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

A closed conductor in induction cookers is metal utensils, which are heated by vortex electric currents. In general, the principle of operation of such devices is not complicated, and with little knowledge in physics and electrical engineering, it will not be difficult to assemble an induction heater with your own hands.

The following devices can be made independently:

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

Do-it-yourself induction cooker must be made in compliance with all norms and rules for the operation of these devices. If electromagnetic radiation dangerous for humans is emitted outside the case in the lateral directions, then it is strictly forbidden to use such a device.

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

1. Ideal for conducting electromagnetic radiation.
2. Not conductive.
3. Withstand high temperature stress.

In household hobs induction surfaces expensive ceramics are used, when made at home induction cooker, to find a worthy alternative to such material is quite difficult. Therefore, to begin with, you should design something simpler, for example, an induction furnace for hardening metals.

Manufacturing instructions

Blueprints

For the manufacture of the furnace you will need the following materials and tools:

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

Additional materials and their features:

1. To make a coil, which will emit an 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.
2. Powerful power transistors are the most expensive part of a homemade induction installation. To mount the frequency generator circuit, it is necessary to prepare 2 such elements. For these purposes, transistors of brands are suitable: IRFP-150; IRFP-260; IRFP-460. In the manufacture of the circuit, 2 identical of the listed field-effect transistors are used.
3. 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 a high-power alternating current to form in the coil, 7 such capacitors are required.
4. During the operation of such an induction device, field-effect transistors will get very hot and if radiators from aluminum alloy, then after a few seconds of operation at maximum power, these elements will fail. Install transistors on heat sinks through thin layer thermal paste, otherwise the efficiency of such cooling will be minimal.
5. Diodes, which are used in an induction heater, must be of ultra-fast action. The most suitable for this circuit, diodes: MUR-460; UV-4007; HER-307.
6. Resistors used in circuit 3: 10 kOhm with a power of 0.25 W - 2 pcs. and 440 ohm power - 2 watts. Zener diodes: 2 pcs. with an operating voltage of 15 V. The power of the zener diodes must be at least 2 watts. A choke for connecting to the power outputs of the coil is used with induction.
7. To power the entire device, you will need a power supply unit with a capacity of up to 500. W. and voltage 12 - 40 V. You can power this device from a car battery, but you will not be able to get the highest power readings at this voltage.

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

1. From copper pipe a spiral with a diameter of 4 cm is made. To make a spiral, a copper tube should be wound onto a rod with flat surface 4 cm in diameter. The spiral should have 7 turns, which should not touch. Mounting rings are soldered to the 2 ends of the tube for connection to the transistor radiators.
2. The printed circuit board is made according to the scheme. If it is possible to supply 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 work much more stable. The capacitors in the circuit are installed in parallel, forming an oscillatory circuit with a copper coil.
3. Metal heating occurs inside the coil, after the circuit is connected to a power supply or battery. When heating the metal, it is necessary to ensure that there is no short circuit of the spring windings. If you touch the heated metal 2 turns of the coil at the same time, then the transistors fail instantly.

Nuances

1. When conducting experiments on heating and hardening metals, inside the induction coil the temperature can be significant and amounts to 100 degrees Celsius. This heating effect can be used to heat domestic water or to heat a house.
2. Scheme of the heater discussed above (Figure 3), at maximum load it is able to provide the radiation of magnetic energy inside the coil equal to 500 watts. Such power is not enough to heat a large volume of 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.
3. A budget solution for organizing induction heating of a liquid, is the use of several devices described above, arranged in series. In this case, the spirals must be on the same line and not have a common metal conductor.
4. 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. With the simultaneous inclusion of 4 such devices, the heating power will be about 2 kW, which is already enough for flow heating liquids with a small circulation of water, up to values ​​that allow the use this design in supply warm water small house.
5. If we connect such heating element with well insulated tank, which will be located above the heater, the result will be a boiler system in which the heating of the liquid will be carried out inside the stainless pipe, the heated water will rise up, and a colder liquid will take its place.
6. If the area of ​​the house is significant, the number of induction coils can be increased up to 10 pieces.
7. The power of such a boiler can be easily adjusted by turning off or on the spirals. The more sections that are simultaneously turned on, the greater the power of the heating device operating in this way will be.
8. To power such a module, you need a powerful power supply. If there is an inverter welding machine direct current, then it is possible to make a voltage converter of the required power from it.
9. Due to the fact that the system operates on direct 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.
10. It is possible to organize “free” heating of the house in this way, subject to installation for powering induction devices batteries, which will be charged by solar and wind energy.
11. Batteries should be combined in 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 operation of the boiler at high power. In addition, series connection will reduce the current in the circuit and increase the battery life.

1. Exploitation homemade devices induction heating, does not always make it possible to exclude the spread of electromagnetic radiation harmful to humans, therefore the induction boiler should be installed in a non-residential area and shielded with galvanized steel.
2. Mandatory when working with electricity safety regulations must be followed and especially for networks alternating current voltage 220 V.
3. As an experiment can be made hob for cooking food according to the scheme indicated in the article, but it is not recommended to constantly operate this device due to imperfections self-manufacturing shielding of this device, because of this, exposure to the human body of harmful electromagnetic radiation that can adversely affect health.

To heat up to red or even melt a small metal object at home, it is not at all necessary to fire up the stove and transfer fuel - modern technologies allow for this to use high-frequency currents (HF). And the simplest (and most common) circuit for an induction heater of metals will be a multivibrator based on field-effect transistors. By at least these modules are assembled from Chinese sites just . Next, see 2 models that differ in power and, of course, in price.

ZVS50- induction heating module entry level, the module can be powered even from batteries with a voltage of up to 12 volts, that is, both from autonomous power supply, and from the network PSU. The price on www.banggood.com is about $8.

• Input voltage: 5-12V
• Board dimensions: 5.5 x 4 x 2 cm
• Coil size: length 2.8, diameter 2 cm

ZVS1000- a module for induction heating of metals by high frequency currents, with a power of up to 1000w. The average price is $35.

This induction heating unit uses 12-48V DC power supply, maximum current 20A, maximum power 1000W. Can be used for processing small parts: hardening, annealing and other heat treatment. It can also be used with a crucible to melt gold, silver, copper, aluminum and other metals. Fast and uniform heating, which is very convenient for jewelers.

• Coil Inner Diameter: 40mm
• Coil Height: 50mm
• At 48 V no load current 5 A

The higher the voltage, the greater the heating current, and hence the power transmitted to the metal. The coil can accept inside 40 mm crucible. It is necessary to use the device with power supplies of the appropriate power and put a cooling cooler on the radiator.

The size of the object that is heated inside the induction coil cannot exceed 1/4 of the volume, otherwise overload and combustion of the circuit may occur. While this circuit can handle 30A temporarily - for long term operation the current should not exceed 20A for safe operation.

Induction heating is a method of non-contact heating by high-frequency currents (eng. RFH - radio-frequency heating, heating by radio-frequency waves) of electrically conductive materials.

Description of the method.

Induction heating is the heating of materials by electric currents that are induced by an alternating magnetic field. Therefore, this is the heating of products made of conductive materials (conductors) by the magnetic field of inductors (sources of an alternating magnetic field). Induction heating is carried out as follows. An electrically conductive (metal, graphite) workpiece is placed in the so-called inductor, which is one or more turns of wire (most often copper). Powerful currents of various frequencies (from tens of Hz to several MHz) are induced in the inductor using a special generator, as a result of which an electromagnetic field arises around the inductor. The electromagnetic field induces eddy currents in the workpiece. Eddy currents heat the workpiece under the action of Joule heat (see the Joule-Lenz law).

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

At a high frequency, eddy currents are displaced by the magnetic field formed by them into thin surface layers of the workpiece Δ ​​(Surface-effect), as a result of which their density increases sharply, and the workpiece is heated. The underlying layers of the metal are heated due to thermal conductivity. It is not the current that is important, but the high current density. In the skin layer Δ, the current density decreases by a factor of e relative to the current density on the workpiece surface, while 86.4% of heat is released in the skin layer (of the total heat release. The depth of the skin layer depends on the radiation frequency: the higher the frequency, the thinner skin layer It also depends on the relative magnetic permeability μ of the workpiece material.

For iron, cobalt, nickel and magnetic alloys at temperatures below the Curie point, μ has a value from several hundreds to tens of thousands. For other materials (melts, non-ferrous metals, liquid low-melting eutectics, graphite, electrolytes, electrically conductive ceramics, etc.), μ is approximately equal to one.

For example, at a frequency of 2 MHz, the skin depth for copper is about 0.25 mm, for iron ≈ 0.001 mm.

The inductor gets very hot during operation, as it absorbs its own radiation. In addition, it absorbs heat radiation from a hot workpiece. They make inductors from copper tubes cooled by water. Water is supplied by suction - this ensures safety in case of a burn or other depressurization of the inductor.

Application:
Ultra-clean non-contact melting, soldering and welding of metal.
Obtaining prototypes of alloys.
Bending and heat treatment of machine parts.
Jewelry business.
Machining small parts that can be damaged by flame or arc heating.
Surface hardening.
Hardening and heat treatment of parts of complex shape.
Disinfection of medical instruments.

Advantages.

High-speed heating or melting of any electrically conductive material.

Heating is possible in a protective gas atmosphere, in an oxidizing (or reducing) medium, in a non-conductive liquid, in a vacuum.

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

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

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

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

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

It is easy to carry out local and selective heating.

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

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

Induction heating units:

On installations with an operating frequency of up to 300 kHz, inverters on IGBT assemblies or MOSFET transistors are used. Such installations are designed for heating large parts. To heat small parts, high frequencies are used (up to 5 MHz, the range of medium and short waves), high-frequency installations are built on electronic tubes.

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

The inductor for heating small parts has small size and a small inductance, which leads to a decrease in the quality factor of the working oscillatory circuit at low frequencies and a decrease in efficiency, and also poses a danger to the master oscillator (the quality factor of the oscillatory circuit is proportional to L / C, an oscillatory circuit with a low quality factor is “pumped” too well with energy, forms a short circuit through the inductor and disables the master oscillator). To increase the quality factor of the oscillatory circuit, two ways are used:
- increasing the operating frequency, which leads to the complexity and cost of the installation;
- the use of ferromagnetic inserts in the inductor; pasting the inductor with panels of ferromagnetic material.

Since the inductor works most efficiently at high frequencies, industrial application received induction heating after the development and start of production of powerful generator lamps. Prior to World War I, induction heating was of limited use. At that time, high-frequency machine generators (works by V.P. Vologdin) or spark discharge installations were used as generators.

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

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

Select a scheme for the following criteria: reliability; fluctuation stability; stability of the power released in the workpiece; ease of manufacture; ease of setup; minimum number of parts to reduce cost; the use of parts that in total give a reduction in weight and dimensions, etc.

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

Disadvantages of three dots:

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

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

When the workpieces are changed (for example, from a smaller one to a larger one), the inductance of the inductor-workpiece system changes, which also leads to a change in the frequency and depth of the skin layer.

When changing single-turn inductors to multi-turn ones, to larger or smaller ones, the frequency also changes.

Under the leadership of Babat, Lozinsky and other scientists, two- and three-circuit generator circuits were developed that have a higher efficiency (up to 70%), and also better keep the operating frequency. The principle of their action is as follows. Due to the use of coupled circuits and the weakening of the connection between them, a change in the inductance of the working circuit does not entail a strong change in the frequency of the frequency setting circuit. Radio transmitters are constructed according to the same principle.

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

A) constant frequency
b) constant power released in the workpiece
c) maximum efficiency.

For example, when a magnetic material is heated above the Curie point, the thickness of the skin layer increases sharply, the current density drops, and the workpiece begins to heat up worse. The magnetic properties of the material also disappear and the magnetization reversal process stops - the workpiece begins to heat up worse, the load resistance abruptly decreases - this can lead to the "spacing" of the generator and its failure. The control system monitors the transition through the Curie point and automatically increases the frequency with an abrupt decrease in load (or reduces power).

Remarks.

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

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

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

When calculating the inductor, it is necessary to take into account the inductance of the tires leading to the inductor, which can be much greater than the inductance of the inductor itself (if the inductor is made in the form of a single turn of a small diameter or even part of a turn - an arc).

There are two cases of resonance in oscillatory circuits: voltage resonance and current resonance.
Parallel oscillatory circuit - resonance of currents.
In this case, the voltage on the coil and on the capacitor is the same as that of the generator. At resonance, the resistance of the circuit between the branching points becomes maximum, and the current (I total) through the load resistance Rn will be minimal (the current inside the circuit I-1l and I-2s is greater than the generator current).

Ideally, the loop impedance is infinity - the circuit draws no current from the source. When the generator frequency changes in any direction from the resonant frequency, the circuit impedance decreases and the linear current (Itotal) increases.

Series oscillatory circuit - voltage resonance.

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

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