Electromagnetic induction, simply induction, is a heating technique for electrical conductive materials (metals). Induction heating is frequently applied in several thermal processes such as the melting and the heating of metals. Induction heating has the important characteristic that the heat is generated in the material to be heated itself. Because of this, induction has a number of intrinsic trumps, such as a very quick response and a good efficiency. Induction heating also allows heating very locally. The heating speeds are extremely high because of the high power density. 2. Physical principles
The principle of induction heating is mainly based on two well-known physical phenomena: 1. Electromagnetic induction 2. The Joule effect 2.1 Electromagnetic induction The energy transfer to the object to be heated occurs by means of electromagnetic induction. It is known that in a loop of conductive material an alternating current is induced, when this loop is placed in an alternating magnetic field (see Figure 1a). The formula is the following:
If a 'massive' conductor (e.g. a cylinder) is placed in the alternating magnetic field instead of the sort- circuited loop, than eddy currents (Foucault currents) will be induced in here (see Figure 2). The eddy currents heat up the conductor according to the Joule effect.
Induction Heater Installations
3.1 General aspects The inductor and the load behave as an inductive load and are compensated with capacitors. A frequency converter feeds the entirety with a single-phase current at the desired frequency. An induction installation also contains a cooling system (for frequency converter, inductor), a transport system and the necessary control and measuring apparatus. 3.2 Power supply and generators The electrical supply can occur in different ways, depending on the frequency at which the installation has to work. 50Hz-installations: The compensated load is directly connected to the transformer. The transformer can be regulated so that the current can be adjusted to the impedance of the load. Frequency converters with thyristors: • efficiency: 90-97% • frequency range: 100Hz – 10kHz • power range: up to 10MW Frequency converters with transistors: • efficiency: 75-90% • frequency range: tot 500kHz • power range: tot 500kW Frequency converters with vacuum tubes: • Efficiency 55-70% • Frequency range: up to 3000kHz • Power range: up to 1200kW 3.3 Inductors In most applications the inductor consists of a copper hollow tube. The most simple, often applied configuration consists of one or more windings that surround the workpiece. However, the inductor can be placed in many ways, depending on the application. The inductor is usually made of copper in order to limit the electric losses. Nevertheless, the inductor is in almost all cases internally water-cooled.
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