Electromagnetic induction heating: principles

Electromagnetic induction heating is one of the electrothermal techniques used to heat a material without direct contact with an electrical energy source. It involves immersing the body to be heated in a time-varying electromagnetic field, and dissipating the energy entering the body in the form of heat. However, it differs markedly from other techniques (infrared and microwave) in the nature of the materials heated and in the electrical frequency band used, i.e. in the depth of penetration and the heating power densities obtained. In fact, by virtue of its principle, it only applies to electrically conductive materials, i.e. materials with an electrical resistivity of between 10 ^–8 Ω.m (copper) and 10 ^–1 Ω.m (molten glass). The frequency band used is between the industrial frequency of 50 Hz and a few megahertz, so penetration depths range from a few micrometers to a few centimeters. Power flux densities can reach 10 ⁵ kW/m ² .

• The performance of this heating technique can be characterized by the frequency-power ratio, and its evolution can be tracked. Since the days when induction heating relied on rotating generators, this product has increased significantly. In fact, these early generators were gradually replaced by static converters and, until recent years, the frequency-power product was typically 100 to 1,000 kHz.kW. The current trend is to increase the frequency and power of installations. This can result in frequency-power values of the order of 250,000 kHz.kW! (as in the case of high-powered inductively coupled plasma torches or steel tube flow welding).

  All this presupposes the parallel development of the necessary electrotechnical equipment, such as capacitors, transformers, water-cooled inductors, anti-inductive wiring (commonly referred to as "aselfic"), etc., which make up modern induction heating technology.

• As with any heating problem, the power required for the type of heat treatment required is dictated by the mass to be heated, the temperature to be reached and the heating time. For the engineer in charge of designing or operating an induction heating system, the issues to be resolved are electromagnetic, to optimize power transfer between the source and the material, and thermal, to determine the temperature field and its evolution over time.

• Once the required power has been defined, there are generally three steps to be taken.

  First of all, there's the question of choosing the right working frequency....