What determines the power of the inductor. Induction heating, basic principles and technologies. Inductive discharge without external magnetic field

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 HFI 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 enclosing the casing, installed inside the discharge chamber in its end part, the input nodes of the main and thermal protective gases. The thermal protective gas inlet unit is made in the form of one or more coaxial annular rows of longitudinal metal tubes with the number in each row equal to the number of longitudinal profiled metal sections. The tubes on the side of the inductor 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 inner diameter of the discharge chamber, counting from the nearest coil 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 annular row, and the longitudinal metal tubes of the row closest to the longitudinal profiled metal sections are connected along the side surface with the adjacent section by soldering or welding. The main gas inlet unit on the side of the inductor is equipped with a diaphragm located at a distance of at least one inner diameter of the discharge chamber from the nearest coil of the inductor and having at least one hole for the passage of gas. The ends of the longitudinal metal tubes for gas outlet in each row are located outside the inductor zone and equidistant from its nearest turn, and the distance of the ends of the longitudinal metal tubes for gas outlet from the nearest coil of the inductor increases with the distance of the coaxial annular row from the longitudinal profiled metal sections. Longitudinal metal tubes are located on the surface of adjacent, radially located longitudinal metal tubes, and 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 side of the inductor forms an annular gap for the passage of gas with longitudinal metal tubes of the nearest coaxial annular row, and the height of the annular gap for the passage of gas is less than the height of the profiled gap for the gas outlet of the longitudinal metal tubes of the nearest coaxial annular row. The use of the proposed design of the RFI plasma torch as a generator of low-temperature plasma in jet-plasma processes for processing dispersed materials 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 RFI- plasmatrons more than 80%. 15 z.p. f-ly, 5 ill.

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

• Faraday-Maxwell;
• Joule-Lenz.

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

Induction heating device

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

EMF acts in such a way that eddy currents flow inside the bodies, which release heat in full accordance with the Joule-Lenz law. Also, the EMF generates an 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 reaching very high temperatures at which it is possible to process

To provide induction heating, it is required to create a certain voltage and frequency in electromagnetic fields. You can do this 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 small frequency inductor is a spiral (insulated conductor), which can be placed inside metal pipe or wrapped around it. The passing currents heat the pipe, which, in turn, transfers heat to environment.

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

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

Where are they used

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

• melting of metals, their soldering in a non-contact way;
• obtaining new metal alloys;
• mechanical engineering;
• jewelry business;
• making small parts that can be damaged by other methods;
• (moreover, details can be of the most complex configuration);
• heat treatment (processing of parts for machines, hardened surfaces);
• medicine (disinfection of devices and instruments).

Induction heating: positive characteristics

This method has many advantages:

• With it, you can quickly heat and melt any conductive material.
• Allows heating in any environment: in vacuum, atmosphere, non-conductive liquid.
• Due to the fact that only the conductive material is heated, the walls that weakly absorb waves remain cold.
• In specialized areas of metallurgy, obtaining ultrapure alloys. This is an entertaining process, because the metals are mixed in a shell of protective gas.

• Compared with other types, induction does not pollute the environment. If in the case of gas burners pollution is present, as well as in arc heating, then induction eliminates this, due to "pure" electromagnetic radiation.
• Small dimensions of the inductor device.
• The possibility of manufacturing an inductor of any shape, this will not lead to local heating, but will contribute to a uniform distribution of heat.
• It is irreplaceable if it is necessary to heat only a certain area of ​​the surface.
• It is not difficult to set up such equipment for 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 is better to turn to specialists.
• The need to accurately match the inductor and the workpiece, otherwise induction heating will be insufficient, its power can reach small values.

Heating with induction equipment

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

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

How does it work

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

In order for water to pass through such a boiler, two pipes are brought to it: for cold, which enters, and at the exit warm water- the second pipe. Due to the pressure, the 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 in maintenance will be inexpensive. The main plus is that the device works silently. You can install it in any room.

Making equipment yourself

Installation of induction heating will not be very difficult. Even those who do not have experience, after careful study, will cope with the task. Before starting work, you need to stock up on the following necessary items:

• inverter. It can be used from welding machine, it is inexpensive and will need high frequency. You can make it yourself. But this is a time consuming task.
• Heater housing (a piece of plastic pipe, induction heating of the pipe in this case will be the most effective).
• Material (a wire with a diameter of no more than seven millimeters will fit).
• Devices for connecting the inductor to the heating network.
• Grid to hold the wire inside the inductor.
• An induction coil can be created from (it must be enameled).
• Pump (in order for water to be supplied to the inductor).

Rules for the manufacture of equipment independently

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, it can be more).

• The wire should be cut into pieces no more 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 net 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 design is closed, an adapter is placed.
• 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 apparatus. The coil is connected to the prepared inverter.
• A water pump is installed.
• The temperature controller 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 tires leading to the inductor, which can be much higher than the inductor itself.

About cooking surfaces

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

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

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

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

For such stoves, special dishes are needed. Most ferromagnetic metals can interact with an induction field: aluminum, stainless and enameled steel, cast iron. Not suitable for such surfaces only: copper, ceramic, glass and dishes made of 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 unusable dishes. The main advantages of brewers are: safety, ease of cleaning, speed, efficiency, economy. Never burn yourself on the surface of the panel.

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

INDUCTION HEATER- it's electric heater, working with a change in the flux of magnetic induction in a closed conducting circuit. This phenomenon is called electromagnetic induction. Want to know how an induction heater works? ZAVODRR is a trade information portal where you will find information about heaters.

Vortex induction heaters

An induction coil is capable of heating any metal, transistor-based heaters are assembled and have a high efficiency of more than 95%, they have long replaced tube induction heaters, in which the efficiency did not exceed 60%.

The vortex induction heater for non-contact heating has no losses due to the adjustment of the resonant coincidence of the operating parameters of the installation with the parameters of the output oscillatory circuit. Vortex-type heaters assembled on transistors can perfectly analyze and adjust the output frequency in automatic mode.

Metal induction heaters

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

• high frequency;
• average frequency;
• ultra high frequency.

Metal induction heaters make it possible to process parts not only in open areas, but also to place heated objects in isolated chambers, in which any medium, as well as vacuum, can be created.

Electric induction heater

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

Electric heater applied in many areas:

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

Medium frequency induction heaters

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

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

High frequency induction heaters

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

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

Microwave induction heaters

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

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

Forging induction heaters

The main purpose of forge-type induction heaters (IKN) is to heat parts or their parts prior to subsequent forging. The blanks can be different type, alloy and shape. Induction forging heaters allow you to process cylindrical workpieces of any diameter in automatic mode:

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

Induction roller heaters

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

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

Induction pipe heaters

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

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

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

Pyrometer for heating control

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

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

The principle of operation of induction heaters

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

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

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

Advantages of induction heaters

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

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

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 quite reliable if you strictly follow the operating instructions and avoid extreme operating modes - first of all, monitor the temperature and proper water cooling.

The details of the operation of all types of induction heaters are often not fully published in the manufacturers' documentation; they should be repaired by qualified specialists who are well acquainted with the detailed principle of operation of such equipment.

Video of the work of induction medium-frequency heaters

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

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

Intensive heating can only be obtained in magnetic fields of high intensity and frequency, 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 it were, 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 the ideas about the absorption of electromagnetic field energy by a conducting medium.

Frequency has a significant effect 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 are developed that are sufficient to melt the most refractory metals.

At the same time, the higher the frequency, the smaller 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 carry out deep or even through heating, which is the same throughout the entire cross section of the body. Thus, by choosing the frequency, it is possible to obtain the character of heating and its intensity required by the technological conditions. The ability to heat products to almost any thickness is one of the main advantages of induction heating, which is widely used for hardening surfaces of parts and tools.

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

The inductor is the working body of the induction heating installation. The heating efficiency is the higher, the closer the type of electromagnetic wave emitted by the inductor to the shape of the heated surface. 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, the goals and conditions of heating. The simplest inductor is an insulated conductor placed inside a metal pipe, stretched or coiled. When an industrial frequency current is passed through the conductor, eddy currents heating it are induced in the pipe. AT agriculture Attempts have been made to use this principle for heating indoor soil, perches for birds, etc.

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

During induction drying of wood, a stack of boards is shifted with metal meshes and placed (rolled up on a special trolley) inside a cylindrical inductor made of large-section conductors wound on a frame of insulating material. Boards are heated by metal grids 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 performance and low heating intensity. Low-frequency induction heating is quite effective for direct heating of massive metal workpieces and a certain ratio between their size and current penetration depth (see below).

The inductors of high-frequency installations are made uninsulated, they consist of two main parts - an inductive wire, with which an alternating magnetic field is created, and current leads for connecting the inductive 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 blanks - 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 right direction, supply cooling and quenching water, etc.

To create high-tension fields, large currents, in the hundreds and thousands of amperes, are passed through the inductors. In order to reduce losses, inductors are made with as little active resistance as possible. Despite this, they are still intensively heated 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, round or rectangular section, inside which flowing 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 the formula (11)

given the expression

In practical calculations, the dimension D is used R in W / cm 2, then

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

Thus, the power dissipated 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 intensity, the greater the heating intensity, the greater the resistivity r, the magnetic permeability of the material m and the current frequency f.

Formula (208) is valid for a plane electromagnetic wave (see Section 2 of Chapter I). When cylindrical bodies are heated in solenoid inductors, the pattern of wave propagation becomes more complicated. Deviations from the ratios for a plane wave are the greater, the smaller the ratios r/z a, where r is the radius of the cylinder, z a- depth of penetration of currents.

In practical calculations, however, they still use a 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 turns. In the presence of gaps, the 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 ratio of powers to the limit

From expression (3.13) it follows that the efficiency decreases with an increase in the air gap and the resistivity of the inductor material. Therefore, inductors are made of massive copper tubes or tires. As follows from expression (214) and Figure 43, the efficiency value approaches its limit already at r/z a>5÷10. This makes it possible 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 illustrative dependences (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 the heating inductor is determined by the ratio of the active and inductive resistances of the inductor-product system. At high frequency, the active and internal inductive resistances of the product are equal, since the phase angle between the vectors and is 45° and |D R| = |D Q|. Therefore, the maximum value of the power factor

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, so an unreasonable overestimation of the frequency reduces the energy performance of the installations. You should always strive to reduce the air gap, but there is a limit due to the breakdown air strength. 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, dropping to 0.1-0.01. To unload the networks and the generator from reactive currents and increase cosf, compensating capacitors are usually included in parallel with the inductor.

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

Deep heating ("low frequencies") is carried out at such a frequency f when the 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 heat transfer by thermal conductivity into the body is negligible.

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

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

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

The induction method can be used for both deep and surface heating. With external heat sources (plasma heating, electric resistance furnaces), deep heating is not possible.

According to the principle of operation, two types of induction heating are distinguished: simultaneous and continuous-sequential.

With 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 you to simultaneously heat all its sections. With continuous-successive heating, the product moves relative to the inductive wire, and the heating of its individual sections occurs as it passes working area inductor.

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

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

During through heating of steel cylindrical billets with a diameter d(mm) the required frequency is determined by the formula

In the process of heating, the specific resistance of metals r increases. In ferromagnets (iron, nickel, cobalt, etc.), with increasing temperature, the value of magnetic permeability m decreases. When the Curie point is reached, the magnetic permeability of ferromagnets drops to 1, that is, they lose their magnetic properties. The usual heating temperature for hardening is 800-1000 ° C, for pressure treatment 1000 - 1200 ° C, that is, above the Curie point. 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 heating (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 P start, and then, due to the loss of steel magnetic properties, drops to the minimum D Pmin. To maintain heating in the 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 control the heating mode.

If we compare the through heating of workpieces for plastic deformation by induction and electrocontact methods (both are direct heating), then we can say that in terms of power consumption, electrocontact 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 rather cumbersome and is associated with the involvement 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 the modes of induction heating it follows that the same thickness of the hardened layer x k can be obtained at different values ​​of the power density D R and duration of heating t. The optimal mode is determined not only by the thickness of the layer x k, but also by the value 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. In the process of heating, the value of pc changes and by 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 overburning. In the presence of control devices, power D R may be equal to or even less than D Pmin(Fig. 3.5), which allows, due to the lengthening of 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 a high-frequency (hardening) transformer.

The power consumed from the network

determined by specific power consumption a(kWh/t) and productivity G(t/h):

for surface heating

,

(3.26)

where D i- increase 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 weight, kg;

S3- surface of the hardened layer, m 2;

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

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

The rest of the designations are explained above.

Approximate values ​​of the specific power consumption during induction heating: tempering - 120, hardening - 250, carburizing - 300, through heating for machining - 400 kWh / t.

Electrical calculation. The electrical calculation is based on dependence (3.7). 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 inductive conductor b(Fig. 3.1). For this case, the inductance L with system inductor - the product can be expressed by the formula

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

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

for the heated state

Inductor power factor

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

U and- voltage on 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 reactance of the transformer and the connecting conductors.

Example. Calculate the inductor and choose a high-frequency installation for surface hardening of cylindrical billets made of carbon steel with a diameter of d a= 30 mm and height h a= 90 mm. Hardened layer depth x k = 1mm, inductor voltage U and = 100 V. We find the recommended frequency according to the formula (218):

Hz.

We stop at the nearest usable frequency. f=67 kHz.

From the graph (Fig. 3.7) we take D R\u003d 400 W / cm 2.

By formula (3.33) we find al for cold state:

cm 2.

Accept a= 0.5 cm, then the diameter of the inductor

cm.

Length of inductive conductor

cm

Number of turns of the inductor

Inductor height

The power supplied to the inductor, according to

kW

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

Oscillatory generator power

kW.

We choose the high-frequency installation LPZ-2-67M, which has an oscillatory 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 higher (up to 10 kHz) - machine generators of the inductor type.

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

High frequency currents from 60 kHz and above are obtained exclusively with the help of lamp generators. Machines 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 generators for heating.

Heating generators are performed, as a rule, with self-excitation (self-excited generators). Compared to generators of independent excitation, they are simpler in design and have better energy and economic performance.

The schemes of lamp generators for heating do not fundamentally differ from radio engineering ones, but they have some features. These circuits do not require strict frequency stability, which greatly simplifies them. circuit diagram the simplest generator for induction heating is shown in Figure 3.10.

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

where R- reduced loop loss resistance.

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

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

Continuous oscillations in the oscillator occur when there is sufficient positive feedback from the grid to the circuit and certain conditions are met that relate the parameters of the lamp and the circuit.

Grid Feedback Coefficient

where U c , U to , U a- voltages 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 characteristics of the lamp.

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

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

Oscillations of the second kind are excited in generators. The anode current flows through the lamp in pulses, only during part (1/2-1/3) of the period. This reduces the constant component of the anode current, reduces the heating of the anode and increases the efficiency of the generator. The grid current also has a pulse shape. 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 a voltage drop across the grid resistance R g during the flow of the constant component of the grid current.

Generators for heating have a load that changes during the heating process, caused by a change in the electrical properties of the heated materials. To ensure the operation of the generator in the optimal mode, characterized by highest values output power and efficiency, installations are equipped with load matching devices. The optimal mode is achieved by selecting the appropriate value of the network 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 in the circuits, it is possible to adjust the resonant resistance of the circuit R a and change the voltage on the grid 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 Selkhoztekhnika enterprises.

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

A special place is occupied by surface hardening of parts. The ability to concentrate power in a given place of the part makes it possible to obtain a combination of the outer hardened layer with the 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 harden parts and tools to any required thickness, if necessary, processing only working surfaces;

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

3) the specific energy consumption is usually lower 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 a hardening and reduction of marriage;

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

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

Induction heating installations are selected according to the following main parameters: purpose, rated vibrational power, operating frequency. Plants manufactured by the industry have a standard power scale with the following steps: 0.16; 0.25; 0.40; 0.63; 1.0 kW and beyond when multiplying these numbers by 10, 100 and 1000.

Installations for induction heating have capacities from 1.0 to 1000 kW, including those with lamp generators up to 250 kW, and above - with machine generators. The operating frequency determined by the calculation is specified on the scale of frequencies permitted for use in electrothermy.

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

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

1. Explain the principle of induction heating. The scope of its application.

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

3. How is the heater winding done?

4. What are the benefits of a heater?

5. What is the phenomenon of the surface effect?

6. Where can the induction air heater be applied?

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

8. What determines the efficiency of a ring inductor?

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

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

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

12. What steel parameters are affected by temperature measurement?

Induction heating is a method of non-contact heating by high-frequency currents (eng. 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. Therefore, this is the heating of products made of conductive materials (conductors) by the magnetic field of inductors (sources of an alternating magnetic field). Induction heating is carried out as follows. An electrically conductive (metal, graphite) workpiece is placed in the so-called inductor, which is one or more 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 arises around the inductor. The electromagnetic field induces eddy currents in the workpiece. Eddy currents heat the workpiece under the action of Joule heat (see the Joule-Lenz law).

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

At a high frequency, eddy currents are displaced by the magnetic field formed by them into thin surface layers of the workpiece Δ ​​(Surface-effect), as a result of which their density increases sharply, and the workpiece is heated. The underlying layers of the 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 a factor of e relative to the current density on the workpiece surface, while 86.4% of heat is released in the skin layer (of the total heat release. The depth of the skin layer depends on the radiation frequency: the higher the frequency, the thinner skin layer It also depends on the relative magnetic permeability μ of the workpiece material.

For iron, cobalt, nickel and magnetic alloys at temperatures below the Curie point, μ has a value from several hundreds 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 skin depth for copper is about 0.25 mm, for iron ≈ 0.001 mm.

The inductor gets very hot during operation, as it absorbs its own radiation. In addition, it absorbs heat radiation from a hot workpiece. They make inductors from copper tubes cooled by water. Water is supplied by suction - this ensures safety in case of a burn 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 business.
Machining small parts that can be damaged by flame or arc heating.
Surface hardening.
Hardening and heat treatment of parts of complex shape.
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) medium, in a non-conductive liquid, in a vacuum.

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

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

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

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

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

It is easy to carry out local and selective heating.

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

Easy automation of equipment - heating and cooling cycles, temperature control and holding, feeding and removal of workpieces.

Induction heating units:

On installations with an operating frequency of up to 300 kHz, inverters 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, the range of medium and short waves), high-frequency installations are built on electronic tubes.

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

The inductor for heating small parts is small in size and small inductance, which leads to a decrease in the quality factor of the working resonant circuit at low frequencies and a decrease in efficiency, and also presents a danger to the master oscillator (the quality factor of the resonant circuit is proportional to L / C, the resonant circuit with a low quality factor is too good "pumped" with energy, forms a short circuit in the inductor and disables the master oscillator). To increase the quality factor of the oscillatory circuit, two ways are used:
- increasing the operating frequency, which leads to the complexity and cost of the installation;
- the use of ferromagnetic inserts in the inductor; pasting the inductor with panels 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 powerful generator lamps. Prior to World War I, induction heating was of limited use. At that time, high-frequency machine generators (works by V.P. Vologdin) or spark discharge installations were used as generators.

The generator circuit can, in principle, be any (multivibrator, RC generator, independently excited generator, various relaxation generators) that operates on a load in the form of an inductor coil and has sufficient power. It is also necessary that the oscillation frequency be sufficiently high.

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

The scheme is selected according to the following criteria: reliability; fluctuation 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 in total give a reduction in weight and dimensions, etc.

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

Disadvantages of three dots:

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

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

When the workpieces are changed (for example, from 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 keep the operating frequency. The principle of their action is as follows. Due to the use of coupled circuits and the weakening of the connection between them, a change in the inductance of the working circuit does not entail a strong change in the frequency of the frequency setting circuit. Radio transmitters are constructed according to the same principle.

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

A) constant frequency
b) constant power released in the workpiece
c) maximum 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 magnetization reversal process stops - the workpiece begins to heat up worse, the load resistance abruptly decreases - this can lead to the "spacing" of the generator and its failure. The control system monitors the transition through the Curie point and automatically increases the frequency with an abrupt decrease in load (or reduces power).

Remarks.

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

Increasing the frequency dramatically reduces the power factor (in proportion to the cube of the frequency).

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

When calculating the inductor, it is necessary to take into account the inductance of the tires 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 a single turn of a small diameter or even part of a turn - an arc).

There are two cases of resonance in oscillatory circuits: voltage resonance and current resonance.
Parallel oscillatory circuit - resonance of currents.
In this case, the voltage on the coil and on the capacitor is the same as that of the generator. At resonance, the resistance of the circuit between the branching points becomes maximum, and the current (I total) through the load resistance Rn 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 linear current (Itotal) increases.

Series oscillatory circuit - voltage resonance.

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

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