Induction heating is a method of electrical heating in which an alternating current passing through an inductor (the "work coil" ) sets up an electromagnetic field. A conductive object to be heated is placed inside of the inductor and the field sets up eddy currents in the object which circulate against the object's resistance, generating heat. Because of the many windings of the work coil compared with the single "winding" that is the object, the eddy currents are huge. And it is current - not voltage - that is responsible for heat production. Voltage serves only to push the current through the resistance, and since the object is usually a very good conductor, very little voltage is needed.
The inductor itself produces little heat - the heat is produced directly inside of the target object, which makes it faster and more energy efficient than other forms of electrical heating. I wanted to see if I could reproduce induction heating.
With limited resources available, I made a coil from some 24 AWG wire that I had from the Circuits Lab, and I made this coil to have an inductance (L) of about 0.25 millihenries, which corresponds to an 8 ohm impedance at 5000 Hz, 8 ohms being the output impedance of the amplifier I used as the power supply.
impedance = 2 * pi * 5000 * L
Inductance heating requires an AC current with a high frequency - 5000 Hz or higher. I used the signal generator software SigJenny to crank out a 5000 Hz sine wave signal, which fed into one channel of my 20 x 20 watt RMS TDA-2005 based audio amplifier. The output of this channel was connected to the work coil, and after inserting a bent-out steel paper clip into the coil, the amplification was turned on and the output cranked up. Superfluous buzzing and whining came from the other speaker (which was connected to the amplifier but no input signal was going in). I am sure that this noise came from inductive/capacitive coupling inside the amplifier circuit and the buzzing and chirping is coming from the computer. Within 10 seconds the "hot metal" odor appeared and bluing of the steel made it obvious that the induction heating had worked. I probably could have heated it red-hot but the 24 gauge wire wasn't taking it too well and I didn't want to mess up the amplifier.
I think that my coil inductance was inaccurate and the impedance was much lower than 8 ohms, due to the massive amount of heat coming from the vents on the amplifier. Since the right amplifier was not operating, the entire power supply was available to drive the left amplifier and the work coil. Thank goodness for the massive copper heat sinks that I put on the TDA-2005 chips!
Practical applications of induction heating are used in induction cooktops, where the eddy currents are induced in steel cookware causing it to heat directly. Induction heating is also used in foundries to melt metals. No impurities from fuel combustion enter the metal, and the magnetic fields can be used to stir the molten material, which is useful when alloys are being mixed.
