Induction cooking: Difference between revisions

An induction cooker uses a type of induction heating for cooking. A coil of copper wire is placed underneath the cooking pot. An oscillating current is applied to this coil, which produces an oscillating magnetic field. This magnetic field creates heat in two different ways. It induces a current in an electrically conductive pot, which produces Joule (${\displaystyle I^{2}R}$) heat. It also creates magnetic hysteresis losses in a ferromagnetic pot. The former effect dominates; hysteresis losses typically account for less than ten percent of the total heat generated.^[1]

It would be possible to build an induction cooker that worked with any conductive pot (for example, an aluminum or copper pot), whether or not the pot was ferromagnetic. But the increased permeability of an iron or steel pot makes the system more practical, by increasing the inductance seen at 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; consumers are generally advised that the cooker will work only with pots that will stick to a magnet. It would not be possible to build an induction cooker that worked with an electrically insulating (for example, glass or ceramic) pot under any conditions.

Induction cookers are faster and more energy-efficient than traditional cooktops. Unlike traditional cooktops, the pot itself is heated to the desired temperature rather than heating the stovetop, reducing the possibility of injury. Skin can be burned if it comes into contact with the pot, or by the stovetop after a pot is removed. Unlike a traditional cooktop, the maximum temperature in the system is that of the pot, which is much less capable of causing serious injury than the high temperatures of flames or red-hot electric heating elements. The induction cooker does not warm the air around it, resulting in added energy efficiency.

Since heat is being generated from an induced electric current, the range can detect when cookware is removed or its contents boil out by monitoring the voltage drop caused by resistance in the circuit. This allows additional functions, such as keeping a pot at minimal boil or automatically turning off when the cookware is removed.

Induction heating could be considered to have reached mainstream status in the USA when in 2008 Consumer Reports reviewed induction appliances alongside gas and thermal heating. The induction units outscored the other types.

This form of flameless cooking has some advantages over conventional gas flame and electric cookers as it provides rapid heating, improved thermal efficiency, greater heat consistency, plus the same or greater degree of controllability as gas.^{[citation needed]} In situations in which a hotplate would typically be dangerous or illegal, an induction plate is ideal as it creates no heat itself.

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. However, boiling water is a process largely dependent on the amount of water; the speed benefits of induction cooking are most often seen when stir-frying: a thin pan with 3 tablespoons of oil may heat up to stir-frying temperature in as little as 10 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 becomes hot. Induction cookers are 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.

Induction cookers have some drawbacks. For example, cookware must generally be made of ferrous materials. Copper or aluminum pans are in some ways better than steel or iron, since their higher thermal conductivity will produce a more even temperature distribution, but they will not work on a typical induction cooker. Some pans are designed specifically for induction, with a thin layer of ferrous material laminated to the aluminum or copper bottom. Induction cookers do not work at all with nonconducting (glass or ceramic) cookware.

Since the heat up time is almost instantaneous, cooking with thin-bottomed pans requires additional attention to avoid burning food. The temperature is controlled by switching on and off the magnetic field, rather than changing its intensity. Hence, the cookware heats up and cools rapidly, causing temperature extremes. An induction cooker works well with a flat-bottomed pan. Curved pans, such as woks (despite companies selling 'induction compatible' ones), require a curved surface Chinese Style Induction Cooker.

Pans placed on an induction cooker must contain oil or a liquid to absorb the heat; otherwise, the rapid increase in temperature will cause food to burn. Anyone with a pacemaker or defibrillator should not use one of these stoves as it may cause complications with such electrical devices.

Also, induction cookers are on average more expensive than electric cooktops.

Induction cookers are getting popular and less expensive than traditional cookers. According to the 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% savings in energy for the same amount of heat transfer.[1] See Table 1.7 of the DoE reference.

There are 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 available only in few retail outlets in North America, but are widely available through online stores and auction sites; some induction units sell for as low as $60 USD in supermarkets. ^{[citation needed]} Twin burner units also made available these days and they are gradually gaining momentum in Japan, Taiwan, and Hong Kong.

When the environment is taken into consideration, a more appropriate measure should be from the source to output. It needs to be noted that even though induction cooking is efficient, the overall efficiency from the energy source to the food is comparable to cooking with gas. Currently electricity generation efficiency from a coal or gas fired power plants (responsible for 80% of total electricity) is about 33%, and the energy lost during transmission is usually about 5%, therefore the overall source to food efficiency is 28%. While cooking using a gas burner has about 30% efficiency at the stove and the gas transmission loss is about 6%, leading to the overall efficiency for gas cooking over a range to be about 27.9%.^{[citation needed]} However, if 100% "green energy" is being used (i.e. produced wholly from renewable sources, such as wind or solar), then an induction cooker has zero effect on the environment.

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^{[citation needed]}. In Hong Kong, power companies recommend a number of models for ready selection which are safe, clean, energy efficient and easy to install^{[citation needed]}.

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 an 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 due to the geometry, and 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. The pot is also ferromagnetic. Since the increased permeability of the material decreases the skin depth, the resistance will be further increased. Also, the copper is formed into a coil with many turns. The bottom of the pot effectively forms a single shorted turn. So the "transformer" steps down the voltage, and steps up the current, which means that the resistance of the pot, as viewed from the primary, appears larger. This means that most of the energy will become heat in the high-resistance steel, and the driving coil will stay cool.

Aluminum or copper cookware is more conductive, and the skin depth in these materials is larger since they are nonmagnetic. This means that the effective resistance of these pots will be much lower. The lower permeability also decreases the inductance seen at the drive coil. The inductive cooker will therefore not work efficiently; 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.^[3] The reasons why iron or steel cookware work on an induction cooker (but aluminum or copper do not) relate mostly to the materials' permeability and resistivity. The hysteresis losses are a much smaller effect.

These 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 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.

The concept of using high frequency magnetic fields to cook with is an old one; first patents date from the early 1900s.^{[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 PyroCeram 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. This elegant circuit design, largely by Ray MacKenzie, successfully dealt with some bothersome overload problems.

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.

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 self-cleaning oven, solid-state kitchen timer and capacitive-touch buttons (very advanced for its time). The units were more expensive than standard cook tops, but were still affordable for a middle-class family.

Market for induction stoves is dominated by German players, such as AEG, Bosch, Miele, Schott AG and Siemens. The Spanish-French company Group Fagor-Brandt, 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.

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[2], many local brands like icMagIC[3], Zanussi, iLighting, German Pool [4]also emerged. Their power and ratings are high, more than 2800 W. They are multiple zone and capable to perform 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; 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 in US, two is most common in Hong Kong, three is most common in Japan.. Some have touch-sensitive controls. Some induction stoves have a memory setting, one per hob, to time the amount of heat required.

• Glass-ceramic
• Microwave oven
• Electrodeless lamp

• Technical Support Document for Residential Cooking Products
• Cooking Inductively: ADI iCoupler Technology Isolates the Hob and the User Interface
• Comparison of Induction cooking vs. that of gas in terms of efficiency