Fundamentals of induction heating
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.
A typical induction heating system
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.
Operating frequency
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 ra
Magnetic vs non magnetic materials
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.
Depth of penitrarion
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 efficiency
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.
Importance of coil design
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".
Applied power
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.
Power required
For your application, you must consider: the degree of temperature change required, the mass,
specific heat and electrical properties of the workpiece, the coupling efficiency of the coil design
and thermal losses due to conduction of heat into workpiece fixturing, convection and radiation.
NOTE: our Applications Lab Engineers have extensive experience in balancing these variables
and are ready to assist you - keep reading!
Will induction work for your application
At our Applications Lab in Scottsville, NY, we are constantly evaluating and
developing new uses for precision induction heating with our advanced solid state
technology. We invite you to contact us about sending samples of your parts to our
lab for a NO CHARGE evaluation and system recommendation. We may already
have a solution for you! Send us your parts, describe your process, tell us what is
most important to you, and we will provide you with our best advice. For more
information, visit our Applications Laboratory page or send us an e-mail with your
questions. We'll look forward to working with you on a precision heating solution!
