Induction heaters use an electrical source to drive alternating current (AC) through a coil to heat metal objects. The passage of current through the electrically conductive coil generates an intense and rapidly changing magnetic field that causes eddy currents to flow through the workpiece. The high frequency used in induction heating also forces AC to flow in a thin layer towards the surface of the workpiece, increasing the effective resistance of the metal object to the passage of the large current. For ferrous materials, the alternating magnetic field inside the coil repeatedly magnetizes and de-magnetizes the iron crystals, causing considerable friction and heating. Heating due to hysteresis loss is greatest for workpiece materials that have a large area. Induction heaters usually incorporate the coil with a tank circuit so that either the current or the voltage becomes sinusoidal. This minimizes inverter losses and radio frequency (RF) interference.

Induction heaters vary in terms of output, power requirements, and features. Output power, the primary specification, is usually measured in watts (W). Other important ratings include the output current supplied to the heating coil and the output frequency, which is often expressed in kilohertz (kHz). In terms of power requirements, induction heaters can use either single-phase or three-phase AC voltage. AC input voltages of 50 Hz, 60 Hz, and 400 Hz are commonly available. In terms of special features, some induction heaters include a microprocessor that controls the voltage applied to the workpiece and the speed at which the object heats. Other induction heaters feature a temperature probe that provides feedback to help regulate heating. Induction heaters with auto-demagnetization eliminate the magnetic field induced by the heating of ferrous materials. Induction heaters with dynamic power regulation and multiple or independent timing controls are also available.

There are many applications for induction heaters. Some devices are designed to heat adhesive bonds to curing or setting specifications. Others are used for bolt or rivet removal, brazing or soldering, melting or casting, and welding. Process annealing is used to treat work-hardened parts made out of low-carbon steels. Hardening causes metals to achieve higher levels of stress-resistance to surface deformation. Induction heaters are used in metal forming and forging, shrink fitting and assembly processes, and welding. They are also used to seal the anti-tamper seals on consumer products and to remove contamination from evacuated vessels such as TV picture tubes, vacuum tubes, and various gas discharge lamps.


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