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. For details about using induction heating with non-conductive materials, please visit susceptor heating.

Components of 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 alternating current and the depth to which it penetrates in the workpiece; low frequencies of 5 to 30kHz are effective for thicker materials requiring deep heat penetration, while higher frequencies of 100 to 400kHz are effective for smaller parts or shallow penetration. For heating microscopic parts, Ameritherm offers power supplies which operate at up to 485 kHz. The higher the frequency, the higher the heat rate; a good analogy is the act of rubbing your hands together for warmth. The faster your rub your hands together, the more warmth you produce.

Magnetic Vs. Non-Magnetic Materials

Magnetic materials are easier to heat than non-magnetics, due to the effects of hysteresis heating. Magnetic materials naturally resist the rapidly changing magnetic fields within the induction coil. The resulting friction produces its own additional heat - hysteresis heating - in addition to eddy current heating. A metal which offers high resistance is said to have high magnetic "permeability". Permeability can vary on a scale of 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 Penetration

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.

The Importance of Coil Design

The induction coil, typically made from copper tubing with a 1/8" to 3/16" diameter, is normally cooled with water. The size and shape of the coil - single or multiple turn; helical, round or square; internal or external - should reflect the shape of your workpiece and variables of your process.

With good coil design, the proper heat pattern is achieved and the efficiency of the induction heating power supply is maximized without making it difficult to insert and retrieve your part. You can read more about this important aspect of induction heating in our free tech note, "Coil Design and Fabrication".

The RF Power Supply

The RF power supply produces a magnetic field around the workpiece by sending an alternating current through the induction coil. The power supply's output determines the relative speed at which the workpiece can be heated. For example, a typical brazing process accomplished with a 3 kW power supply could be completed more quickly with a 5 kW power supply. However, additional power capability may increase the power supply's, size and weight and utility requirements; larger power supplies typically require 3-phase electrical connections and facilities for water cooling. For more information about RF power supplies, go to our Product Catalog.

Key Factors to Consider for Induction Heating

Determining Your Power Requirements

Several variables must be considered to determine the amount of heat energy required for a specific application: the degree of temperature change required; the mass, specific heat and electrical properties of the workpiece; and the coupling efficiency of the coil design. In addition, thermal losses due to conduction of heat into workpiece fixturing, convection and radiation must also be considered. 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 Induction Heating 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!

Small Production Runs & Process Development Available

We offer the facilities and staff of our Applications Lab for brazing, heat treating, metal bonding, adhesive curing, melting, heat staking and many other processes. Our experienced engineers will process your parts with speed, accuracy and consistency. Contact us for a free induction heating quotation!