Induction cooking is a cooking process using direct electrical induction heating of cookware, rather than relying on flames or heating elements. Induction cooking allows high power and very rapid increases in temperature to be achieved: changes in heat settings are instantaneous.
An induction electric stove (also induction hob or induction cooktop) generally has a heat-resistant glass-ceramic surface. Below each cooking position there is a coil of copper wire with an alternating electric current passing through it. The resulting oscillating magnetic field induces an electrical current in the metal bottom of the cookware, which produces heat by flowing through resistance. Typically, cookware must contain a ferromagnetic metal such as cast iron or some stainless steels. Induction tops typically will not heat copper or aluminum cookware because the magnetic field cannot produce a concentrated current (see skin effect).
Induction cooking is among the most efficient ways of cooking, which means it produces less waste heat and it can be quickly turned on and off. Some chefs and restaurants have a preference for induction stoves, as their precise settings allow for greater consistency and cooking to exact specifications. Induction has safety advantages compared to gas stoves and emits 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. Heavy cookware may scratch the surface if dragged across the cook top surface. History
The first patents were issued in the early 1900s, Arthur Berry applying for a UK patent in 1906, and Simon Hohlfeld, a German patent in 1909. Demonstration stoves were shown by the Frigidaire division of General Motors in the mid-1950s on a touring showcase. The induction cooker was shown heating a pot of water with a newspaper placed between the stove and the pot, to demonstrate the convenience and safety. This unit was never put into production.
Modern implementations came in the early 1970s, with work done at the Research & Development Center of Westinghouse Electric Corporation. That work was first put on display at the 1971 National Association of Home Builders convention in Houston, Texas, as part of the Westinghouse Consumer Products Division display. The stand-alone single-burner range was named the Cool Top Induction Range. It used parallel Delco Electronics transistors developed for automotive electronic ignition systems to drive the current.
Westinghouse decided to make a few hundred production units to develop the market. Those were named Cool Top 2 (CT2) Induction ranges. The development work was done by a team led by Bill Moreland and Terry Malarkey. The ranges were priced at , including a set of high quality cookware made of Quadraply, a new laminate of stainless steel, carbon steel, aluminum and another layer of stainless steel (outside to inside). Production began in 1973 and stopped in 1975.Moreland, W. C. (1973). "The Performance of Induction Range Cooking Appliances". IEEE Transactions on Industry Applications, IA-9(6), 810–814.
CT2 had four "burners" of about each. The surface was a Pyroceram ceramic sheet surrounded by a stainless-steel bezel, upon which four magnetic sliders adjusted four corresponding potentiometers below. That design, using no through-holes, made the range impervious to spills. The electronics section was made of four identical modules cooled by a single quiet, low-speed, high-torque fan.
In each of the electronics modules, the , domestic line power was converted to between of continuously variable DC by a phase-controlled rectifier, which was then converted to (peak) AC by two arrays of six paralleled Motorola automotive-ignition transistors in a half-bridge configuration driving a series-resonant LC oscillator, of which the inductor component was the induction-heating coil and its load, the cooking pan. The circuit design successfully dealt with overload problems.
Control electronics included functions such as protection against over-heated pans and overloads. Provision was made to reduce radiated electrical and magnetic fields. Magnetic pan detection was provided.
CT2 was UL Listed and received Federal Communications Commission (FCC) approval, both firsts. Numerous patents were issued. CT2 won several awards, including Industrial Research Magazine's IR-100 1972 best-product award and a citation from the United States Steel Association. Raymond Baxter demonstrated the CT2 on the BBC TV series Tomorrow's World. He showed how the CT2 could cook through a slab of ice.
Sears Kenmore sold a free-standing oven/stove with four induction-cooking surfaces in the mid-1980s (Model Number 103.9647910). The unit also had a self-cleaning oven, solid-state kitchen timer and capacitive-touch control buttons, advanced for the time. The units were more expensive than standard cooking surfaces.
In 2009 Panasonic developed an all-metal induction cooker that used frequencies up to , three to five times higher than other cooktops, to work with non-ferrous metal cookware.
Theory
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 (typically ) 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 cookware then in turn heats its contents by heat conduction.
For high efficiency, electrical resistance should be minimized in the coil and maximized in the pan to allow most of the heat to develop 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. Conversely, increased skin effect in the cookware results in more efficient coupling, which is one of the factors making ferrous materials preferable.
Features
Induction cooking provides fast heating, improved thermal efficiency, and more consistent heating than cooking by thermal conduction. Generally, the higher the power rating, the faster the cooking time. Induction cooktop power ratings are generally quoted for power delivered to the pan, whereas gas ratings are specified in terms of gas use, but gas is much less efficient. In practice, induction cook zones commonly have heating performance more comparable to a commercial gas burner than domestic burners. Often a thermostat is present to measure the temperature of the pan. This helps prevent the pan from severely overheating if accidentally heated empty or boiled dry, but some models can allow the induction cooker to maintain a target temperature.
Induction cooker tops are generally a low-thermal expansion glass-ceramic. The surface of the cooker is heated only by the pot and so does not usually reach a high temperature. The thermal conductivity of glass ceramics is poor so the heat does not spread far. Induction cookers are easy to clean because the cooking surface is flat and smooth and does not usually get hot enough to make spilled food burn and stick. The surface is brittle and can be damaged by sufficient impact although they must meet specified impact standards.
Noise is generated by an internal cooling fan. Electromagnetically induced acoustic noise and vibration (a high-pitched hum or buzz) may be produced, especially at high power, if the cookware has loose parts or if the layers of the pot are not well bonded to each other; cookware with welded-in cladding layers and solid riveting is less likely to produce this type of noise. Some users are more capable of hearing (or more sensitive to) this high-frequency sound.
Some cooking techniques available when cooking over a flame are not applicable. Persons with artificial pacemakers or other electronic medical implants are usually instructed to avoid sources of magnetic fields. Radio receivers near the induction-cooking unit may pick up some electromagnetic interference.
Energy lost from gas cooking heats the kitchen, whereas with induction cooking, energy losses are much lower. This results in less heating of the kitchen and reduces the required amount of ventilation. Gas stoves are a significant source of indoor air pollution. Gas cooking efficiencies are lower once waste heat generation is taken into account. Especially in restaurants, gas cooking can significantly increase air temperature in localized areas. Extra cooling and zoned venting may be needed to adequately condition hot areas without overcooling other areas. In a commercial setting, induction cookers do not require safety interlocks between the fuel source and the ventilation, as may be required with gas systems.
Some units have touch-sensitive controls. Some have a memory setting, one per element, to control the time that heat is applied. At least one manufacturer makes a "zoneless" induction cooking surface with multiple induction coils. This allows up to five pots to be used at once anywhere on the cooking surface, rather than in pre-defined spots.
Cookware
Cookware must be compatible with induction heating; generally, only ferrous metal can be heated. Cookware should have a flat bottom since the magnetic field strength (heating power) drops rapidly with distance from the surface. (Wok-shaped cooktops are available for use with round-bottom woks.) Induction disks are metal plates that are heated by induction and heat non-ferrous pots by thermal contact, but these are much less efficient than ferrous cooking vessels.
Induction-compatible cookware can nearly always be used on other stoves. Some cookware or packaging is marked with symbols to indicate compatibility with induction, gas, or electric heat. Induction cooking surfaces work well with any pans with a high ferrous metal content at the base. Cast iron pans and any black metal or iron pans are compatible. Stainless steel pans are compatible if the base of the pan is a magnetic grade of stainless steel. If a magnet sticks well to the bottom of the pan, it is compatible. Non-ferrous cookware is compatible with "all-metal" cookers.
Aluminum and copper are desirable in cookware, since they conduct heat well. Because of this, 'tri-ply' pans often have an induction-compatible skin of stainless steel containing a layer of thermally conductive aluminum.
For frying, a pan base must be a good heat conductor to spread heat quickly and evenly. The sole of the pan will be either a steel plate pressed into aluminum, or a layer of stainless steel over the aluminum. Aluminum's high thermal conductivity makes the temperature more uniform across the pan. Stainless frying pans with an aluminum base do not have the same temperature at their sides as an aluminum sided pan. Cast iron frying pans work well with induction cooking surfaces, although the material is not as good a thermal conductor as aluminum.