DIY induction melting furnaces. Do-it-yourself induction oven: diagram, assembly

Nowadays, induction furnaces are widely used in the metal smelting process. The current produced in the field of the inductor contributes to the heating of the substance, and this feature of such devices is not only basic, but also the most important. Processing causes the substance to undergo several transformations. The first stage of transformation is the electromagnetic stage, followed by the electrical stage, and then the thermal stage. The temperature generated by the stove is used practically without any residue, so this solution is the best among all others. Many may be interested in a manufactured stove. Next we will talk about the possibilities of implementing such a solution.

Types of furnaces for melting metals

This type of equipment can be divided into main categories. The first has a heart channel as its base, and the metal is placed in such furnaces in a ring manner around the inductor. The second category does not have such an element. This type is called a crucible, and the metal is placed inside the inductor itself. It is technically impossible to use a closed core in this case.

Basic principles

The melting furnace in this case operates on the basis of the phenomenon of magnetic induction. And there are several components. The inductor is the most important component of this device. It is a coil, the conductors of which are not ordinary wires, but copper tubes. This requirement is imposed by the design of the melting furnaces itself. The current that passes through the inductor generates a magnetic field that affects the crucible inside which the metal is located. In this case, the material plays the role of a secondary transformer winding, that is, a current passes through it, heating it. This is how melting occurs, even if you make an induction furnace yourself. How to build this type of furnace and increase its efficiency? This is an important question that has an answer. The use of high-frequency currents can significantly increase the efficiency of equipment. For this, it is appropriate to use special power supplies.

Features of induction furnaces

This type of equipment has certain characteristic features that are both advantages and disadvantages.

Since the distribution of the metal must be uniform, the resulting material is characterized by a good homogeneous mass. This type of furnace works by transporting energy through zones, while also introducing the function of focusing energy. Parameters such as capacitance, operating frequency and lining method are available for use, as well as regulation of the temperature at which the metal melts, which significantly facilitates the work process. The existing technological potential of the furnace creates a high melting rate; the devices are environmentally friendly, completely safe for humans and ready for use at any time.

The most noticeable disadvantage of such equipment is the difficulty of cleaning it. Since the slag is heated solely due to the heat generated by the metal, this temperature is not enough to ensure its full use. The high difference in temperature between the metal and the slag does not allow the waste removal process to be as simple as possible. As another disadvantage, it is customary to highlight the gap, due to which it is always necessary to reduce the thickness of the lining. Due to such actions, after some time it may become faulty.

Use of induction furnaces on an industrial scale

In industry, crucible and channel induction furnaces are most often found. In the first, melting of any metals in arbitrary quantities is carried out. Containers for metal in such variants can hold up to several tons of metal. Of course, induction melting furnaces In this case, it is impossible to do it yourself. Channel furnaces are designed for smelting non-ferrous metals different types, as well as cast iron.

This topic is often of interest to fans of radio design and radio technology. Now it is becoming clear that creating induction furnaces with your own hands is quite possible, and many people have managed to do this. However, to create such equipment, it is necessary to implement the operation of an electrical circuit that would contain the prescribed actions of the furnace itself. Such solutions require the involvement of those capable of producing wave oscillations. A simple do-it-yourself induction furnace according to the circuit can be built using four electronic lamps in combination with one neon lamp, which gives a signal that the system is ready for operation.

In this case, the capacitor handle alternating current is not located inside the device. Thanks to this, you can create an induction furnace with your own hands. The device diagram describes in detail the location of each individual element. You can make sure that the device is powerful enough by using a screwdriver, which should reach a red-hot state in just a few seconds.

Peculiarities

If you create an induction furnace with your own hands, the operating principles and assembly of which are studied and carried out according to corresponding scheme, you should be aware that the melting rate in this case may be affected by one or more factors listed below:

Pulse frequency;

Hysteresis losses;

Generating power;

The period of heat release;

Losses associated with the occurrence of eddy currents.

If you are planning to build an induction stove with your own hands, then when using lamps you need to remember that their power should be distributed so that four pieces are enough. When using a rectifier, you will get a network of approximately 220 V.

Household use of stoves

In everyday life, such devices are used quite rarely, although similar technologies can be found in heating systems. They can be seen in the form of microwave ovens and In the environment of new technologies, this development has found wide application. For example, the use of eddy currents in induction cookers allows you to cook a huge variety of dishes. Since they take very little time to heat up, the burner cannot be turned on if nothing is standing on it. However, special utensils are required to use such special and useful cookers.

Build process

Do-it-yourself induction consists of an inductor, which is a solenoid made from a water-cooled copper tube and a crucible, which can be made from ceramic materials, and sometimes made of steel, graphite and others. In such a device you can smelt cast iron, steel, precious metals, aluminum, copper, magnesium. Induction furnaces they are made with their own hands with a crucible capacity from a couple of kilograms to several tons. They can be vacuum, gas-filled, open and compressor. The furnaces are powered by high, medium and low frequency currents.

So, if you are interested in making your own induction furnace, the scheme involves the use of the following main components: a melting bath and an induction unit, which includes a hearth stone, an inductor and a magnetic core. A channel furnace differs from a crucible furnace in that electromagnetic energy is converted into thermal energy in the heat release channel, in which there must always be an electrically conductive body. To make the initial start-up of a channel furnace, molten metal is poured into it or a template made of a material that can be straightened in the furnace is inserted. When the melting is completed, the metal is not completely drained, but a “swamp” remains, intended to fill the heat release channel for future start-up. If you are going to build an induction furnace with your own hands, then to make it easier to replace the hearth stone for the equipment, it is made detachable.

Furnace components

So, if you are interested in making a mini induction oven with your own hands, then it is important to know that its main element is the heating coil. In the case of a homemade version, it is enough to use an inductor made of a bare copper tube, the diameter of which is 10 mm. For the inductor, an internal diameter of 80-150 mm is used, and the number of turns is 8-10. It is important that the turns do not touch, and the distance between them is 5-7 mm. Parts of the inductor should not come into contact with its screen; the minimum gap should be 50 mm.

If you are planning to make an induction furnace with your own hands, then you should know that on an industrial scale, water or antifreeze is used to cool the inductors. In the case of low power and short-term operation of the device being created, you can do without cooling. But during operation, the inductor gets very hot, and scale on copper can not only sharply reduce the efficiency of the device, but also lead to a complete loss of its performance. It is impossible to make a cooled inductor on your own, so it will need to be replaced regularly. You cannot use forced air cooling, since the fan housing placed close to the coil will “attract” EMF, which will lead to overheating and a decrease in the efficiency of the furnace.

Generator

When assembling an induction furnace with your own hands, the diagram involves the use of such important element, like an alternator. You should not try to make a stove if you do not know the basics of radio electronics at least at the level of a semi-skilled radio amateur. The choice of generator circuit should be such that it does not produce a hard current spectrum.

Using Induction Furnaces

This type of equipment is widely used in areas such as foundries, where the metal has already been cleaned and needs to be given a specific shape. You can also get some alloys. They have also become widespread in jewelry production. The simple principle of operation and the possibility of assembling an induction furnace with your own hands allow you to increase the profitability of its use. For this area, devices with a crucible capacity of up to 5 kilograms can be used. For small productions this option will be optimal.

Induction furnaces were invented back in 1887. And within three years the first industrial development appeared, with the help of which various metals were smelted. I would like to note that in those distant years these stoves were a novelty. The whole point is that scientists of that At the time, they did not quite understand what processes were taking place in it. Today we figured it out. In this article we will be interested in the topic - do-it-yourself induction furnace. How simple is its design, is it possible to assemble this unit at home?

Principle of operation

You need to start assembling by understanding the principle of operation and structure of the device. Let's start with this. Pay attention to the figure above, we will understand it according to it.

The device includes:

Generator G, which produces alternating current.
Capacitor C, together with coil L, creates an oscillating circuit, which provides the installation with high temperature.

Attention! Some designs use a so-called self-oscillating generator. This makes it possible to remove the capacitor from the circuit.
The coil in the surrounding space forms a magnetic field in which there is a voltage, indicated in our figure by the letter “H”. The magnetic field itself exists in free space, and can be closed through a ferromagnetic core.
It also acts on the charge (W), in which it creates a magnetic flux (F). By the way, instead of the charge, some kind of blank can be installed.
The magnetic flux induces a secondary voltage of 12 V. But this only happens if W is an electrically conductive element.
If the heated workpiece is large and solid, then the so-called Foucault current begins to operate inside it. It is of the vortex type.
In this case, eddy currents are transmitted from the generator through a magnetic field thermal energy, thereby heating the workpiece.

The electromagnetic field is quite wide. And even the multi-stage energy conversion, which is present in homemade induction furnaces, has maximum efficiency - up to 100%.

Crucible furnace

Varieties

There are two main designs of induction furnaces:

Duct.
Crucible.

We will not describe all their distinctive features here. Just note that the duct option is a design that is similar to welding machine. In addition, in order to melt metal in such furnaces, it was necessary to leave a little melt, without which the process simply would not work. The second option is an improved scheme that uses technology without residual melt. That is, the crucible is simply installed directly into the inductor.

How it works

Why do you need such a stove at home?

In general, the question is quite interesting. Let's look at this situation. There is enough a large number of Soviet electrical and electronic devices that used gold or silver contacts. These metals can be removed different ways. One of them is an induction stove.

That is, you take the contacts, put them in a narrow and long crucible, which you install in the inductor. After 15-20 minutes, reducing the power, cooling the apparatus and breaking the crucible, you will get a rod, at the end of which you will find a gold or silver tip. Cut it off and take it to a pawnshop.

Although it should be noted that with this homemade unit Various processes can be carried out with metals. For example, you can harden or temper.

Coil with battery (generator)

Stove components

In the Working Principle section, we have already mentioned all the parts of an induction furnace. And if everything is clear with the generator, then the inductor (coil) needs to be sorted out. A copper tube is suitable for it. If you are assembling a device with a power of 3 kW, then you will need a tube with a diameter of 10 mm. The coil itself is twisted with a diameter of 80-150 mm, with a number of turns from 8 to 10.

Please note that the turns of the copper tube should not touch each other. Optimal distance there are 5-7 mm between them. The coil itself should not touch the screen. The distance between them is 50 mm.

Typically, industrial induction furnaces have a cooling unit. It is impossible to do this at home. But for a 3 kW unit, working for up to half an hour is not dangerous. True, over time, copper scale will form on the tube, which reduces the efficiency of the device. So the coil will have to be changed periodically.

Generator

In principle, making a generator with your own hands is not a problem. But this is only possible if you have sufficient knowledge in radio electronics at the level of an average radio amateur. If you don’t have such knowledge, then forget about the induction stove. The most important thing is that you also need to skillfully operate this device.

If you are faced with the dilemma of choosing a generator circuit, then take one piece of advice - it should not have a hard current spectrum. In order to make it clearer what we are talking about, we offer the most simple diagram generator for an induction furnace in the photo below.

Generator circuit

Required knowledge

The electromagnetic field affects all living things. An example is microwaved meat. Therefore, it is worth taking care of safety. And it doesn’t matter whether you are assembling the stove and testing it or working on it. There is such an indicator as energy flux density. So it depends on the electromagnetic field. And the higher the frequency of radiation, the worse it is for the human body.

Many countries have adopted safety measures that take into account energy flux density. There are developed permissible limits. This is 1-30 mW per 1 m² of the human body. These indicators are valid if exposure occurs no more than one hour per day. By the way, the installed galvanized screen reduces the density of the ceiling by 50 times.

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Induction heaters work on the principle of “derived current from magnetism”. An alternating magnetic field is generated in a special coil high power, which generates eddy electric currents in a closed conductor.

The closed conductor in induction cookers is a metal cookware, which is heated by eddy electric currents. In general, the operating principle of such devices is not complicated, and if you have a little knowledge of physics and electrical engineering, you can assemble induction heating It won't be difficult to do it yourself.

The following devices can be made independently:

Devices for heating in a heating boiler.
Mini ovens for melting metals.
Plates for cooking food.

A do-it-yourself induction cooker must be manufactured in compliance with all standards and regulations for the operation of these devices. If electromagnetic radiation dangerous to humans is emitted outside the housing in lateral directions, then the use of such a device is strictly prohibited.

In addition, the great difficulty in designing a stove lies in the selection of material for the base of the hob, which must meet the following requirements:

Ideally conduct electromagnetic radiation.
Not a conductive material.
Withstand high temperature load.

In household cookers induction surfaces Expensive ceramics are used to make an induction cooker at home, find a worthy alternative this kind of material is quite difficult. Therefore, first you should design something simpler, for example, an induction furnace for hardening metals.

Manufacturing instructions

Blueprints

Picture 1. Electrical diagram induction heater

Figure 2. Device.

Figure 3. Schematic of a simple induction heater

To make a stove you will need the following materials and tools:

solder;
textolite board.
mini drill.
radioelements.
thermal paste.
chemical reagents for etching the board.

Additional materials and their features:

For making a coil, which will emit the alternating magnetic field necessary for heating, it is necessary to prepare a piece of copper tube with a diameter of 8 mm and a length of 800 mm.
Powerful power transistors are the most expensive part of homemade induction installation. For mounting the circuit frequency generator You need to prepare 2 of these elements. Transistors of the following brands are suitable for these purposes: IRFP-150; IRFP-260; IRFP-460. When manufacturing the circuit, 2 identical of the listed field-effect transistors are used.
For the manufacture of an oscillatory circuit you will need ceramic capacitors with a capacity of 0.1 mF and an operating voltage of 1600 V. In order for high-power alternating current to form in the coil, 7 such capacitors will be required.
When operating such an induction device, field-effect transistors will get very hot and if radiators from aluminum alloy, then after just a few seconds of working on maximum power, these elements will fail. Transistors should be placed on heat sinks through a thin layer of thermal paste, otherwise the effectiveness of such cooling will be minimal.
Diodes, which are used in an induction heater, must be ultra-fast acting. The most suitable diodes for this circuit are: MUR-460; UF-4007; HER – 307.
Resistors used in circuit 3: 10 kOhm power 0.25 W – 2 pcs. and 440 Ohm power - 2 W. Zener diodes: 2 pcs. with an operating voltage of 15 V. The power of the zener diodes must be at least 2 W. A choke for connecting to the power terminals of the coil is used with induction.
To power the entire device you will need a power supply with a power of up to 500 W. and voltage 12 - 40 V. You can power this device from a car battery, but you won’t be able to get the highest power readings at this voltage.

The manufacturing process of the electronic generator and coil itself takes a little time and is carried out in the following sequence:

From copper pipe a spiral with a diameter of 4 cm is made. To make a spiral, a copper tube should be screwed onto a rod with a flat surface with a diameter of 4 cm. The spiral should have 7 turns, which should not touch. Fastening rings are soldered to the 2 ends of the tube for connection to the transistor radiators.
The printed circuit board is made according to the diagram. If it is possible to install polypropylene capacitors, then due to the fact that such elements have minimal losses and stable operation at large amplitudes of voltage fluctuations, the device will operate much more stable. The capacitors in the circuit are installed in parallel to form an oscillating circuit with a copper coil.
Heating the metal occurs inside the coil after the circuit is connected to the power supply or battery. When heating the metal, it is necessary to ensure that there is no short circuit in the spring windings. If you touch 2 turns of the coil at the same time with heated metal, the transistors will fail instantly.

Nuances

When conducting experiments on heating and hardening of metals, inside the induction coil the temperature can be significant and amounts to 100 degrees Celsius. This thermal heating effect can be used to heat water for domestic use or for heating a home.
Diagram of the heater discussed above (Figure 3), at maximum load capable of providing radiation of magnetic energy inside the coil equal to 500 W. This power is not enough for heating large volume water, and the construction of a high-power induction coil will require the manufacture of a circuit in which it will be necessary to use very expensive radio elements.
Budget solution for organizing induction heating of liquids, is the use of several devices described above, located in series. In this case, the spirals must be on the same line and not have a common metal conductor.
AsA stainless steel pipe with a diameter of 20 mm is used. Several induction spirals are “strung” onto the pipe, so that the heat exchanger is in the middle of the spiral and does not come into contact with its turns. When 4 such devices are turned on simultaneously, the heating power will be about 2 kW, which is already sufficient for flow-through heating of liquid with a small circulation of water, to values allowing the use this design in supply warm water small house.
If you connect such a heating element to a well-insulated tank, which will be located above the heater, the result will be a boiler system in which the liquid will be heated inside a stainless pipe, the heated water will rise upward, and a colder liquid will take its place.
If the area of the house is significant, then the number of induction coils can be increased to 10 pieces.
The power of such a boiler can be easily adjusted by turning off or turning on the spirals. The more sections that are turned on at the same time, the greater the power of the heating device operating in this way.
To power such a module you will need a powerful power supply. If you have a DC inverter welding machine, you can use it to make a voltage converter of the required power.
Due to the fact that the system operates on constant electric current, which does not exceed 40 V, the operation of such a device is relatively safe, the main thing is to provide a fuse block in the generator power circuit, which in the event of a short circuit will de-energize the system, thereby eliminating the possibility of a fire.
You can organize “free” home heating in this way., subject to installation for powering induction devices batteries, which will be charged using solar and wind energy.
The batteries should be combined into sections of 2, connected in series. As a result, the supply voltage with such a connection will be at least 24 V, which will ensure the boiler operates at high power. In addition, a series connection will reduce the current in the circuit and increase the service life of the batteries.

Exploitation homemade devices induction heating, does not always eliminate the spread of electromagnetic radiation harmful to humans, therefore the induction boiler should be installed in non-residential premises and shielded with galvanized steel.
Mandatory when working with electricity safety regulations must be followed and, especially this applies to AC networks with a voltage of 220 V.
As an experiment you can make a hob for cooking according to the scheme specified in the article, but it is not recommended to use this device constantly due to imperfections self-made shielding of this device, because of this, the human body may be exposed to harmful electromagnetic radiation that can adversely affect health.

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

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

Physical basis and principle of operation of the furnace

Fig.1. Induction furnace diagram

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

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

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

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

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

Making an inductive heater

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

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

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

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

Generator manufacturing

Fig.3. Lamp circuit

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

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

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

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

Universal generator circuit

Modern induction furnaces operate on more advanced elements - microcircuits and transistors. The universal circuit of a push-pull generator, developing power up to 1 kW, has enjoyed great success. The operating principle is based on an independent excitation generator, with the inductor switched on in bridge mode (Fig. 4).

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

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

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

Conclusion on the topic

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

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

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

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

The 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 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 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 to... mobile communications. Its development required the availability of inexpensive transistors capable of delivering power of several kW per high frequencies. They, in turn, were created on the basis of semiconductor heterostructures, for the research of which Russian physicist Zhores Alferov received the Nobel Prize.

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

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

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

Operating principle

The operating principle of an induction furnace is illustrated in Fig. 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 of a non-ideal dielectric with a dielectric constant >1 potentially achievable Efficiency of induction furnaces decreases, and in an environment with 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:

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;
Secondary short-circuited winding – melt;
A ring-shaped crucible made of heat-resistant dielectric in which the melt is placed;
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 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 an inductor stove will help out, and the electricity consumption will be feasible for the family budget: after all, the main share of heating energy comes from the latent heat of metal melting. And by changing the power, frequency and location of the part in the inductor, you can heat exactly the right place exactly as it should, see fig. higher.

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

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

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

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

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

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

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

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

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

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

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

Note: for comparison, an inductor for a melting furnace for 150 kg of steel is bent from a copper pipe with an outer diameter of 40 mm and an inner diameter of 30 mm. The number of turns is 7, the inside diameter of the coil is 400 mm, and the height is also 400 mm. To boost it in zero mode you need 15-20 kW if available closed loop cooling 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 - air condenser variable capacity C, with a gap between the plates of at least 3 mm. Works only on zero mode. The setting indicator is a neon lamp L. The peculiarity of the circuit is a very soft, “lamp” emission spectrum, so you can use this generator without special measures 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 from short circuit to DC 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 similar ones lying around somewhere, then you can start experiments on induction heating with this circuit. The transistor must be installed on a radiator with an area of at least 400 square meters. see with blowing from a computer or similar fan. Adjustment of the capacity in the inductor, up to 0.3 kW, by changing the supply voltage within 6-24 V. Its source must provide a current of at least 25 A. The power dissipation of the resistors of the basic voltage divider is at least 5 W.

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

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

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

Video: homemade induction heater in action

Induction boilers

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

Let's say this 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.

An industrial induction heating boiler is designed in a completely different way: 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:

It never burns out - there is no hot electric coil in it.
The massive labyrinth reliably shields the inductor: PES in the immediate vicinity of the 30 kW induction boiler is zero.
Efficiency – more than 99.5%
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.
The breakdown itself, due to the “oakiness” of the structure, is extremely unlikely.
Does not require separate grounding.
Indifferent to lightning strikes; It cannot burn a massive coil.
The large surface of the labyrinth ensures effective heat exchange with a minimum temperature gradient, which almost eliminates the formation of scale.
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. A diagram of a low-power induction heater for hot water systems with a storage tank is shown. 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 doesn’t matter, but metal must be isolated from the rest of the system dielectric inserts, as shown in Fig., so that the secondary current is closed only through water.

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

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

Video: “induction” boiler based on household tiles

Inductor in the kitchen

Induction hobs 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 hob- 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, according to sanitary requirements, the current in the inductor must be a pure sinusoid, and its magnitude and frequency must vary in a complex way according to the degree of readiness of the dish. That is, the generator must have digital generation of the output current, controlled by the same microcontroller.

There is no point in making a kitchen induction hob yourself: more money will be spent on electronic components alone at retail prices than on 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.”

However, induction cookers have many advantages over others:

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

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