Fig. 12.5A shows the temperature responses for points A, B, and C. Even in the best result ( Fig. 12.4A), the characterization of corrosion is difficult. However, as time increases, the lateral blur effect severely affects the detectability of corrosion. In Fig. 12.4A–B, the profile of corrosion can be seen. Fig. 12.4A–C show thermograms for a coated sample with 6-month corrosion, at 50, 200, and 500 ms, respectively. The sampling frequency of the IR camera was 200 Hz and the sampling interval was 5 ms. The heating time was set as 200 ms, followed by a cooling time of 300 ms. In this experiment, both the induction coil and camera were placed on the same side of the sample. Thus, they are preferred if such an option is available. In higher-power systems, inductors are less expensive and more reliable than capacitors. The inductive coupled method shown in Figure 7.28B is used in situations where a fixed load provides the opportunity to use a fixed coupling inductor to save the expense and maintenance of issues related to capacitors. The capacitor-coupled method shown in Figure 7.28A is very similar in operation and design to that showed in Figure 3.15 for E-field applicators. (C) The dual variable capacitor case is normally used for automated tuning.Ĭapacitive coupling is usually preferred, particularly in low-power systems, because variability of coupling is often needed and variable capacitors are more readily available. (B) In the inductive coupled case, the coupling inductor is usually fixed for a specific load. (A) In the capacitor-coupled method, the coupling capacitor, C c, provides variable coupling. Direct coupling methods in H-field applicators. The heat is concentrated in the outer layer ( S di) of the work piece.įigure 7.28.

This property is useful when we want to surface harden a work piece. Due to the skin-effect the heating will be greatest on the outside of the work piece. We limit our discussion to these installations. Installations using higher frequencies (2 to 500 kHz) use the skin-effect to locally heat metals or to get them to glow. In this case the skin effect plays virtually no role.

Induction ovens whose purpose is to melt metals operate with lower frequencies. As we saw in chapter 5 depending on the frequency we have pronounced or non pronounced skin-effect. In ferromagnetic materials hysteresis losses also play a role. The eddy currents in the work piece cause eddy current losses which result in heating. The (cool) water connections of the induction coil are visible among others at the top of the photo Notice the integrated spray installation to shock the work piece. Photo Plustherm gmbh: Special induction coil for case hardening objects.

Making an induction coil