How to make an induction heater with your own hands from the welding inverter. Induction heating, basic principles and technology budget module induction heating

Induction heating (induction heating) - the method of contactless heating of high frequency currents (English. RFH - Radio-Frequency Heating, heating by the waves of the radio frequency range) of electrically conductive materials.

Description of the method.

Induction heating is the heating of materials electric currentswhich are induced by a variable magnetic field. Therefore, it is the heating of products from conductive materials (conductors) by a magnetic field of inducers (sources of an alternating magnetic field). Induction heating is carried out as follows. Electrically conductive (metallic, graphite) Billet is placed in the so-called inductor, which is one or more turns of the wire (most often copper). In the inductor, with the help of a special generator, powerful currents of different frequencies are subject to (from a dozen Hz to several MHz), as a result of which an electromagnetic field occurs around the inductor. The electromagnetic field suggests vortex currents in the workpiece. The vortex currents warm up the workpiece under the action of Joulehe heat (see the Joule-Lenza law).

The "inductor-blank" system is a non-dedicated transformer in which the inductor is a primary winding. The workpiece is a secondary winding, closed spice. The magnetic flow between the windings is closed through the air.

At high frequency, the vortex currents are displaced by the magnetic field formed by the same magnetic field into thin surface layers of the blank δ (surface effect), as a result of which their density increases sharply, and the workpiece is heated. The following metal layers are heated due to thermal conductivity. It is not current current, but a large current density. In the skin-layer Δ The current density decreases in e times relative to the current density on the surface of the workpiece, while 86.4% of heat is released into the skin-layer. The depth of the skin layer depends on the frequency of the radiation: the higher the frequency, the thinner Skin layer. It also depends on the relative magnetic permeability of μ material of the workpiece.

For iron, cobalt, nickel and magnetic alloys at temperatures below the point of Curie μ has a value from several hundred 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 depth of the skin layer for copper is about 0.25 mm, for iron ≈ 0.001 mm.

The inductor is strongly heated during operation, as it absorbs itself its own radiation. In addition, it absorbs thermal radiation from a split billet. Make inductors from copper tubes cooled by water. Water is satisfied with suction - this ensures safety in the event of a burner or other depressurization of the inductor.

Application:
Ultrapy contactless smelting, soldering and welding of metal.
Obtaining prototypes of alloys.
Bending and heat treatment of machine parts.
Jewelry.
Processing small parts that may be damaged during gas-flame or arc heating.
Surface hardening.
Hardening and heat treatment of parts of complex shape.
Disinfection of medical instrument.

Benefits.

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

It is possible to heat in an atmosphere of protective gas, in an oxidative (or reducing) medium, in a non-conductive fluid, in vacuo.

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

Due to the arising MHD, the efforts occurs intensive mixing of the liquid metal, up to hold of it in a suspended state in air or protective gas - so far be obtained alloys in small quantities (levitational smelting, smelting in an electromagnetic crucible).

Because warming up through electromagnetic radiation, There is no pollution of the preparation of the combustion products of the torch in the case of gas flame heating, or the material of the electrode in the case of arc heating. Placing samples into an atmosphere of inert gas and high speed Heating will eliminate the scale.

Ease of operation due to a small inductor size.

The inductor can be made of a special form - this will allow it evenly to warm over the entire surface of the detail of a complex configuration, without leading to their warping or local non-rest.

Easy to carry out local and selective heating.

Since the most intense warm-up goes in thin upper layers Billets, and the underlying layers warmer more gently due to thermal conductivity, the method is ideal for carrying out surface hardening of parts (the core remains viscous).

Easy equipment automation - heating and cooling cycles, adjustment and deterring temperature, feed and eat blanks.

Induction heating installations:

At installations with a working frequency of up to 300 kHz, inverters are used on IGBT assemblies or MOSFET transistors. Such installations are designed for heating large parts. High frequencies are used to warm up small parts (up to 5 MHz, medium and short wave range), high frequency settings are built on electronic lamps.

Also, for warming up small parts, the installation of increased frequency on the MOSFET transistors on operating frequencies to 1.7 MHz is being built. Transistor management and their protection at elevated frequencies represent certain difficulties, so the installation of an increased frequency is still quite expensive.

Inductor for heating small parts has small sizes and a small inductance, which leads to a decrease in the quality of the working oscillatory circuit at low frequencies and a reduction in efficiency, and is also a danger to the specifying generator (the voltage of the oscillating circuit is proportional to the L / C, the oscillatory loop with low quality is too good "pumped" with the energy forms a short circuit in the inductor and displays the specifying generator). To increase the voluntaryness of the oscillatory circuit, use two ways:
- increasing the operating frequency, which leads to the complication and appreciation of the plant;
- the use of ferromagnetic inserts in the inductor; Pluging inductor with ferromagnetic material panels.

Since the most efficient inductor works on high frequencies, Industrial use of induction heating received after the development and start of production of powerful generator lamps. Prior to World War I, induction heating had limited use. As generators, then used machine generators of an increased frequency (work V. P. Vologdin) or spark discharge settings.

The generator scheme can be in principle any (multivibrator, RC generator, an independent excitation generator, various relaxation generators) operating on the load in the form of an inductor coil and with sufficient power. It is also necessary that the frequency of oscillations is high enough.

For example, to "cut" in a few seconds steel wire with a diameter of 4 mm, a oscillatory capacity is needed at least 2 kW at a frequency of at least 300 kHz.

Choose a scheme for the following criteria: Reliability; stability of oscillations; stability of power secreted in the billet; simplicity; convenience of setting; minimum number of parts to reduce cost; Application of parts, in the amount of mass reduction and dimensions, etc.

For many decades, an inductive trimmer was used as a high-frequency oscillation generator (Hartley Generator, an autotransformed feedback generator, a circuit on an inductive contour voltage divider). This is a self-exciting scheme of parallel power of an anode and a frequency-selective chain, made on the oscillatory circuit. It was successfully used and continues to be used in laboratories, jewelry workshops, on industrial enterprisesas well as in amateur practice. For example, during the Second World War on such installations, a surface hardening of the rollers of T-34 tank was performed.

Disadvantages of three points:

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

The strong deviation of the frequency at the time of heating the billets from magnetic materials above the Curie point (≈700C) (changes μ), which changes the depth of the skin-layer and unpredictably changes the heat treatment mode. When thermal processing of responsible parts, it may be unacceptable. Also, powerful TDHs should operate in a narrow range of frequencies permitted Rossvyazokhrankulture, since with poor shielding are actually radio transmitters and can interfere with television and radio broadcasting, coastal and rescue services.

When changing billets (for example, smaller to larger) changes the inductance of the inductor-blank system, which also leads to a change in the frequency and depth of the skin layer.

When changing single inductors on multi-ski, on larger or more small-sized frequencies also changes.

Under the leadership of Babat, Lozinsky and other scientists, two- and three-constructive schemes of generators with a higher efficiency (up to 70%) were developed, as well as better retaining operating frequency. The principle of their action is as follows. Due to the use of related contours and weakening the relationship between them, the change in the inductance of the working circuit does not entail strong change frequency frequency circuit. By the same principle, radio transmitters are designed.

Modern TVH-generators are inverters on IGBT assemblies or powerful MOSFET transistors, usually made according to the bridge or semi-sequence. Work at frequencies up to 500 kHz. Transistor shutters are open using a microcontroller control system. The control system, depending on the task, allows you to automatically hold

A) constant frequency
b) constant power allocated in the workpiece
c) the highest efficiency.

For example, when the magnetic material is heated above the Curie point, the skin-layer thickness increases sharply, the current density drops, and the billet begins to warm worse. Also disappear magnetic properties The material and the process of magnetization stops - the billet begins to heat worse, the load resistance is abruptly decrease - this can lead to a "separation" of the generator and failure. The control system monitors the transition through the Curie point and automatically increases the frequency with a hopping load reduction (or reduces power).

Comments.

Inductor if possible, it is necessary to have as close as possible to the workpiece. This not only increases the density of the electromagnetic field near the workpiece (in proportion to the square of the distance), but also increases the power coefficient COS (φ).

Increasing the frequency sharply reduces the power coefficient (proportional to the frequency cube).

When the magnetic materials are heated, additional heat is also highlighted due to reclamation, their heating to the point Curie is much more efficient.

When calculating the inductor, it is necessary to take into account the inductance of the entrance to the inductor of the tire, which can be much more inductance of the inductor itself (if the inductor is made in the form of a single turn of a small diameter or even parts of the turn - arcs).

There are two cases of resonance in oscillatory circuits: stress resonance and current resonance.
Parallel oscillatory circuit - reasons.
In this case, on the coil and on the condenser, the voltage is the same as the generator. With resonance, the contour resistance between branching points becomes the maximum, and the current (I total) through the load resistance RN will be minimal (current inside the loop I-1L and I-2C larger than the generator current).

In the ideal case, the total contour resistance is equal to infinity - the diagram does not consume the current from the source. When the generator frequency changes, to any side of the resonance frequency, the total contour resistance decreases and the linear current (I Society) increases.

Sequential oscillating circuit - stress resonance.

The main feature of the serial resonance contour is that its full resistance is minimally with resonance. (Zl + zc - minimum). When adjusting the frequency by a value exceeding or the resonant frequency below, the imperative increases.
Output:
In a parallel circuit, with a resonance, the current through the conclusions of the contour is 0, and the voltage is maximum.
In a sequential circuit, on the contrary, the voltage tends to zero, and the current is maximum.

The article is taken from the site http://dic.academic.ru/ and recycled to a more understandable text for the reader, the company LLC "Prominductor".

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