Induction Heating Principle and It’s Applications

Induction heating principle has been used in manufacturing processes since the 1920s. As it’s said that – necessity is the mother of invention, during world war-2, the need for a fast process to harden the parts of the metal engine, has developed induction heating technology rapidly. Today we see the application of this technology in our day to day requirements. Recently, the need for improved quality control and safe manufacturing techniques has brought this technology into the limelight once again. With today’s high-end technologies, new and reliable methods for implementation of induction heating are being introduced.

What is Induction Heating?

The working principle of the induction heating process is a combined recipe of Electromagnetic induction and Joule heating. Induction heating process is the non-contact process of heating an electrically conductive metal by generating eddy currents within the metal, using electromagnetic induction principle. As the generated eddy current flows against the resistivity of the metal, by the principle of Joule heating, heat is generated in the metal.

How Induction Heating Works?

Knowing the Faraday’s law is very useful for understanding the working of induction heating. According to Faraday’s law of electromagnetic induction, changing the electric field in the conductor gives rise to an alternating magnetic field around it, whose strength depends on the magnitude of the applied electric field. This principle also works vice-versa when the magnetic field is changed in the conductor.

So, the above principle is used in the inductive heating process. Here a solid state RF frequency power supply is applied to an inductor coil and the material to be heated is placed inside the coil. When Alternating current is passed through the coil, an alternating magnetic field is generated around it as per Faraday’s law. When the material placed inside the inductor comes in the range of this alternating magnetic field, eddy current is generated within the material.

Now the principle of Joule heating is observed. According to this when a current is passed through a material heat is generated in the material. So, when current is generated in the material due to the induced magnetic field, the flowing current produces heat from within the material. This explains the process of non-contact inductive heating.

Induction Heating Circuit Diagram

The setup used for the induction heating process consists of an RF power supply to provide the alternating current to the circuit. A copper coil is used as inductor and current is applied to it. The material to be heated is placed inside the copper coil.

By altering the strength of the applied current, we can control the heating temperature. As the eddy current produced inside the material flows opposite to the electrical resistivity of the material, precise and localized heating is observed in this process.

Besides eddy current, heat is also generated due to hysteresis in magnetic parts. The electrical resistance offered by a magnetic material, towards the changing magnetic field within the inductor, cause internal friction. This internal friction creates heat.

As the induction heating process is a non-contact heating process, the material to be heated can be present away from the power supply or submerged in a liquid or in any gaseous environments or in a vacuum. This type of heating process doesn’t require any combustion gases.

Factors to be Considered While Designing Induction Heating System

There are some factors that should be considered while designing an induction heating system for any type of applications.

• Normally, the induction heating process is used for metals and conductive materials. The non –conductive material can be heated directly.
• While applied on magnetic materials, heat is generated both by eddy current and hysteresis effect of magnetic materials.
• Small and Thin materials are heated quickly compared to large and thick materials.
• Higher the frequency of the alternating current, lower the heating depth of penetration.
• Materials of higher resistivity are heated quickly.
• The inductor in which the heating material is to be placed should allow easy insertion and removal of the material.
• While calculating power supply capacity, the specific heat of the material to be heated, the mass of the material and the temperature rise required are to be considered.
• The heat loss due to conduction, convection, and radiation should also be taken into account for deciding the power supply capacity.

Induction Heating Formula

The depth penetrated by eddy current into the material is determined by the frequency of the inductive current. For current carrying layers, the effective depth can be calculated as

Here d indicates depth (cm), the relative magnetic permeability of the material is denoted by µ, ρ  the resistivity of the material in ohm-cm, f indicates ac field frequency in Hz.

Induction Heating Coil Design

The coil used as an inductor, to which power is applied comes in various forms. The induced current in the material is proportional to the number of turns in the coil. Thus, for the effectiveness and efficiency of the induction heating, the coil design is important.

Usually, induction coils are water-cooled copper conductors. There are different shapes of coils used, based on our applications.  The multi-turn helical coil is most commonly used. For this coil, the width of the heating pattern is defined by the number of turns in the coil. Single-turn coils are useful for applications where heating of narrow band of workpiece or tip of the material is required.

The multi-position helical coil is used for heating more than one workpieces. Pancake coil is used when it is required to heat only one side of the material. The internal coil is used for heating internal bores.

Applications of Inductive Heating

• Targeted heating for surface heating, melting, soldering is possible with the inductive heating process.
• Besides metals, heating of liquid conductors and gaseous conductors is possible by inductive heating.
• For heating of silicon in semiconductor industries, the inductive heating principle is used.
• This process is used in inductive furnaces for to heat metal to its melting point.
• As this is a contactless heating process, vacuum furnaces make use of this process for making specialized steel and alloys that would get oxidized when heated in the presence of oxygen.
• Induction heating process is used for welding of metals and sometimes plastics when they are doped with ferromagnetic ceramics.
• Induction stoves used in the kitchen works on the inductive heating principle.
• For brazing carbide to shaft induction heating process is used.
• For tamper resistant cap sealing on bottles and pharmaceuticals, the induction heating process is used.
• Plastic injection modeling machine uses induction heating to improve energy efficiency for injection.

For manufacturing industries, induction heating provides a powerful pack of consistency, speed, and control. This is a neat, rapid and non-polluting heating process. The heat loss observed during inductive heating can be solved using Lenz’s law. This law showed a way of productively using the heat loss occurring in the process of inductive heating. Which of the application of inductive heating has amazed you?