Induction furnace
Induction furnaces are furnaces that can be used to heat and melt metals using inductive heating using an electric current. The heating takes place by means of electromagnetic induction and generation of an eddy current in the metallic and thus electrically conductive melt material.
The induction furnace transfers the energy to the material to be melted by means of an inductor , a coil specially constructed for this purpose and usually cooled . These inductors are available in many designs and variations and they are usually specially adapted and manufactured in shape for the respective furnace design or individual workpieces. Induction furnaces are often designed as so-called crucible furnaces . The water-cooled inductor is located outside the electrically non-conductive crucible.
General
Induction furnaces are more efficient than other heating methods despite the losses of the inverter / converter, which is often required for the supply, because the energy is induced directly into the melting material, the heat is generated directly in the melting material and does not have to be, as with other methods of heating, by conduction, - radiation or convection are transmitted.
The inductors have a very low power factor. Therefore almost all induction furnaces contain large capacitors for reactive power compensation . This ensures that the power grid or the converter only has to provide active power.
Comparison / classification of the achievable power density:
| Type of heating | Power flux density in W / cm 2 |
|---|---|
| Convection (heat transfer through molecular movement ) | 0.5 |
| Radiation (electric furnace, muffle furnace ) | 8th |
| Touch, heat conduction ( hotplate , salt bath) | 20th |
| Flame ( burner ) | up to> 1,000 |
| Induction heating | 30,000 |
| Laser (welding, hardening, cutting) | > 100,000 |
Application types
Industrial induction heating is mainly used for the following applications:
- Hardening
- glow
- Starting
- Melt
- Soldering and welding
- Hot forming
- Surface treatment (coating with plastics etc.)
Induction furnaces for heating are mostly systems with an open coil mounted on one side or with an inductor into which the parts to be heated are inserted or passed through. Devices with outputs of a few kW are available as table-top devices, while large systems, for example for heating forgings or for heating aluminum extrusion billets, have outputs in the range of several MW and can be the size of a container . The advances in power electronics have made considerable improvements in terms of size, efficiency and process control possible over the past few decades.
Induction furnaces are used in foundries as crucible furnaces for melting cast iron and other metals. Here, the field of the furnace coil not only causes heating, but also, if the frequency is selected appropriately, a movement of the melt, for example in order to be able to mix in alloy components. The power range of such systems extends from 50 kW to 16 MW, with cast iron power densities of up to 1 MW per ton of furnace content.
There are induction furnaces for melting high-alloy steels or for vacuum hardening, which work under complete exclusion of air or vacuum and thus with the exclusion of oxidation .
Channel furnaces are inductively heated units that are used both for melting non-ferrous metals and as storage and holding units for cast iron.
functionality
Induction furnaces consist of a coil and an alternating current source.
Induction coil
The core of the induction furnace is the induction coil. The induction coils are also called inductors. They are usually water-cooled, since reactive currents of up to 1,000 A occur even in small systems (P> 5 kW) . Alternating current flows through the inductor, which means that an alternating magnetic field builds up in its interior. The material to be heated or melted forms, as it were, a second, short-circuited coil in which a voltage is induced which results in eddy currents. This current leads to a heating of the material, whereby the heat does not get into the material from the surface, but arises in it itself.
The shape of the inductor is similar to a coil, but is shaped according to requirements. Often the coil is made of tube (water cooling); In the case of an induction hob , it consists of high-frequency stranded wire .
Magnetic materials ( permeability μ = 100 to 500) heat up faster than non-magnetic materials (permeability μ = 1) due to the hysteresis heating that occurs in addition to eddy current heating ( loss of magnetic reversal ).
AC power source
The electrical power is drawn from the power grid . In contrast to pure mains frequency systems and the rotating frequency converters previously used for higher frequencies, the mains current is z. B. converted with a thyristor three-phase rectifier bridge into a controllable or adjustable direct current and fed via a direct current smoothing choke to a thyristor inverter , which converts the direct current into single-phase alternating current at the desired frequency. By switching on an appropriately designed capacitor battery, the phase shift between current and voltage in the furnace coil, which is dependent on the magnetic coupling between coil and charge, is compensated, so that the inverter only has to deliver the required active power.
Since the current penetration depth depends on the operating frequency, higher frequencies are generated depending on the application. The range of applicable frequencies is divided into low frequency (50 to 500 Hz), medium frequency (0.5 to 50 kHz) and high frequency (50 kHz to 30 MHz), with penetration depths> 8 mm in the low frequency range and around 0.1 mm in the High frequency range. The dependence of the penetration depth as a function of the frequency is described by the skin effect .
Converters for low frequency work from the mains frequency by means of thyristor converters. In the medium to lower high frequency range, converters with power MOSFETs , MOS transistors or IGBT converters are used. Transmitter tubes are still used in the MHz range .
In the ISM band around 27 MHz, power oscillators with the Huth-Kühn circuit are used.
Depending on the design of the induction generator or converter, the inductor must have a certain inductance . If this is not the case, the converter runs inappropriately and the power cannot be transferred correctly to the workpiece to be heated.
Web links
- Performance calculation; Induction furnace manufacturer as well as further information
- Inductive hardening and heating, more information