induction heating
Method of raising the temperature of an electrically conductive material by subjecting it to an alternating electromagnetic field. Energy in the electric currents induced in the object is dissipated as heat. Induction heating is used in metalworking to heat metals for soldering, tempering, and annealing, and in induction furnaces for melting and processing metals. The principle of the induction-heating process resembles that of the transformer. A water-cooled coil (inductor), acting as the primary winding of a transformer, surrounds the material to be heated (the workpiece), which acts as the secondary winding. Alternating current flowing in the primary coil induces eddy currents in the workpiece, causing it to become heated. The depth to which the eddy currents penetrate, and therefore the distribution of heat within the object, depend on the frequency of the primary alternating current and the magnetic permeability, as well as the resistivity, of the material.
induction heating [in′dək·shən ¦hēd·iŋ]
Induction heating
The heating of a nominally electrical conducting material by eddy currents induced by a varying electromagnetic field. The principle of the induction heating process is similar to that of a transformer. In the illustration, the inductor coil can be considered the primary winding of a transformer, with the workpiece as a single-turn secondary. When an alternating current flows in the primary coil, secondary currents will be induced in the workpiece. These induced currents are called eddy currents. The current flowing in the workpiece can be considered as the summation of all of the eddy currents.
In the design of conventional electrical apparatus, the losses due to induced eddy currents are minimized because they reduce the overall efficiency. However, in induction heating, their maximum effect is desired. Therefore close spacing is used between the inductor coil and the workpiece, and highcoil currents are used to obtain the maximum induced eddy currents and therefore high heating rates. See Core loss
Induction heating is widely employed in the metalworking industry for a variety of industrial processes. While carbon steel is by far the most common material heated, induction heating is also used with many other conducting materials such as various grades of stainless steel, aluminum, brass, copper, nickel, and titanium products. See Brazing, Heat treatment (metallurgy), Soldering
The advantages of induction heating over the conventional processes (like fossil furnace or salt-bath heating) are the following: (1) Heating is induced directly into the material. It is therefore an extremely rapid method of heating. It is not limited by the relative slow rate of heat diffusion in conventional processes using surface-contact or radiant heating methods. (2) Because of skin effect, the heating is localized and the heated area is easily controlled by the shape and size of the inductor coil. (3) Induction heating is easily controllable, resulting in uniform high quality of the product. (4) It lends itself to automation, in-line processing, and automatic-process cycle control. (5) Startup time is short, and standby losses are low or nonexistent. (6) Working conditions are better because of the absence of noise, fumes, and radiated heat. See Electric heating
Induction Heating
the heating of current carriers through the generation of electric currents in them by an alternating electromagnetic field. The power released in a conductor by induction heating depends on the dimensions and physical properties of the conductor (specific electric resistivity and relative magnetic permeability), as well as on the frequency and intensity of the magnetic field. In induction heating the electromagnetic field is supplied by induction heaters.
Induction heating is characterized by the nonuniform release of power in the object being heated. Eighty-six percent of the power is released in the surface layer (the so-called penetration). The penetration of the current Δ (in meters) is Δ = 
where ρ is the specific electrical resistivity in ohms • m, μ, is the relative magnetic permeability, and f is the frequency in hertz (Hz).
Low-frequency (50 Hz), medium-frequency (up to 10 kHz) and high-frequency (over 10 kHz) currents are used in induction heating to generate an alternating electromagnetic field. Mechanical and static converters, as well as tube oscillators, are used to supply medium- or high-frequency current to induction heaters.
Induction heating is most widely used in the melting of metals, zone melting, and heating for pressure shaping. Induction heating is the most advanced contactless method of transmitting electrical energy to the object being heated, converting electrical energy directly into thermal energy. A schematic diagram of a device using induction heating is shown in Figure 1.
REFERENCES
Babat, G. I. Induktsionnyi nagrev melallov i egopromyshlennoe primenenie. 2nd ed. Moscow-Leningrad, 1965.Vysokochastotnaia elektrotermiia: Spravochnik. Moscow-Leningrad, 1965.
Elektrotermicheskoe oborudovanie: Spravochnik. Moscow, 1967.
A. B. KUVAIDIN
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