Induction furnaces working with metals: principle of operation, homemade device diagram, safety rules

Developed over a century ago, induction ovens are a part of our everyday life. This became possible thanks to the development of electronics. Explosive growth in the power of controllers made on the basis of silicon semiconductors and the appearance in widespread sale transistors capable of providing high power (several kilowatts) in recent years have acquired the character avalanches. All this gave mankind incredibly great prospects in the development of miniature installations, comparable in power with industrial devices of the near past.

Use and structure of the device

The use of induction furnaces in the household allows you to avoid the appearance of foci in the room open flame and is a fairly effective way of melting and controlled heating of metals and alloys. This is due to the fact that the metal heats up, heats up and melts not under the influence of high-temperature burners, and by passing high-frequency currents through themselves, stimulating the active movement of particles in the structure material.

It became possible to appear in everyday life:

• Compact duct induction furnaces, in which you can melt metal and create various products by casting and structures, purify various precious alloys from impurities and temper products, giving them additional strength.
• Hot water boilers, whose efficiency already lies far beyond the limits of conventional boilers.
• Stoves for cooking, which are not only safer than gas in terms of operational characteristics, but also more effective than some microwave ovens in the area of ​​heating food and maintaining its temperature.
• Crucible smelters, which have gained the most admirers among people engaged in self-production and repair of electrical devices.

In addition, electric induction stoves are becoming more widespread, which work not only with conductive material. Their device is slightly different from conventional induction furnaces, since it is based on heating by electric induction of a material that does not conduct current (they are also called dielectrics) between capacitor plates, that is, its leads of different polarity. At the same time, the temperatures reached are not very high (about 80-150 degrees Celsius), therefore such installations are used to melt plastic or heat it.

Design features and principle of operation

An induction furnace works on the basis of the formation of eddy electric currents in it. For this, an inductor consisting of turns of a thick wire is used, to which an alternating current source is supplied. It is the alternating current that forms a magnetic field that is constantly changing depending on the current frequency. It also provokes the transfer of these currents to the substance placed inside the coil, along with a large amount of heat. In this case, even the most ordinary welding inverter can act as a generator.

There are two types of induction furnaces:

1. With a magnetic core, the peculiarity of which is the location of the inductor inside the volume of the metal that can be melted.
2. Without magnetic core - when the inductor is outside.

The design with the presence of a magnetic circuit is used, for example, in channel furnaces. They use an unopened metal (most often steel) magnetic circuit, inside which there are a melting crucible and an inductor, which form the primary winding circuit. The crucible material can be graphite, refractory clay, or any other non-conductive material with suitable heat resistance. The metal that needs to be melted is placed in it. These are, as a rule, all kinds of non-ferrous alloys, duralumin and cast iron.

The generator of such a furnace must provide alternating current frequency within 400 hertz. It is also possible to use a conventional electrical network instead of a generator and to power the furnace using a current with a frequency at 50 hertz, but in this case the heating temperature will be lower and for more refractory alloys such an installation is not will do.

Crucible furnaces, which do not have a magnetic circuit in their design, have become much more widespread among enthusiasts. They use currents of much higher frequency to achieve higher field density. This is due precisely to the absence of a magnetic circuit - too much of the field energy is dissipated in space. To counteract this, it is necessary to fine-tune the oven:

• Ensure equal frequency of the induction circuit and the voltage from the generator (when using an inverter, this is easiest to do).
• Select the diameter of the melting crucible so that it is close to the wavelength of the received radiation of the magnetic field.

In this way, losses can be minimized by up to 25% of the total power. To achieve the best result, it is recommended to set twice or even three times the frequency of the AC source than the resonant one. In this case, the diffusion of metals that make up the alloy will be maximum, and its quality will be much better. If you increase the frequency and further, you can achieve the effect of pushing the high-frequency field to the surface of the product and thus quench it.

Vacuum Melting Furnaces

This type of installation can hardly be called domestic, but it is worth considering them due to the fact that vacuum melting has a number of technological advantages over other types. By its design, it resembles a crucible, with the difference that the furnace itself is in a vacuum chamber. This makes it possible to achieve greater purity of the metal melting process, reduce its oxidizability during processing and speed up the process, achieving significant energy savings.

In addition, the limited and closed space helps to avoid the release of harmful vapors of melting metals into the surrounding space and to maintain the purity of the process of their processing. The ability to control the composition and processing process is also one of the advantages of this type of oven.

Duct induction units

Another type of industrial ovens that are more widely used than others. They can be used not only as smelters, but also as distributors of prepared material and mixers of several types of raw materials. Typical designs for such devices include:

• The presence of a bath in which there is a raw material that has reached or is reaching a given temperature.
• The channel through which the molten mass passes through the magnetic field.
• A magnetic circuit that provides constant circulation of liquid metal.
• Primary winding coil that drives a magnetic field.

The slightest opening of the circuit, which is formed by the liquid metal, the magnetic core and the coil, leads to an increase in its own resistance and instantaneous ejection of the entire mass of raw materials from the channel. To counteract this phenomenon, a "swamp" is left inside the channel - a small mass of metal, which is maintained in a liquid state.

Advantages of channel-type inductive furnaces:

• Low cost of installations.
• Economical - to maintain the temperature inside the bath, which does not dissipate heat well, you need a small amount of electricity.
• The efficiency of the inductor during operation is very high.

Disadvantages:

• The slow movement of the molten metal through the channel makes it difficult to control its quality and oxidation.
• The need to leave a certain amount of raw materials inside lowers the quality of the chemical composition of the next charge and the possibility of its finer control.
• The need to maintain the tightness of the installation due to the threat of rupture of the magnetic field and the formation of vortex radiation. Difficulty maintaining isolation when lining the internal walls of the installation with some compounds.

The main elements of the furnace circuit

In order to assemble the installation and carry out work on it, it is necessary to find a suitable induction furnace circuit and parts for it. To find the latter, it is very useful to have one or more unnecessary power supplies from a computer, since most of the details can be found in them. A typical diagram of a simple oven with a homemade inverter will include elements such as:

• Field transistors, you can use IRFZ46N or analogs (IRFZ44V, which has a current strength of 55 amperes at the drain leg, is even better). It is advisable to select field workers with the maximum possible value of the breakdown voltage, so they will last much longer.
• Chokes, resistors with a resistance of 470 Ohm (you can use one watt or two half watt, connected in circuit in series) and nine small-capacity capacitors (up to 1 microfarad) that can be evaporated from the unit nutrition.
• Radiators for cooling transistors - field workers in TO-220AV cases are very hot during operation and can explode from a lack of heat removal from them.
• A copper wire with a diameter of about a millimeter to create ferrite rings and a diameter of 2 millimeters to create an inductor.
• Diodes of UF4007 brands, 2 pieces, but it is better to have a couple of spare ones in case you collect something wrong for the first time - they will fly out first.
• A battery with a capacity of about 8-10 ampere-hours. These are usually taken from old uninterruptible power supplies and have an output voltage of 12 volts.
• As a crucible, you can mold and burn on a fire or with a burner a clay pot of the diameter you need.

The inverter for installation is assembled according to the scheme proposed by S. V. Kukhtetsky for laboratory tests. It can be easily found on the Internet. The power of the inverter, which is powered from a voltage in the range of 12-35 volts, will be 6 kilowatts, and its operating frequency will be 40-80 kilohertz, this will be more than enough for home projects.

Safety at work

Since working with an induction furnace involves close contact with molten metal and currents high frequency and strength, it is worth taking care of the high-quality grounding of the installation and reliable means protection. In this case, the clothes must strictly comply with all the requirements:

• To be made of dense, non-consumable and non-combustible material.
• The basic protective suit should include an apron and gloves. Whenever possible, you should wear shoes with rubberized soles on your feet, while your feet and socks should be dry.
• To protect your eyes, you should purchase special glasses, this will save you from accidentally getting a red-hot piece of metal in your eyes.

Do not forget about the good ventilation of the room in which they will work. The molten metal releases chemical compounds into the air that are not good for your lungs.