When an electrically conducting material is exposed to a magnetic field, eddy currents are induced in the material. This phenomenon is called "Induction Heating". The eddy currents are concentrated at the surface of the material.
The generation of heat in the material occurs due to the induced eddy currents. When the magnetic flux surrounded by the conductor loop changes, an induced potential is generated in the loop. Similarly, when a conductor is exposed to an alternating magnetic field, it also generates an induced potential under the action of electromagnetic induction, resulting in the formation of induced current or eddy current in the conductor.
These induced currents overcome the resistance of the conductor itself and generate joule heat. This heat is used to heat the conductor itself, causing it to warm up, melt, and achieve various purposes of thermal processing. This is the principle of medium-frequency induction heating.
The physical principles governing the process of induction heating are quite simple. An alternating current flows in a solenoid or coil, which generates a transient magnetic field. According to Maxwell's equations, this magnetic field induces electric currents (eddy currents) in nearby conductor materials. Due to the Joule effect, heat is generated in the conductor material, reaching the melting point of the metal being heated. By adjusting the current parameters, the molten metal can be maintained as a liquid or its solidification can be precisely controlled.
The generated eddy currents flow against the resistivity of the metal, resulting in precise localized heat without any direct contact between the part and the inductor. Both magnetic and non-magnetic parts can generate this heat, which is often referred to as the "Joule effect".
In addition to the Joule effect, additional heat is generated internally by hysteresis. Magnetic parts create internal friction as they pass through an inductor. Magnetic materials naturally resist the rapidly changing magnetic field inside the inductor, creating internal friction, which generates heat.
The operation of an induction furnace involves a nonconductive crucible holding the metal to be melted, surrounded by a coil of copper wire. A powerful alternating current flows through the wire, creating a rapidly reversing magnetic field that penetrates the metal. This magnetic field induces eddy currents inside the metal, which heat it through Joule heating. In ferromagnetic materials like iron, the material can also be heated by magnetic hysteresis, which involves the reversal of molecular magnetic dipoles in the metal. The eddy currents also cause vigorous stirring of the melt, ensuring good mixing.
An advantage of induction heating is that the heat is generated within the furnace's charge itself, rather than applied by a burning fuel or other external heat source. This is particularly important in applications where contamination is a concern.
When the charge material is molten, the interaction of the magnetic field and the electrical currents flowing in the induction coil produces a stirring action within the molten metal. This stirring action forces the molten metal to rise upwards in the center, creating a characteristic meniscus on the surface. The degree of stirring action depends on factors such as the power and frequency applied, the size and shape of the coil, and the density and viscosity of the molten metal. The stirring action is important for mixing alloys, melting turnings, and achieving temperature homogeneity throughout the furnace. However, excessive stirring can lead to increased gas pick-up, lining wear, and oxidation of alloys.
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