Induction high frequency heating. Induction heating calculation High frequency induction

The invention relates to electrical engineering and is aimed at increasing the service life of RF plasma torches and increasing their thermal efficiency. The problem is solved by the fact that the RF plasma torch contains a cylindrical discharge chamber made in the form of water-cooled longitudinal profiled metal sections placed in a protective dielectric casing, an inductor covering the casing, and input units for the main and thermal protective gases installed inside the discharge chamber at its end part. The thermal protective gas input unit is made in the form of one or more coaxial annular rows of longitudinal metal tubes with a number in each row equal to the number of longitudinal profiled metal sections. The tubes on the inductor side have a profiled gap for gas outlet, as well as a longitudinal gap relative to adjacent tubes in a row up to a distance of at least one internal diameter discharge chamber, counting from the nearest turn of the inductor. The tubes are connected along the side surface by soldering or welding with radially located longitudinal metal tubes of the adjacent coaxial ring row, and the longitudinal metal tubes of the row closest to the longitudinal profiled metal sections are connected along the side surface to the adjacent section by soldering or welding. The main gas input unit on the inductor side is equipped with a diaphragm located at a distance of at least one internal diameter of the discharge chamber from the nearest turn of the inductor and having at least one hole for gas passage. The ends of the longitudinal metal tubes for the gas outlet in each row are located outside the inductor zone and are equidistant from its nearest turn, and the distance of the ends of the longitudinal metal tubes for the gas outlet from the nearest turn of the inductor increases with the distance of the coaxial ring row from the longitudinal profiled metal sections. Longitudinal metal tubes are located on the surface of adjacent, radially located longitudinal metal tubes, and the longitudinal metal tubes of the coaxial annular row closest to the longitudinal profiled metal sections are located on the surface of adjacent sections. The diaphragm on the inductor side forms an annular gap for the passage of gas with the longitudinal metal tubes of the nearest coaxial ring row, and the height of the annular gap for the passage of gas is made less than the height of the profiled gap for the gas outlet of the longitudinal metal tubes of the nearest coaxial ring row. The use of the proposed design of an RF plasma torch as a generator of low-temperature plasma in jet-plasma processes for processing dispersed materials has made it possible to create effective plasma reactor devices for opening finely ground ore raw materials, spheroidizing dispersed materials and obtaining highly dispersed oxide powders by generating untwisted plasma jets at the thermal efficiency of the RFI- plasma torches more than 80%. 15 salary f-ly, 5 ill.

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

Description of the method.

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

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

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

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

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

The inductor becomes very hot during operation, as it absorbs its own radiation. In addition, it absorbs thermal radiation from the hot workpiece. They make inductors from copper tubes, cooled with water. Water is supplied by suction - this ensures safety in the event of a burn-through or other depressurization of the inductor.

Application:
Ultra-clean non-contact melting, soldering and welding of metal.
Obtaining prototypes of alloys.
Bending and heat treatment of machine parts.
Jewelry making.
Processing of small parts that can be damaged by gas flame or arc heating.
Surface hardening.
Hardening and heat treatment of parts with complex shapes.
Disinfection of medical instruments.

Advantages.

High-speed heating or melting of any electrically conductive material.

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

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

Due to the MHD forces that arise, intensive mixing of the liquid metal occurs, up to keeping it suspended in air or a protective gas - this is how ultra-pure alloys are obtained in small quantities (levitation melting, melting in an electromagnetic crucible).

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

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

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

It is easy to carry out local and selective heating.

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

Easy automation of equipment - heating and cooling cycles, temperature adjustment and maintenance, feeding and removal of workpieces.

Induction heating units:

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

Also, to heat small parts, high-frequency installations are being built using MOSFET transistors for operating frequencies up to 1.7 MHz. Controlling transistors and protecting them at higher frequencies presents certain difficulties, so higher frequency settings are still quite expensive.

The inductor for heating small parts has small sizes and small inductance, which leads to a decrease in the quality factor of the working oscillatory circuit at low frequencies and a decrease in efficiency, and also poses a danger to the master oscillator (the quality factor of the oscillatory circuit is proportional to L/C, an oscillatory circuit with a low quality factor is “pumped” too well with energy and forms a short circuit on the inductor and disables the master oscillator). To increase the quality factor of the oscillatory circuit, two ways are used:
- increasing the operating frequency, which leads to more complex and expensive installations;
- use of ferromagnetic inserts in the inductor; pasting the inductor with panels made of ferromagnetic material.

Since the inductor operates most efficiently at high frequencies, induction heating received industrial application after the development and start of production of high-power generator lamps. Before World War I, induction heating had limited use. High-frequency machine generators (works by V.P. Vologdin) or spark-discharge installations were then used as generators.

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

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

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

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

Disadvantages of three points:

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

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

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

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

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

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

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

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

Notes.

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

Increasing the frequency sharply reduces the power factor (proportional to the cube of the frequency).

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

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

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

Ideally, the loop impedance is infinity—the circuit draws no current from the source. When the generator frequency changes in any direction from the resonant frequency, the circuit impedance decreases and the line current (I total) increases.

Series oscillatory circuit – voltage resonance.

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

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

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 be achieved only 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 a frequency, you can obtain the required technological conditions the nature of heating and its intensity. 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 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 heat up from metal mesh, 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

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 - the 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 the 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 consumption electricity increases. 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. At external sources heat (plasma heating, in 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 working area inductor.

Frequency selection. A sufficiently high efficiency can be obtained only with a certain ratio between the size of the body and the frequency of the current. The selection 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 of steel cylindrical blanks 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 ferromagnetic materials 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. Change physical properties 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 by 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 is suitable 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. There is a kind of self-switching off of the steel product, which ensures high quality hardening without burnout. 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 of 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):

Hz

We stop at the closest used frequency 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

cm

Number of inductor turns

Inductor height

Power supplied to the inductor, according to

kW

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.

IN lately machine generators are being 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. Schematic diagram The simplest 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 DC 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 in the presence of sufficient positive feedback mesh with a contour and the fulfillment of certain conditions connecting the parameters of the lamp and the contour.

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 highest values 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 to perform induction heaters Is it necessary to use ferromagnetic pipes at industrial frequencies?

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 achieve very high temperatures, at which it is possible to process

To ensure induction heating, it is necessary to create electromagnetic fields specific voltage and frequency. This can be done in special device- 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 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 should be close in shape.

Where are they used?

Application of induction heating in modern world widespread. 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 like 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

For arrangement 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 it works

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

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. warm water- 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 welding machine, it 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, 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, this type of heating was found in hobs slabs

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.