Induction cooker

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Induction Stove (Top View)

An induction cooker uses induction heating for cooking. A ferromagnetic or ferrimagnetic coated pot is placed above an induction coil for the heating process to take place. This type of 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 cooktops. Additionally, the stove top does not become dangerously hot; however, skin can still be burned if it comes into contact with the inside of the pot. Also, the induction cooker does not warm the air around it as other cookers do.

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 voltage drop caused by resistance to the current. This creates possible additional functions, such as keeping a pot at minimal boil or automatically turning off when the cookware is removed.

1 Benefits
2 Economic considerations
3 Common usage
4 Heat generation
5 Early production
6 Vendors
7 See also
8 External links

[edit] Benefits

Induction Stove (Side View)

This form of flameless cooking has an edge over conventional gas flame and electric cookers as it provides rapid heating, vastly improved thermal efficiency, greater heat consistency, plus the same or greater degree of controllability as gas.^{[citation needed]}

The amount of time that it takes a pot to boil depends on the power of the induction cooktop. Thus, the time can be from three minutes for 3600 watt induction stove tops, to around ten minutes for 1200 watt ones: much faster than conventional electric coil or radiant cookers.^{[citation needed]}

Induction cookers are safer to use than conventional stoves because there are no open flames and the "element" itself reaches only the temperature of the cooking vessel; only the pan becomes hot. However, it must be remembered that that pan was at 100 °C (212 °F) and in deep fat frying could be as hot as 200 °C (392 °F). Induction cookers are also easier to clean because the cooking surface is flat and smooth, even though it may have several zones of heating induction. In addition, food cannot burn onto the cooking surface as it is not hot.

[edit] Economic considerations

Induction cookers are considerably more expensive than traditional cookers, but consume half as much electricity as electric-resistance elements 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%.^{[citation needed]}

There are, however, cheaper single-induction-zone cooktops available largely from Asian suppliers. This is due to Asia's more densely populated cities, therefore making this type of induction cooker popular where living space is at a premium.^{[citation needed]} Single-zone induction cookers are not available in retail outlets in North America, but are widely available through online stores and auction sites; some induction hobs sell for as low as $60 USD in supermarkets.^{[citation needed]}

[edit] Common usage

Most induction cooking is done on stovetop units, which may be built into a countertop or may be a portable unit. In this style of cooking, the electromagnet is usually sealed beneath a heat-resisting glass-ceramic sheet that is easily cleaned. The pot is placed on the glass coating, and begins to heat up along with its contents. In Japan, a large percentage of rice cookers are powered by induction heating.

[edit] Heat generation

An induction cooker works like an electrical transformer: it transfers electrical energy into the pot, using a time-varying magnetic field. A coil of wire is mounted underneath the cooking surface, and a large alternating current is made to flow through that wire. This current creates a changing magnetic field. When an electrically conductive pot is brought close to the cooking surface, this magnetic field induces an electrical current in the pot.

The metal pot is not a perfect conductor, and as a result these eddy currents encounter some electrical resistance. This resistance converts the current into heat. The result is that the metal pot, and only the metal pot, heats up. Heat is transferred from the pot to the food inside the pot by conduction. The cooking surface is designed to be a good thermal insulator, so that a minimum of heat is transferred from the pot to the cooking surface (and thus wasted). In normal operation, the cooking surface stays cool enough to touch without injury.

If the pot is made from a non-magnetic or electrical insulator, then no current can flow through the pot. This means that no heat will be generated. Inductive cookers do not work with Pyrex glass or ceramic.

Current is flowing through both the pot and the driving coil; but the pot should heat up, and the coil should stay cool, since any energy that does not go in to heating the pot is wasted. This occurs because the coil and the pot are made of different metals. The coil is typically made from copper, or another metal with high electrical conductivity. The pot is typically made from stainless steel or iron, which is much less conductive. This means that most of the energy will become heat in the high-resistance steel, and the driving coil will stay cool. (The partition of heat is not completely determined by the materials' electrical conductivities. The effective resistance that the eddy current sees is affected by the ferromagnetic steel's permeability, which determines the skin depth. The geometry of the pot and driving coil are also important.)

With aluminum or copper cookware, this is not the case; the current will heat the driving coil just as much as it will the pot. The inductive cooker will therefore not work efficiently. With iron or steel cookware, some heat is also generated due to the ferromagnetic material's magnetic hysteresis. This is a smaller component (typically less than 7%) of the total heat generated. These two contributions, I²R losses from eddy currents and hysteresis losses, or "copper losses" and "iron losses", are similar to the two identically-named loss mechanisms in an electrical transformer. In a transformer, these losses are undesired, because the useful output is electrical power; in an inductive cooker, the useful output is heat, so these "losses" are what is desired.

[edit] Early production

The concept of using high frequency magnetic fields to cook with is an old one; first patents date from the early 20th century^{[citation needed]}.

Modern implementation in the USA dates from the early 1970s, with work done at the Research & Development Center of Westinghouse Electric Corporation at Churchill Borough, near Pittsburgh, PA, USA.^{[citation needed]}

This work was first put on public display at the 1971 National Association of Home Builders convention in Houston, TX, as part of the Westinghouse Consumer Products Division display.^{[citation needed]} The stand-alone single burner range was named the Cool Top Induction Range. It used transistors developed for automotive electronic ignition systems to drive the 25 kHz current.

Westinghouse decided to make a few hundred production units further to develop the market. These were named Cool Top 2 (CT2) Induction ranges. The development work was done at the same R&D location by a team led by Bill Moreland and Terry Malarkey. The ranges were priced at $1500 each. This price included a set of high quality cookware made of Quadraply, a stainless steel/carbon steel/aluminum/stainless steel laminate (outside to inside).

Production took place in 1973 through 1975, and stopped coincidentally with Westinghouse Consumer Products Division being sold to White Consolidated Industries Inc.

CT2 had four burners of sufficient power, about 1600 Watts. The range top was a ceramic sheet surrounded by a stainless steel bezel upon which four magnetic sliders adjusted four corresponding potentiometers set below. This design, using no through-holes, made the range proof against spills. The electronic section was made in four identical modules. Provision was made for fan cooling of the electronics.

In each of the electronics modules the 240V 60Hz domestic line power was converted to 20V to 200V continuously variable DC by a phase-controlled rectifier. This DC power was in turn converted to 27 kHz 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.

Control electronics included functions such as protection against over-heated cook-pans and overloads. Provision was made to reduce radiated electrical & magnetic fields. There was magnetic pan detection also.

CT2 was UL Listed and received FCC approval, both firsts. Numerous patents were also issued.

Raymond Baxter demonstrated the CT2 on his BBC series, Tomorrow’s World. He showed how the CT2 could cook through a slab of ice.

[edit] Vendors

Market for induction stoves is dominated by German players, such as AEG, Bosch, Miele, Schott AG and Siemens. The Italian firm Smeg and Sweden's Electrolux are also key players in the European market. Prices range from about GB£250 to 1000 within the UK. In 2006, Stoves launched the UK's first domestic induction hob on a range cooker at a slightly lower cost than those imported.

Taiwanese and Japanese electronics companies are the dominant players in induction cooking for East Asia. Certain companies have also started marketing in the West; such as Tatung, Sunpentown, Panasonic and Hitachi. However, their products available in Western markets are a small fraction of what is available in their home markets. Interestingly, some Japanese electronics giants only sell domestically. Some of the brands on the retail market in the Western US are Wolf, Viking, Thermador, GE Profile, KitchenAid, and Jenn-Air (Whirlpool Corp), all with 30" and 36" kitchen counter-top models.

Small stand-alone induction cookers are relatively inexpensive, around US$60.

Units may have two, three, four, or five induction zones, but four is the most common. Some have touch-sensitive controls. Some induction stoves have a memory setting, one per hob, to time the amount of heat required.