What is the difference between electromagnetic induction furnace and traditional resistance furnace?

An electric furnace in which current passes through a conductor to generate Joule heat as a heat source. According to the electric heating mode, resistance furnaces are divided into direct heating and indirect heating. In a directly heated resistance furnace, the current passes directly through the material. Because the electric heating power is concentrated in the material itself, the material is heated quickly, and it is suitable for processes that need rapid heating, such as the heating of forging blanks. This kind of resistance furnace can heat the material to a very high temperature, such as graphitization electric furnace of carbon material, which can heat the material to more than 2500□. Directly heated resistance furnace can be made into vacuum resistance heating furnace or shielding gas resistance heating furnace, which is often used to sinter tungsten, tantalum, niobium and other products in powder metallurgy. Attention should be paid when heating with this furnace: ① In order to make the material heated evenly, the conductive cross section and conductivity of each part of the material should be consistent; (2) Because the resistance of the material itself is quite small, in order to achieve the required electric heating power, the working current is quite large, so the contact between the transmission electrode and the material is better, avoiding arcing and burning the material, and the resistance of the transmission bus is small, thus reducing the circuit loss; (3) When supplying alternating current, short network should be reasonably configured to avoid too large inductive reactance and too low power factor.

Most resistance furnaces are indirect heating resistance furnaces, which are equipped with resistors specially used to realize electric-thermal conversion, called electrothermal bodies, and transfer heat energy to materials in the furnace (Figure 1 Indirect heating resistance furnace). The shell of this electric furnace is made of steel plate, and the hearth is lined with refractory and filled with material. The most commonly used electric heaters are iron-chromium-aluminum electric heaters, nickel-chromium electric heaters, silicon carbide rods and molybdenum disilicide rods. According to requirements, the atmosphere in the furnace can be ordinary atmosphere, protective atmosphere or vacuum. The general power supply voltage is 220 volts or 380 volts, and an intermediate transformer with adjustable voltage is provided when necessary. Small furnace (< 10 kW) single-phase power supply, large furnace three-phase power supply. For materials with single variety and large batch, continuous furnace should be used for heating. Most resistance furnaces with the furnace temperature below 700□ are equipped with blowers to strengthen heat transfer in the furnace and ensure uniform heating. Resistance furnace for melting fusible metals (lead, lead-bismuth alloy, aluminum, magnesium and their alloys, etc.). ) can be made into a crucible furnace; Or make a reverberatory furnace with a molten pool, and install an electric heater on the top of the furnace. Electroslag furnace is a resistance furnace which uses slag to realize electrothermal transformation (see electroslag remelting).

An electric furnace that heats or melts materials by using the inductive electrothermal effect of materials. The basic component of induction furnace is induction coil wound with copper tube. An AC voltage is applied to both ends of the induction coil to generate an alternating electromagnetic field. The conductive material is placed in the induction coil, and eddy current is generated in the material due to electromagnetic induction, which is converted into heat energy through resistance to heat the material. Therefore, induction heating can also be considered as a kind of direct heating resistance heating.

The characteristic of induction electrothermal is that the electrothermal power (current distribution) transformed in the heated material is very uneven, with the largest surface and the smallest center, which is called skin effect. In order to improve the electric heating efficiency of induction heating, the power frequency should be appropriate. Small melting furnaces use high frequency electricity to heat the surface of materials, while large melting furnaces use medium frequency or power frequency electricity to heat the materials deeply. Induction coil is a load with considerable inductance, and its power factor is generally very low. In order to improve the power factor, induction coils are generally connected in parallel with capacitors, which are called compensation capacitors. The gap between the induction coil and the material should be small. The induction coil should be made of square copper tube, and the tube should be cooled with water. The turn-to-turn gap of induction coil should be as small as possible and the insulation should be good. Induction heating devices are mainly used for melting and casting steel, copper, aluminum, zinc, etc. It has the advantages of fast heating speed, less burning loss, high mechanization and automation, and is suitable for automatic production lines.

Industrial induction melting furnaces include crucible furnace (coreless induction furnace) and trough furnace (cored induction furnace), as shown in Figure 2. Crucibles are made of refractory materials or steel, and the capacity varies from several kilograms to dozens of tons. Its melting characteristic is that the melt in the crucible is subjected to electrodynamics, which forces the liquid level of the molten pool to rise, and the melt flows around from the center of the liquid level, resulting in circulation. This phenomenon is called electrodynamic effect, which can make the melt composition uniform, but the disadvantage is that the slag is biased to the periphery and the coverage is poor. Compared with trough furnace, crucible furnace has flexible operation and high melting temperature, but low power factor and high power consumption. The inductor of trough smelting furnace consists of iron core, induction coil and trough lining. The trough is one or two belt-shaped annular troughs, and the melt communicated with the molten pool is filled in the trough. In principle, the trough smelting furnace can be regarded as an iron core transformer with only one turn of coil in secondary short circuit. The induced current flows in the melt channel to realize electrothermal transformation.