The induction furnace is no longer a new product - this invention has existed since the 19th century, but only in our time, with the development of technology and elemental base, is it finally beginning to enter everyday life everywhere. Previously, there were many questions about the intricacies of the operation of induction furnaces, not all physical processes were fully understood, and the units themselves had a lot of shortcomings and were used only in industry, mainly for melting metals.

Now, with the advent of powerful high-frequency transistors and cheap microcontrollers, which have made breakthroughs in all areas of science and technology, truly effective induction furnaces have appeared that can be freely used for household needs(cooking, heating water, heating) and even assemble it yourself.

Physical basis and principle of operation of the furnace

Fig.1. Induction furnace diagram

Before choosing or making an induction heater, you should understand what it is. IN lately There is a surge of interest in this topic, but few people have a full understanding of the physics of magnetic waves. This has given rise to many misconceptions, myths and a lot of ineffective or unsafe homemade products. You can make an induction furnace with your own hands, but before that you should get at least basic knowledge.

The operating principle of an induction stove is based on the phenomenon of electromagnetic induction. The key element here is the inductor, which is a high-quality inductor. Induction furnaces are widely used for heating or melting electrically conductive materials, most often metals, due to the thermal effect of inducing an eddy electric current into them. The diagram presented above illustrates the structure of this furnace (Fig. 1).

Generator G produces a variable frequency voltage. Under the influence of its electromotive force, an alternating current I 1 flows in the inductor coil L. Inductor L together with capacitor C represents an oscillatory circuit tuned to resonance with the frequency of the source G, due to which the efficiency of the furnace increases significantly.

In accordance with physical laws, an alternating magnetic field H appears in the space around the inductor L. This field can also exist in the air, but to improve the characteristics, special ferromagnetic cores that have better magnetic conductivity compared to air are sometimes used.

Power lines magnetic field pass through an object W placed inside the inductor and induce a magnetic flux F in it. If the material from which the workpiece W is made is electrically conductive, an induced current I 2 arises in it, closing inside and forming vortex induction flows. In accordance with the law of the thermal effect of electricity, eddy currents heat up the object W.

Making an inductive heater

An induction furnace consists of two main functional blocks: an inductor (heating induction coil) and a generator (source AC voltage). The inductor is a bare copper tube, rolled into a spiral (Fig. 2).

To make a furnace with a power of no more than 3 kW with your own hands, the inductor must be made with the following parameters:

• tube diameter – 10 mm;
• spiral diameter – 8-15 cm;
• number of coil turns – 8-10;
• the distance between the turns is 5-7 mm;
• The minimum clearance in the screen is 5 cm.

Do not allow adjacent turns of the coil to come into contact; maintain the specified distance. The inductor must not in any way come into contact with the protective screen of the furnace; the gap between them must be no less than specified.

Generator manufacturing

Fig.3. Lamp circuit

It is worth noting that an induction furnace for its manufacture requires at least average radio engineering skills and abilities. It is especially important to have them to create the second key element - a high-frequency current generator. You won’t be able to assemble or use a home-made stove without this knowledge. Moreover, it can be life-threatening.

For those who take on this matter with knowledge and understanding of the process, there are various ways and diagrams according to which an induction furnace can be assembled. When choosing a suitable generator circuit, it is recommended to abandon options with a hard radiation spectrum. These include a widely used circuit using a thyristor switch. High-frequency radiation from such a generator can create powerful interference for all surrounding radio devices.

Since the mid-20th century, an induction furnace assembled with 4 lamps has enjoyed great success among radio amateurs. Its quality and efficiency are far from the best, and radio tubes are difficult to obtain nowadays, however, many continue to assemble generators using this particular design, since it has a great advantage: a soft, narrow-band spectrum of the generated current, thanks to which such a furnace emits a minimum of interference and is as safe as possible (Fig. 3).

The operating mode of this generator is adjusted using a variable capacitor C. The capacitor must have an air dielectric, the gap between its plates must be at least 3 mm. The diagram also contains a neon lamp L, which serves as an indicator.

Universal generator circuit

Modern induction furnaces operate on more advanced elements - microcircuits and transistors. Enjoys great success universal scheme two-stroke generator, developing power up to 1 kW. The operating principle is based on an independent excitation generator, with the inductor switched on in bridge mode (Fig. 4).

Advantages of a push-pull generator assembled according to this scheme:

1. Ability to work on the 2nd and 3rd modes in addition to the main one.
2. There is a surface heating mode.
3. Regulation range 10-10000 kHz.
4. Soft emission spectrum throughout the entire range.
5. Does not require additional protection.

Frequency adjustment is carried out using a variable resistor R2. The operating frequency range is set by capacitors C 1 and C 2. The interstage matching transformer must have a ring ferrite core with a cross-section of at least 2 sq.cm. The winding of the transformer is made of enameled wire with a cross section of 0.8-1.2 mm. Transistors must be placed on a common radiator with an area of ​​400 sq.cm.

Conclusion on the topic

The electromagnetic field (EMF) emitted by an inductor stove affects all conductors around it. This includes effects on the human body. Under the influence of EMF, the internal organs are evenly warmed up, and the overall body temperature rises throughout the entire volume.

Therefore, when working with the stove, it is important to take certain precautions to avoid negative consequences.

First of all, the generator housing must be shielded using a casing made of galvanized iron sheets or a mesh with small cells. This will reduce the radiation intensity by 30-50 times.

It should also be borne in mind that in the immediate vicinity of the inductor the energy flux density will be higher, especially along the winding axis. Therefore, the induction coil should be positioned vertically, and it is better to observe the heating from afar.

The very principle of operation of an induction furnace is that heat for melting is obtained from electricity, which is generated by an alternating magnetic field. In such furnaces, energy is converted from electromagnetic, then into electrical and ultimately into heat. How to make an induction furnace with your own hands?

Such furnaces are divided into two types:

1. Crucible. In such furnaces, the inductor and the core are located inside the metal. This type of furnace is used in industrial smelters for melting copper, aluminum, cast iron, steel, as well as in jewelry factories for melting precious metals.
2. Duct. In this type of furnace, the inductor and the core are located around the metal.

Compared to boilers or other stoves, induction stoves have a number of advantages:

• instantly warm up;
• focus energy in a given range;
• environmentally friendly device and relative safety;
• there is no waste;
• huge possibilities for adjusting temperature and capacity;
• homogeneity of the metal that is melting.

Induction furnaces are also used for heating. This is a convenient and at the same time silent heating method.

Does not require a special room for the boiler. Scale does not accumulate on the heating element, and any liquid can be used for circulation through the heating system, be it oil, water, or others. The stove is also durable, as it wears out minimally. As mentioned earlier, it is very environmentally friendly, because there are no harmful emissions into the air, and also meets all fire safety requirements.

Collection of information

A person who understands how to read and understand an electrical diagram will not find it difficult to figure out how to make an induction furnace like this. On the Internet you will see dozens, or even hundreds of options for making various induction furnaces using household trash, for example, from an old microwave or welding inverter.

Be sure to remember that electric current is a dangerous thing. And to make an induction furnace, you need to have an idea of ​​what heating using induction is. It is advisable to have someone with you who has a good understanding of at least the basics of electrical engineering or has experience working with electrical equipment.

Operating principle

The basis of the operation of such a stove is the extraction of heat from an electric current, which produces an alternating magnetic field using an inductor. It turns out that we get heat first from electromagnetic energy, and then from electrical energy. The closedness of the currents that flow through the turns of the inductor (inductor) generates heat and warms the metal from the inside.

Such a stove can have a simplified version and run from a 220V home network. But this requires a rectifier, that is, an adapter.

Furnace structure

The design of an induction device is similar to a transformer. In it, the primary winding is powered by alternating current, and the secondary winding serves as a heated body.

The simplest inductor is considered to be an insulated conductor (shaped like a spiral or core), which is located on the surface metal pipe or inside it.

Here are some nodes that work by induction:

• inductor;
• melting furnace compartment;
• a heating element for a heating furnace;
• generator;
• frame.

Nowadays, stoves operating on inductive principle which are commonly used in industry. In order for induction stoves to be used in domestic conditions, their design was significantly transformed; only the principle of energy conversion remained unchanged. Such a device can be made with your own hands from available materials. The main thing is to understand the design and understand how this stove works.

Working principle of induction furnace

The operation of such a furnace is based on the principle of induction heating. In other words, thermal energy is obtained from the electric current generated by the electromagnetic field. Thanks to this feature, this device differs from ordinary electric heaters.

The design of the inductor is quite simple. Its center is graphite or metal an electrically conductive piece around which the wire should be wound. Using the power of the generator, currents of different frequencies begin to be launched into the inductor, creating a powerful electromagnetic field around the inductor. Due to the effect of such a field on the workpiece and the creation of eddy currents in it, the graphite or metal begins to heat up very much and gives off heat to the surrounding air.

Inductors began to be used in everyday life relatively recently.

Types of induction devices

According to their purpose, such devices are domestic and industrial. However, such a classification is considered incomplete. There is also several types of stoves:

• Crucible. The most common type of units used in metallurgy. This design does not contain a core. These devices are mainly used for processing and melting any metals. They have proven themselves remarkably well in other areas.
• Duct. Their design is similar to a transformer.
• Vacuum. They are used when it is necessary to remove impurities from metal.

Household ovens are divided into two groups:

• Units used for heating. They are small induction boiler units that are installed in autonomous heating systems.
• Induction cookers for cooking food. The main difference from an ordinary electric stove is the economical consumption of electricity.

Is it possible to make an induction furnace designed for melting metals with your own hands? Although it is, on the one hand, a complex piece of equipment, on the other hand, due to the relative simplicity and clarity of the operating principle, it becomes possible to make an induction heating device with your own hands. In addition, many specialists with the necessary knowledge and skills are able to create high-quality units from ordinary materials. To make an induction furnace with your own hands, you will need a diagram and a good knowledge of physics.

Do-it-yourself induction furnaces mainly used for heating rooms. Small crucible structures are best suited for melting metals in small volumes, for example, in the manufacture of jewelry or jewelry. Induction hobs are considered an excellent solution for country houses. And in a city apartment they are used as an additional heater if there is any failure in the central system heating.

A circuit diagram of such a simple induction heater will be needed to complete the work. You can work without it, but it is undesirable, since such a heater is complex electrical equipment. Its design and internal contents are developed in advance. The scheme combines all the master’s ideas into a single whole. If you need to design a stove rather than a simple heater, you won’t be able to do without a diagram at all.

The design of a do-it-yourself induction furnace is quite simple: a heating element, a common housing, an inductor. If the unit is required for processing materials, a melting chamber should be additionally designed. The heart of an induction furnace is a workpiece that conducts current and can heat up to high temperatures. A nichrome spiral or graphite brushes do an excellent job of this task. When choosing between them, you should focus on the tasks that the heater faces. For a melting furnace, the best option would be to use graphite brushes, for a heating device - a nichrome spiral. The use of nichrome makes it possible to connect the unit to a regular electrical network.

How to make an induction oven with your own hands

To create an effective unit it is necessary take into account the following parameters:

• generator frequency and power;
• the rate at which heat is lost;
• amount of losses in eddy currents.

First, you need to correctly select all the necessary details of the circuit to obtain sufficient conditions for melting in the workshop. If the unit is assembled with your own hands, the generator frequency should be 27.12 MHz. The coil should be made of wire or thin copper tube, and there should not be more than 10 turns.

Power vacuum tubes should be big. The scheme provides for the installation of a neon lamp, which will be used as an indicator of the readiness of the device. The circuit also provides for the use of chokes and ceramic capacitors. The connection to the home outlet is made through a rectifier.

A home-made induction furnace looks like this: a small stand on legs to which a generator with all the necessary circuit parts is attached. And the inductor is connected to the generator.

Advantages and disadvantages of induction furnaces

Induction units can have different powers and this depends on the design features. It is very difficult to assemble an industrial frequency device with your own hands, and this is not necessary. It's better to buy them.

Induction furnaces may have both pros and cons:

Safety

When working with the stove, you should be careful of getting thermal burns. In addition, such a device has high fire danger. These units must not be moved under any circumstances during operation. You need to be very careful when such stoves are installed in an apartment.

Alternating electromagnetic field begins to heat up the room surrounding it, and this feature is directly dependent on the power and frequency of the device’s radiation. Powerful industrial ovens can impact items in clothing pockets, nearby metal parts, and human tissue.

Conclusion

You can make an induction furnace yourself, but this is not always advisable. It is better not to take on such work if you have absolutely no knowledge in the field of electrical equipment and physics. Before starting to design even the most simple device, it should be developed, designed and diagrammed. If you have no experience in manufacturing electrical appliances, it is best to purchase such a factory-made unit.

© When using site materials (quotes, images), the source must be indicated.

The induction furnace was invented a long time ago, back in 1887, by S. Farranti. The first industrial installation started operating in 1890 at the Benedicks Bultfabrik company. For a long time, induction furnaces were exotic in the industry, but not due to the high cost of electricity; then it was no more expensive than now. There were still many unknowns in the processes occurring in induction furnaces, and element base electronics did not allow the creation effective schemes managing them.

In the induction furnace industry, a revolution has occurred literally before our eyes today, thanks to the emergence, firstly, of microcontrollers, the computing power of which exceeds that of personal computers ten years ago. Secondly, thanks to... mobile communications. Its development required the availability of inexpensive transistors capable of delivering power of several kW per high frequencies. They, in turn, were created on the basis of semiconductor heterostructures, for the research of which Russian physicist Zhores Alferov received the Nobel Prize.

Ultimately, induction stoves not only completely transformed the industry, but also became widely used in everyday life. Interest in the subject gave rise to a lot of homemade products, which, in principle, could be useful. But most authors of designs and ideas (there are many more descriptions of which in the sources than functional products) have a poor understanding of both the basics of the physics of induction heating and the potential danger of poorly executed designs. This article is intended to clarify some of the more confusing points. The material is based on consideration of specific structures:

1. An industrial channel furnace for melting metal, and the possibility of creating it yourself.
2. Induction-type crucible furnaces, the simplest to use and the most popular among home-made furnaces.
3. Induction hot water boilers are rapidly replacing boilers with heating elements.
4. Household induction cookers competing with gas stoves and in a number of parameters superior to microwaves.

Note: All devices under consideration are based on magnetic induction created by an inductor (inductor), and therefore are called induction. Only electrically conductive materials, metals, etc. can be melted/heated in them. There are also electric induction capacitive furnaces, based on electrical induction in the dielectric between the capacitor plates; they are used for “gentle” melting and electrical heat treatment of plastics. But they are much less common than inductor ones; consideration of them requires a separate discussion, so we’ll leave them for now.

Operating principle

The operating principle of an induction furnace is illustrated in Fig. right. In essence, it is an electrical transformer with a short-circuited secondary winding:

• The alternating voltage generator G creates an alternating current I1 in the inductor L (heating coil).
• Capacitor C together with L form an oscillatory circuit tuned to the operating frequency, this in most cases increases the technical parameters of the installation.
• If the generator G is self-oscillating, then C is often excluded from the circuit, using the inductor’s own capacitance instead. For the high-frequency inductors described below, it is several tens of picofarads, which exactly corresponds to the operating frequency range.
• In accordance with Maxwell's equations, the inductor creates an alternating magnetic field with intensity H in the surrounding space. The magnetic field of the inductor can either be closed through a separate ferromagnetic core or exist in free space.
• The magnetic field, penetrating the workpiece (or melting charge) W placed in the inductor, creates a magnetic flux F in it.
• F, if W is electrically conductive, induces a secondary current I2 in it, then the same Maxwell equations.
• If Ф is sufficiently massive and solid, then I2 closes inside W, forming an eddy current, or Foucault current.
• Eddy currents, according to the Joule-Lenz law, release the energy they receive through the inductor and the magnetic field from the generator, heating the workpiece (charge).

Electromagnetic interaction from the point of view of physics is quite strong and has a fairly high long-range effect. Therefore, despite the multi-stage energy conversion, an induction furnace is capable of showing an efficiency of up to 100% in air or vacuum.

Note: in a medium made of a non-ideal dielectric with a dielectric constant >1, the potentially achievable efficiency of induction furnaces drops, and in a medium with a magnetic permeability >1, it is easier to achieve high efficiency.

Channel furnace

The channel induction melting furnace is the first one used in industry. It is structurally similar to a transformer, see fig. right:

1. The primary winding, powered by a current of industrial (50/60 Hz) or high (400 Hz) frequency, is made of a copper tube cooled from the inside by a liquid coolant;
2. Secondary short-circuited winding – melt;
3. A ring-shaped crucible made of heat-resistant dielectric in which the melt is placed;
4. Magnetic circuit assembled from transformer steel plates.

Channel furnaces are used for melting duralumin, non-ferrous special alloys, and producing high-quality cast iron. Industrial channel furnaces require priming with a melt, otherwise the “secondary” will not short-circuit and there will be no heating. Or arc discharges will appear between the crumbs of the charge, and the entire melt will simply explode. Therefore, before starting the furnace, a little melt is poured into the crucible, and the remelted portion is not poured completely. Metallurgists say that a channel furnace has residual capacity.

A channel furnace with a power of up to 2-3 kW can be made from an industrial frequency welding transformer yourself. In such a furnace you can melt up to 300-400 g of zinc, bronze, brass or copper. You can melt duralumin, but the casting needs to be allowed to age after cooling, from several hours to 2 weeks, depending on the composition of the alloy, so that it gains strength, toughness and elasticity.

Note: duralumin was actually invented by accident. The developers, angry that they could not alloy aluminum, abandoned another “nothing” sample in the laboratory and went on a spree out of grief. We sobered up, returned - and no one had changed color. They checked it - and it gained the strength of almost steel, while remaining as light as aluminum.

The “primary” of the transformer is left standard; it is already designed to operate in the short-circuit mode of the secondary with a welding arc. The “secondary” is removed (it can then be put back and the transformer can be used for its intended purpose), and a ring crucible is put in its place. But trying to convert an HF welding inverter into a channel furnace is dangerous! Its ferrite core will overheat and shatter into pieces due to the fact that the dielectric constant of ferrite is >>1, see above.

The problem of residual capacity in a low-power furnace disappears: a wire of the same metal, bent into a ring and with twisted ends, is placed in the seeding charge. Wire diameter – from 1 mm/kW furnace power.

But a problem arises with a ring crucible: the only material suitable for a small crucible is electroporcelain. It is impossible to process it yourself at home, but where can you get a suitable one? Other refractories are not suitable due to high dielectric losses in them or porosity and low mechanical strength. Therefore, although a channel furnace produces smelting of the highest quality, does not require electronics, and its efficiency already at a power of 1 kW exceeds 90%, they are not used by home-made people.

For a regular crucible

The residual capacity irritated metallurgists - the alloys they melted were expensive. Therefore, as soon as sufficiently powerful radio tubes appeared in the 20s of the last century, an idea was immediately born: throw a magnetic circuit onto (we will not repeat the professional idioms of tough men), and put an ordinary crucible directly into the inductor, see fig.

You can’t do this at an industrial frequency; a low-frequency magnetic field without a magnetic circuit concentrating it will spread out (this is the so-called stray field) and give off its energy anywhere, but not into the melt. The stray field can be compensated by increasing the frequency to a high one: if the diameter of the inductor is commensurate with the wavelength of the operating frequency, and the entire system is in electromagnetic resonance, then up to 75% or more of the energy of its electromagnetic field will be concentrated inside the “heartless” coil. The efficiency will be corresponding.

However, already in the laboratories it became clear that the authors of the idea overlooked an obvious circumstance: the melt in the inductor, although diamagnetic, is electrically conductive, due to its own magnetic field from eddy currents, it changes the inductance of the heating coil. The initial frequency had to be set under the cold charge and changed as it melted. Moreover, the range is greater, the larger the workpiece: if for 200 g of steel you can get by with a range of 2-30 MHz, then for a blank the size of a railway tank, the initial frequency will be about 30-40 Hz, and the operating frequency will be up to several kHz.

It is difficult to make suitable automation on lamps; to “pull” the frequency behind the blank requires a highly qualified operator. In addition, the stray field manifests itself most strongly at low frequencies. The melt, which in such a furnace is also the core of the coil, to some extent collects a magnetic field near it, but still, to obtain acceptable efficiency it was necessary to surround the entire furnace with a powerful ferromagnetic screen.

Nevertheless, due to their outstanding advantages and unique qualities (see below), crucible induction furnaces are widely used both in industry and by home-made people. Therefore, let’s take a closer look at how to properly make one with your own hands.

A little theory

When designing a homemade “induction”, you need to firmly remember: the minimum power consumption does not correspond to the maximum efficiency, and vice versa. The stove will take the minimum power from the network when operating at the main resonant frequency, Pos. 1 in Fig. In this case, the blank/charge (and at lower, pre-resonant frequencies) operates as one short-circuited turn, and only one convective cell is observed in the melt.

In the main resonance mode, up to 0.5 kg of steel can be melted in a 2-3 kW furnace, but heating the charge/workpiece will take up to an hour or more. Accordingly, the total electricity consumption from the network will be high, and the overall efficiency will be low. At pre-resonant frequencies it is even lower.

As a result, induction furnaces for melting metal most often operate at the 2nd, 3rd, and other higher harmonics (Pos. 2 in the figure). The power required for heating/melting increases; for the same half a kilo of steel, the 2nd one will need 7-8 kW, and the 3rd one 10-12 kW. But warming up occurs very quickly, in minutes or fractions of minutes. Therefore, the efficiency is high: the stove does not have time to “eat” much before the melt can be poured.

Furnaces using harmonics have the most important, even unique advantage: several convective cells appear in the melt, instantly and thoroughly mixing it. Therefore, it is possible to conduct melting in the so-called mode. rapid charge, producing alloys that are fundamentally impossible to smelt in any other melting furnaces.

If you “raise” the frequency 5-6 or more times higher than the main one, then the efficiency drops somewhat (not much), but another thing appears wonderful property induction on harmonics: surface heating due to the skin effect, displacing EMF to the surface of the workpiece, Pos. 3 in Fig. This mode is rarely used for melting, but for heating workpieces for surface cementation and hardening it is a nice thing. Modern technology would be simply impossible without this method of heat treatment.

About levitation in an inductor

Now let’s do a trick: wind the first 1-3 turns of the inductor, then bend the tube/bus 180 degrees, and wind the rest of the winding in the opposite direction (Pos. 4 in the figure). Connect it to the generator, insert a crucible in the charge into the inductor, and give current. Let's wait until it melts and remove the crucible. The melt in the inductor will gather into a sphere, which will remain hanging there until we turn off the generator. Then it will fall down.

The effect of electromagnetic levitation of the melt is used to purify metals by zone melting, to obtain high-precision metal balls and microspheres, etc. But for a proper result, melting must be carried out in a high vacuum, so here levitation in the inductor is mentioned only for information.

Why an inductor at home?

As you can see, even a low-power induction stove for apartment wiring and consumption limits is too powerful. Why is it worth doing it?

Firstly, for the purification and separation of precious, non-ferrous and rare metals. Take, for example, an old Soviet radio connector with gold-plated contacts; They didn’t spare gold/silver for plating back then. We put the contacts in a narrow, high crucible, put them into the inductor, and melt them at the main resonance (professionally speaking, at the zero mode). After melting, we gradually reduce the frequency and power, allowing the blank to harden for 15 minutes to half an hour.

Once it cools down, we break the crucible and what do we see? A brass post with a clearly visible gold tip that just needs to be cut off. Without mercury, cyanide and other deadly reagents. This cannot be achieved by heating the melt from the outside in any way; convection in it will not do so.

Well, gold is gold, and now there is no black scrap metal lying on the road. But the need for uniform or precisely dosed heating of metal parts over the surface/volume/temperature for high-quality hardening will always be found by a homemaker or individual entrepreneur. And here again the inductor stove will help out, and the electricity consumption will be feasible for the family budget: after all, the main share of heating energy comes from the latent heat of melting of the metal. And by changing the power, frequency and location of the part in the inductor, you can heat exactly the right place exactly as it should, see fig. higher.

Finally, by making a specially shaped inductor (see figure on the left), you can release the hardened part into in the right place, on breaking carburization with hardening at the end/ends. Then, where necessary, use bending, ivy, and the rest remains hard, viscous, elastic. At the end, you can reheat it again where it was released and harden it again.

Let's get to the stove: what you need to know

An electromagnetic field (EMF) affects the human body, at least warming it up in its entirety, like meat in a microwave. Therefore, when working with an induction furnace as a designer, craftsman or operator, you need to clearly understand the essence of the following concepts:

PES – electromagnetic field energy flux density. Determines the general physiological impact of EMF on the body, regardless of the frequency of radiation, because The PES of an EMF of the same intensity increases with increasing radiation frequency. By sanitary standards different countries permissible PES value is from 1 to 30 mW per 1 sq. m. of body surface with constant (more than 1 hour per day) exposure and three to five times more with a single short-term, up to 20 minutes.

Note: The USA stands apart; its permissible power consumption is 1000 mW (!) per square meter. m. body. In fact, Americans consider the beginning of physiological effects to be external manifestations, when a person already becomes ill, and the long-term consequences of EMF exposure are completely ignored.

The PES decreases with distance from a point source of radiation by the square of the distance. Single-layer shielding with galvanized or fine-mesh galvanized mesh reduces the PES by 30-50 times. Near the coil along its axis, the PES will be 2-3 times higher than at the side.

Let's explain with an example. There is a 2 kW and 30 MHz inductor with an efficiency of 75%. Therefore, 0.5 kW or 500 W will go out of it. At a distance of 1 m from it (the area of ​​a sphere with a radius of 1 m is 12.57 sq. m.) per 1 sq. m. will have 500/12.57 = 39.77 W, and per person - about 15 W, this is a lot. The inductor must be positioned vertically, before turning on the furnace, put a grounded shielding cap on it, monitor the process from a distance, and immediately turn off the furnace when it is completed. At a frequency of 1 MHz, the PES will drop by a factor of 900, and a shielded inductor can be operated without special precautions.

Microwave – ultra high frequencies. In radio electronics, microwave frequencies are considered to be so-called. Q-band, but according to microwave physiology it starts at about 120 MHz. The reason is electrical induction heating of cell plasma and resonance phenomena in organic molecules. Microwave has a specifically targeted biological effect with long-term consequences. It is enough to receive 10-30 mW for half an hour to undermine health and/or reproductive capacity. Individual susceptibility to microwaves is extremely variable; When working with him, you need to regularly undergo a special medical examination.

It is very difficult to suppress microwave radiation; as the pros say, it “siphons” through the slightest crack in the screen or with the slightest violation of the grounding quality. Effective fight with microwave radiation of equipment is possible only at the level of its design by highly qualified specialists.

The most important part of an induction furnace is its heating coil, the inductor. For homemade stoves For a power of up to 3 kW, an inductor made of a bare copper tube with a diameter of 10 mm or a bare copper bus with a cross-section of at least 10 square meters will be used. mm. The internal diameter of the inductor is 80-150 mm, the number of turns is 8-10. The turns should not touch, the distance between them is 5-7 mm. Also, no part of the inductor should touch its shield; the minimum gap is 50 mm. Therefore, in order to pass the coil leads to the generator, it is necessary to provide a window in the screen that does not interfere with its removal/installation.

The inductors of industrial furnaces are cooled with water or antifreeze, but at a power of up to 3 kW, the inductor described above does not require forced cooling when operating for up to 20-30 minutes. However, it itself becomes very hot, and scale on copper sharply reduces the efficiency of the furnace until it loses its functionality. It is impossible to make a liquid-cooled inductor yourself, so it will have to be changed from time to time. You cannot use forced air cooling: the plastic or metal fan housing near the coil will “attract” EMFs to itself, overheat, and the efficiency of the furnace will drop.

Note: for comparison, an inductor for a melting furnace for 150 kg of steel is bent from a copper pipe with an outer diameter of 40 mm and an inner diameter of 30 mm. The number of turns is 7, the inside diameter of the coil is 400 mm, and the height is also 400 mm. To power it up in zero mode, you need 15-20 kW in the presence of a closed cooling circuit with distilled water.

Generator

The second main part of the furnace is the alternator. It’s not worth even trying to make an induction furnace without knowing the basics of radio electronics at least at the level of an average radio amateur. Operating is the same, because if the stove is not under computer control, you can set it to mode only by feeling the circuit.

When choosing a generator circuit, you should in every possible way avoid solutions that give a hard current spectrum. As an anti-example, we present a fairly common circuit using a thyristor switch, see Fig. higher. A calculation available to a specialist based on the oscillogram attached to it by the author shows that the PES at frequencies above 120 MHz from an inductor powered in this way exceeds 1 W/sq. m at a distance of 2.5 m from the installation. Deadly simplicity, to say the least.

As a nostalgic curiosity, we also present a diagram of an ancient tube generator, see fig. right. These were made by Soviet radio amateurs back in the 50s, Fig. right. Setting to mode - with an air capacitor of variable capacitance C, with a gap between the plates of at least 3 mm. Works only on zero mode. The setting indicator is a neon light bulb L. The peculiarity of the circuit is a very soft, “lamp” radiation spectrum, so this generator can be used without special precautions. But - alas! – you can’t find lamps for it now, and with a power in the inductor of about 500 W, the power consumption from the network is more than 2 kW.

Note: The frequency of 27.12 MHz indicated in the diagram is not optimal; it was chosen for reasons of electromagnetic compatibility. In the USSR, it was a free (“junk”) frequency, for which permission was not required to operate, as long as the device did not interfere with anyone. In general, C the generator can be tuned in a fairly wide range.

In the next fig. left - simple generator with self-excitation. L2 – inductor; L1 – coil feedback, 2 turns of enameled wire with a diameter of 1.2-1.5 mm; L3 – blank or charge. The inductor's own capacitance is used as a loop capacitance, so this circuit does not require adjustment, it automatically enters the zero mode mode. The spectrum is soft, but if the phasing of L1 is incorrect, the transistor instantly burns out, because it turns out to be in active mode with a DC short circuit in the collector circuit.

Also, the transistor can burn out simply from changing outside temperature or self-heating of the crystal - no measures are provided to stabilize its mode. In general, if you have old KT825 or similar ones lying around somewhere, then you can start experiments on induction heating with this circuit. The transistor must be installed on a radiator with an area of ​​at least 400 square meters. see with blowing from a computer or similar fan. Adjustment of the capacity in the inductor, up to 0.3 kW, by changing the supply voltage within 6-24 V. Its source must provide a current of at least 25 A. The power dissipation of the resistors of the basic voltage divider is at least 5 W.

The diagram follows. rice. on the right is a multivibrator with an inductive load using powerful field-effect transistors (450 V Uk, at least 25 A Ik). Thanks to the use of capacitance in the oscillatory circuit circuit, it produces a rather soft spectrum, but out-of-mode, therefore suitable for heating parts up to 1 kg for quenching/tempering. Main disadvantage circuits - the high cost of components, powerful field switches and high-speed (cutoff frequency of at least 200 kHz) high-voltage diodes in their base circuits. Bipolar power transistors in this circuit do not work, overheat and burn out. The radiator here is the same as in the previous case, but airflow is no longer needed.

The following scheme already claims to be universal, with a power of up to 1 kW. This is a push-pull generator with independent excitation and bridge-connected inductor. Allows you to work in mode 2-3 or in surface heating mode; the frequency is regulated by a variable resistor R2, and the frequency ranges are switched by capacitors C1 and C2, from 10 kHz to 10 MHz. For the first range (10-30 kHz), the capacitance of capacitors C4-C7 should be increased to 6.8 μF.

The transformer between the stages is on a ferrite ring with a cross-sectional area of ​​the magnetic core of 2 square meters. see Windings - made of enameled wire 0.8-1.2 mm. Transistor radiator – 400 sq. see for four with airflow. The current in the inductor is almost sinusoidal, so the radiation spectrum is soft and no additional protective measures are required at all operating frequencies, provided that it works for up to 30 minutes a day after 2 days on the 3rd.

Video: homemade induction heater in action

Induction boilers

Induction hot water boilers will undoubtedly replace boilers with heating elements wherever electricity is cheaper than other types of fuel. But their undeniable advantages have also given rise to a lot of homemade products, which sometimes literally make a specialist’s hair stand on end.

Let's say this construction: propylene pipe With running water surrounds the inductor, and it is powered by a 15-25 A HF welding inverter. An option is to make a hollow donut (torus) from heat-resistant plastic, pass water through the pipes, and wrap it in a tire for heating, forming an inductor rolled into a ring.

EMF will transfer its energy to water well; It has good electrical conductivity and an abnormally high (80) dielectric constant. Remember how the remaining droplets of moisture on the dishes shoot out in the microwave.

But, firstly, to fully heat an apartment in winter, you need at least 20 kW of heat, with careful insulation from the outside. 25 A at 220 V provide only 5.5 kW (how much does this electricity cost according to our tariffs?) with 100% efficiency. Okay, let's say we're in Finland, where electricity is cheaper than gas. But the consumption limit for housing is still 10 kW, and for excess you have to pay at an increased tariff. And the apartment wiring will not withstand 20 kW; you need to pull a separate feeder from the substation. How much will such work cost? If the electricians are still far from overpowering the area, they will allow it.

Then, the heat exchanger itself. It should either be massive metal, then only induction heating of the metal will work, or made of plastic with low dielectric losses (propylene, by the way, is not one of these, only expensive fluoroplastic is suitable), then the water will directly absorb the EMF energy. But in any case, it turns out that the inductor heats the entire volume of the heat exchanger, and only its inner surface transfers heat to the water.

As a result, at the cost of a lot of work and risk to health, we get a boiler with the efficiency of a cave fire.

An industrial induction heating boiler is designed in a completely different way: simple, but impossible to do at home, see fig. right:

• The massive copper inductor is connected directly to the network.
• Its EMF also heats a massive metal labyrinth-heat exchanger made of ferromagnetic metal.
• The labyrinth simultaneously isolates the inductor from water.

Such a boiler costs several times more than a conventional one with a heating element, and is suitable only for installation on plastic pipes, but in return it provides a lot of benefits:

1. It never burns out - there is no hot electric coil in it.
2. The massive labyrinth reliably shields the inductor: PES in the immediate vicinity of the 30 kW induction boiler is zero.
3. Efficiency – more than 99.5%
4. Absolutely safe: the intrinsic time constant of the high-inductance coil is more than 0.5 s, which is 10-30 times longer than the response time of the RCD or machine. It is further accelerated by the “recoil” from the transient process when the inductance breaks down on the housing.
5. The breakdown itself, due to the “oakiness” of the structure, is extremely unlikely.
6. Does not require separate grounding.
7. Indifferent to lightning strikes; It cannot burn a massive coil.
8. The large surface of the labyrinth ensures effective heat exchange with a minimal temperature gradient, which almost eliminates the formation of scale.
9. Enormous durability and ease of use: the induction boiler, together with a hydromagnetic system (HMS) and a sediment filter, operates without maintenance for at least 30 years.

About homemade boilers for hot water supply

Here in Fig. shows a diagram of a low-power induction heater for DHW systems with storage tank. It is based on any power transformer of 0.5-1.5 kW with a primary winding of 220 V. Dual transformers from old tube color TVs - “coffins” on a two-rod magnetic core of the PL type - are very suitable.

The secondary winding is removed from such windings, the primary is rewound onto one rod, increasing the number of its turns to operate in a mode close to a short circuit (short circuit) in the secondary. The secondary winding itself is water in a U-shaped pipe bend that encloses another rod. Plastic pipe or metal - at industrial frequency it makes no difference, but the metal must be isolated from the rest of the system with dielectric inserts, as shown in Fig., so that the secondary current is closed only through water.

In any case, such a water heater is dangerous: a possible leak is adjacent to the winding under mains voltage. If you are going to take such a risk, then you need to drill a hole in the magnetic circuit for the grounding bolt, and first of all, tightly ground the transformer and the tank with a steel busbar of at least 1.5 square meters. cm (not sq. mm!).

Next, the transformer (it should be located directly under the tank), with a double-insulated power cable connected to it, a ground electrode and a water-heating coil, is poured into one “doll” with silicone sealant, like a pump motor aquarium filter. Finally, it is highly advisable to connect the entire unit to the network via a high-speed electronic RCD.

Video: “induction” boiler based on household tiles

Inductor in the kitchen

Induction hobs have become commonplace in the kitchen, see fig. According to the principle of operation, this is the same induction stove, only the bottom of any metal cooking vessel acts as a short-circuited secondary winding, see fig. on the right, and not just from ferromagnetic material, as the ignorant often write. Just aluminum cookware goes out of use; doctors have proven that free aluminum is a carcinogen, and copper and tin have long been out of use due to toxicity.

Household induction hob- a product of the age of high technology, although its idea originated simultaneously with induction melting furnaces. Firstly, to isolate the inductor from the cooking, a durable, resistant, hygienic and EMF-free dielectric was needed. Suitable glass-ceramic composites have come into production relatively recently, and the top plate of the slab accounts for a significant portion of its cost.

Then, all cooking vessels are different, and their contents change their electrical parameters, and the cooking modes are also different. A specialist will not be able to do this by carefully tightening the knobs to the desired fashion; you need a high-performance microcontroller. Finally, according to sanitary requirements, the current in the inductor must be a pure sine wave, and its magnitude and frequency must vary in a complex way according to the degree of readiness of the dish. That is, the generator must have digital generation of the output current, controlled by the same microcontroller.

There is no point in making a kitchen induction hob yourself: the electronic components alone at retail prices will cost more money than a finished one good tiles. And it’s still quite difficult to control these devices: anyone who has one knows how many buttons or sensors there are with the inscriptions: “Stew”, “Roast”, etc. The author of this article saw a tile that separately listed “Navy Borscht” and “Pretanier Soup.”

However, induction cookers have many advantages over others:

• Almost zero, unlike microwave ovens, PPE, even if you sit on this tile yourself.
• Possibility of programming for preparing the most complex dishes.
• Melting chocolate, rendering fish and poultry fat, preparing caramel without the slightest sign of burning.
• High efficiency as a result of fast heating and almost complete concentration of heat in the cooking vessel.

To the last point: take a look at fig. on the right, there are schedules for heating up cooking on an induction stove and a gas burner. Anyone who is familiar with integration will immediately understand that an inductor is 15-20% more economical, and there is no need to compare it with a cast-iron “pancake”. The cost of money on energy when preparing most dishes for an induction cooker is comparable to that of a gas cooker, and even less for stewing and cooking thick soups. The inductor is so far inferior to gas only during baking, when uniform heating is required on all sides.

Features of induction furnaces

• Advantages
• Flaws

• Furnace Basics

Use of stoves in everyday life

• Fireplaces planika
• Guca ovens
• Thorma stoves

Video on the topic

Related articles

Leave a comment on the article

Nowadays, this type of furnace with an induction system is widely in demand in the process of metal smelting. With the help of current, the process of heating the metal occurs; this is not only the main, and perhaps the main, feature of such furnaces. The product of current occurs in the field of the inductor. During the processing process, electric current goes through several stages of transformation. The first stage of transformation is the electromagnetic stage, then the electrical stage and, as a final result, the thermal stage. The heat generated by the stove is almost completely used and therefore induction stoves are the best among competitors. Many people ask the question, how to make such a device yourself and is it possible? In this article we will reveal the features of the equipment and help solve some problems and find the right answers.

Types of ovens with induction system

There are two main types that induction furnaces are divided into.

In one of them, the base is the cardiac channel; in such furnaces, metal is placed around the inductor in a ring manner.

The second type does not have such a channel. This type is called a crucible, and in these furnaces the metal is located inside the inductor itself. In the latter case, the use of a closed core is technically impossible.

Features of induction furnaces

The obvious advantages of induction furnaces are some of the features listed below:

Advantages

Since the metal is evenly distributed, the resulting material has a good homogeneous mass during the process. This type of furnace works by moving energy through zones and also has an energy focusing function. It is possible to widely use such parameters as operating frequency, capacity and lining method, regulation of the melting temperature of the metal, which is also a positive component of the work process. The technological potential of the furnaces creates a very high melting rate; the furnaces are environmentally friendly, safe for humans and are always ready for use.

Flaws

The obvious downside to these ovens is cleaning them. Since only the heat generated by the metal heats the slag, and such a temperature, as a rule, is not enough to ensure its full use. Due to the high temperature difference between the slag and metal, the waste removal process becomes very difficult. Another disadvantage is the gap, due to which the thickness of the lining must always be reduced. As a result of such actions, it usually becomes faulty after some time.

Induction furnaces in industry

The industry mainly uses such types of furnaces as channel and crucible induction furnaces. Crucible furnaces melt many metals in any quantity. The container for metal in such furnaces can accommodate volumes from several hundred to several tons of metal. As for channel furnaces, they smelt non-ferrous metals of various types and quantities, and also have the ability to melt cast iron.
Induction furnaces in industry come in three frequencies: medium frequency furnace, industrial frequency furnace and resistance furnace.

The first type of equipment is usually used in various workshops, such as foundries of metal smelting plants and mechanical engineering plants.

In addition to melting non-ferrous metals, their capabilities include melting steel. This is due to the fact that such furnaces are equipped with graphite crucibles, which will also allow for melting and overheating of cast iron.

As for resistance furnaces, they are used to superheat and remelt aluminum and zinc alloys, as well as many other metals.

Furnace Basics

There is one interesting fact. The basis of the operation and functioning of ovens with an induction system were taken to design the microwave ovens that are so necessary today.
Microwave ovens carry most of the capabilities of induction furnaces, only with less force and heating temperature produced. Today, there are mini ovens, such as delonghi, which act as ovens and can cook food quickly and tasty. They incorporate heating elements with a specific effect, which not only, for a long time, simplify life modern people, and also make it more comfortable.

Making a stove with your own hands is possible today

Very often, fans of radio technology and radio design are interested in this topic. Today it should be clear that creating such equipment with your own hands is quite possible, and many have already done it. But to create such a furnace you need to implement the action electrical diagram, in which the actions of this very furnace would be prescribed. Such circuits require the use of RF generators that are capable of producing oscillation waves. To create the desired circuit, you need to take advantage of the capabilities of four vacuum tubes, as well as one neon lamp, which signals that the system is ready for operation.
Due to the fact that the handle of the AC capacitor is not located inside the oven, you can construct a mini induction oven yourself using the diagram described above. A clear advantage of such furnaces is that even a small variable capacity performs very well in operation. Metal placed in its coil melts surprisingly quickly, for example, a piece of aluminum melts in about thirty seconds. You can verify the power of the device using a screwdriver, which reaches a red-hot state in a few seconds.
When you are designing this type of furnace, you need to know that the melting rate in such furnaces depends on several factors, which are listed below:

• Generating power;
• Hysteresis losses;
• Losses due to eddy currents.
• Pulse frequency;
• The period during which heat is transferred from the furnace to the outside.

When you use lamps, you must remember that they must distribute their power so that only 4 lamps are enough. You will get a network of approximately 220 V if a rectifier is used.

Use of stoves in everyday life

The use of such stoves in everyday life is quite rare, although similar technologies, of course, are found in heating systems almost everywhere. They come in the form of microwaves, induction cookers, and also in the form electric ovens. It's pretty wide application in an environment of new technologies.
For example, the use of eddy induction currents, which are used in the operation of induction cookers, makes it possible to prepare many different dishes. Since they take very little time to heat up, and you won't be able to turn on the burner, eating on it costs nothing. But in order to use these quite productive and useful stoves, you need to have special utensils.

Types of heating devices

This type of furnace, such as induction, of course, should not be classified as heating stoves. But since they are used along with many other devices, we will do it.

Fireplaces planika

But thanks to modern technology, we can use the latest discoveries in the field of use of these devices. Heating elements such as planika fireplaces not only heat the house, but also play an important decorative role in decorating the room. They are biologically friendly to the external environment, since their fuel uses liquid, the processing of which produces water and heat. Also, one of their advantages is that after their work there is no dust or ash left.

Guca ovens

Nowadays, guca stoves are very popular, the main difference of which is high quality and reliability in operation, as well as a very beautiful external design. If you do not have experience in developing such stoves, it is better to trust specialized factories for the manufacture of these products. Because not only the warmth and comfort of your premises will depend on this, but also, above all, safety.

Thorma stoves

Thorma stoves are very famous these days. They are manufactured in a factory with many years of experience in producing reliable and proven products. This plant is considered the leader in productivity in Europe. Furnaces manufactured at the factory, as a rule, are light in weight and can be installed not only on the foundation, but also on the walls. This is very useful if your premises small sizes and you need to save space. They are also equipped with a special mode of long-term fuel combustion. Thanks to this mode, you can: leave the oven on for a long time and not worry about the safety of the room. This combustion lasts about 10 hours. The furnaces have convection chambers that completely burn the fuel in the furnace. This feature saves money and time.
So, the use of stoves of this type is becoming popular, but still they are not able to replace their competitors. Although they are the most advanced among the currently known electric furnaces, they make it possible to fully utilize the generated heat. Creating such furnaces manually is possible if you have some experience in such activities, since the safety of the premises will depend on the performance and safety of such furnaces. If you do not have this experience, it is better to use the services of factories and various workshops that are ready to help you at any moment. We hope that this article helped you if you are still going to construct this miracle of technology and you will use all the tips listed above. Good luck in your endeavors.

Video on the topic