PRINCIPLE OF OPERATION OF INDUCTION FURNACES. INDUCTION HEATING

Induction heating and the principle of operation of induction furnaces is to convert the energy of the electromagnetic field absorbed by the electrically conductive heated object into thermal energy.

In induction heating installations, the electromagnetic field is created by an inductor, which is a multi-turn cylindrical coil (solenoid). An alternating electric current is passed through the inductor, as a result of which an alternating magnetic field that changes in time arises around the inductor. This is the first energy conversion of the electromagnetic field, described by the first Maxwell equation.

The heated object is placed inside or near the inductor. The varying (in time) flux of the vector of magnetic induction created by the inductor penetrates the heated object and induces an electric field. The electric lines of this field are located in a plane perpendicular to the direction of the magnetic flux, and are closed, that is, the electric field in the heated object has a vortex character. Under the action of an electric field, according to Ohm’s law, conduction currents (eddy currents) arise. This is the second energy conversion of the electromagnetic field, described by the second Maxwell equation.

In a heated object, the energy of the induced alternating electric field irreversibly transforms into heat. Such thermal dissipation of energy, resulting in the heating of the object, is determined by the existence of conduction currents (eddy currents). This is the third transformation of the energy of the electromagnetic field, and the energy ratio of this transformation is described by the Lenz – Joule law.

The described energy conversion of the electromagnetic field makes it possible:

transfer the electrical energy of the inductor to the heated object without resorting to the contacts (unlike resistance furnaces)
allocate heat directly in the heated object (the so-called “furnace with an internal heat source” in the terminology of Prof. N. V. Okorokov), as a result of which the use of thermal energy turns out to be the most perfect and the heating rate increases significantly (compared to the so-called “stoves with external heat source “).
The magnitude of the electric field in a heated object is influenced by two factors: the magnitude of the magnetic flux, i.e. the number of magnetic field lines penetrating the object (or coupled with the heated object), and the frequency of the supply current, i.e. the frequency of changes (in time a) magnetic flux coupled with a heated object.

This makes it possible to perform two types of installations of induction heating, which differ in their design and operational properties: induction installations with a core and without a core.

According to the technological purpose, installations of induction heating are divided into melting furnaces for melting metals and heating installations for heat treatment (quenching, tempering), for through heating of blanks before plastic deformation (forging, stamping), for welding, brazing and surfacing, for chemical heat treatment products, etc.

The frequency of change of the current feeding the installation of induction heating, distinguish:

installations of industrial frequency (50 Hz), powered from the network directly or through step-down transformers;
installations of high frequency (500-10000 Hz), powered by electric or semiconductor frequency converters;
high-frequency installations (66 000-440 000 Hz and above), powered by tube electronic generators.

Induction heating – core installation

In a melting furnace (Fig. 1), a cylindrical multi-turn inductor, made of a copper shaped tube, is placed on a closed core made of electrical steel sheet (0.5 mm sheet thickness). Around the inductor place a refractory ceramic lining with a narrow annular channel (horizontal or vertical), where the liquid metal. A necessary condition for work is a closed electrically conductive ring. Therefore, it is not possible to melt individual pieces of solid metal in such a furnace. To start the furnace, it is necessary to pour a portion of liquid metal from another furnace into the channel or to leave part of the liquid metal from the previous melting (residual capacity of the furnace).

induction heating and induction channel furnace devices

Image1. Diagram of the induction channel furnace device: 1 – indicator; 2 – metal; 3 – channel; 4 – magnetic core; Ф – the main magnetic flux; F1p and F2p – magnetic scattering fluxes; U1 and I1 – voltage and current in the inductor circuit; I2 – conduction current in metal

In a steel magnetic core of an induction channel furnace, a large working magnetic flux closes and only a small part of the total magnetic flux created by the inductor closes through the air in the form of a scattering flux. Therefore, such furnaces successfully operate at an industrial frequency (50 Hz).

Currently, there are a large number of types and designs of such furnaces developed at VNIIETO (single-phase and multi-phase with one and several channels, with a vertical and horizontal closed channel of different shapes). These furnaces are used to melt non-ferrous metals and alloys with a relatively low melting point, as well as to produce high-quality cast iron. In the smelting of cast iron, the furnace is used either as a stoker (mixer) or as a melting unit. Designs and technical characteristics of modern induction channel furnaces are given in the special literature.

Induction Heating – Coreless Installations

In a melting furnace (Img. 2), the metal to be melted is in a ceramic crucible placed inside a cylindrical multi-turn inductor. The inductor is made of copper shaped tube through which cooling water is passed. Learn more about the design of the inductor here.

The absence of a steel core leads to a sharp increase in the magnetic flux of scattering; The number of magnetic field lines linked to the metal in the crucible will be extremely small. This circumstance requires a corresponding increase in the frequency of change (in time) of the electromagnetic field. Therefore, for efficient operation of induction crucible furnaces, it is necessary to feed them with increased currents and, in some cases, high frequencies from the corresponding current transducers. Such furnaces have a very low natural power factor (cos φ = 0.03-0.10). Therefore, it is necessary to use capacitors to compensate for reactive (inductive) power.

Currently, there are several types of induction crucible furnaces, developed at VNIIETO in the form of corresponding size ranges (in terms of capacity) of high, increased and industrial frequencies, for steel melting (IST type).

induction crucible furnace devices

Image 2. Diagram of the induction crucible furnace: 1 – inductor; 2 – metal; 3 – crucible (arrows indicate the trajectory of the circulation of the liquid metal as a result of electrodynamic phenomena)

The advantages of crucible furnaces are the following: the heat released directly in the metal, the high uniformity of the metal in chemical composition and temperature, the absence of sources of metal contamination (besides the crucible lining), ease of control and regulation of the smelting process, hygiene of working conditions. In addition, induction crucible furnaces are characterized by: higher productivity due to high specific (per unit of capacity) heating power; the ability to melt the solid charge, leaving no metal from the previous melting (as opposed to channel furnaces); low lining mass compared to metal mass, which reduces the accumulation of thermal energy in the lining of the crucible, reduces the heat inertia of the furnace and makes smelting furnaces of this type extremely convenient for periodic work with intervals between melts, in particular for shaped-casting shops of engineering plants; compactness of the furnace, which allows you to simply isolate the working space from the environment and to melt in vacuum or in a gas environment of a given composition. Therefore, in metallurgy vacuum induction crucible furnaces (type WIS) are widely used.

Along with the advantages of induction crucible furnaces, there are the following disadvantages: the presence of relatively cold slags (slag temperature less than the temperature of the metal), which impede the refining processes in the smelting of high-quality steels; complex and expensive electrical equipment; low durability of the lining with sharp temperature fluctuations due to the small thermal inertia of the crucible lining and the eroding action of the liquid metal during electrodynamic phenomena. Therefore, such furnaces are used to remelt alloyed waste in order to reduce the burnout of elements.

1. Egorov A.V., Morzhin A.F. Electric furnaces (for the production of steel). M .: Metallurgy, 1975, 352 p.

This article was taken from this source.