The concept of induction

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What is induction heating?

Electromagnetic induction is unique because it actually generates heat inside the material that is heated, has an immediate effect (no inertia), high-power densities and adjustable penetration depths.  Compared to other heating techniques, it takes less time to heat and is more efficient and accurate.

Induction heating is a direct application of two laws of physics: the Lenz law and the Joule law. When immersed in a variable magnetic field (generated by an induction coil or inductor), any substance that conducts electricity carries the electrical current induced, also called Foucault currents. According to the Joule effect, the movement of the electrons creating these currents dissipates the heat in the substance where they were generated.

In practical terms, electromagnetic induction involves putting a part (usually made of electrical conductive material) inside a magnetic field, which is kept variable with an "inductor coil" (inductance), which itself is connected to a power source and a capacitor bank, and the assembly forms an oscillating circuit at a so-called "resonance" frequency.

The quality of energy transmission to the part being treated depends on the placement of the inductors and the parts (connection, respective lengths), the power frequency and the skin effect, which characterizes the distribution of currents induced in the part on the surface or at the core.

The higher the frequency, the more the induced currents concentrate on the surface, which determines a "penetration" depth (skin thickness).

In addition, magnetic properties (the relative permeability), electrical properties (resistance) and thermal properties (conductibility) of the parts to be heated with the temperature and the type of inductor (geometric, type of conductor, technology), and the impedance of the load also changes accordingly, which requires choosing the right power source in terms of the "impedance range", that should be as wide as possible.

This technology started being introduced for industrial applications in the 1960s. Since then, it has constantly been developed and generated in innovative solutions.

The range of induction heating techniques

The part is moving through the inductor and the current is distributed on its surface – the magnetic field runs parallel to direction of the movement, which is called longitudinal flux.

The part is moving next to the coil – the magnetic field is perpendicular to the part, which is called transverse flux.

Induction heating equipment typically includes:

 

  • A medium- or high-frequency power converter (generator or inverter), connected to a compensation box, so called "capacitor bank"
  • One or more heating inductors
  • A water-based cooling system for the inverter, the capacitor box and possibly the inductor
  • A system to feed or to handle/feed the parts to be heated
  • A control and command system for the heating equipement, or integrated a part of the control system of the line or machinery.

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