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Induction heating is a result of electromagnetic induction which refers to the occurrence of electric current generation in a closed circuit by the fluctuation of current in another circuit placed next to it.
Practical induction heating has been used since the 1920’s. Growth in the induction heating industry expanded very rapidly during World War II. Surface hardening, or case hardening, was a very large growth area during WWII. Military vehicles and weaponry using case hardening on axles and engine components could outlast those without case hardening. After the war, the technology improvements moved rapidly into the civilian sector as the demand for reliable automobiles increased. Today induction heating is used in a wide variety of industrial processes including: forging of metals, melting and casting, surface coatings, cap sealing for food and drug products, metal surface hardening, bonding metal parts and the manufacturing of semi-conductors.
The phenomenon of electromagnetic induction was discovered in 1831 by Michael Faraday. The basic principle of induction heating, which is a result of Faraday’s discovery, is the fact that an alternating electrical current flowing through a circuit affects the magnetic movement of a secondary circuit located near it.
Faraday’s discovery led to the development of electric motors, generators, and transformers. The applications, however, were not flawless. Heat loss, which occurs during the electromagnetic induction process, was a significant problem with the overall functionality of a system.
The heat loss, it was recognized later, could be turned into productive heat energy in an electric heating system. Many industries benefited from this realization by implementing intentional induction heating for numerous applications. In these applications, induction heating made it easier to meet heating parameters without the requirement of an additional external heat source. The removal of external heat sources substantially reduced overall system heat loss while maintaining a convenient working environment. Absence of the physical contact of a heated object to a heating device provided a safer working environment. Thus the primary benefit of induction heating was the fact that a safe, high density energy could be achieved by within a short period of time.
The fundamental theory of induction heating is similar to that of a transformer. Modeled as a transformer, electromagnetic induction uses a multi turn primary, (the inductive coil), and a single turn shorted secondary, (the heated part), which causes a substantial heat in the secondary due to the increased current.
The higher the frequency of the alternating current administered to a coil, the more intensive is the induced current flowing around the surface of the heated object. The density of the induced current diminishes when flowing closer to the center of the heated object, which is called the skin effect. From this effect, one can determine the majority of the electric energy is concentrated in the skin depth dimension.
Induction heating equipment must create alternating currents at frequencies from 60 Hz to over 1 MHz. In the beginning, spark gap oscillators, motor driven generators and vacuum tubes were used to create the alternating current. Technology advanced and soon SCR, (Silicon Controlled Rectifier), based power supplies were used to replace older generators. Very large and powerful transistors are now used in power supplies for induction heating.
An induction heating system is comprised of several major components.
Power Supply (generates the high frequency current)
Load Matching or Heat Station (matches the impedance of the coil to the power supply)
Induction Coil (copper coil wrapped around object to be heated)
Water Cooling (high power systems are water cooled to remove waste heat)
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