Principle and application of induction cooker

Principle

An induction cooker uses induction heating for cooking. Usually a ferromagnetic or ferrimagnetic coated pot is placed above an induction coil for the heating process to take place. This type of hob (or, in American English stove top or cooktop) does not work with non-ferromagnetic cookware, such as glass, aluminum, and most stainless steel, nor with ferromagnetic material covered with a conductive layer, such as a copper-bottomed pan.

Induction cookers are faster and more energy-efficient than traditional hobs. Additionally, the risk of accidental burning is diminished since the hob itself only gets marginally hot (due to heat conduction down from cookware), allowing direct contact with a reduced chance of harm. Also, no heat is lost to the air directly from the hob, keeping the kitchen containing the cooker cooler.

Since heat is being generated from an electric current induced by an electric coil, the range can detect when cookware is removed or its contents boil out by monitoring the resistance to the current. In theory, this creates additional functions, such as keeping a pot at minimal boil or automatically turning off when the cookware is removed.

Economic considerations
Induction cookers are considerably more expensive than traditional cookers, but consume half as much electricity as electric cookers and are more efficient in heat transfer, achieving an absolute efficiency of 84% in US Dept of energy tests (compared to a typical 40% for a gas cooker).[citation needed] According to CEG Electric Glass Company, "[Induction cooking] power savings of 40-70% are realistically achievable in comparison to conventional cooktops." CEG Electric Glass Company also states induction cooking has an efficiency rate of 90%, while Electric and Gas have efficiency rates of less than 50%.[2]

Heat Generation
In induction cooking, heat is generated in two ways. In magnetic hysteresis, the rapidly oscillating magnetic field causes power in the magnetic field to be converted to heat in the ferromagnetic base of the pot due to hysteresis. The amount of heat produced is proportional to the area of the hysteresis loop. This source of heat is typical 7 % or less, and gives therefore a small amount of heat.

The primary source of heat is the current produced by the electic field (known as eddy current). The current has its greatest value in the base of the pot, and gradualy decrease with a rate dependent on frequency and material propeties (relative permeability and electrical conduction). When the current amplitude has decreased e-1 or 37 % of its start value, its distance is called the skin depth. If the skin depth is one forth of the thickness of the pot base, almost all eddy current has transformd to heat (97 %).

The heat from the pot is then transferred to the food by conduction, but very little heat is transferred to the actual stovetop. After boiling a pot of water, the surface of an induction stove is warm to touch but not so warm as to burn human flesh.

A pot or saucepan with a copper base or made of aluminium will not work properly on an induction cooktop. The primary reason is probably of the thickness of the pot. A calculation of the skin depth in aluminium gives ≈ 12 mm, which gives 48 mm thickness of the pot (10 mm for a ferromagnetic pot).

http://en.wikipedia.org/wiki/Induction_cooker

http://theinductionsite.com/how-induction-works.shtml