How to make a furnace for melting metal. Homemade induction furnace for melting metal Induction furnace for melting metal operating principle

Attach the outer cylinder to the metal bottom. This can be done by welding or screwing. If the size of the bottom is significantly larger than the diameter of the cylinder, this will make the structure more stable and safe.

Place the bottom of the outer cylinder on fire bricks, achieving as much stability as possible. These heat-resistant bricks will support your furnace during melting and insulate its hot bottom.

Insert the inner cylinder into the outer one, making sure it fits exactly in the middle. The space between the walls of the cylinders can be filled with refractory lime mortar or dry sand, which will give the structure a O greater stability; you can simply fix the cylinders relative to each other with metal wedges.

Drill or cut a hole about 6 cm (2 1/4 inches) in diameter in the outer and inner cylinders near the bottom, slanting inward and upward, so that air can flow freely into the crucible, providing oxygen to the burning fuel.

Cut a metal tube with a diameter of 6 cm and a length of half a meter or more (a thin-walled metal tube for wires will do) - it will serve to supply air to the melting chamber; weld it to the hole in the outer cylinder or attach it with screws.

Cut the circle sheet metal, large enough to completely cover the top of the camera. Cut a 15X15 cm (6X6 inch) hole in this circle to allow air to circulate freely and to add metal to the crucible; the cut piece will serve as a lid. For convenience, you can attach the lid with a chain to the outer wall of the oven, and also attach a handle to the lid.

Make a crucible (melting pot). You can use the appropriate size metal cylinder from an old thermos, or a stainless steel boiler. In order to be able to pour molten metal from the crucible, attach a steel handle to it, which would protrude from the top of the melting chamber.

Connect the blower to the metal tube installed earlier near the bottom of the housing. You can use an old hairdryer or low-power leaf blower, attaching them to the tube with tape. If you don’t have a hair dryer or a machine, any device that will provide the necessary air flow through the tube will do. However, remember that too much air flow can lead to intense and rapid combustion of coal, and insufficient air flow will suppress combustion and will not provide you with the required temperature.

The induction furnace is often used in the metallurgy field, so this concept well known to people who are, to one degree or another, involved in the process of smelting various metals. The device allows you to convert electricity generated by a magnetic field into heat.

Similar devices are sold in stores for quite a high price, but if you have minimal skills in using a soldering iron and can read electronic circuits, then you can try to make an induction furnace with your own hands.

A homemade device is unlikely to be suitable for performing complex tasks, but it will cope with basic functions. The device can be assembled on the basis of a working welding inverter made of transistors, or using lamps. The most productive device is the one based on lamps due to its high efficiency.

Working principle of induction furnace

Heating of the metal placed inside the device occurs by converting electromagnetic pulses into heat energy. Electromagnetic pulses are generated by a coil of copper wire or pipe.

Scheme induction furnace and heating schemes

When the device is connected, an electric current begins to flow through the coil, and a electric field changing its direction over time. The functionality of such an installation was first described by James Maxwell.

The object to be heated must be placed inside or close to the coil. The target object will be penetrated by a flow of magnetic induction, and a vortex-type magnetic field will appear inside. Thus, inductive energy will turn into thermal energy.

Varieties

Induction coil stoves are usually divided into two types depending on the type of construction:

• Duct;
• Crucible.

In the first devices, the metal to be melted is located in front of the induction coil, and in the second type of furnace it is placed inside it.

You can assemble the oven by following these steps:

1. We bend the copper pipe in the form of a spiral. In total, you need to make about 15 turns, the distance between which should be at least 5 mm. The crucible should be freely located inside the spiral, where the smelting process will take place;
2. We produce a reliable housing for the device, which should not conduct electric current and must withstand high air temperatures;
3. Chokes and capacitors are assembled according to the above diagram;
4. A neon lamp is connected to the circuit, which will signal that the device is ready for operation;
5. A capacitor is also soldered to adjust the capacitance.

Use for heating

Induction furnaces of this type can also be used to heat a room. Most often they are used in conjunction with a boiler, which additionally heats cold water. In fact, the designs are used extremely rarely due to the fact that, as a result of losses of electromagnetic energy, the efficiency of the device is minimal.

Another disadvantage is based on the device’s consumption of large amounts of electricity during operation, which is why the device falls into the category of economically unprofitable.

System cooling

A device assembled independently must be equipped with a cooling system, since during operation all components will be exposed to high temperatures, and the structure may overheat and break. In store-bought ovens, cooling is done with water or antifreeze.

When choosing a cooler for your home, preference is given to options that are most profitable for implementation from an economic point of view.

For home ovens, you can try using a regular paddle fan. Please note that the device should not be placed too close to the furnace, as metal parts of the fan negatively affect the performance of the device and can also open vortex flows and reduce the performance of the entire system.

Precautions when using the device

When working with the device, you should adhere to the following rules:

• Some elements of the installation, as well as the metal that is melting, are subject to intense heat, resulting in a risk of burns;
• When using a lamp oven, be sure to place it in a closed housing, otherwise there is a high risk of electric shock;
• Before working with the device, remove all metal elements and complex electronic devices from the operating area of ​​the device. The device should not be used by people who have a pacemaker.

Induction type metal melting furnace can be used in tinning and forming metal parts.

A homemade installation can be easily adjusted to work in specific conditions, changing some settings. If you adhere to the indicated diagrams when assembling the structure, and also follow basic safety rules, homemade device will practically not be inferior to store-bought household appliances.

Induction melting furnace has been used for melting metals and alloys for the past few decades. The device has become widespread in the metallurgical and mechanical engineering fields, as well as in jewelry. If you wish, you can make a simple version of this equipment yourself. Let's take a closer look at the operating principle and features of using an induction furnace.

Induction heating principle

In order for a metal to move from one state of aggregation to another, it must be heated to sufficiently high temperature. Moreover, each metal and alloy has its own melting point, which depends on the chemical composition and other factors. An induction melting furnace heats the material from the inside by creating eddy currents that pass through crystal lattice. The process under consideration is associated with the phenomenon of resonance, which causes an increase in the strength of eddy currents.

The operating principle of the device has the following features:

1. The space that is formed inside the coil serves to accommodate the workpiece. This heating method can be used in industrial conditions only if it is created large device, into which it will be possible to place a mixture of various sizes.
2. The installed coil can have a different shape, for example, a figure of eight, but the most common is the spiral. It is worth considering that the shape of the coil is selected depending on the characteristics of the workpiece being heated.

In order to create an alternating magnetic field, the device is connected to a household power supply network. To improve the quality of the resulting alloy with high fluidity, high-frequency generators are used.

Design and use of an induction furnace

If desired, you can create an induction furnace for melting metal from scrap materials. The classic design has three blocks:

1. A generator that creates high frequency alternating current. It is this that creates an electric current, which is converted into a magnetic field passing through the material and accelerating the movement of particles. Due to this, the transition of metal or alloys from solid to liquid occurs.
2. The inductor is responsible for creating a magnetic field, which heats the metal.
3. The crucible is designed for melting material. It is placed in an inductor, and the winding is connected to current sources.

The process of converting electric current into a magnetic field is used today in a wide variety of industries.

The main advantages of the inductor include the following points:

1. A modern device is capable of directing a magnetic field, thereby increasing efficiency. In other words, the charge is heated, not the device.
2. Due to the uniform distribution of the magnetic field, the workpiece is heated evenly. In this case, from the moment the device is turned on until the charge is melted, a small amount of time takes place.
3. The homogeneity of the resulting alloy, as well as its high quality.
4. When heating and melting the metal, no evaporation is formed.
5. The installation itself is safe to use and does not cause the formation of toxic substances.

There are simply a huge number of different options for homemade induction furnaces, each with its own specific features.

Types of induction furnaces

Considering the classification of devices, we note that the workpieces can be heated both inside and outside the coil. That is why there are two types of induction furnaces:

1. Channel. This kind of device has small channels that are located around the inductor. To generate an alternating magnetic field, a core is located inside.
2. Crucible. This design is characterized by the presence of a special container called a crucible. It is made of refractory metal with a high melting point.

It is important that channel induction furnaces have large overall dimensions and are intended for industrial metal melting. Due to the continuous melting process, a large volume of molten metal can be obtained. Channel induction furnaces are used for melting aluminum and cast iron, as well as other non-ferrous alloys.

Crucible induction furnaces are characterized by relatively small in size. In most cases, this kind of device is used in jewelry, as well as when melting metal at home.

When creating a furnace with your own hands, you can adjust the power by changing the number of turns. It is worth considering that as the power of the device increases, a larger battery is required, as the energy consumption increases. In order to reduce the temperature of the main structural elements, a fan is installed. During long-term operation of the stove, its main elements can heat up significantly, which is worth taking into account.

Lamp-based induction furnaces have become even more widespread. You can make a similar design yourself. The assembly process has the following features:

1. A copper tube is used to create an inductor, for which it is bent in a spiral. The ends must also be large, which is required to connect the device to a current source.
2. The inductor should be placed in the housing. It is made of heat-resistant material that can reflect heat.
3. The lamp cascades are connected according to a circuit with capacitors and chokes.
4. The neon indicator lamp is connected. It is included in the circuit to indicate that the device is ready for operation.
5. A variable capacitor is connected to the system.

An important point is how the system can be cooled. When operating almost all induction furnaces, the main structural elements can heat up to high temperatures. Industrial equipment has a forced cooling system that runs on water or antifreeze. In order to create a water cooling design with your own hands, quite a lot of money is required.

At home, an air cooling system is installed. For this purpose, fans are installed. They should be positioned so as to ensure a continuous flow of cold air to the main structural elements of the furnace.

Induction melting is a widely used process in ferrous and non-ferrous metallurgy. Induction smelting is often superior to combustion furnace smelting in terms of energy efficiency, product quality and production flexibility. These pre-

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property are determined by specific physical characteristics induction furnaces.

In induction melting, a solid material is converted into a liquid phase under the influence of electromagnetic field. As in the case of induction heating, heat is released in the melted material due to the Joule effect from induced eddy currents. The primary current passing through the inductor creates an electromagnetic field. Regardless of whether the electromagnetic field is concentrated by magnetic cores or not, the coupled inductor-load system can be represented as a transformer with a magnetic core or as an air transformer. The electrical efficiency of the system is highly dependent on the field-influencing characteristics of the ferromagnetic components.

Along with electromagnetic and thermal phenomena in the process induction melting Electrodynamic forces play an important role. These forces must be taken into account, especially in the case of melting in powerful induction furnaces. The interaction of induced electric currents in the melt with the resulting magnetic field causes a mechanical force (Lorentz force)

Pressure Melt flows

Rice. 7.21. Action electromagnetic forces

For example, the force-induced turbulent movement of the melt is very important both for good heat transfer and for the mixing and adhesion of non-conducting particles in the melt.

There are two main types of induction furnaces: induction crucible furnaces (IFC) and induction channel furnaces (ICF). In ITP, the molten material is usually loaded in pieces into a crucible (Fig. 7.22). The inductor covers the crucible and the melted material. Due to the absence of a concentrating field of the magnetic circuit, the electromagnetic connection between

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inductor and loading strongly depends on the wall thickness of the ceramic crucible. To ensure high electrical efficiency, the insulation must be as thin as possible. On the other hand, the lining must be thick enough to withstand thermal stresses and

metal movement. Therefore, a compromise should be sought between electrical and strength criteria.

Important characteristics of induction melting in ITP are the movement of the melt and the meniscus as a result of the influence of electromagnetic forces. The movement of the melt ensures both uniform temperature distribution and homogeneous chemical composition. The mixing effect at the surface of the melt reduces material losses during additional loading of small-sized charge and additives. Despite the use of cheap material, the reproduction of a melt of constant composition ensures high quality casting.

Depending on the size, type of material being melted and the field of application, ITPs operate at industrial frequency (50 Hz) or medium frequency.

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at frequencies up to 1000 Hz. The latter are becoming increasingly important due to their high efficiency in melting cast iron and aluminum. Since the melt motion at constant power is weakened with increasing frequency, higher power densities and consequently greater productivity become available at higher frequencies. Due to the higher power, the melting time is reduced, which leads to an increase in the efficiency of the process (compared to furnaces operating at industrial frequency). Taking into account other technological advantages, such as flexibility in changing melted materials, mid-frequency ITPs are designed as the high-power melting plants that currently dominate the iron foundry industry. Modern powerful mid-frequency ITS for cast iron melting have a capacity of up to 12 tons and a power of up to 10 MW. Industrial-frequency ITPs are developed for larger capacities than medium-frequency ones, up to 150 tons for cast iron melting. Intensive mixing of the bath is of particular importance when smelting homogeneous alloys, such as brass, therefore, industrial frequency ITPs are widely used in this area. Along with the use crucible furnaces For smelting, they are currently also used for holding liquid metal before casting.

In accordance with the energy balance of IHP (Fig. 7.23), the level of electrical efficiency for almost all types of furnaces is about 0.8. Approximately 20% of the initial energy is lost in the inductor in the form of Joe heat. Ratio of heat loss through the crucible walls to that induced in the melt electrical energy reaches 10%, so the total efficiency of the furnace is about 0.7.

The second widely used type of induction furnace is the IKP. They are used for casting, aging and, especially, melting in ferrous and non-ferrous metallurgy. The ICP generally consists of a ceramic bath and one or more induction units (Fig. 7.24). IN

In principle, the induction unit can be represented as a transform

The operating principle of the IKP requires the presence of a constantly closed secondary loop, so these furnaces operate with a liquid residue of the melt. Useful heat is generated mainly in the channel, which has a small cross-section. The circulation of the melt under the influence of electromagnetic and thermal forces ensures sufficient heat transfer into the bulk of the melt located in the bath. Until now, ICPs have been designed for industrial frequency, however research papers are also carried out for higher frequencies. Thanks to the furnace's compact design and very good electromagnetic coupling, its electrical efficiency reaches 95%, and its overall efficiency reaches 80% and even 90%, depending on the material being melted.

According to the technological conditions in different fields of application, ICPs are required various designs induction channels. Single-channel furnaces are mainly used for aging and casting,

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steel melting is less common at installed capacities of up to 3 MW. For melting and holding non-ferrous metals, two-channel designs are preferable, providing best use energy. In aluminum melting plants, the channels are made straight for ease of cleaning.

The production of aluminum, copper, brass and their alloys is the main area of ​​application of IKP. Today, the most powerful ICPs with a capacity

up to 70 tons and a power of up to 3 MW are used for aluminum smelting. Along with high electrical efficiency, low melt losses are very important in aluminum production, which predetermines the choice of ICP.

Promising applications of induction melting technology include the production of high-purity metals such as titanium and its alloys in cold crucible induction furnaces and the melting of ceramics such as zirconium silicate and zirconium oxide.

When melting in induction furnaces, the advantages of induction heating are clearly demonstrated, such as high energy density and productivity, homogenization of the melt due to stirring, precise

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energy and temperature control, as well as simplicity of automatic process control, ease manual control and greater flexibility. High electrical and thermal efficiency combined with low melt losses and, therefore, raw material savings result in low specific consumption energy and environmental competitiveness.

The superiority of induction melting devices over fuel ones is continuously increasing thanks to practical research supported by numerical methods for solving electromagnetic and hydrodynamic problems. As an example, we can note the internal coating of the IKP steel casing with copper strips for copper smelting. Reducing eddy current losses increased the efficiency of the furnace by 8%, and it reached 92%.

Further improvement in the economics of induction melting is possible through the use of modern control technologies, such as tandem or dual feed control. Two tandem ITPs have one power source, and while melting is underway in one, the molten metal is held in the other for casting. Switching the power source from one furnace to another increases its utilization. Further development This principle is dual power control (Fig. 7.25), which ensures long-term simultaneous operation of furnaces without switching using special automatic process control. It should also be noted that an integral part of the economics of smelting is the compensation of total reactive power.

In conclusion, to demonstrate the advantages of energy- and material-saving induction technology, we can compare fuel and electrothermal methods for melting aluminum. Rice. 7.26 shows a significant reduction in energy consumption per ton of aluminum when melting in

Chapter 7. Energy-saving capabilities of modern electrical technologies

□ metal loss; Shch melting

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induction channel furnace with a capacity of 50 tons. The final energy consumption is reduced by approximately 60%, and the primary energy by 20%. At the same time, CO2 emissions are significantly reduced. (All calculations are based on typical German energy conversion and CO2 emission coefficients for mixed power plants). The results obtained highlight the special influence of metal losses during melting associated with its oxidation. Their compensation requires a large additional expenditure of energy. It is noteworthy that in copper production, metal losses during smelting are also large and must be taken into account when choosing a particular smelting technology.

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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 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 the electronics element base did not allow creating effective schemes managing them.

In the induction furnace sector, 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 illiterately 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), which is why they 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.
• The inductor, in accordance with Maxwell's equations, 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. 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 it was impossible to alloy aluminum, abandoned another “no” 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 workers. 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-resonance 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 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 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. By sanitary standards different countries valid value PES from 1 to 30 mW per 1 sq. m. of body surface with constant (over 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 combating of microwave radiation from equipment is possible only at the level of its design by highly qualified specialists.

Furnace components

Inductor

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.

Inductors industrial furnaces 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. Make your own inductor with liquid cooled impossible, 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

Second main part furnaces - alternating current generator. 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. on the left is a simple self-excited generator. 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 a change in the external temperature or self-heating of the crystal - no measures are provided to stabilize its mode. In general, if you have old KT825 or the like 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 emission spectrum is soft at all operating frequencies additional measures protection is not required, provided that you work 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 from a 15-25 A HF welding inverter. Option - a hollow donut (torus) is made from heat-resistant plastic, water is passed through the pipes through it, and for heating it is wrapped in a tire, 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 or in winter you need at least 20 kW of heat, with careful insulation 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 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 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 at 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 these, the primary is rewound onto one rod, increasing the number of its turns to operate in a mode close to short circuit ( short circuit) on 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, the transformer (it should be located directly under the tank), with a double-insulated network cable connected to it, a grounding conductor and a water-heating coil, is 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 through 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. Aluminum cookware is simply falling 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 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 output current generation, 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 familiar with integration will immediately understand that an inductor is 15-20% more economical, and it cannot be compared with a cast-iron “pancake”. Cost of money on energy when preparing most dishes for induction cooker comparable to gas, 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.

Video: failed induction heater from a kitchen stove

In conclusion

So, it’s better to buy induction electrical appliances for heating water and cooking ready-made; they’ll be cheaper and easier. But it won’t hurt to have a homemade induction crucible furnace in your home workshop: they will become available subtle ways melting and heat treatment of metals. You just need to remember about PES with microwaves and strictly follow the rules of design, manufacturing and operation.