Induction heating is based on three basic effects: electromagnetic induction, skin effect and heat transfer.
Electromagnetic induction was first discovered by Michael Faraday in 1831. An electrically conducting object (usually a metal) can be heated when placed in an inductor that is part of a resonant circuit. An alternating current flowing through the inductor’s coil generates an oscillating magnetic field. This in turn induces Eddy currents (also called Foucault currents) in the object which, by means of resistive (Joule) heating, heats up the object. According to Lentz’s law the direction of the inductive current is opposite that current that generated the magnetic field which induced the inductive current. In addition to this, the high frequency used in induction heating applications gives rise to a phenomenon called skin effect. This skin effect forces the alternating current to flow in a thin layer close to the surface of the object. The effective resistance of the object is increased which greatly increases the heating effect. As the skin effect is frequency dependent, the
frequency can be used to specify the heating depth. Although the heating due to Eddy currents is desirable in this application, it is interesting to note that transformer manufacturers need to avoid this phenomenon in their transformers. Laminated transformer cores, powdered iron cores and ferrites are all used to prevent Eddy currents from flowing inside transformer cores.
The effects described above render induction heating many advantages compared to other heating methods. Induction heating is cost effective due to the lower energy consumption. Heat is generated directly in the heated object without any interfaces. Heating is contactless and therefore very clean. Induction heating is a fast process offering improved productivity with higher volumes. The heating energy can be easily and precisely regulated and the whole object or just parts of it can be heated by frequency adjustments.
Application Examples
There are many applications applying the principles of induction heating. The most powerfull applications are melting and pouring in the heavy metal industry with furnace capacities of up to 50 tons and inverter powers of up to 50 MW.
Some applications in the medium power range are surface hardening or quenching for the production of cogwheels, arbors, valves, rails, etc. Further examples are preheating for material forming or before welding, induction welding, stress revealing after welding, steel tempering or annealing. Also induction bending (800 kW level) to shape big tubes for powerplants or „I-shape“ steal support forming are frequent applications.
