Induction furnace

Induction furnaces are appliances (ovens ) with which metals can be heated and melted using the inductive heating.

An induction furnace transmits the power by means of an inductor or a coil. These inductors and coils are available in many designs and variations. They are mostly for workpieces specially adapted and manufactured.

Possible to achieve this heating is through the induction of an eddy current in a metallic conductor - to be heated workpiece.

Induction, despite the loss of often required to power inverter / inverter efficient than other heating methods, since the energy is directly induced into the workpiece, so the heat is produced exclusively in the work piece and does not have, as with other methods, by conduction, radiation or convection are transferred.

The inductors have a very low power factor. Therefore, almost all plants contain large capacitors for reactive power compensation. This ensures that the power or the inverter needs to provide active power.

Comparison / classification of the achievable power density:

Types of applications

The industrial induction heating is mainly used for the following applications:

• Melt
• Soldering and Welding
• Hot stamping
• Surface treatment ( coating with plastics, etc. )

Induction Furnaces for heating systems are usually mounted with one side open coil or with an inductor, in which the parts to be heated to be performed on or through him. Devices with capacities ranging from a few kW are available as a desktop unit, while large installations, such as for heating of forgings or for heating of aluminum billets, services have a range of several MW and can have the size of a container. The advances in power electronics have in recent decades enables significant improvements in terms of size, efficiency and process control.

In foundries, induction furnaces are used as crucible furnaces for smelting cast iron and other metals. Here, the field coil of the furnace not only causes heating, but with suitable choice of frequency, a movement of the melt, with mixing, to approximately alloy components. The power range of such systems extends from 50 kW to 16 MW, which in cast iron power densities of up to 1 MW per ton of furnace capacity are possible.

The melting of high alloy steels or for vacuum hardening, induction furnaces exist that work under complete exclusion of air or vacuum and thus the absence of oxidation.

Furnaces are inductively heated aggregates, which both are used in the smelting of non-ferrous metals as well as storage and holding unit for cast iron.

Operation

Induction furnaces consist of a coil and an AC power source.

Induction coil

The heart of the induction furnace is the induction coil. The induction coils are also called inductors. They are water cooled as a rule, as even in small plants (P > 5 kW) occur reactive currents up to 1,000 A. The inductor is flowed through by alternating current, thereby, in particular, an alternating magnetic field builds up in its interior. To be heated or to be melted material forms almost a second short-circuited coil in which a voltage is induced, the eddy currents result. This current results in heating of the material, the heat does not come forth from the surface of the material, but is formed in this itself.

The shape of the inductor is similar to a coil is formed, however, depending on requirements. Often, the coil consists of pipe (water cooling); the induction hob it consists of Litz.

Magnetic materials (permeability μ = 100 to 500 ) heat up due to the additionally occurring for eddy current heating hysteresis heating ( core loss ) faster than non-magnetic (permeability μ = 1).

AC power source

The electrical power is drawn from the power grid. In contrast to pure power frequency equipment and the rotating frequency converters previously used for higher frequencies, the power is with a thyristor three-phase bridge rectifier converts, for example, in a controlled or regulated DC current and fed via a DC smoothing choke a thyristor inverters, which converts the direct current into single-phase alternating current having the desired frequency. Due to the inclusion of an appropriately designed capacitor bank which is dependent on the magnetic coupling between the coil and batch phase shift between current and voltage in the furnace coil is compensated so that the inverter must deliver only the active power required.

Since the current penetration depth is dependent on the operating frequency, the higher frequencies are generated depending on the application. The range of applicable frequencies in the low frequency (50 to 500 Hz), medium-frequency (0.5 to 50 kHz ), and high frequency is divided (50 kHz to 30 MHz ), the penetration > 8 mm in the low- frequency range, and approximately 0.1 mm in amount of high frequency range. The dependency of penetration depth as a function of frequency is described by the skin effect.

Converter for low frequency work using thyristor inverter from the mains frequency. In the medium to lower the high frequency range converter with power MOSFETs, MOS transistors or IGBT converters are used. In the MHz range still come transmitting tubes are used.

In the ISM band by 27 MHz power oscillators are used with the Huth - Kühn- circuit.

Depending on the model of the induction generator or inverter, the inductor must have a certain inductance. If this is not the case, the inverter runs nonconformist and the power can not be transmitted correctly to the workpiece to be heated.