Induction Heating Fundamentals

Induction heating is a method of providing fast, consistent heat for manufacturing applications which involve bonding or changing the properties of metals or other electrically-conductive materials. The process relies on induced electrical currents within the material to produce heat. Although the basic principles of induction are well known, modern advances in solid state technology have made induction heating a remarkably simple, cost-effective heating method for applications which involve joining, treating, heating and materials testing.

The basic components of an induction heating system are an AC power supply, induction coil, and workpiece (material to be heated or treated). The power supply sends alternating current through the coil, generating a magnetic field. When the workpiece is placed in the coil, the magnetic field induces eddy currents in the workpiece, generating precise amounts of clean, localized heat without any physical contact between the coil and the workpiece.

There is a relationship between the frequency of the RF field and the depth to which it penetrates your workpiece; low frequencies (up to 30kHz) are effective for thicker materials requiring deep heat penetration, while higher frequencies (100 to 400kHz) are effective for smaller parts or shallow penetration. The higher the frequency, the higher the heat rate.

Due to the effects of hysteresis , magnetic materials are easier to heat than non-magnetics; these materials naturally resist the rapidly changing magnetic fields within the induction coil. The resulting friction produces hysteresis heating in addition to eddy current heating. A metal which offers high resistance is said to have high magnetic permeability which can vary from 100 to 500 for magnetic materials; non-magnetics have a permeability of 1. Hysteresis heating occurs at temperatures below the "Curie" point - the temperature at which a magnetic material loses its magnetic properties.

The induced current flow within the part is most intense on the surface, and decays rapidly below the surface. So the outside will heat more quickly than the inside; 80% of the heat produced in the part is produced in the outer "skin". This is described as the "skin depth" of the part. The skin depth decreases when resistivity decreases, permeability increases or frequency increases.

Coupling refers to the proportional relationship between the amount of current flow in the workpiece and the distance between the workpiece and the coil. Close coupling generally increases the flow of current and therefore increases the amount of heat produced in the workpiece.

The induction coil, made from copper tubing, is water-cooled. The size and shape of the coil (single or multiple turn; helical, round or square; internal or external) follows the shape of your workpiece and variables of your process so that the proper heat pattern is achieved and the efficiency of the induction system is maximized. You can read more about this important aspect of induction heating in our free tech note, "Coil Design and Fabrication".

The system generates an RF field in the induction coil,producing a magnetic field around your workpiece. System output determines the relative speed at which the workpiece is heated: a brazing process accomplished with a 3 kW system could be completed more quickly with a 5 kW system. However, additional power capability may increase the system's, size and weight and utility requirements; larger ones require 3-phase electrical connections and facilities for water cooling. For more information about RF power supplies, go to our Product Catalog.