Induction Heating Working Principles

Michael Faraday in 1831, discovered the principle of electromagnetic induction, during his experiments related to electromagnetism. He found that when an electric current was passed through one coil, a momentary current flow was generated on the surface of another coil placed inside the first one. The newly discovered principle was extensively worked on and was soon used in the manufacturing industry on a large-scale basis.

What is Induction Heating
Induction heating can be defined as a non-contact heating process used to heat a conductive metal by inducing it with electric current. Along with efficiency, it generates clean, accurate and internal heating of an object. The process of induction heating is based on two principles: electromagnetism and the Joule effect.

The process of induction heating is not as complex as it seems. Once the meaning and the principles of induction heating are clear, the applications and their process become easier to understand.

Simply put, the process involves passage of electric current through a coil within which the metal that is to be heated is placed. The coil becomes the transformer primary and the metal (usually known as the workpiece or work head) becomes the transformer secondary.

Theoretically, only three things are required for induction heating:

1. Electric Current Supply
2. Induction Coil
3. Workpiece

But in practice, few other things are also required, like:

• Water Cooling Process
• Impedance Matching Network (for better power transfer between the induction coil and high frequency current supply)
• Control Electronics (to control heat intensity and to time the heating cycles)

The induction heating process can be explained in the following steps:
Step 1: Electric current is passed through the coil. An electromagnetic force field or flux is produced around it due to the current flow by the principle of electromagnetism.

Step 2: An alternate electric current is produced on the surface of the workpiece placed inside the coil. This alternate current flow is called the 'eddy current'.
Step 3: The metal offers resistance to the eddy current and then due to the Joule Effect, the energy dissipated produces internal heat.

The working process of induction heating is influenced by certain factors. Some of them are listed below.

• Characteristics of the Workpiece: As stated earlier, induction heating can effectively be done on objects that are conducive to electricity. 

Most of the induction heat is found to be on the surface of the workpiece. So, the overall amount of induction heat of the workpiece depends on the thickness of the object. Also, higher the resistivity of the workpiece, higher the amount of induction heat.

• Power Density (or Amount of Power Supply): Higher supply of electric current to the coil increases the electric current. This causes the workpiece to heat up faster, though only the surface may get heated. Ideally, longer heating cycles of lower electric supply, insures even heating of the workpiece.

• Coil Design: The alternating magnetic field is created due to the current flow in the coil. That is why, proper heating of the workpiece depends on proper and effective designing of the coil. A coil should be designed with the shape and size of the workpiece in mind.

The process of induction heating is governed by some fundamental principles.

• Reference or Penetration Depth: The 'skin effect' of heating refers to the natural tendency of the eddy currents to flow on the surface of a material, thus producing heat there. 

The thickness of the material, measured from the surface, in which 80 to 90 percent of the total heat is concentrated is called the penetration depth or the reference depth. It depends on the frequency of the force field, resistivity and magnetic property of the workpiece.

• Proximity Effect: It is the effect that one magnetic field has on the other, causing disturbance in the uniform flow of charge in both. This increases the resistivity of the materials, which in turn increases induction heat. 

This principle is even known as the Principle of Least Inductivity. Also, the total loop inductance, created by the outgoing and returning current, is minimized by the distribution pattern of current flow at high frequencies.

• Coupling Efficiency: It is the principle which determines the distance between the coil and the workpiece such that the amount of heat produced is maximum. Closer the coupling of the electric current in the coil and the workpiece, greater the induction heat.

• Hysteresis: If a workpiece has magnetic properties, natural electric resistance is offered to the rapid change in the force field and additional heat is produced by the flow of magnetic material in the inductor. 

This phenomenon is called hysteresis. But induction heating caused due to this occurs only below the Curie temperature. Curie temperature is the temperature after which magnetic properties are lost.

The use of induction heating increased during the World War II. In the modern times, the applications and usage of induction heating have vastly increased, both in quantity and quality.

From production of anti-tamper seals for bottles used in pharmacies to the process of 'getter firing' for removal of contamination from picture tubes and vacuum tubes, induction heating is widely in demand. Few such applications are given below.

• Metallurgy (or Heat Treatment)
• Soldering

• Annealing
• Induction Furnace
• Welding
• Induction Cooking
• Plastic Processing
• Brazing
• Sealing
• Heating-to-fit
• Steel Hardening

Induction heating is advantageous for many reasons.

It is precise, well-controlled, very efficient and improves the quality of the product. On the flip side, huge investment is required, for installation of induction heaters and it can be effectively used only for objects with simple shapes.

Yet, many people prefer induction heating due to the consistency and better productivity it offers. Apart from that, it is economically beneficial and eco-friendly.