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Induction coil

The basic requirements for penetration heating induction coils are tight coupling with the workpiece under full load and no-load conditions, satisfactory thermal and electrical insulation, and sufficient rigidity.

Since the induction coil works in a harsh environment and must also have high reliability, the induction coil must be designed very carefully. Most induction coil designs are combined with operating devices and matching parts such as transformers, capacitors, etc. After the main parameters have been determined, the layout of the induction coil must take into account the effects of water cooling systems, vibrations, and magnetic field leakage.

Induction coil

Copper with high conductivity is always used as a conductor material, and the induction coil is made of water-cooled hollow copper tubes. Small heaters can be made of copper wire coated with nylon or PVC plastic in a fluidized bed and then cast in refractory cement.

A water-cooled stainless steel guide covered with a hard tungsten-chromium-cobalt alloy coating is passed through the air gap along the axial direction of the induction coil to prevent the workpiece from wearing the refractory cement on the induction coil. Although this structure is simple, inexpensive, and durable, it does not have the best thermal and electrical insulation properties. Concrete is susceptible to cracking due to rapid cooling and heating, so when the induction coil reaches the end of its expected life and needs to be renovated, it is difficult to repair it without damaging the coil.

Power supply frequency

At power supply frequencies, the thickness of the coil conductor is usually 12 mm, bearing in mind that at 50/60 Hz the current layer depth of cold copper is 10 mm. Therefore the induction coil is made of a tube or of a square section water pipe (D-section) extruded as a whole. In the past, the square conductor and the cooling water pipe were soldered together with silver solder to form a composite conductor, which is not only expensive but also less reliable than a solid tube. The induction coil is wound on a mandrel using a large lathe.

After forming, the induction coil is cleaned and prepared for insulation. Figure Although the induction coil is cooled by water and limited to a relatively low temperature, its outer surface may still be subjected to high temperatures. For this reason, the use of H-type insulation materials is satisfactory, but of course this is not an absolute requirement. Modern high-temperature insulation materials are of very high quality and the designer can choose from a wide range.

In the past, a 1.5 mm thick glass braid was wrapped around the conductor and then vacuum impregnated with high temperature organic paint, but the life of this organic paint is necessarily short. Silicone-impregnated glass ribbon can be used to wrap half of the outer periphery of the copper tube like a sleeve. A machinery manufacturer uses high temperature ceramic insulation material that can meet the insulation requirements and has excellent high temperature performance to spray on the induction coil. When manufacturing the induction coil, some intermediate taps can be equipped to obtain different heating specifications according to the load conditions.

Under the power frequency state, the number of volts per turn is low, so the insulation between the turns can be very small. The voltage used in the penetration heating furnace rarely exceeds 1200 volts and is generally very low, so high-grade insulation materials are not required. Almost all penetration heating induction coils require an insulation layer between the induction coil and the workpiece. This protects the coil and its insulation layer from radiant heat and acts as a heat-insulating material around the workpiece. Figure 2.31 shows some typical insulation methods used in the construction of through-heating induction coils at frequencies of 50/60 Hz and 1, 2, and 10 kHz.

Stainless steel linings can only be used at industrial frequencies because they are heated in medium-frequency furnaces. To prevent circular induced currents, these linings can be separated longitudinally to form gaps. Refractory materials can be asbestos boards with a refractoriness of 760°C; however, above this temperature, refractory materials must be used to cast the induction coils. Modern aluminum silicate ceramic materials such as silicon carbide aluminum silicon ceramic fibers are suitable, and this material can be made into various forms. Removable stainless steel guide plates can be used to reduce wear of the furnace lining.