Induction cooking: Difference between revisions

An induction cooker uses a type of induction heating for cooking. It is chiefly distinguished from other common forms of stovetop cooking by the fact that the heat is generated directly in the cooking vessel, as opposed to being generated in the stovetop (as by electrical coils or burning gas) and then transferred to the cooking vessel.

In an induction stovetop, a coil of copper wire is placed underneath the cooking pot. An alternating electric current is made to flow through the coil, which produces an oscillating magnetic field which creates heat in the cooking vessel over it, which must be made of a magnetic material (ferromagnetic) and electrically conductive, in two different ways. Principally, it induces an electric current in the pot, which produces resistive heating proportional to the square of the current and to the electrical resistance of the vessel. Secondly, it also creates magnetic hysteresis losses in the pot due to its ferromagnetic nature. The first effect dominates: hysteresis losses typically account for less than ten percent of the total heat generated.^[1]

Induction cookers are faster and more energy-efficient than traditional electric cooktops; moreover, they allow instant control of cooking energy, which no energy source other than gas offers. Because induction heats the cooking vessel itself, the possibility of burn injury is significantly less than with other methods: only skin contact with the cooking vessel itself (or, when high heat has been used, the stovetop for a while after the vessel has been removed) can cause harm. There are not the high temperatures of flames or red-hot electric heating elements found in traditional cooking equipment, which generates heat independent of the cooking vessel. Further, induction cookers do not themselves warm the surrounding air, resulting in further energy efficiencies.

It is possible to build an induction cooker that works with any pot that conducts electricity (a pot made of any metal, but not of an electrically insulating material such as glass or ceramic), even if the pot is not ferromagnetic, but the increased permeability of an iron or steel pot makes the system substantially more practical by increasing the inductance seen by the drive coil and by decreasing the skin depth of the current in the pot, which increases the AC resistance for the ${\displaystyle I^{2}R}$ heating. ^[2] Most practical induction cookers are designed for ferromagnetic pots; users are advised that the cooker will work only with pots to which a magnet will stick.

Since heat is being generated by an induced electric current, the unit can detect whether cookware is present (or whether its contents have boiled dry) by monitoring the voltage drop caused by resistance in the circuit (which reflects how much energy is being absorbed). That allows such functions as keeping a pot at minimal boil or automatically turning an element off when cookware is removed from it.

Induction heating could be considered to have reached mainstream status in the USA when in 2008 Consumer Reports reviewed induction appliances alongside gas and other forms of electrical heating.

This form of flameless cooking has certain advantages over conventional gas flame and electric cookers, as it provides rapid heating, improved thermal efficiency, and greater heat consistency, yet similar precise control to gas.^[3] In situations in which a hotplate would typically be dangerous or illegal, an induction plate is ideal, as it creates no heat itself.

The time to boil a certain amount of water is proportional to the power; a 3600-watt induction element is three times faster than a 1200-watt ones. The actual time depends upon the amount of water, but is typically a few minutes. Induction heating is much faster without water, e.g., for stir-frying—a thin pan containing three tablespoons of oil may heat up to stir-fry temperature in as little as ten seconds.

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 generates heat. Induction cookers are easier to clean because the cooking surface is flat and smooth, even though it may have several zones of heating induction. Food tends not to burn onto the cooking surface as it is not as hot as the pot and contents.

Cookware used on induction cookers must be made with a significant amount of ferrous materials. "Clad" multi-layer cookware, with an outer layer of ferrous material bonded to interior layers of aluminum or copper then stainless steel, combine the better thermal conductivity (than iron) of these metals with the magnetic permeability of the ferrous layer. Cast-iron cookware (including ceramic- or porcelain-coated cast iron) is suitable, as is much—but by no means all—stainless-steel or steel-clad cookware. (Most cookware appropriate for use on induction cooktops will be so labelled.)

An induction cooker must match the shape of the pan; the usual flat cooker works well with flat-bottomed pans, but "induction-compatible" woks, for example, with rounded bottoms need an induction cooker that fits the shape of the wok

Induction cookers are more expensive than other electric cooktops.

Induction cookers require especially careful installation, because the electronics within an induction cooker must not be heated excessively during operation. Units are usually equipped with fan-cooled heat sinks attached to components requiring cooling, and proper flow channels and venting for heated air, which reduces the likelihood of premature failure. Clearances and positioning specified by the maker must be scrupulously observed.

Also owing to heat-venting requirements, not all induction cooktops can safely be installed over all ovens; it is necessary to refer to the cooktop maker's installation instructions to determine which combinations are considered acceptable, and many makers' instructions seek to steer buyers to using an oven of the same brand (even if others would function satisfactorily).

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According to the U.S. Department of Energy the efficiency of energy transfer for an induction cooktop is 90%, versus 71% for a smooth-top non-induction electrical unit, for an approximate 20% saving in energy for the same amount of heat transfer.^[4]

Asian manufacturers have taken the lead in producing inexpensive single-induction-zone cooktops; efficient, low-waste-heat units are advantageous in densely populated cities with little living space per family, as are many Asian cities. Induction cookers are less frequently used in other parts of the world.

Energy efficiency is the ratio between energy delivered to the food and that consumed by the cooker. The environmental impact of cookers needs to take into account the efficiency of the process of generating and distributing electricity. Cooking with gas has an energy efficiency of about 30%, plus distribution losses leading to an overall efficiency of around 27.9%.^[5] If electricity is generated by burning hydrocarbons, generation efficiency and transmission loss lead to about the same overall efficiency, so that an induction cooker with efficiency close to 100% has about the same environmental impact as a gas cooker, and other electric cookers have more. The environmental impact of electricity from other sources (nuclear, wind, hydroelectric, solar, etc.) varies (the impact of construction, maintenance, etc. should be taken into account for all power sources for the most meaningful comparison).

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Note: This table considers only the efficiency of the cooktop; it does not consider the efficiency of distribution systems for gas and electricity. Electricity production from burning gas has an efficiency of about 1/3, consequently, the entries for electric efficiencies should be reduced by a factor of 3. However, not all electricity is produced through burning gas.

Induction equipment may be a build-in stovetop, part of a range, or a standalone countertop unit; build-in and rangetop units typically have multiple elements (what on a gas unit would be called "burners"), while standalones are usually single-element, though occasionally dual-element. All such elements share a basic design: an electromagnet 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 ^{[citation needed]}. In Hong Kong, power companies recommend a number of models for ready selection that are safe, clean, energy-efficient, and easy to install^{[citation needed]}.

An induction cooker transfers electrical energy by induction from a coil of wire into a pot made of material which must be electrically conductive, and is usually ferromagnetic. The heat generated is analogous to the unwanted heat dissipated in an electric transformer; most of the heat is due to resistive heating like a transformer's copper losses and eddy currents, and the rest is analogous to a transformer's other iron losses.

A coil of wire, usually litz wire, is mounted underneath the cooking surface, and a large alternating current is passed through it. The current creates a changing magnetic field. When an electrically conductive pot—an electrically insulating pot made of, say, glass or ceramic will not work—is brought close to the cooking surface, the magnetic field induces an electrical current, called an "eddy current", in the pot. The eddy current, flowing through the electrical resistance, causes electrical power to be dissipated as heat; the pot gets hot, and heats its contents by heat conduction. The cooking surface is made of a material which is a bad heat conductor, so only minimal 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 after the cooking vessel is removed.

Some energy will be dissipated wastefully by the current flowing through the resistance of the coil; wasted energy is minimised by the geometry of the design and by the coil having low resistance (it is usually made of copper, a metal with high electrical conductivity); the pot is typically made from stainless steel or iron, which is much less conductive. The pot is also ferromagnetic. Since the increased magnetic permeability of the material decreases the skin depth, the resistance will be further increased. The copper coil, on the other hand, is made from wire known as litz wire, which is a specially configured bundle of many tiny wires in parallel, which effectively have a very thick skin depth at the high frequencies present in an induction heater. Furthermore, the copper is formed into a coil with many turns, while the bottom of the pot effectively forms a single shorted turn. That "transformer" thus steps down the voltage and steps up the current, which means that the resistance of the pot—as viewed from the primary—appears larger. That in turn means that most of the energy becomes heat in the high-resistance steel, while the driving coil stays cool.

The reasons iron or steel cookware work on an induction cooker but aluminum or copper do not relate mostly to the materials' permeability and resistivity. Aluminum or copper cookware is more conductive, and the skin depth in these materials is larger since they are nonmagnetic. That means that the effective resistance of such pots will be much lower. That lower permeability also decreases the inductance seen at the drive coil. Thus, the inductive cooker will not work efficiently with such pots: more heat will be dissipated in the drive coil, and less in the pot. With iron or steel cookware, some heat is also generated due to the ferromagnetic material's magnetic hysteresis. This is a smaller component of the total heat generated.^[8] The differences in hysteresis losses are a much smaller effect.

Those two contributions, I²R losses (Joule heating) from eddy currents and hysteresis losses, correspond to the two types of losses in the core of a transformer. In a normal transformer, those losses are undesired, because the useful output is electrical power; in an inductive cooker, the useful output is heat, so such "losses" are exactly what is desired.

First patents date from the early 1900s.^[9] Demonstration stoves were shown by the Frigidaire division of General Motors in the mid-1950s on a touring GM showcase in North America. The induction stove 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, however, was never put into production. 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]}

That work was first put on public display at the 1971 National Association of Home Builders convention in Houston, Texas, 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 to develop the market. Those 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 $1500. That 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 the sale of Westinghouse Consumer Products Division to White Consolidated Industries Inc.

CT2 had four burners of sufficient power, about 1600 watts. The range top was a PyroCeram ceramic sheet surrounded by a stainless-steel bezel upon which four magnetic sliders adjusted four corresponding potentiometers set below. That 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 between 20V and 200V of continuously variable DC by a phase-controlled rectifier. That 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. That elegant circuit design, largely by Ray MacKenzie, successfully dealt with certain bothersome overload problems.

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

CT2 was UL Listed, and received Federal Communications Commission (FCC) approval, both firsts. Numerous patents were also 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 his BBC 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 featured a self-cleaning oven, solid-state kitchen timer, and capacitive-touch control buttons (advanced for its time). The units were more expensive than standard cooktops.

The market for induction stoves is dominated by German players, such as AEG, Bosch, Miele, Schott AG and Siemens. The Spanish company Fagor, Italian firm Smeg and Sweden's Electrolux are also key players in the European market. Prices range from about GBP250 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.

The European induction cooking market for Hotels, Restaurants and other caterers is primarily satisfied by smaller more specialist commercial induction catering equipment manufacturers such as Adventys of France [1] Control Induction of the UK [2] and Scholl of Germany [3]

Taiwanese and Japanese electronics companies are the dominant players in induction cooking for East Asia. After aggressive promotions by utilities in HK like CLP Power HK Ltd [4], many local brands like icMagIC [5], Zanussi, iLighting, German Pool [6] also emerged. Their power and ratings are high, more than 2800 W. They are multiple zone and capable of performing better than their gas counterpart. The efficiency is as high as 90% and saves a lot of energy and environmentally friendly. Their use by local Chinese for wok cooking is getting popular. Some of these companies have also started marketing in the West. However, the product range sold in Western markets is a subset of that in their domestic market; some Japanese electronics manufacturers only sell domestically.

Small stand-alone induction cookers are relatively inexpensive, priced from around $60 USD.

Units may have two, three, four, or five induction zones, but four (normally in a 30-inch-wide unit) is the most common in the US and Europe, two is most common in Hong Kong, and three is most common in Japan. Some have touch-sensitive controls. Some induction stoves have a memory setting, one per element, to time the amount of heat required.

Induction hobs (cooktops) work well with any pans with a high ferrous metal content at the point where the base is in contact with the Induction hob cooking surface. Cast iron pans and any black metal or iron pans will work on an induction hob. Stainless steel pans will often work on an induction hob provided the sole of the pan is a grade of stainless steel that has a high iron content, to check this do the magnet test: if a magnet sticks well to the sole of the pan it will work on an induction hob.

The types of pans that you use on an induction hob, provided that they have a ferritic (sticky to a magnet) base will be generally the same as those you would use on a conventional electric or gas hob.

For frying on an induction hob you need a pan with a base that is a good heat conductor. Because of the high heat used for frying it is vital to spread the heat quickly and evenly. Many frying pans are made of aluminium with a non stick lining, or have an aluminium base with a stainless steel pan liner with or without a non stick coating. For induction hob use, the sole of the pan will be either a ferritic plate pressed into the aluminium or a cladding of ferritic stainless steel over the aluminium. This ferritic sole is the part which actually generates the heat on your induction hob. The advantage of the aluminium pans is that because aluminium is such a fantastic conductor it carries the heat to all areas of the pan, stainless frying pans with an aluminium base will not have the same level of heat at the sides as the aluminium sided pan. Some frying pans are made of iron which does not perform as well as aluminium as a conductor and does not provide even heat, but it is cheap. Being ferritic, iron pans work well with induction.

Saucepans and Stockpots for use on Induction Hobs – For use on an induction hob the sole of the pan must be ferritic, the rest of the pan materials are dependent on the product being heated/cooked on the induction hob. For boiling of items such as vegetables in water the heat is spread around the pan by the water and the 100C boiling point stops any hot spots on the base of the pan from overcooking the food. For products such as sauces it is important that at least the base of the pan incorporates a good heat conducting material such as aluminium to spread the heat evenly across the base. For really delicate products e.g. thick sauces, such as saballon, a pan with aluminium incorporated on all surfaces of the pan is better as the heat is pulled up the sides by the conductivity of the aluminium letting the chef heat the sauce rapidly but gently and evenly without overheating or hotspots. Again to work on Induction hobs the pans need a ferritic sole.

Wikimedia Commons has media related to Induction cookers.

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• Technical Support Document for Residential Cooking Products
• Cooking Inductively: ADI iCoupler Technology Isolates the Hob and the User Interface
• Guide to Induction Cookers