Induction cooking is performed using direct electrical induction heating of cooking vessels, rather than relying on indirect radiation, convection, or thermal conduction. Induction cooking allows high power and very rapid increases in temperature to be achieved: changes in heat settings are instantaneous.
Cooking vessels with a suitable base are placed on an induction electric stove (also "induction hob" or "induction cooktop") which generally has a heat-proof glass-ceramic surface above a coil of copper wire with a low radio frequency alternating electric current passing through it. The resulting oscillating magnetic field induces an electrical current in the vessel. This large eddy current flowing through the resistance of a thin layer of metal in the base of the vessel results in resistive heating.
For nearly all models of induction cooktops, a cooking vessel must be made of, or contain, a ferrous metal such as cast iron or some stainless steels. The iron in the pot concentrates the current to produce heat in the metal. If the metal is too thin, or does not provide enough resistance to current flow, heating will not be effective. Induction tops typically will not heat copper or aluminum vessels because the magnetic field cannot produce a concentrated current, but cast iron, enameled, carbon steel and stainless steel pans usually work. Any vessel can be used if placed on a suitable metal disk which functions as a conventional hotplate.
Induction cooking has good coupling between the pan and the coil and is thus quite efficient, which means it puts out less waste heat and it can be quickly turned on and off. Induction has safety advantages compared to gas stoves and outputs no air pollution into the kitchen. Cooktops are also usually easy to clean, because the cooktop itself has a smooth surface and does not get very hot.
Design
Principles
An induction cooker wirelessly transfers electrical energy by induction from a coil of wire into a metal vessel. The coil is mounted under the cooking surface, and a low radio frequency (typ. ~25-50 kHz) alternating current is passed through it. The current in the coil creates a dynamic electromagnetic field which is strongly magnetic. When a suitable electrically conductive pot is brought close to the cooking surface, the oscillating field induces large eddy currents in the pot. The coil has many turns, while the bottom of the pot effectively forms a single shorted turn. This forms a transformer that steps down the voltage and steps up the current. This large current flowing through the base of the pot produces heat through Joule heating; the hot pot then in turn heats its contents by heat conduction.
For high efficiency there should be as little electrical resistance in the coil and as much as possible in the pan so that most of the heat is developed in the pan.
At the frequencies typically used in induction cooking, currents flow mostly on the outside of conductors (the skin effect). Reducing the skin effect in the coil reduces its resistance and the heat wasted in the coil. Therefore, the coil is made from litz wire, which is a bundle of many smaller insulated wires woven together in parallel. Litz wire reduces skin effect, and coil resistance, so that the coil stays cool.
Materials
For induction cooking, the base of a suitable vessel is typically made of a steel or iron. These ferromagnetic materials have a high magnetic permeability which greatly decreases the skin depth, concentrating the current in a very thin layer at the surface of the metal bottom of the pan. This makes the electrical resistance in the pan relatively high, efficiently heating the pan.
However, for non ferrous metals, such as aluminum, the skin depth in the pans with typical induction cooktops is too large, and thus efficiency with a standard induction cooker is poor: the resistive heating in the coil and pan are similar. This could damage the cooktop, which detects it and rejects the pan.
The heat that can be produced in a pot is a function of the surface resistance. A higher surface resistance produces more heat for similar currents. This is a “figure of merit” that can be used to rank the suitability of a material for induction heating. The surface resistance in a thick metal conductor is proportional to the resistivity divided by the skin depth. Where the thickness is less than the skin depth, the actual thickness can be used to calculate surface resistance.