DIY induction oven. Assembling induction melting furnaces with your own hands Assemble an induction furnace

For many years people have been smelting metal. Each material has its own melting point, which can only be achieved using special equipment. The first furnaces for melting metal were quite large and were installed exclusively in the workshops of large organizations. Today, a modern induction furnace can be installed in small workshops when setting up jewelry production. It is small, easy to use and highly effective.

Operating principle

The melting unit of an induction furnace is used to heat a wide variety of metals and alloys. The classic design consists of the following elements:

1. Drain pump.
2. Water cooled inductor.
3. Frame made of of stainless steel or aluminum.
4. Contact area.
5. The hearth is made of heat-resistant concrete.
6. Support with hydraulic cylinder and bearing unit.

The operating principle is based on the creation of Foucault eddy induction currents. As a rule, when working household appliances Such currents cause failures, but in this case they are used to heat the charge to the required temperature. Almost all electronics begin to heat up during operation. This negative factor in the use of electricity is used to its full capacity.

Advantages of the device

The induction melting furnace began to be used relatively recently. The famous open-hearth furnaces, blast furnaces and other types of equipment are installed at production sites. Such a furnace for melting metal has the following advantages:

It is this last advantage that determines the spread of induction furnaces in jewelry, since even a small concentration of impurities can negatively affect the result obtained.

Depending on the design features, floor-standing and tabletop induction furnaces are distinguished. Regardless of which option was chosen, there are several basic rules for installation:

The device may become very hot during operation. That is why there should be no flammable or explosive substances nearby. In addition, according to technology fire safety close should a fire shield must be installed.

Only two types of furnaces are widely used: crucible and channel. They have similar advantages and disadvantages, the differences lie only in the method of operation used:

The most popular type of induction furnace is the crucible type. This is due to their high performance and ease of operation. In addition, if necessary, such a design can be made independently.

Homemade versions are quite common. To create them you need:

1. Generator.
2. Crucible.
3. Inductor.

An experienced electrician, if necessary, can make an inductor with his own hands. This structural element is represented by a winding of copper wire. The crucible can be purchased at the store, but a lamp circuit, a self-assembled battery of transistors, or a welding inverter are used as a generator.

Using a welding inverter

An induction furnace for melting metal with your own hands can be created using welding inverter as a generator. This option is the most widely used since the efforts made concern only the manufacture of the inductor:

1. Thin-walled copper tube is used as the main material. The recommended diameter is 8-10 cm.
2. The tube is bent according to the desired pattern, which depends on the characteristics of the housing used.
3. There should be a distance of no more than 8 mm between the turns.
4. The inductor is placed in a textolite or graphite housing.

After creating the inductor and placing it in the housing, all that remains is to install the purchased crucible in its place.

Such a circuit is quite complex in execution; it involves the use of resistors, several diodes, transistors of various capacities, a film capacitor, a copper wire with two different diameters and throttle rings. Assembly recommendations are as follows:

The created circuit is placed in a textolite or graphite case, which are dielectrics. Scheme, involving the use of transistors, quite difficult to implement. Therefore, you should undertake the manufacture of such a stove only if you have certain work skills.

Lamp stove

IN Lately stoves using lamps are being created less and less often, as they require care when handling. The circuit used is simpler compared to the case of using transistors. Assembly can be carried out in several stages:

The llamas used must be protected from mechanical impact.

Equipment cooling

When creating an induction furnace with your own hands, the biggest problem that arises is cooling. This is due to the following points:

1. During operation, not only the molten metal is heated, but also some elements of the equipment. That's why for long work efficient cooling is required.
2. Application based method air flow, characterized by low efficiency. In addition, it is not recommended to install fans near the stove. This is due to the fact that metal elements may influence the generated eddy currents.

Typically, cooling is carried out by supplying water. Creating a water cooling circuit at home is not only difficult, but also economically unprofitable. Industrial versions of the furnace already have a built-in circuit, to which it is enough to connect cold water.

Safety precautions

When using an induction furnace, you must observe certain technique security. Basic recommendations:

When installing equipment, you should consider how the charge will be loaded and the molten metal will be extracted. It is recommended to set aside a separate prepared room for installing an induction furnace.

A home induction furnace can handle melting relatively small portions of metal. However, such a forge does not need either a chimney or bellows pumping air into the smelting zone. And the entire structure of such a furnace can be placed on desk. Therefore, heating by electrical induction is in the best possible way melting metals at home. And in this article we will look at the designs and assembly diagrams of such stoves.

In factory workshops you can find channel induction furnaces for melting non-ferrous and ferrous metals. These installations have very high power, set by an internal magnetic circuit, which increases the electrical density magnetic field and temperature in the furnace crucible.

IN industrial scale production of channel induction furnaces for melting non-ferrous and ferrous metals

However, channel structures consume large portions of energy and take up a lot of space, so at home and in small workshops an installation without a magnetic circuit is used - a crucible furnace for melting non-ferrous/ferrous metals. You can even assemble such a structure with your own hands, because the crucible installation consists of three main components:

• A generator that produces alternating current at high frequencies, which are necessary to increase the density of the electromagnetic field in the crucible. Moreover, if the diameter of the crucible can be compared with the wavelength of the alternating current frequency, then such a design will allow transforming into thermal energy up to 75 percent of the electricity consumed by the installation.
• The inductor is a copper spiral created based on an accurate calculation of not only the diameter and number of turns, but also the geometry of the wire used in this process. The inductor circuit must be configured to amplify power as a result of resonance with the generator, or more precisely with the frequency of the supply current.
• The crucible is a refractory container in which all the melting work takes place, initiated by the occurrence of eddy currents in the metal structure. In this case, the diameter of the crucible and other dimensions of this container are determined strictly according to the characteristics of the generator and inductor.

Any radio amateur can assemble such a stove. To do this he needs to find correct scheme and stock up on materials and parts. You can find a list of all this below in the text.

The design of a homemade crucible furnace is based on the simplest laboratory Kukhtetsky inverter. The circuit diagram of this transistor installation is as follows:

Transistor installation diagram

Based on this diagram, you can assemble an induction furnace using the following components:

• two transistors - preferably field-effect type and brand IRFZ44V;
• copper wire with a diameter of 2 millimeters;
• two diodes of the UF4001 brand, or even better - UF4007;
• two throttle rings - they can be removed from the old desktop power supply;
• three capacitors with a capacity of 1 μF each;
• four capacitors with a capacity of 220nF each;
• one capacitor with a capacity of 470 nF;
• one capacitor with a capacity of 330 nF;
• one 1 watt resistor (or 2 resistors of 0.5 watt each), designed for a resistance of 470 ohms;
• copper wire with a diameter of 1.2 millimeters.

Additionally, you'll need a couple of heatsinks - these can be removed from old motherboards or CPU coolers, and accumulator battery with a capacity of at least 7200 mAh from an old source uninterruptible power supply at 12 V. Well, the crucible container in in this case in fact, it is not needed - bar metal will melt in the furnace, which can be held by the cold end.

Print out and hang the drawing of Kukhtetsky’s laboratory inverter above your desk. After this, arrange all the radio components by type and brand and heat up the soldering iron. Attach two transistors to the radiators. And if you will be working with the stove for more than 10-15 minutes at a time, attach computer coolers to the radiators, connecting them to a working power supply. The pinout diagram for transistors from the IRFZ44V series is as follows:

Transistor pinout diagram

Take 1.2 millimeter copper wire and wrap it around ferrite rings, making 9-10 turns. As a result, you will get chokes. The distance between the turns is determined by the diameter of the ring, based on the uniformity of the pitch. In principle, everything can be done “by eye”, varying the number of turns in the range from 7 to 15 revolutions. Assemble a battery of capacitors by connecting all parts in parallel. As a result, you should have a 4.7 uF battery.

Now make an inductor using 2mm copper wire. The diameter of the turns in this case can be equal to the diameter of a porcelain crucible or 8-10 centimeters. The number of turns should not exceed 7-8 pieces. If during testing the furnace power seems insufficient to you, redesign the inductor by changing the diameter and number of turns. Therefore, in the first couple of stages, it is better to make the inductor contacts not soldered, but detachable. Next, assemble all the elements on a PCB board, based on the drawing of Kukhtetsky’s laboratory inverter. And connect a 7200 mAh battery to the power contacts. That's all.

Now you can test the furnace by selecting correct parameters inductor for each type of metal or crucible. However, during testing or melting, you need to remember safety precautions when working with electric furnaces.

The induction installation generates a very high temperature, sufficient to melt metal weighing up to 10-20 grams. Therefore, when working with a crucible, you need to use an apron made of dense material and the same mittens. They will protect you from burns if metal accidentally spills from the container.

It is better to hide the assembled furnace structure in an insulated housing, leaving only the inductor behind its walls. This will protect both the user and fragile radio components. And for ventilation, it is necessary to cut or drill several holes in the housing, ensuring the inflow and outflow of air.

The residual magnetic field can heat metal parts on the wearer's clothing, causing them to burn the skin. Therefore, it is better to approach the crucible in simple clothes, without zippers or metal buttons. In addition, it is better to remove all electrical appliances from the inductor at least a meter away.

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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 because of the high cost of electricity; then it was no more expensive than now. There was still a lot of unknowns in the processes occurring in induction furnaces, and the electronics element base did not allow the creation of effective control circuits for them.

In the induction furnace industry, a revolution has occurred literally before our eyes, thanks to the emergence, firstly, of microcontrollers, the computing power of which exceeds that of personal computers ten years ago. Secondly, thanks... mobile communications. Its development required the availability of inexpensive transistors capable of delivering power of several kW at 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 designs:

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 cooking appliances that compete with gas stoves and are superior to microwaves in a number of parameters.

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. on 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 received 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. on 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 welding transformer industrial frequency. 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 remarkable property of harmonic induction appears: 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 did not 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 here is the need for uniform or precisely dosed heating over the surface/volume/temperature metal parts For high-quality hardening, a homemaker or individual entrepreneur will always have it. And here again the inductor stove will help out, and the electricity consumption will be feasible for 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, having made the inductor special form(see figure on the left), you can release the hardened part in the desired place, without breaking the 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. According to sanitary standards of different countries, the 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 when the slightest violation grounding quality. Effective combating of microwave radiation from 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 150 kg steel melting furnace is bent from copper pipe 40 mm outer diameter and 30 inner. 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. on right. These were made by Soviet radio amateurs back in the 50s, Fig. on 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. on the left is a simple self-excited generator. L2 – inductor; L1 – feedback coil, 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. The main disadvantage of the circuit is 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, without a doubt, will 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 design: a propylene pipe with running water is surrounded by an 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 around it for heating bus, 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.

Induction heating boiler industrial production is designed completely differently: simple, but impossible to do at home, see fig. on 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 highly inductive 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 minimum temperature gradient, which almost eliminates the formation of scale.
9. Great durability and ease of use: induction boiler Together with a hydromagnetic system (HMS) and a settling filter, it 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 surrounding 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 is the transformer (it should be located directly under the tank), with a network cable in double insulation, ground electrode and hot water coil are poured into one “doll” silicone sealant, like an aquarium filter pump motor. 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 for the kitchen have already become familiar, 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 hobs - the product of the century high technology, although the 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, the current in the inductor must be sanitary requirements a pure sinusoid, and its magnitude and frequency must change 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: electronic components alone will cost retail prices It will take more money than for ready-made 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.”

Nevertheless, induction cookers have a lot of 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.

Induction furnaces are used for smelting metals and are distinguished by the fact that heating in them occurs through electric current. The current is excited in the inductor, or more precisely in a constant field.

In such structures, energy is converted several times (in this sequence):

• into electromagnetic;
• electrical;
• thermal

Such stoves allow you to use heat with maximum efficiency, which is not surprising, because they are the most advanced of all existing models that run on electricity.

Note! Induction designs come in two types - with or without a core. In the first case, the metal is placed in a tubular trough, which is located around the inductor. The core is located in the inductor itself. The second option is called crucible, because in it the metal and the crucible are already inside the indicator. Of course, there can be no talk of any core in this case.

In today's article we will talk about how to makeDIY induction oven.

Among the many advantages, the following are worth highlighting:

• environmental cleanliness and safety;
• increased homogeneity of the melt due to active movement of metal;
• speed – the oven can be used almost immediately after switching on;
• zonal and focused energy orientation;
• high melting rate;
• no fumes from alloying substances;
• possibility of temperature adjustment;
• numerous technical possibilities.

But there are also disadvantages.

1. The slag is heated by the metal, as a result of which it has a low temperature.
2. If the slag is cold, then it is very difficult to remove phosphorus and sulfur from the metal.
3. The magnetic field is dissipated between the coil and the melting metal, so a reduction in the thickness of the lining will be required. This will soon lead to the lining itself failing.

Video – Induction oven

Industrial Application

Both designs are used in the smelting of cast iron, aluminum, steel, magnesium, copper and precious metals. The useful volume of such structures can range from several kilograms to several hundred tons.

Industrial furnaces are divided into several types.

1. Constructions mid frequency Typically used in mechanical engineering and metallurgy. With their help, steel is melted, and when using graphite crucibles, non-ferrous metals are melted.
2. Industrial frequency designs are used in iron smelting.
3. Resistance structures are intended for melting aluminum, aluminum alloys, zinc.

Note! It was induction technology that formed the basis of more popular devices - microwave ovens.

Household use

Due to obvious reasons, an induction furnace for melting is not often used in everyday life. But the technology described in the article is found in almost all modern houses and apartments. These include the above-mentioned microwave ovens, induction cookers, and electric ovens.

Consider, for example, slabs. They heat the dishes due to induction eddy currents, as a result of which heating occurs almost instantly. It is typical that it is impossible to turn on a burner that has no cookware on it.

The efficiency of induction cookers reaches 90%. For comparison: for electric stoves it is approximately 55-65%, and for gas stoves it is no more than 30-50%. But in fairness, it is worth noting that special utensils are required to operate the described stoves.

Homemade induction furnace

Not long ago, domestic radio amateurs clearly demonstrated that you can make an induction furnace yourself. Today there are a lot various schemes and manufacturing technologies, we have listed only the most popular of them, which means the most effective and easy to implement.

Induction furnace made from high frequency generator

Below is an electrical circuit for making a homemade device from a high-frequency (27.22 megahertz) generator.

In addition to the generator, assembly will require four high-power light bulbs and a heavy lamp for the readiness indicator.

Note! The main difference between a stove made according to this scheme is the condenser handle - in this case it is located outside.

In addition, the metal located in the coil (inductor) will melt in a device of the smallest power.

When making it is necessary to remember some important points, affecting the speed of metal control. This:

• power;
• frequency;
• eddy losses;
• heat transfer intensity;
• hysteresis losses.

The device will be powered from a standard 220 V network, but with a pre-installed rectifier. If the furnace is intended for heating a room, then it is recommended to use a nichrome spiral, and if for melting, then graphite brushes. Let's take a closer look at each of the designs.

Video - Construction of a welding inverter

The essence of the design is as follows: a pair of graphite brushes is installed, and granite powder is poured between them, after which the connection is made to the step-down transformer. It is characteristic that during smelting there is no need to fear electric shock, since there is no need to use 220 V.

Assembly technology

Step 1. The base is assembled - a box made of fireclay bricks measuring 10x10x18 cm, laid on fire-resistant tiles.

Step 2. The box is finished with asbestos cardboard. After wetting with water, the material softens, which allows it to be given any shape. If desired, the structure can be wrapped with steel wire.

Note! The dimensions of the box may vary depending on the power of the transformer.

Step 3. The best option for a graphite furnace - a transformer from a welding machine with a power of 0.63 kW. If the transformer is designed for 380 V, then it can be rewound, although many experienced electricians claim that you can leave everything as is

Step 4. The transformer is wrapped with thin aluminum - this way the structure will not get too hot during operation.

Step 5. Graphite brushes are installed, a clay substrate is installed on the bottom of the box - this way the molten metal will not spread.

The main advantage of such a furnace is its high temperature, which is even suitable for smelting platinum or palladium. But among the disadvantages are the rapid heating of the transformer, small volume (no more than 10 g can be smelted at a time). For this reason, a different design will be required for larger volume melts.

So, to smelt large volumes of metal you will need a furnace with nichrome wire. The operating principle of the design is quite simple: electric current is supplied to a nichrome spiral, which heats up and melts the metal. There are a lot of different formulas on the Internet for calculating the length of a wire, but they are all, in principle, the same.

Step 1. For the spiral, nichrome ø0.3 mm with a length of about 11 m is used.

Step 2. The wire must be wound. To do this, you will need a straight copper tube ø5 mm - the spiral is wound on it.

Step 3. A small ceramic pipe ø1.6 cm and 15 cm long is used as a crucible. One end of the pipe is plugged with asbestos thread - this way the molten metal will not flow out.

Step 4. After checking the functionality, the spiral is laid around the pipe. In this case, the same asbestos thread is placed between the turns - it will prevent short circuits and limit the access of oxygen.

Step 5. The finished coil is placed in a high power lamp socket. Such cartridges are usually ceramic and have the required size.

Advantages of this design:

• high productivity (up to 30 g per pass);
• fast heating (about five minutes) and long cooling;
• ease of use - it is convenient to pour metal into molds;
• prompt replacement of the spiral in case of burnout.

But there are, of course, disadvantages:

• nichrome burns out, especially if the spiral is poorly insulated;
• insecurity - the device is connected to a 220 V power supply.

Note! You cannot add metal to the stove if the previous portion has already been melted there. Otherwise, all the material will scatter throughout the room, moreover, it can injure your eyes.

As a conclusion

As you can see, you can still make an induction furnace yourself. But to be frank, the described design (like all those available on the Internet) is not exactly a stove, but a Kukhtetsky laboratory inverter. It is simply impossible to assemble a full-fledged induction structure at home.

Metal smelting by induction is actively used in various industries, such as mechanical engineering, metallurgical and jewelry production. The material is heated under the influence of electric current, which allows the heat to be used with maximum efficiency. Large factories have special industrial units for this, while at home you can assemble a simple and small induction furnace with your own hands.

Such furnaces are popular in production

Self-assembly of the stove

There are many technologies and schematic descriptions of this process presented on the Internet and magazines, but when choosing, it is worth choosing one model that is most effective in operation, as well as affordable and easy to implement.

Homemade melting furnaces have a fairly simple design and usually consist of only three main parts housed in a sturdy casing. These include:

• element generating high frequency alternating current;
• a spiral-shaped part created from a copper tube or thick wire, called an inductor;
• crucible - a container in which calcination or melting will be carried out, made of refractory material.

Of course, such equipment is not often used in everyday life, because not all craftsmen need such units. But the technologies found in these devices are present in household appliances, which many people deal with almost every day. This includes microwaves, electric ovens and induction cookers. You can make various equipment using diagrams with your own hands if you have the necessary knowledge and skills.

In this video you will learn what this oven consists of

Heating in this technique is carried out thanks to induction eddy currents. The temperature rise occurs instantly, unlike other devices of a similar purpose.

For example, induction cookers have an efficiency of 90%, but gas and electric cookers cannot boast of this value, it is only 30-40% and 55-65%, respectively. However, HDTV cookers have a drawback: to use them you will have to prepare special dishes.

Transistor design

There are many different schemes for assembling induction melters at home. A simple and proven furnace made from field-effect transistors is quite easy to assemble; many craftsmen familiar with the basics of radio engineering can handle its manufacture according to the diagram shown in the figure. To create an installation need to prepare following materials and details:

• two IRFZ44V transistors;
• copper wires (for winding) in enamel insulation, 1.2 and 2 mm thick (one piece each);
• two rings from chokes, they can be removed from the power supply of an old computer;
• one 470 Ohm resistor per 1 W (you can connect two 0.5 W each in series);
• two UF4007 diodes (can easily be replaced with the UF4001 model);
• 250 W film capacitors - one piece with a capacity of 330 nF, four - 220 nF, three - 1 µF, 1 piece - 470 nF.

Before assembling such a stove, do not forget about the tools

Assembly takes place according to the schematic drawing; it is also recommended to check the step-by-step instructions, this will protect you from mistakes and damage to elements. Creating an induction melting furnace with your own hands is carried out according to the following algorithm:

1. Transistors are placed on fairly large heatsinks. The fact is that circuits can get very hot during operation, which is why it is so important to select parts of the appropriate size. All transistors can be placed on one radiator, but in this case you will have to insulate them, preventing them from coming into contact with metal. Washers and gaskets made of plastic and rubber will help with this. The correct pinout of transistors is shown in the picture.
2. Then they start making chokes; you will need two of them. To do this, take copper wire 1.2 millimeters in diameter and wrap it around rings taken from the power supply. These elements contain ferromagnetic iron in powder form, so it is necessary to make at least 7-15 turns, leaving a small distance between them.
3. The resulting modules are assembled into one battery with a capacity of 4.6 μF, and the capacitors are connected in parallel.
4. Copper wire 2 mm thick is used to wind the inductor. It is wrapped 7-8 times around any cylindrical object, its diameter should correspond to the size of the crucible. The excess wire is cut off, but rather long ends are left: they will be needed for connecting to other parts.
5. All elements are connected on the board, as shown in the figure.

If necessary, you can build a housing for the unit; for this purpose, only heat-resistant materials, such as textolite, are used. The power of the device can be adjusted, for which it is enough to change the number of turns of wire on the inductor and their diameter.

There are several variations of the induction furnace that can be assembled

With graphite brushes

The main element of this design is assembled from graphite brushes, the space between which is filled with granite, crushed to a powder state. Then the finished module is connected to a step-down transformer. When working with such equipment, you do not have to worry about electric shock, since it does not need to use 220 volts.

Manufacturing technology of an induction furnace from graphite brushes:

1. First, the body is assembled; for this, fire-resistant (fireclay) bricks measuring 10 × 10 × 18 cm are laid on tiles that can withstand high temperatures. The finished box is wrapped in asbestos cardboard. To give this material the required form, it is enough to moisten it with a small amount of water. The size of the base directly depends on the power of the transformer used in the design. If desired, the box can be covered with steel wire.
2. An excellent option for graphite furnaces would be a 0.063 kW transformer taken from a welding machine. If it is designed for 380 V, then for safety reasons it can be subjected to winding, although many experienced radio technicians believe that this procedure can be abandoned without any risk. However, it is recommended to wrap the transformer with thin aluminum so that the finished device does not heat up during operation.
3. A clay substrate is placed at the bottom of the box so that the liquid metal does not spread, after which graphite brushes and granite sand are placed in the box.

The main advantage of such devices is considered to be the high melting point, which can change the state of aggregation of even palladium and platinum. The disadvantages include too rapid heating of the transformer, as well as small area a furnace that will not allow smelting more than 10 g of metal at a time. Therefore, every master should understand that if the device is assembled to process large volumes, then it is better to make a furnace of a different design.

Lamp-based device

A powerful melting stove can be assembled from electronic light bulbs. As can be seen in the diagram, to obtain high-frequency current, beam lamps must be connected in parallel. Instead of an inductor, this device uses a copper tube with a diameter of 10 mm. The design is also equipped with a tuning capacitor in order to be able to regulate the power of the furnace. For assembly you need to prepare:

• four lamps (tetrodes) L6, 6P3 or G807;
• trimmer capacitor;
• 4 chokes at 100-1000 µH;
• neon indicator light;
• four 0.01 µF capacitors.

To start copper tube give the shape of a spiral - this will be the inductor of the device. In this case, a distance of at least 5 mm is left between the turns, and their diameter should be 8-15 cm. The ends of the spiral are processed for attachment to the circuit. The thickness of the resulting inductor should be 10 mm greater than that of the crucible (it is placed inside).

The finished part is placed in the housing. For its manufacture, you should use a material that will provide electrical and thermal insulation for the filling of the device. Then a cascade is assembled from lamps, chokes and capacitors, as shown in the figure, the latter being connected in a straight line.

It's time to connect the neon indicator: it is needed so that the master can find out when the device is ready for work. This light bulb is connected to the furnace body along with the handle of the variable capacitor.

Cooling system equipment

Industrial units for melting metal are equipped with special cooling systems using antifreeze or water. Equipping these important installations in homemade HDTV stoves will require additional costs, which is why the assembly can put a significant dent in your wallet. Therefore, it is better to provide a household unit with a cheaper system consisting of fans.

Air cooling with these devices is possible when they are located remotely from the furnace. Otherwise, the metal windings and fan parts can serve as a loop for short-circuiting eddy currents, which will significantly reduce the efficiency of the equipment.

Tube and electronic circuits also tend to become hot during operation of the unit. Heat sinks are usually used to cool them.

Terms of use

For experienced radio technicians, assembling an induction furnace according to the diagrams with your own hands may seem like an easy task, so the device will be ready quite quickly, and the master will want to try his creation in action. It is worth remembering that when working with homemade installation It is important to follow safety precautions and not forget about the main threats that may arise during operation of an inertial furnace:

1. Liquid metal and heating elements devices can cause severe burns.
2. Tube circuits consist of parts with high voltage, therefore, during assembly of the unit, they must be placed in a closed box, thus eliminating the possibility of accidentally touching these elements.
3. The electromagnetic field can influence even those things that are outside the installation box. Therefore, before turning on the device, you need to remove all complex technical devices, such as Cell phones, digital cameras, MP3 players, and also remove all metal jewelry. People with pacemakers are also at risk: they should never use such equipment.

These furnaces can be used not only for smelting, but also for quickly heating metal objects during forming and tinning. By changing the output signal of the installation and the parameters of the inductor, you can configure the device for a specific task.

For melting small volumes of iron they will be used homemade stoves, these efficient devices can operate from regular outlets. The device does not take up much space, it can be placed on a desktop in a workshop or garage. If a person knows how to read simple electrical diagrams, then he does not need to purchase such equipment in a store, because he can assemble a small stove with his own hands in just a few hours.

Radio amateurs have long discovered that they can make induction furnaces for melting metal with their own hands. These simple diagrams will help you make a HDTV installation for home use. However, it would be more correct to call all the described designs “Kukhtetsky’s laboratory inverters,” since it is simply impossible to independently assemble a full-fledged stove of this type.