May 2024 Volume 6

EQUIPMENT & TECHNOLOGY

Operating Frequency Induction heating differs from other types of heating processes as it generates heat below the surface of the part. The depth at which the heat is generated is known as the depth of current penetration and is calculated using the equation below:

The depth in which the current penetrates the material is directly related to the material being heated and inversely proportional to the operating frequency. As carbon steel is heated above the Curie temperature, the electrical resistivity of the material increases, resulting in a deeper depth of current penetration at different frequencies as shown in Figure 2. Frequency (Hz) Depth (mm) Depth (in) 500 25.1 0.988 1000 17.7 0.697 3000 10.4 0.410 To achieve the best electrical efficiency for the heating process, the diameter of the billet should be 3 to 4 times the thickness of the depth of current penetration. Figure 3 shows an example of the electrical efficiency for heating different diameter billets at 1kHz & 3kHz. As the diameter of the billets decreases at lower frequencies, the efficiency starts to drop off as the ratio of diameter to depth of current penetration decreases. Figure 2: Depth of Current Penetration for Carbon Steel Billets heated above Curie Temperature

Figure 4: Coil Electrical vs Thermal Efficiency Unfortunately, since induction heating systems are designed to heat a range of billet diameters, choosing the best frequency is a compro mise. However, the smallest diameter billet will dictate the operating frequency so that this billet is effectively heated at the best possible efficiency. Coil Design The design of the induction heating coil is complex and has the largest effect on operating efficiency. One of the most important design factors affecting efficiency is the coupling gaps between the billet and the coil winding. The coupling gap design is a balance between the coil copper diameter and the thickness of the refrac tory that requires a balance between electrical and thermal efficiency. Figure 4 is a simple graph that shows if the refractory thickness is too thick, the electrical efficiency is sacrificed and vice versa. For most forging companies, the induction heating system runs a range of billet diameters. Therefore, the coil design must be capable of properly heating different billet sizes while maximizing efficiency. Unfortunately, as stated earlier, coupling between the billet and coil winding greatly affects efficiency. As the billet diameter decreased, the coupling gap increased resulting in lower efficiency, as shown in Figure 5.

Figure 3: Efficiency vs. Diameter

Figure 5: Coil Layout

FIA MAGAZINE | MAY 2024 13