth or even through heating, the same body throughout the bodice. Thus, choosing the frequency, it is possible to obtain the nature of heating and intensity necessary for technological conditions. The possibility of heating products is practically any thickness - one of the main advantages of induction heating, which is widely used for hardening parts and tools. Surface hardening after induction heating significantly increases the wear resistance of products compared to thermal processing in the furnaces. Induction heating is also successfully used for melting, heat treatment, metal deformations and in other processes. Inductor is a working unit for installing induction heating. The heating efficiency is higher than the closer the view emitted by the inductor electromagnetic wave to the form of the heated surface. The form of a wave (flat, cylindrical, etc.) is determined by the inductor form. Constructive design of inductors depends on the shape of heated bodies, goals and conditions of heating. The simplest inductor is an isolated conductor placed inside a metal pipe, elongated or rolled into a spiral. When using an industrial frequency conductor in the pipe, weighing its vortex currents are thrown into the pipe. In agriculture, attempts were made to use this principle for soil heating in a closed soil, poultry and dr. In induction water heaters and pasteurizers of milk (work on them was not extended by the framework of experimental samples) Inductors are performed by the type of three-phase electric motors. Inside the inductor placed a metal vessel of a cylindrical shape. Rotating (or pulsating with a single-phase version) The magnetic field created by the inducer leads in the walls of the vessel vortex currents and heats them. From the walls, heat is transmitted in a liquid vessel. With the induction drying of wood, the boards of the boards are shifted with metal grids and placed (rolled out on a special trolley) inside the cylindrical inducer from the conductor of large sections wound on the frame from the insulating material. Boards are heated from metal grids, which are induced by vortex currents. The examples explained the principle of plants of indirect induction heating. The disadvantages of such installations include low energy indicators and small heating intensity. Low-frequency induction heating is quite effective with direct heating of massive metal blanks and a certain ratio between their dimensions and the depth of the current penetration (see below). The inductors of high-frequency settings are performed by uninsulated, they consist of two main parts of the inducer wire, with which an alternating magnetic field is created, and the currentwater to connect the induced wire to the source of electrical energy. Constructive completion of the inductor can be very diverse. Flat inductors, cylindrical blanks are used for heating flat surfaces - cylindrical (solenoid) inductors, etc. (Fig. 3.1). Inductors may have a complex form (Fig. 3.2), due to the need to concentrate electromagnetic energy in the desired direction, the supply of cooling and hardening water, etc. To create high-tension fields in inductors, large currents are passed, calculated by hundreds and thousands of amps. In order to reduce losses, inductors are manufactured with small active resistance. Despite this, they are still intensively heated both by their own current and due to heat transfer from blanks, so they are equipped with forced cooling. Inductors are usually performed from copper tubes of round or rectangular section, inside of which flow water is passed for cooling. Specific surface power. The electromagnetic wave emitted by the inductor falls on the metal body and, absorbing in it, causes heating. The power of the energy flow occurring through the unit of the body surface is determined by formula (11) taking into account the expression In practical calculations, use the dimension d R in W / cm 2, then Substituting the value of H 0
In formula (207), we get Thus, the power secreted in the product is proportional to the square of the ampere turns of the inductor and the power absorption coefficient. With the constant tension of the magnetic field, the heating intensity is the greater, the greater the resistance R, the magnetic permeability of the material M and the frequency of the current f.. Formula (208) is valid for a flat electromagnetic wave (see § 2 of chapters I). When the cylindrical bodies are heated in solenoid inductors, the wave propagation pattern is complicated. Deviations from ratios for a flat wave the greater the less relationship r / z A,where r. - cylinderradius, z A. - Depth of the penetration of currents. In practical calculations, they still use a simple dependence (208), introducing correction coefficients in it - Bercha functions dependent on the relationship r / z A (Fig. 43). Then Formula (212) is valid for a solid inducer without gaps between the turns. In the presence of loss gaps in the inductor increase. As an increase in the frequency of the function F A (R a, z a) and F and (r and z a)tend to units (Fig. 43), and the ratio of power is to the limit From the expression (3.13) it follows that the k. P. D. Decreases with an increase in the air gap and the resistivity of the inductor material. Therefore, inductors are performed from massive copper tubes or tires. As follows from the expression (214) and Figure 43, the value of the k. P. D. Approaching its limit already r / z a \u003e 5 ÷ 10. This allows you to find a frequency that provides a high enough to. P. D. Taking advantage of the inequality and formula (15) for the depth of penetration z a,receive It should be noted that simple and visual dependences (3.13) and (3.14) are valid only for a limited number of relatively simple cases of induction heating. Inductor power coefficient. The power coefficient of the heating inductor is determined by the ratio of the active and inductive resistance of the inductor system - the product. At high frequency, the active and internal inductive resistance of the product is equal, since the phase angle between vectors is 45 ° and | D R| \u003d | D. Q.|. Consequently, the maximum value of the power factor where but -air gap between inductor and product, m. Thus, the power factor depends on the electrical properties of the material of the product, air gap and frequency. With an increase in the air gap, the inductance of scattering increases and the power factor is reduced. The power factor is inversely proportional to the root square of the frequency, so the unreasonable overestimation of the frequency reduces the power supply facilities of the installations. It should always strive to reduce the air gap, however, there is a limit due to the penetrating air tension. In the heating process, the power factor does not remain constant, since R and M (for ferromagnets) change with a change in temperature. In actual conditions, the power factor of induction heating rarely exceeds a value of 0.3, decreasing to 0.1-0.01. To unload networks and generator from reactive currents and increasing the creation, in parallel inductor usually includes compensating capacitors. The main parameters characterizing the induction heating modes are the frequency of the current and to. P. D. Depending on the frequencies used, two induction heating modes are conditionally distinguished: deep heating and superficial. Depth heating ("small frequencies") is carried out at such a frequency f. when the depth of penetration z A.approximately equal to the thickness of the heated (ordered) 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 heat transfer to the heat conduction into the bodies was insignificant. Since in this mode, the depth of the penetration of currents z A. relatively large ( z A. »
X K.),
that is according to the formula: Surface heating ("large frequencies") is carried out at relatively high frequencies. In this case, the depth of the penetration of currents z A.significantly less than the thickness of the heated layer x K. (Fig. 3.4.6). Warming up for the whole thickness x K.comes due to the thermal conductivity of the metal. When heated along this mode, smaller generator power is required (in Figure 3.4, the useful power is proportional to shaded areas having a double hatching), but the heating time and the specific consumption of electricity are increasing. The latter is suspended due to the thermal conductivity of the deep layers of metal. Kpd. Heating, proportional to the ratio of the area with double hatching to the entire area, limited curve t.and coordinate axes, in the second case below. At the same time, it should be noted that the heating to a certain temperature of the metal layer of a thickness B, lying on the layer of hardening and called the transition layer, is absolutely necessary for the reliable connection of the hardened layer with the main metal. With superficial heating, this layer is thicker and the connection is more reliable. With a significant drop in the frequency, heating becomes generally impracticable, since the penetration depth will be very large and energy absorption in the product is insignificant. The induction method can be carried out both deep and surface heating. With external heat sources (plasma heating, in the electrobrix of resistance), the deep heating is not possible. According to the principle of operation, two types of induction heating are distinguished: simultaneous and continuously consistent. With simultaneous heating, the area of \u200b\u200bthe induced wire facing the heated surface of the product is approximately equal to the area of \u200b\u200bthis surface, which allows you to simultaneously heat all its sections. With continuous-sequential heating, the product moves relative to the inducer wire, and the heating of its individual sites occurs as the industrial work area passes. Frequency selection. A high enough to. P. D. Can be obtained only at a certain ratio between body sizes and current frequency. The choice of optimal current frequency was mentioned above. In the practice of induction heating, the frequency is chosen according to empirical dependencies. When heating parts for surface hardening on depth x K.(mm) The optimal frequency (Hz) is found from the following dependencies: for parts of a simple form (flat surfaces, rotation body) With a cross-cutting heat of steel cylindrical billets with a diameter d.(mm) The required frequency is determined by the formula In the process of heating, the resistivity of Metals R increases. Ferromagnets (iron, nickel, cobalt, etc.) with an increase in temperature decreases the magnetic permeability value m. When the point of Curie is reached, the magnetic permeability of ferromagnets drops to 1, that is, they lose their magnetic properties. The usual heating temperature for hardening 800-1000 ° C, under pressure processing 1000 - 1200 ° C, that is, above the point of Curie. The 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 heating process (Fig. 3.5). Initially, due to the increase in R specific power D R increases and reaches the maximum value D P Makh \u003d (1.2 ÷ 1.5) D R NCH and then due to the loss steel magnetic properties drops to the minimum D P MIN. . To maintain heating in optimal mode (with sufficiently high to. P. D.) Installations provide devices to match the parameters of the generator and load, that is, the possibility of regulating the heating mode. If we compare through the heating of the blanks under the plastic deformation with an induction method and an electrocontact (both refer to direct heating), it can be said that the electricity consumption of electrocontact heating is appropriate for long billets of a relatively small section, and induction - for short-acting blanks relatively large diameters. The strict calculation of inductors is quite cumbersome and is associated with the involvement of additional semi-empirical data. We will look at the simplified calculation of cylindrical inductors for surface hardening, based on the above dependencies. Thermal calculation. From consideration of induction heating regimens, it follows that the same thickness of the hardened layer x K.can be obtained at different values \u200b\u200bof the specific power D R and heating duration t. The optimal mode is determined not only the layer thickness x kbut the magnitude of the transition zone B binding the tempered layer with the depth layers of the metal. In the absence of control devices of the generator power, the nature of the change in the specific power consumed by the steel product is depicted by the graph shown in Figure 3.5. In the process of heating, the RC value changes both by the end of the heating, after switching through the Curie point, decreases sharply. It happens like the self-off of the steel product, which ensures high quality quenching without facing. In the presence of control devices power D R may be equal or even less d P MIN. (Fig. 3.5), which allows the lengthening of the heating process to reduce the specific power required for this thickness of the hardening layer x k Graphs of heating modes under surface hardening for carbon and unleaned steel with a thickness of the transition zone, constituting 0.3-0.5 of the hardening layer, are shown in Figures 3.6 and 3.7. Selecting D. R, it is not difficult to find the power supplying to the inductor, where H. Tr. - k. p. d. high-frequency (hardening) transformer. Power consumed from the network, determined by the specific consumption of electricity but(kW-h / t) and performance G. (t / h): for surface heating where D. i. - increment of the heat generation of the workpiece as a result of heating, KJ / kg; D. -liness of the material of the workpiece, kg / m 3; M 3 -mass of the workpiece, kg; S 3. - surface of the hardening layer, m 2; B. - Metal ugar (with induction heating 0.5-1.5%); h TP - k. p. d. heat transfer due to thermal conductivity inside the workpiece (with surface hardening H TP = 0,50). The remaining designations are explained above. Exemplary values \u200b\u200bof specific electricity consumption in induction heating: vacation-120, hardening - 250, cementation - 300, through heating under mechanical processing - 400 kWh / t. Electrical calculation. An electrical calculation is based on dependence (3.7). Consider the case when the depth of penetration z A. significantly less inductor size and details, and the distance butthere is little between the inductor and the product compared to the width of the inducer conductor b.(Fig. 3.1). For this case inductance L S.inductor system - product can be expressed by the formula Substituting the value of the current in formula (3.7) and bearing in mind that Formula (3.30) gives a relationship between the specific power, electrical parameters and the geometric dimensions of the inductor, the physical characteristics of the heated metal. Taking for the function of the inductor size, we get for heated state Power coefficient inductor where P is the active power of the inductor, W; U I. - voltage in the inductor, in; F. - frequency Hz. When connecting capacitors to the primary chain of a high-frequency transformer, capacitors must be increased to compensate for the transformer reactivity and connecting conductors. Example. Calculate inductor and choose high-frequency installation for surface hardening of cylindrical billets made of carbon steel diameter d A. \u003d 30 mm and height h a. \u003d 90 mm. Depth of the hardening layer x K \u003d. 1mm, inductor voltage U and \u003d.100 V. We find the recommended frequency by Formula (218): Stand on the nearest of the frequencies used f. \u003d 67 kHz. From the graph (Fig. 3.7) accept D R \u003d 400 W / cm 2. According to formula (3.33) we find al For cold condition: Accept but \u003d 0.5 cm, then the diameter of the inductor Length of the induced conductor The number of turns of the inductor Inductor height Power supplied to the inductor where 0,66 - k. p. inductor (Fig. 3.8). Oscillatory power generator Select the high-frequency installation of LPZ-2-67M, having a vibrational power of 63 kW and the operating frequency of 67 kHz. The induction heating technique uses current (industrial) frequency of 50 Hz, an average frequency of 150-10000 Hz and high frequency from 60 kHz to 100 MHz. Currents of medium frequency are obtained using machine generators or static frequency converters. In the 150-500 Hz range, generators of the usual synchronous type are used, and above (up to 10 kHz) - engineered inductor type generators. Recently, machine generators are outstanding with more reliable static frequency converters performed on transformers and thyristors. High frequency currents from 60 kHz and above are obtained exclusively using lamp generators. Installations with tube generators are used to perform a variety of heat treatment operations, surface hardening, melting metals, etc. Without affecting the theories of the issue set out in other courses, consider only some features of generators for heating. Heating generators are executed, as a rule, with self-excitation (autogenerators). Compared to independent excitation generators, they are easier on the device and have better energy and economic indicators. The diagrams of lamp generators for heating are not fundamentally different from radio engineering, but have some features. From these schemes does not require strict frequency stability, which will noticeably simplify them. The concept of the simplest generator for induction heating is shown in Figure 3.10. The main element of the scheme is the generator lamp. In heating generators, three-electro lamps are most often used, which are easier compared to tetroles and feed and ensure sufficient reliability and sustainability of generation. The load of the generator lamp serves an aode oscillating circuit, the parameters of which inductance L.and container FROMthey are selected from the condition of the circuit in the resonance at the operating frequency: where R -the reduced contour loss resistance. Contour parameters R., L, S.defined, taking into account changes introduced by the electrophysical properties of heated tel. The power of the anode chains of the generator lamps is carried out by direct current from rectifiers collected on thiratron or gasotron (Fig. 3.10). Powering alternating current for economic considerations is applied only for small capacities (up to5 kW). The secondary voltage of the power (anodic) transformer feeding the rectifier is 8 - 10 kV, straightened voltage 10 - 13 square meters. Unlucky fluctuations in the autogenerator occur if there is sufficient positive feedback of the grid with a circuit and performing certain conditions connecting the parameters of the lamp and contour. Return Return Coefficient where U S. , U K. , U A. - Nonflows, respectively, on the grid, the oscillatory circuit and the anode of the generator lamp; D.- permeability of the lamp; S D. - Dynamic steepness of the anode-grid characteristic of the lamp. Reverse grid communication in the generators for induction heating is most often performed according to the three-point scheme, when the grid voltage is taken from the part of the inductance of an anode or heating circuit. Figure 3.10 The voltage on the mesh is supplied from the part of the turns of the communication coil L2,which is an element of the inductance of the heating circuit. Heating generators, in contrast to radio engineering, are most often performed by two-circuit (Fig. 3.10) or even one-contact. Two-kinning generators are easier to be configured in resonance and more resistant to work. The generators are excited by second-sort oscillations. Anode current flows through the lamp with pulses, only during part (1/2-1 / 3) of the period. Due to this, the constant component of the anode current is reduced, the heating of the anode decreases and increases to. P. Generator. The form of pulses has a grid current. Cut-off of the anode current (within the cut-off angle q \u003d 70-90 °) is carried out by supplying a constant negative displacement to the grid, which is created by a voltage drop on grid resistance R G.when the constant component of the mesh current is flowing. Generators for heating have a load varying during heating process caused by a change in the electrophysical properties of heated materials. To ensure the operation of the generator in the optimal mode, characterized by the highest values \u200b\u200bof the output of the power and to. P. D., Installations are equipped with load matching devices. The optimal mode is achieved by the selection of the corresponding value of the reverse mesh coefficient k S. and the fulfillment of the condition where E A -power supply voltage; E C -constant displacement on the grid; I A1. - The first harmonic of the anode current. To match the load in schemes, it is possible to adjust the contour resonance resistance R A.and change voltage on the grid U s.The change in these values \u200b\u200bis achieved by introducing additional containers into the circuit or inductivities and the switching of anodic, cathode and grid clamps (probes) connecting the contour with the lamp. Installations of induction heating are very common at repair factories and agricultural machinery enterprises. In the repair production, the currents of medium and high frequency are used for through and surface heating of the parts made of cast iron and steel under hardening, before hot deformation (forging, stamping), when the parts are restored, the methods of surfacing and high-frequency metallization, when soldering with solid solders, etc. Special place is occupied by surface hardening of parts. The possibility of a power concentration in a specified location of the part allows you to obtain a combination of an outer hardened layer with plasticity of deep layers, which significantly increases wear resistance and resistance to alternating and shock loads. The advantages of surface hardening using induction heating are as follows: 1) the ability to order parts and tools to any needed thickness, if necessary, processing only work surfaces; 2) a significant acceleration of the hardening process, which ensures high performance of the installations and reduces the cost of heat treatment; 3) is usually smaller than other methods of heating specific energy consumption due to selectivity of heating (only on a given depth) and the frequency of the process; 4) high quality hardening and marriage reduction; 5) the possibility of organizing the flow of production and automation of processes; 6) High culture of production, improving sanitary and hygienic working conditions. The installation of induction heating is chosen according to the following basic parameters: assignment, nominal oscillatory power, operating frequency. The installed industries have a standard capacity scale with the following steps: 0.16; 0.25; 0.40; 0.63; 1.0 kW and further with multiplication of these numbers by 10, 100 and 1000. Installations for induction heating have power from 1.0 to 1000 kW, including tube generators up to 250 kW, and above - with machine generators. The operating frequency determined by the calculation is specified by the frequency scale allowed for use in electrothermia. High-frequency installations for induction heating have a single indexation: OPECs (high-frequency induction). After the letters through the dash, the oscillatory power (kW) is denoted in the numerator, in the denominator - the frequency (MHz). After the numbers are written letters denoting technological purpose. For example: VCI-40 / 0.44-zP - high-frequency installation of induction heating, oscillatory power 40 kW, frequency 440 kHz; Screen Runs - for quenching surfaces (NS - for end-to-end heating, stroke welding, etc.). 1. Explain the principle of induction heating. The area of \u200b\u200bits use. 2. List the main elements of the installation of induction heating and specify their purpose. 3. How is the heater winding? 4. What are the advantages of the heater? 5. What is the phenomenon of the surface effect? 6. Where can the induction air heater be used? 7. What depends the depth of current penetration into heated material? 8. What is determined by the Ring Inductor efficiency? 9. Why is ferromagnetic pipes need to be used to perform induction heaters at an industrial frequency? 10. What is most significantly affected by the inductor COS? 11. How does heating rate change with increasing the temperature of the heated material? 12. What parameters have the temperature measurement affect? Induction heater - This is electric heaterworking when changing the magnetic induction flow in a closed conductive circuit. This phenomenon is called electromagnetic induction. Want to know how the induction heater works? Zavodrr. - This is a trading information portal, where you will find information about heaters. The induction coil is capable of heating any metal, the heaters are collected on transistors and have a high efficiency of more than 95%, they have long replaced the lamp induction heaters, which did not go out for 60%. The vortex induction heater for contactless heating does not have losses to set the resonant coincidence of the operating parameters of the installation with the parameters of the output oscillating circuit. Vortex-type heaters collected on transistors are able to perfectly analyze and adjust the output frequency in automatic mode. Heaters for the induction heating of the metal have a contactless way due to the action of the vortex field. Different types of heaters penetrate the metal to a certain depth of 0.1 to 10 cm, depending on the selected frequency: Induction metal heaters allow you to process parts not only on open areas, but also to place heated objects in isolated cameras in which you can create any environment, as well as a vacuum. High Frequency Electric Induction Heater Every day she gains new ways to use. The heater operates on an alternating electric current. Most often, induction electric heaters are used to bring metals to the necessary temperatures at the following operations: forging, soldering, welding, bending, hardening, etc. Electrical induction heaters operate at a high frequency of 30-100 kHz and are used to heat different types of media and coolants. Electric heater Applied in many areas: When a deeper heating is required, induction heaters of the mid-frequency type, operating average frequencies from 1 to 20 kHz. The compact inductor for all types of heaters is the most different shape, which is selected so as to ensure the uniform heating of the samples of the most diverse shape, and the specified local heating can be carried out. The mid-frequency type will treat materials for forging and quenching, as well as through heating under stamping. Light in management, with efficiency up to 100%, induction mid-frequency heaters are used for a large circle of technologies in metallurgy (also for smelting of various metals), mechanical engineering, instrument making and other areas. The widest range of high-frequency induction heaters. Heaters are characterized by a high frequency of 30-100 kHz and a wide range of 15-160 kW capacities. The high-frequency type provides a small heating depth, but this is enough to improve the chemical properties of the metal. High-frequency induction heaters are easy to manage and economical, and at the same time their efficiency can reach 95%. All types work continuously for a long time, and a two-bit version (when a high frequency transformer is put into a separate unit) allows 24-hour work. The heater has 28 types of protection, each of which is responsible for its function. Example: water control in the cooling system. Ultrahigh-frequency induction heaters are over frequency (100-1.5 MHz), and penetrate the warm-up depth (up to 1 mm). A superhigh frequency type is indispensable for treating thin, small, with a small diameter of parts. The use of such heaters avoids unwanted deformations associated with heating. Ultrahigh-frequency induction heaters on JGBT modules and MOSFET transistors have a power limits - 3.5-500 kW. Used in electronics, in the production of high-precision tools, hours, jewelry, for the production of wire and for other purposes involving special accuracy and filigree. The main purpose of the induction heaters of a blacksmith type (ICN) is heated by parts or their parts preceding the subsequent forging. Billets can be the most different type, alloy and forms. Induction blacksmith heaters allow you to handle cylindrical billets by any diameter in automatic mode: Induction heaters for hardening shafts Work together with the hardening complex. The processed item is in a vertical position and rotates inside a fixed inductor. The heater allows you to use all types of shafts for serial local heating, the depth of injection can be the shares of millimeters in depth. As a result of the induction heating of the shaft along the entire length with an instant cooling, its strength and durability increase repeatedly. All types of pipes can be treated with induction heaters. The pipe heater can be with an air or water type of cooling, with a capacity of 10-250 kW, with the following parameters: Each thermoproining option is used to improve the quality of any steel pipes. One of the most important parameters of the work of induction heaters - temperature. For more careful control over it, in addition to embedded sensors, infrared pyrometers are often used. These optical devices allow you to quickly and easily determine the temperature of it difficult to access (due to high heating, the probability of exposure to electricity, etc.) of surfaces. If you connect a pyrometer to the induction heater, you can not only monitor the temperature mode, but also automatically maintain the heating temperature for a specified time. In the inductor during operation, a magnetic field is formed in which the part is placed. Depending on the task assigned (the heating depth) and the parts (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 reads: when the conductor is found in a variable electromagnetic field, it is formed by EMF (electromotive force). The amplitude schedule shows that it moves in proportion to the change in the speed of the magnetic flux. Due to this, vortex currents are formed in the circuit, the magnitude of which depends on the resistance (material) of the conductor. Under the law, Joule-Lenz, the current leads to the heating of the conductor, which has resistance. The principle of operation of all types of induction heaters is similar to a transformer. Conductive billet, which is located in the inductor, is similar to a transformer (without magnetic pipeline). Primary winding is an inductor, the secondary inductance of the part, and the load is metal resistance. When tvch, the heating is formed "skin-effect", the vortex currents that are formed inside the workpiece, displacing the main current to the surface of the conductor, because the heating of the metal on the surface is stronger than inside. The induction heater has undoubted advantages and is the leader among all types of instruments. This advantage is folded in the following: Repair of induction heaters is made from spare parts from our warehouse. At the moment we can repair all types of heaters. Induction heaters are sufficiently reliable if it is strictly followed by operating instructions and not allow the extensive mode of operation - 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 documentation of manufacturers, their repair should be engaged in qualified specialists who are familiar with the detailed principle of work of such equipment. You can familiarize yourself with the video operation of the mid-frequency induction heater .. The average frequency is used for deep penetration into all types of metal products. The mid-frequency heater is a reliable and modern equipment that works around the round for the benefit of your enterprise.
The main feature of induction heating is the conversion of electrical energy to heat using an alternating magnetic flux, i.e. inductive path. If, on a cylindrical spiral coil (inductor), passes an alternating electric current I, then an alternating magnetic field F M is formed around the coil, as shown in Fig. 1-17, in. The magnetic flux has the greatest density inside the coil. When the metallic conductor is placed in the cavity of the metallic conductor in the material, an electromotive force occurs, the instant value of which is equal to: Under the influence of ED. In the metal placed in a fast-acting magnetic field, an electric current occurs, the magnitude of which depends primarily on the magnitude of the magnetic flux, crossing the contour of the heated material, and the frequency of the current F forming the magnetic flow. Heat release during induction heating occurs directly in the volume of heated material, with most of the heat allocated in the surface layers of the heated part (surface effect). The thickness of the layer in which the most active heat release occurs, is equal to: where ρ is a resistivity, Ohm * cm; μ - relative magnetic permeability of material; F - frequency, Hz. From the above formula, it can be seen that the thickness of the active layer (the depth of penetration) decreases for this metal with increasing frequency. The frequency selection depends mainly on the technological requirements. For example, when weaving metals, the frequency of 50 - 2500 Hz will be required, when heated is up to 10,000 Hz, with a surface hardening - 30,000 Hz and more. When smelting the cast iron, an industrial frequency is used (50 Hz), which allows you to increase the total kp. Installations, as they exclude energy loss on frequency conversion. Induction heating is high-speed, since heat is highlighted directly into the thickness of the heated metal, which allows the melting of the metal in induction electric hollows 2-3 times faster than in reflective flames. Heating with high frequency currents can be made in any atmosphere; Induction thermal installations do not require time for warming up and easy to embed into automatic and streamlines. Using induction heating, temperatures can be achieved up to 3000 ° C and more. Due to its advantages, high-frequency heating is widely used in the metallurgical, engineering and metalworking industry, where it is used for metal melting, with thermal processing of parts, heating under stamping, etc. Principle of operation of induction furnaces. Principle of induction heating
The principle of induction heating consists in converting the energy of the electromagnetic field, absorbed by the electrically conductive heated object, into thermal energy. In the installations of induction heating, the electromagnetic field is created by an inductor, which is a multi-axis cylindrical coil (solenoid). A variable electric current is passed through the inductor, as a result of which the variable magnetic field variables around the inducer occurs around the inductor. This is the first conversion of the energy of the electromagnetic field described by the first equation of Maxwell. The heated object is placed inside the inductor or next to it. Changing (in time) The vector stream of magnetic induction created by the inductor permeates the heated object and induces an electric field. The electrical lines of this field are located in the plane perpendicular to the direction of the magnetic flux, and are closed, i.e. the electric field in the heated object is vortex. Under the influence of the electric field, according to the law of Ohm, conductivity currents arise (vortex currents). This is the second conversion of the energy of the electromagnetic field described by the second equation of Maxwell. In the heated object, the energy of the induced alternating electric field is irreversibly moving into thermal. Such thermal dispersion of energy, the consequence of which is the heating of the object, is determined by the existence of conductivity currents (vortex currents). This is the third conversion of the energy of the electromagnetic field, and the energy ratio of this transformation is described by the Lenza-Joule law. The described transformations of the electromagnetic field energy make it possible: The magnitude of the electric field strength in the heated object is influenced by two factors: the magnitude of the magnetic flux, i.e. the number of magnetic power lines that permeate the object (or linked to the heated object), and the feed current frequency, i.e. the rate of change (in time ) Magnetic flux captured with a heated object. This makes it possible to perform two types of installations of induction heating, which differ in both the design and operational properties: induction installations with a core and without a core. On the technological purpose of the installation of induction heating is divided into melting furnaces for melting metals and heating installations for thermal processing (quenching, vacation), for through heating of blanks in front of plastic deformation (forging, stamping), for welding, soldering and surfacing, for chemical heat treatment products, etc. By frequency of changes in the current supplying the installation of induction heating, distinguish: Similar documents
Ultrapy contactless smelting, soldering and welding of metal.
Obtaining prototypes of alloys.
Bending and heat treatment of machine parts.
Jewelry.
Processing small parts that may be damaged during gas-flame or arc heating.
Surface hardening.
Hardening and heat treatment of parts of complex shape.
Disinfection of medical instrument.
- increasing the operating frequency, which leads to the complication and appreciation of the plant;
- the use of ferromagnetic inserts in the inductor; Pluging inductor with ferromagnetic material panels.
b) constant power allocated in the workpiece
c) the highest efficiency.
Parallel oscillatory circuit - reasons.
In this case, on the coil and on the condenser, the voltage is the same as the generator. With resonance, the contour resistance between branching points becomes the maximum, and the current (I total) through the load resistance RN will be minimal (current inside the loop I-1L and I-2C larger than the generator current).
Output:
In a parallel circuit, with a resonance, the current through the conclusions of the contour is 0, and the voltage is maximum.
In a sequential circuit, on the contrary, the voltage tends to zero, and the current is maximum..
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Vortex induction heaters
Induction metal heaters
Electric induction heater
Medium-grade induction heaters
High-frequency induction heaters
Ultrahigh-frequency induction heaters
Blacksmith induction heaters
Induction heaters shafts
Induction pipe heaters
Pyrometer for heating control
Principle of operation of induction heaters
Advantages of induction heaters
Video operation of induction mid-frequency heaters
1) Transfer the electrical energy of the inductor into the heated object without resorting to the contacts (unlike resistance furnaces)
2) Select heat directly in the heated object (the so-called "furnace with an internal heating source" by the terminology of prof. N. V. Okorokova), as a result of which the use of heat energy turns out to be the most perfect and heating rate increases significantly (compared to the so-called " furnace with an external heating source ").
1) industrial frequency installations (50 Hz) feed on the network directly or through lower transformers;
2) Installations of increased frequency (500-10000 Hz) receiving power from electromashic or semiconductor frequency converters;
3) High-frequency settings (66,000-440,000 Hz and above) powered by lamp electronic generators.
