Induction heating is heating of an electrically conducting material by inducing eddy currents in it. To understand induction heating, it is first necessary to know what eddy currents are and how they can be induced. So, let’s understand eddy currents first.
Consider a simple arrangement wherein a metal plate can swing freely like a pendulum. Bring it to one of the sides and drive it towards the center. Observe the motion of the plate.
Now place two magnets with opposite poles facing each other such that the metal plate is perpendicular to them. Repeat the action of bringing the plate to one of the sides and driving it towards the center. You’ll notice that the metal plate slows down faster than it did when there were no magnets. Let’ see why this happens.
The basic principle behind this is Faraday's law of electromagnetic induction. Whenever bulk pieces of conductors (a piece of metal in this case) are subjected to changing magnetic fields, circulating currents are induced in them. Literally, electrons flow in a circle in the conductor. These induced currents are the eddy currents. The direction of eddy current tries to oppose the change in magnetic field.
When the plate is entering the region between the poles of the magnet, it starts to experience an increase in magnetic flux. To oppose this increase in magnetic flux, eddy currents are induced in the plate. As you can see from the diagram below, the induced currents are in the clockwise direction on the front side of the plate. Applying the right hand rule on the front face of the plate gives South polarity. The front face of the metal plate now acts like a South pole and repels the South pole of the permanent magnet. A similar behavior can be observed on the back of the metal plate where currents are induced in the counterclockwise direction.
When the plate tries to move out of the region between the two poles, the magnetic flux cut by the plate tends to decrease. Counterclockwise current is induced on the front face of the conducting plate this time and it behaves as a north pole, trying to attract itself to the south pole of the permanent magnet. This effect of the induced eddy currents thus damps out the velocity of the plate faster. The plate comes to rest at the center of the poles faster, and when it does, as it is no longer moving, no induction of eddy currents occurs as the magnetic field as seen by the plate is stationary.
The same thing can be expected if the plate was stationary and the magnets were moved instead. There would still be a change in flux as observed by the metal plate. Eddy currents will still be induced. Simply put, as long as the metal plate experiences a changing magnetic field, eddy currents will be induced in it. This is the very principle behind induction heating.
In induction heating, a helical coil is taken, and it is energized with a high frequency AC supply. The coil produces a high frequency alternating magnetic field. When a conductor is introduced into this alternating magnetic field produced by the helical coil, eddy currents (circulating currents) are induced in the bulk of it. When a current passes through a material, the resistance of the material causes power to be dissipated in the material, heating it up. This is called joule heating. In this case, these eddy currents lead to the joule heating of the metal.
Induction heating is used extensively in industrial processes like metallurgy, forging and welding. The fact that the heat is produced in the material itself via induction rules out the need for any external contacts on the raw material being handled, so contactless heating is possible.
One more common application of induction heating is in the induction cooktop. The induction cooktop consists of a ceramic plate on which the utensils can be placed to cook food. Underneath the ceramic plate is a wound wire coil with many turns. This coil is energized by a high frequency (like 25KHz) AC supply. When a pot is placed on top of the ceramic plates, the dynamic magnetic field created by the coil induces eddy currents in the bottom of the pot. The bottom of the pot undergoes joule heating, and the heat from the bottom is transferred to the contents of the pot.
It might be beneficial to add that the extent of heating of any conducting material depends on the resistance of that material. Copper being an excellent conductor of electricity offers low resistance, and hence exhibits poor induction heating. Iron and stainless steel on the other hand are best suited for applications involving induction heating.
Not all metals respond well to induction heating. The set up of induction heating equipment is costly compared to the other metal heat treatment equipment. However, the fact that heat is generated in the material itself and does not need to be conducted into the material is what makes induction heating so advantageous.