Induction heating is a non contact method of precisely and accurately heating conductive materials. It is used in a variety of engineering and materials processing applications in the automotive, aerospace and engineering sectors.
The induction heater typically consists of three elements.
The process of Induction heating relies on the principle of electromagnetic induction. The principle dictates that when a conductive material is placed in a rapidly changing magnetic field a current will flow in this conductor. In the induction heater the component is placed in a cooper coil which has a rapidly changing magnetic field the component then effectively acts as the short circuit secondary winding of a transformer. The current flow thereby created combined with the resistance properties of the material creates a loss which is expressed as heat. In magnetic materials additional heat is generated below the curie point due to a magnetic loss know as hysterisis.
The depth at which the current flows in the material is determined by various factors. These include the resistivity and relative permeability of the material, the frequency of the magnetic field and the power density. Frequency selection is based on many factors including the type of application (case hardening, forging brazing etc). The higher the frequency the shallower the depth of heat and the lower the frequency the deeper the depth.
As the heat is developed within the component being heated and not the coil the only heat seen in the coil is as result of its own electrical losses and the radiated heat from the component. For this reason the coil is water cooled.
Due to the nature of the induction heating process it has a number of benefits over traditional methods as follows:
Magnetic steels have a temperature at which they lose their magnetic properties this typically occurs at around 750°C to 800°C.
Induction heating can only directly heat conductive materials such as metals, carbon based materials etc. However there are many applications where induction heating is used to heat a metal part prior to insertion into a plastic.
Both magnetic and non magnetic metals can be heated. It is however easier to heat a magnetic material than a non magnetic due to the hysteresis loss. It should however be borne in mind that this effect ceases at temperatures above curie.
This phenomenon is known as current cancellation. Due to changes in the materials relative permeability the depth of penetration in the material pre-curie is much less than post-curie. Once the current penetration is beyond a certain depth the current flow one way in the component cancels out the current going the other way therefore significantly reducing the heating effect. Often the radiated losses then outweigh the heat input leading to the phenomena you are observing.
The frequency of the power supply, resistivity of the material, power density and relative permeability all affect the depth of current flow in the material.
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