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Fundamentals of Induction Heating

michael faraday

 Two key phenomena must be learned to understand the fundamentals of induction heating; Faraday’s Law of Induction and Skin Effect.

Faraday’s Law of Induction

When an electrically conducting material (such as a metal) is placed within a time-varying magnetic field, an electric current (called an “eddy current”) is induced in the part producing a second magnetic field which opposes the applied field (figure below). The reason behind this phenomenon is that a time-varying magnetic field disturbs the relaxed environmental condition of the electrically conducting material. In return, the material tries to oppose this change by producing another magnetic field to cancel the imposed field.

heating element

The induction phenomenon has two important consequences:

     i. Induced force. An example is shown in the figure below, where a permanent magnet is dropped into a copper tube. The induced force according to the Faraday’s law tries to stop the magnet’s motion inside the tube.

induced current magnet

copper magnet demonstration

Video credits: https://www.youtube.com/watch?v=5BeFoz3Ypo4

     ii. Induced heat. When an electrically conductive material is exposed to an alternating magnetic field, depending on the material, heat is induced by two mechanisms; Joule Heating and Magnetic Hysteresis. The latter occurs in the magnetic metals (such as Carbon Steel below Curie temperature) in which the rotation of the adjacent magnetic dipoles due to the direction change of the imposed magnetic field will lead into friction and heat. This effect increases by increasing the frequency of the imposed magnetic field.

 magnetic field rotation

Joule Heating is the main heating effect caused by induction phenomenon. Any current I, ac or dc, passing through an electrically conducting material causes voltage drop V resulting in energy conversion to heat. Heat power is defined by V.I=R.I^2, where R is the electrical resistance of the current path. The resistance of the current path is inversely proportional to the cross-section area in which the current is flowing. 

How is the induced heat generated?

If an electrically conducting material is exposed to a magnetic field, eddy currents are induced in the material. Special characteristics of such currents result in a phenomenon which we call “Induction Heating”. The eddy currents are concentrated at the surface of the material. The reason is that at high frequency, the imposed magnetic field changes its direction very fast. Therefore, the induced currents in one direction do not have enough time to penetrate into the depth of the metal before their time is up. The thickness of the current penetration in the material is called “Skin Depth”. Skin depth depends on the electromagnetic properties of the material and also is inversely proportional to frequency. Figure below shows the dependence of the skin depth to frequency. Here, δ is the skin depth, ρ is the electrical resistivity, ω is the angular frequency and μ is the magnetic permeability.

Using high frequencies in induction heating industry (Mainly 10kHz to 700kHz) implies very thin penetration depths in metals (typically less than 1mm). Passing high current density (big I) through that shallow depth (big R) results in high R.I^2. Consequently, high energy conversion from electrical to heat occurs. 

 heating element induced current

Reference:

  • S. Zinn and S. L. Semiatin, “Elements of Induction Heating, Design, Control and Applications”, A S M International, ISBN-13: 9780871703088, 1988