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Posted on 06 February 2020

High Frequency Inductive Heating

 

Especially in the metal industry it is often necessary to heat objects, possibly for pouring, soldering, case hardening, melting or tempering of these parts.

Mostly the heat is generated outside the object and via radiation or convection the heat is transferred to the object or work piece. A much more elegant method is to generate the heat in the object (or part of the object) using eddy currents. These eddy currents are induced using high frequency magnetic fields produced in induction coils. This is called inductive H.F. heating. It is clear that only electrically conductive material can be heated in this way.

The induction coil that is placed around the object is called the work coil (figure 1). This work coil is often a copper tube through which water flows to cool the coil.

Induction coil

Figure 1. Object with an induction coil

The eddy currents in the work piece cause eddy current losses which result in heating. In ferromagnetic materials hysteresis losses also play a role. As shown in the article "The Skin Effect", depending on the frequency we have pronounced or non pronounced skin-effect. Induction ovens whose purpose is to melt metals operate with lower frequencies. In this case the skin effect plays virtually no role. Installations using higher frequencies (2 to 500 kHz) use the skin-effect to locally heat metals or to get them to glow. We limit our discussion to these installations. Due to the skin-effect the heating will be greatest on the outside of the work piece. This property is useful when we want to surface harden a work piece. The heat is concentrated in the outer layer (Sdi) of the work piece.

The skin depth Sdi follows from:

With μ0 = 4π⋅10-7 H/m this becomes:

Here f  is in Hz and ρ  in Ω mm2/m.

The magnetic field strength in the work coil is extremely high and with ferromagnetic work pieces magnetic saturation often occurs. This means amongst other things that the relative permeability μr is small. At the curie point (for steel at 760°C) the magnetic properties disappear ( μr = 1) and the penetration depth will increase for a specific frequency.

It is obviously also the case that if the high frequency field is maintained long enough that the work piece will quickly heat up.

Special induction coil

Photo Plustherm gmbh: Special induction coil used to harden metal strips at 10m/min. In the middle of the photo six metal strips are visible passing through a special coil composed of 10 rectangular shaped (horizontal) windings

To work with a reasonable efficiency it can be shown that . Here d  is the diameter of the (round) work piece. The frequency is chosen such that the penetration depth is a maximum of . From (2) we find then:

The efficiency decreases with increasing frequencies and in addition the efficiency of an R.F. generator also decreases with rising frequencies so that in most cases fmin is used. Figure 2 shows fmin as a function of the diameter of the work piece for different materials.

Minimum frequency

Figure 2. Minimum frequency related to the diameter of the work piece

One of the most important applications of inductive high frequency heating is hardening steel. The heating duration is often a fraction of a second. Usually the power concentration is from 1 to 5 kW per cm2. Steel types with a carbon content above 0.3% can be hardened in this manner. Shocking the work piece with a liquid is easy with this system since it is possible to spray with the openings between the windings of the work coil (figure 2).

For the R.F. generator a so called resonant converter is used. Figure 3 shows the basic schematic of a DC-AC current source inverter with a parallel resonant load.

Parallel resonant load

Figure 3. Parallel resonant load, controlled by a current source

The induction coil and the load are replaced by an equivalent LR and Rb. The capacitance CR causes parallel resonance. With a sufficiently high Q-factor we obtain a practically sinusoidal voltage v0 across the parallel circuit. The current source inverter (three-phase bridge) and the square wave generator (single phase bridge) can be constructed using thyristors (figure 4). To prevent high di/dt values through the thyristors a small coil L2 is placed in series with the load.

Current source inverter

Figure 4. Current source inverter with a parallel resonant load

 

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This post was written by:

- who has written 7 posts on PowerGuru - Power Electronics Information Portal.

Professor Dr. Jean Pollefliet is the author of several best-selling textbooks in Flanders and the Netherlands

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