Metallic Foil Induction Cooking

Abstract

An ultrasonic frequency induction surface cooking unit heats aluminum foil and other thin metal utensils placed on the cool cooking surface. For optimum heating the aluminum foil has a thickness of 0.5 mils. A uniform heating distribution is obtained by preferably using a rectangular induction heating coil with several series-connected elongated coil sections, or by varying the metal thickness to graduate the energy acceptance. Frozen convenience foods can be defrosted in this manner, and the aluminum foil can be wrapped about the food and shaped by the user into disposable utensils using a set of molds. Disposable foil cooking obviates the clean-up and storage problems of pots and pans.

Description

BACKGROUND OF THE INVENTION

This invention relates to metallic foil cooking on cool-top induction surface cooking units. More particularly, the invention relates to induction cooking appliances suitable for the cooking of food in thin metal containers and wrappings usually made of disposable aluminum foil, and to the various ways that metallic foil and thin metals can be fashioned as disposable and reusable inductively heated utensils.

The heating in an oven of precooked frozen convenience foods packaged in thin metal containers and the baking of potatoes wrapped in aluminum foil are both well known. In an oven, however, the efficiency of heat transfer to the metallic foil is relatively low due in part to the reflection of heat by the shiny metal. It is necessary to use the full oven cavity and the heating source must obtain a high temperature before the warm-up process begins with allowance for loss of heat to the room. Consequently, an extended period of time is required to heat the food to the serving temperature. It is recognized generally in U.S. Pat. No. 3,294,946 that the induction heating of thin metal frozen food containers is inherently more efficient, but the eddy current cooker disclosed is specially designed for this purpose and is not suitable for general cooking or for use with the widely different sizes and shapes of containers now available. It utilizes pairs of motor-driven circular disks with alternately poled permanent magnets with the frozen food container inserted between so as to pass magnetic flux perpendicularly through the container. The reaction forces produced by the low frequency rotating magnetic fields are used to propel the fully heated food container from the cooker. These eddy current cookers are bulky, expensive, not suitable for flat-top cooking units and portable convenience food warmers, and further give unsatisfactory cooking results because of the uneven heating distribution.

The recently developed cool-top induction surface cooking units used in the practice of this invention have been described heretofore mainly as to heating conventional pots and pans and other cooking utensils, both magnetic and non-magnetic. A complete induction range or cooktop unit for domestic appliances is disclosed in copending application Ser. No. 212,351, filed on Dec. 27, 1971 by the present inventors, and in other copending applications referenced therein. The use of aluminum foil for cooktop cooking is described briefly and claimed more broadly. These appliances have such desirable user features as a cool, counter-top, clean wire cooking surface; fast utensil warm-up and responsive heating with lower power requirements; noiseless operation; and complete freedom of movement of the utensil.

SUMMARY OF THE INVENTION

In accordance with the invention, the range of usefulness of cool-top induction surface cooking units is extended by the ability to heat and defrost foods in aluminum foil containers and wrappings on the cooking surface rather than only in the oven as previously. At ultrasonic operating frequencies the reaction forces are insignificant and do not cause movement of the lightweight foil. The invention is applicable to thin metals and metallic foils generally, but aluminum foil with an optimum thickness of 0.5 mils is heated efficiently and is the most common suitable material that is widely available to the user. In addition to defrosting and heating frozen convenience foods packaged in aluminum foil containers, a sheet of foil can be wrapped about the food to be heated (such as soft-boiled egg or a plastic frozen food pouch) to achieve an oven effect, or can be fashioned into user-made disposable utensils with a food receiving cavity (as for a fried egg or hamburger) using a set of standard molds. The use of a two-part mold set is advantageous because one part of the mold can be employed both to shape the aluminum foil utensil and provide support while eating from it. An inductively heated two-part cooker with a plurality of appropriately shaped cavities is appropriate for party and commercial cooking, for example, to heat a batch of hot dogs.

Good cooking results are obtained in foil cooking on a cool-top cooking surface by selecting the static power conversion circuit and induction heating coil to generate an alternating magnetic field distribution that produces relatively uniform heating in the foil or other thin metal. Aluminum foil in particular has such a thin cross section that it has poor lateral heat spreading qualities. Accordingly, to achieve uniform heating it is necessary to inductively heat all areas of the foil directly, or to reduce the extent and location of areas not directly inductively heated so that these latter areas can be heated by the limited lateral heat spreading mechanism. The preferred arrangement here disclosed is to employ a nominally flat even-heating induction coil comprising a plurality of elongated coil sections mounted side-by-side such that adjacent coil sides are aiding. The even-heating coil desirably has a rectangular configuration and a surrounding peripheral coil section, all series-connected. An alternative technique for use with foil cookers that heat non-uniformly such as those with a flat spiral coil, is to employ a disposable container or utensil with a variable metal thickness bottom wall to graduate the energy acceptance and thereby achieve uniform heating. Differential heating for different types of foods can be achieved by contouring the container bottom wall to have different heights so as to vary the spacing to the coil.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of an even-heating multi-section induction heating coil and a block representation of a solid state power conversion circuit for supplying ultrasonic power to the coil;

FIG. 2 is a diagrammatic cross-sectional view showing the relation of the induction heating coil, cooking surface, optional warming tray, and disposable aluminum foil utensils;

FIG. 3 is a perspective view of a portable induction cooking unit especially suitable for metallic foil cooking but useful also for cooking with conventional utensils;

FIG. 4 is a schematic circuit diagram of a preferred form of inverter for the induction cooking unit;

FIG. 5 illustrates temperature distribution characteristics for the rectangular induction heating coil of FIG. 1 (A and B) and the flat spiral coil of FIG. 8 (C);

FIG. 6 shows the power-thickness characteristic for aluminum foil;

FIG. 7 is a modification of the rectangular even-heating induction heating coil of FIG. 1 in a circular or oval configuration;

FIG. 8 is a diagrammatic cross section similar to FIG. 2 showing a flat spiral induction heating coil and a circular thin metal container with a varying bottom thickness to effect even-heating;

FIG. 9 is a cross section of a plastic frozen food pouch wrapped in inductively heated aluminum foil;

FIGS. 10 and 11 respectively are perspective views of a user-made disposable utensil formed by pressing aluminum foil about the circular mold of FIG. 11;

FIG. 12 is a perspective view of a plasticized thin metal cooker for a plurality of hot dogs;

FIG. 13 is a cross section through the loaded and closed hot dog cooker of FIG. 12;

FIG. 14 is a cross section through a glass container with an immersed metallic foil heating element;

FIG. 15 is a cross-sectional view showing a metal foil container with a contoured bottom for differential heating of foods by varying the spacing to the cooking surface or coil; and

FIG. 16 is a diagrammatic side view partly in cross section of a two-part plastic mold used both to shape a disposable utensil and provide support while using it to eat from.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The induction cooking unit shown in FIGS. 1-3 is described with regard to a portable, single coil induction surface unit for metallic foil cooking and warming as well as some general cooking. It is understood that essentially the same mechanical structure and electronic circuitry with power ranges as required can be included at one or more coil positions as an induction surface cooking unit for an electric range or cooktop. The invention is further described primarily with regard to the use of aluminum foil, but the same principles are applicable where appropriate to composites of metal foil with paper, cardboard, and plastic, and to disposable and reusable thin metallic utensils made of stamped tin plate or other suitable thin metals. The static power conversion circuit indicated generally at 12 is preferably energized by a single phase commercially available 60 Hz, 120 or 240 volt source of alternating voltage. Static power converter 12 comprises a solid state a-c to d-c power supply 11 including a full wave rectifier and a filter network for producing a d-c supply voltage that is converted by a solid state inverter 14 to an ultrasonic frequency voltage wave for driving the induction heating coil 15.

Induction heating coil 15 is a multi-section, single layer, air-core or ferromagnetic-core coil, especially built with a rectangular configuration for the even heating of foods packaged in rectangular and square thin metal containers. Precooked frozen convenience foods such as the popular "TV dinner" and various precooked meats, fish, vegetables, and potatoes are commonly packaged in such containers. Coil 15 is comprised by four series-connected coil sections including three elongated, identical central coil sections 15a, 15b, and 15c, all surrounded by a generally rectangular peripheral coil section 15d. Each of the coil sections is wound continuously in spiral fashion, with braided ribbon conductors or solid flat strip conductors. To generate sufficient magnetic flux to heat the metallic foil container or wrapping to the desired level, each coil section is tightly wound with the short cross-sectional dimension of the conductors facing upwards and adjacent turns separated by a strip 20 of insulating material. The current direction in each coil section is indicated by the arrows. Along line B--B in the width direction, the currents in any two adjacent parallel legs flow in the same direction, and thus the magnetic fields reinforce one another. The magnetic field changes polarity as the line B--B is scanned from one side to the other. To obtain this magnetic field distribution, it is evident that there must be an odd number of central coil sections. The magnetic field produced in the longitudinal direction along the line A--A, on the other hand, is approximately equal except at the ends of the coil. This coil arrangement produces relatively even-heating in inductively heated metallic foils as is explained later with regard to FIG. 5.

In the cooking unit (FIG. 2), induction heating coil 15 is appropriately mounted in a horizontal position immediately below a non-metallic or substantially non-metallic support plate 16, typically made of a thin sheet of glass or plastic. Induction heating coil 15 is preferably a flat coil, but can deviate slightly from flatness so as to effect a better field distribution. Plate 16 supports the magnetic or non-magnetic cooking utensil to be heated, and is hereafter referred to as the cooking surface. The cooking utensil can be a pot or pan or other conventional cooking vessel. A novel feature of the invention, however, is that cooking surface 16 is used to support a disposable or reusable utensil made of thin metallic foil, such as the aluminum foil sheet 17a illustrated in FIG. 2 that is slightly dished by the user for the cooking of a hamburger, a fried egg, or other appropriate food. The aluminum foil 17a can also be wrapped about an appropriate food to produce the oven effect, such as to cook a soft boiled egg or warm a piece of pie. Precooked frozen convenience foods in thin metal containers can also be heated by placing them on top of the cooking surface 16. Operation of static power converter 12 to impress an ultrasonic frequency voltage wave on induction heating coil 15 results in the generation of an alternating magnetic field. The magnetic flux is coupled across the air gap through non-metallic cooking surface 16 to disposable aluminum foil utensil 17a. At ultrasonic operating frequencies in the range of 18 to 40 kHz, the cooking unit is inaudible to most people. An even more important consequence from the standpoint of the present invention of the use of this ultrasonic frequency range is that there are insignificant pondermotive forces and other forces of either attraction or replusion which at lower frequencies would cause light weight aluminum foil sheets, wrappings, and containers to move when placed on the cooking surface. A sheet of aluminum foil, without food, can be placed on cooking surface 16 and remains in place while being inductively heated without the fear of it being thrown to the side by the action of pondermotive forces to thereby cause possible injury to the user or nearby persons.

An optional feature in an induction cooking unit is the inclusion in the appliance of a food warming opening and tray beneath the induction heating coil 15. An alternating magnetic field is also produced at the bottom of air-coil induction heating coil 15 as well as at the top, and a portion of this magnetic flux is available for warming and heating food enclosed in an aluminum foil wrapping 17b supported on a non-metallic tray 23. Other articles such as dishes can be warmed by wrapping them in foil. Tray 23 is preferably made of plastic and is slidable in and out of the cooking unit as taught in copending application Ser. No. 200,424, filed Nov. 19, 1971, by the present inventors. The warming of the piece of pie wrapped in aluminum foil is illustrated. Frozen foods in closed aluminum foil containers can also be defrosted in this manner, although not as efficiently when placed on the cooking surface 16.

The single-coil portable metallic foil induction cooker shown in FIG. 3 is intended to stand on a kitchen counter surface and is energized by a 120 volt source. A relatively small box-like housing 19 contains the electronic circuitry and induction heating coil 15, and the unit is controlled by an on-off knob 21 and a power level knob 21'. A pair of combination leg and handle units 22 are attached to each side of housing 19 to facilitate easy handling and storage. As was previously mentioned, the rectangular configuration of induction heating coil 15 is particularly suitable for defrosting and heating the "TV dinner" dinner illustrated at 17c. For defrosting frozen foods prepackaged in metallic foil containers and wrappings, it has been found that faster and more efficient results are produced by enclosing the cooking surface 16 with a cover 24, which conveniently is made of transparent plastic. Of course, this induction cooker can also be used for general purpose cooking using conventional cooking utensils, depending on the power range of the unit. At least a 10:1 power range and a maximum power of 1 to 1.5 kilowatts is required for general purpose cooking, whereas an induction cooker solely for metallic foil cooking can have a maximum power as low as 200-400 watts.

An important feature of the induction cooking equipment is that the cooking surface 16 is relatively smooth and substantially continuous and unbroken. Because of the lack of reaction forces of either attraction or replusion, the user has complete freedom to move the disposable foil utensil or conventional utensil on the cooking surface. Among the other advantages of induction cooking are, briefly, that the cooking surface 16 remains relatively cool since the highest temperatures involved are the temperature of the foils and utensils themselves, and consequently spilled foods do not char and the cooking surface is easy to wipe clean. The transfer of energy to the foil or utensil is relatively efficient and consistent, not degrading as the heating coil ages, since heat is generated only in the foil or utensil where it is wanted. Since this is a low thermal mass system and there is thus a relatively low storage of heat in the foil or utensil, the heating level of cooking temperature can be changed rapidly. The cool, smooth cooking surface 16 is available for other food preparation and cooking tasks, such as cutting and trimming vegetables, opening cans, etc. This feature may be of more utility in a portable induction cooker for a camp or cottage, particularly since inverter 14 can be powered by a battery rather than by alternating voltage. Another advantage of induction cooking that is particularly important for metallic foil cooking is that the foil is heated relatively uniformly. The importance of this feature particularly for aluminum foil cooking in view of the poor lateral heat spreading in the foil will be developed later.

To understand the electrical aspects of the uniform, fast, efficient induction heating of thin aluminum foil and other thin metallic foils, it is of benefit to discuss inverter 14 and the way in which power is coupled to the foil. FIG. 4 shows a preferred form of inverter 14 used in the practice of ultrasonic frequency induction cooking. This one-thyristor series resonant inverter requires a small number of components, only one gating circuit, and produces a wide range of output power levels and corresponding utensil heating levels. Only a brief description of the construction and operation is given in view of the discussion in the aforementioned application as well as in Kornrumpf application, Ser. No. 200,530, filed on Nov. 19, 1971, now U.S. Pat. No. 3,697,716 dated Oct. 10, 1972, which shows the same inverter configuration. Inverter 14 comprises a unidirectional conducting power thyristor 33 connected in series circuit relationship with a reset inductor 35 between d-c input terminals 30 and 31. A constant or variable direct voltage E.sub.dc is supplied to d-c terminals 30 and 31. A diode 34 to conduct power current in the reverse direction is connected across the load terminals of thyristor 33. A series RC circuit is also usually connected across the load terminals of thyristor 33 for dv/dt protection to limit the rate of reapplication of forward voltage to the device. The basic power circuit is completed by a commutating capacitor 32 and induction heating coil 15 connected in series with one another and coupled directly across the terminals of the inverse-parallel combination of thyristor 33 and diode 34. The four coil sections 15a-15d are series-connected. When either of the power devices is conducting, capacitor 32 and induction heating coil 15 form a series resonant circuit for generating damped sinusoidal current pulses that flow through induction heating coil 15, which has the dual function of providing commutating inductance as well as coupling power to the load. Reset inductor 35 functions to reset the commutating capacitor by charging commutating capacitor 32 positively during the non-conducting intervals of the thyristor-diode combination. Each cycle of current flow is initiated by a gating pulse applied to thyristor 33 by a variable repetition rate gating circuit 36. A user control 37, for example, an adjustable potentiometer actuated by control knob 21' on the cooker control panel (FIG. 3), sets the repetition rate of gating circuit 36.

The application of a gating pulse to thyristor 33 by main gating control circuit 36 causes it to turn on, energizing the series resonant circuit essentially comprising commutating capacitor 32 and induction heating coil 15. A damped sinusoidal current pulse flows through induction heating coil 15 and charges commutating capacitor 32 negatively. At this point the current in the series resonant circuit reverses and a damped sinusoidal current pulse of the opposite polarity flows through induction heating coil 15 and diode 34. During the time that feedback diode 34 is conducting, thyristor 33 is reverse biased by the voltage across diode 34 and turns off. When the current in the series resonant circuit again attempts to reverse, thyristor 33 does not conduct since it has regained its forward voltage blocking capabilities, and a gating pulse is not applied to the thyristor at this time. Because of the losses in the electrical circuit due to the heating of the utensil, commutating capacitor 32 at the end of the complete conduction cycle on a steady state basis is left charged to a lower voltage than it had at the beginning of the oscillation.

During the circuit off-time when both of the power devices 33 and 34 are non-conducting, the energy stored in reset inductor 35 is discharged and transferred primarily to commutating capacitor 32, thereby leaving the commutating capacitor 32 with a net positive charge at the end of the circuit off-time or energy transfer period. Waveform 38 in FIG. 4 shows the asymmetrical sinusoidal induction coil current for two complete cycles of operation separated by a time delay interval corresponding to the energy transfer period. A small current circulates in coil 15 during the energy transfer period due to the recharge current of capacitor 32. With practical component choices, the circuit transfers more energy from reset inductor 35 to commutating capacitor 32 as the transfer period is made shorter, relative to the high frequency oscillation period. There are thus two effects that increase the power of watts supplied to the load when the inverter operating frequency or repetition rate is increased. There are larger as well as more frequently applied current pulses in induction heating coil 15.

The load for inverter 14 is provided by the electrical losses in the utensil. With respect to the utensil load, induction heating coil 15 functions as the primary winding of an air-core transformer. In a physical equivalent circuit for the utensil, identified generally at 17, the utensil functions as a single turn secondary winding with a series resistance 17r connected between the ends of the single turn representing the I.sup.2 R or eddy current losses, and hysteresis losses where applicable. The currents and voltages induced in the utensil are determined essentially by transformer laws. An important consideration in induction cooking is the source-to-load impedance matching to enhance efficiency of power transfer. The situation in this case using a commercially available a-c source is that the input circuit impedance is fixed if minimum cost is to result. Ordinary utensils have a fixed range of impedances. Since the utensil functions as a single turn secondary winding, and the primary side voltage is fixed at 120 volts (or 240 volts), 60 Hz, it follows that the primary side impedance is also prescribed. This dictates certain impedance matching characteristics in static power converter 12. It is possible for the primary side circuit to include an impedance matching transformer, however none is used in FIG. 4 in order to reduce the number of components and the circuit cost.

The even-heating, multi-section, rectangular induction heating coil 15 shown in FIG. 1 heats all sections of uniform thickness aluminum foil containers and disposable aluminum foil utensils to a relatively uniform temperature. The temperature distributions A and B shown in FIG. 5 are produced respectively along the length dimension and along the width dimension such as A--A and B--B in FIG. 1. In the length direction the magnetic field distribution and the temperature to which the foil is heated is approximately constant except at the two ends where the amount of heating is reduced. In the width direction, the magnetic field varies cylically, as does the heating of the foil. There is actually no induction heating at the center of any of the coil sections, and it is necessary to rely on lateral heat spreading in the foil in order to heat those portions of the foil over the coil centers. Aluminum is of course a non-magnetic metal and is heated only by the I.sup.2 R losses resulting from the circulation of eddy currents. Another characteristic of aluminum foil due to its thin cross section is poor lateral heat spreading. Conventional thick aluminum cookware is known for its good thermal conductivity, however any foil suffers in its ability to spread or conduct heat laterally due to the thin cross section and large surface area. These considerations underlie the selection of the elongated, relatively narrow central coil sections. Each portion of the aluminum foil is inductively heated directly, such as areas above a coil side or between adjacent coil sections, or if not heated inductively such as those portions overlying the centers of the coil sections, are a relatively short distance (about 1 inch or less) from an area that is heated. At these short distances, the lateral heat spreading mechanism is effective to obtain the over-all result of relatively uniform heating in all sections of the aluminum foil.

Uniform heating of the foil container or disposable utensil is particularly needed in the case of defrosting frozen foods because cool spots develop if the heat is not distributed uniformly. The oven defrosting of frozen foods, it is noted, is an even-heat environment. For other foods such as fried bacon and scrambled eggs, uniform heating of the pan or other cooking utensil is needed for good cooking results, and it is a matter of common knowledge that foods of this type are often turned and moved about as the cooking proceeds to prevent localized burning.

Another important aspect of aluminum foil induction cooking is that the thickness of aluminum foils, contrary to what might be expected, is favorable to the efficient coupling of power to the metal by induction cooking equipment. The conventional approach to obtaining efficient induction heating is based on skin effect considerations, in that the usual rule of the thumb is that the thickness of the inductively heated metal should be at least as great as the skin depth and preferably three to four times the skin depth. The depth of induced current penetration is generally given as being proportional to the square root of resistivity divided by the relative permeability times the frequency. Thus, the depth of penetration varies with the square root of frequency and is dependent both on resistivity and the magnetic property characteristics of the material. Since aluminum is non-magnetic, the latter is not a consideration. For 52S aluminum the depth of penetration at 25 kHz is approximately 30 mils. Based purely on skin effect considerations, then, for most efficient induction heating an aluminum cooking utensil should be at least 30 mils and up to 120 mils in thickness. However, it is found that due to the impedance matching considerations mentioned, club aluminum ware is not heated efficiently by induction surface cooking units operating in the range of 18-40 kHz. Instead, it is found that the amount of power coupled to aluminum utensils is a function of thickness and has an optimum value as shown in FIG. 6. For the frequency range of interest and practical physical dimensions, the power is most efficiently coupled to the utensil when it has a thickness of about 0.5 mils, and there is very poor coupling at thicknesses of less than 0.1 mil and greater than 2 mils. The thickness of heavy duty household aluminum foil is approximately 0.5 mils, and thus it can be concluded that aluminum foil in thicknesses already commonly available to the user is heated efficiently by ultrasonic induction cooking equipment.

In view of the fact that the depth of current penetration at the representative frequency of 25 kHz is 30 mils, it is evident that in aluminum foil induction cooking most of the magnetic flux penetrates completely through the thickness of the metal. The explanation for the power-thickness characteristic shown in FIG. 6 is that at 0.5 mils the resistance of aluminum foil is sufficiently high that there is a good impedance match between source and load (assuming a 120 volt or 240 volt supply). The reader is referred back to the previous discussion on impedance matching in which it was developed that the one turn secondary winding provided by the utensil, and the fixed primary supply voltage, means that the primary impedance characteristic of the induction cooking equipment is also fixed. With this type of analysis, it is determined that at 30 mils the resistance of aluminum utensils is too low to obtain a good impedance match. Efficiency of power coupling is improved by reducing the thickness of the aluminum utensil to obtain a higher effective resistance and therefore a better impedance match. Most efficient heating, therefore, occurs when the thickness is about 0.5 mils, with poorer power coupling as the thickness decreases below this value or appreciably above this value. The explanation of the increased resistance of aluminum as the thickness is reduced is based on the well-known formula that the resistance of the conductor depends on its specific resistivity .rho. and is directly proportional to its length and inversely proportional to its area. It can be shown by known induction surface heating analysis that the area heated by each coil section can be divided up into a number of concentric elongated annuluses having the same shape as a coil section turn. As used in the formula, the length is the circumferential length at the center of each elongated annulus and the area is the product of the width of the elongated annulus and the thickness of the utensil. The circumferential length and the width of the elongated annulus are both fixed by the dimensions of the induction heating coil section. Varying the thickness of the aluminum utensil, then, varies the conductor resistance.

Using the metallic foil induction cooker shown in FIG. 3, precooked frozen convenience foods packaged in aluminum foil containers are defrosted and heated with good results generally in less time than is required for defrosting in an oven. Furthermore, there is less room-heating and no oven preheating time is required because of the general feature of induction cooking of fast utensil warm-up. The power control knob 21' is set to an appropriate position, easily determined, to obtain rapid defrosting and heating without scorching and burning the food. By way of example, a well-known TV dinner bears the instructions to heat in a preheated oven for 25 minutes at 450.degree.F. The corresponding time for the induction cooker is 15 minutes. In general, it can be said that the defrosting and heating time for induction cooking is less than that for oven heating, and in addition there is no preheating requirement. As was previously mentioned, the induction cooker can be used for the warming of foods such as rolls, a piece of pie, and grapefruit by wrapping the food in aluminum foil and placing it on the cooking surface for heating. Temperature feedback control is probably desirable in order to provide the optimum results with minimum care on the part of the user. In either case, the aluminum foil disposable utensil is heated efficiently and uniformly with good cooking results. If desired, the metallic foil induction cooker of FIG. 3 or range with a similar induction cooking surface unit can be provided with built-in storage for aluminum foil, either in the form of rolls, or square pull-out sheets. Thus a complete cooking appliance is provided without the need for pots and pans with their subsequent washing and clean-up problem. Referring to FIG. 7, the even-heating, multi-section induction heating coil 15' is a modification of the FIG. 1 coil in a round or oval configuration. To conform to the peripheral shape of coil section 15d', coil sections 15a' and 15c' are identical to one another but considerably shorter than the other central coil section 15b'. The oval or round configuration is especially suitable as one or more of the heating coils in a conventional four-coil electric range or cooktop. The even-heating coil 15' is used to advantage, of course, for the uniform heating of conventional pots and pans and other cooking utensils.

Another technique for achieving uniform distribution of heat in the utensil is to contour the thickness of the utensil to graduate the energy acceptance. The induction heating coil 15" shown in cross section in FIG. 8 is an annular, flat spiral, single layer, air-core coil of the type illustrated in the aforementioned Harnden and Kornrumpf application, Ser. No. 212,351. As is shown in FIG. 5, the aluminum foil temperature characteristic (C) is relatively non-uniform due to the corresponding flux distribution produced by such a coil. The composite paper-and-aluminum foil container 17d in FIG. 8 by way of example is a circular frozen pie container and has an inner foil layer with a variable thickness to obtain a more uniform temperature distribution. The thickness of the aluminum foil layer is approximately 0.5 mils for optimum coupling of the power with the exception of a thicker circular area 40 at the center of the container and a thicker annular ring 41 about midway between the center and the edges of the container. The thicker ring 41 couples power less efficiently than the optimum thickness of 0.5 mils (see FIG. 6) while at the center of the container there is no coupled magnetic flux and the thicker central area 40 is provided for better thermal conduction of the heat through the metal from the surrounding heated areas. The two techniques for obtaining a uniform heating distribution that have been described can be combined together, that is, designing the induction heating coil to have a suitable magnetic flux distribution and by contouring the thickness of the metallic foil to achieve a graduated energy acceptance.

It is realized that when considering the general subject of defrosting and heating frozen convenience foods, there are many pre-cooked convenience food packages with different types of foods that benefit from heating at different temperatures of different heating rates. The heating considerations for a homogeneous food are not the same as those for "TV dinners" with several different types of food. In an even-heat environment such as an oven, it may be necessary to arrange the food or the foil coversheet to obtain differential heating. An alternative approach made possible by the induction heating of thin metal convenience food containers is shown in FIG. 15. The thin metal container 17c' is made of uniform thickness aluminum foil and is divided into several food cavities 55-58 by raised dividers 59. The innovation illustrated is that the bottom wall of containers 17c' is contoured so that selected food cavities are at different heights from cooking surface 16 and induction heating coil 15. Since the spacing of the food cavities from coil 15 is different, the amount of induction heating is variable according to the food being cooked. Thus, cavities 56 and 57 are at intermediate and high levels as compared to cavities 55 and 58, and are suitable for foods that require a reduced heating level such as applesauce and butter. Pastries are also a problem area with unique heating requirements that make the contoured bottom wall container approach desirable.

Some of the other aspects of metallic foil induction cooking are illustrated in FIGS. 9-14. In FIG. 9, a frozen food pouch-type plastic package 42 is enclosed in an aluminum foil wrapping 17e for thawing and heating by placing it on the cooking surface 16 of the induction cooker shown in FIG. 3. This eliminates the need for boiling water to prepare these foods. FIG. 10 illustrates the forming of a user-made disposable aluminum foil utensil 17f with a functional cavity tailored for the particular food being cooked. A circular cavity is suitable for pancakes, fried eggs, hamburgers, and the like. A simple way to fashion such a disposable aluminum foil utensil 17f is by use of the circular mold or pre-form 43 shown in FIG. 11. A sheet of aluminum foil is easily pressed about mold part 43 which can be made of plastic or wood. Additional standard molds in other shapes can be provided for other types of foods, such as a square mold, a rectangular mold, a mold for hot dogs, etc.

The two-part mold set shown in FIG. 16 has the dual function of shaping the disposable utensil and serving as a support for the vessel while its contents are being consumed by the user. The heat-and-serve disposable utensil 17f' is made slightly deeper so as to be suitable for heating soup and thereafter serving as a disposable soup bowl. The two-part mold 60, 61 is used to stamp out disposable utensil 17f' by pressing the foil into conformity with the cavity on part 60, and is preferably made of plastic. Since there are a number of plastic materials that withstand temperatures of 450.degree.-500.degree.F, the highest temperatures encountered in induction cooking, the female mold section 60 can also be used to support disposable utensil 17f' while the food is being heated. Since heat-and-serve mold section 60 is not directly heated while on the metal foil cooker, it can be picked up by the user and moved to a table where it supports the vessel, which now functions as a soup bowl. Desirably there is a thin bottom wall section on mold 60 to provide support for the food or liquid as it is being cooked and transported.

The reusable stamped metal cooker 45 shown in FIGS. 12 and 13 is used for the simultaneous cooking of a plurality of food items with a repeatable shape such as hamburgers and hot dogs. The two identical halves 46 and 47 of the cooker are made, for instance, of a stamped thin tin plate or steel sheet 48 coated with a layer of plastic 49 to improve its appearance and handling. The thickness of the steel layer 48 can be uniform or can be non-uniform for better heat distribution. As here illustrated, the identical food cavities in each of the cooker halves 46 and 47 are shaped to receive hot dogs. Upon being loaded and closed, the hot dogs 50 are almost completely surrounded as shown in FIG. 13 and are cooked uniformly. This utensil can be washed after use, and is suitable for party cooking in the home or for commercial cooking.

To complete the presentation of the utility of aluminum foil in induction cooking, FIG. 14 illustrates the heating of water in a glass container 51 by means of an inductively heated foil disk 52 dropped into the bottom of the container. Ordinary glass containers can be used without danger of breakage with the only limitation being that the glass be capable of withstanding boiling water without cracking. Pre-cut aluminum disks can be provided, or can be cut from a sheet of heavy duty aluminum foil by the user. Similarly, plastic vessels can be employed effectively as utensils in induction cooking systems.

In summary, the range of usefulness of ultrasonic induction surface cooking units is extended by the ability to cook foods in thin metallic foil utensils on the cooking surface rather than only in the oven as previously known. Aluminum foil with an optimum thickness of 0.5 mils is efficiently heated inductively, and it is desirable for good cooking results to obtain a uniform heating distribution as by proper selection of the induction heating coil or by varying the thickness of the foil to achieve graduated energy acceptance. In addition to defrosting and heating frozen convenience foods packaged in thin metal containers, aluminum foil can be wrapped about food to be heated or fashioned into user-made disposable utensils using a set of standard molds. Stamped-out thin metal cookers with appropriately shaped cavities are suitable for party and commercial cooking.

While the invention has been particularly shown and described with reference to several preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention.

Claims

What we claim as new and desire to secure by Letters Patent of the United States is:

1. A metallic foil induction surface cooking unit comprising

a substantially unbroken, relatively smooth, non-metallic cooking surface,

an induction heating coil mounted adjacent to and beneath said cooking surface, and

a solid state power conversion circuit for producing an ultrasonic frequency wave that drives said induction heating coil and generates an alternating magnetic field distribution which produces relatively uniform heating in a variety of sizes and shapes of food containing metallic foil utensils and containers placed on top of said cooking surface.

2. A surface cooking unit according to claim 1 in which said induction heating coil is a nominally flat even-heating induction coil comprising a plurality of elongated coil sections connected together and arranged side-by-side with adjacent coil sides close to one another and aiding.

3. A surface cooking unit according to claim 1 in which said induction heating coil is a nominally flat even-heating induction heating coil comprising an odd number of series connected elongated coil sections arranged side-by-side and close together such that current in adjacent coil sides flows in the same direction.

4. A surface cooking unit according to claim 3 further including a peripheral coil section closely surrounding said elongated coil sections and also connected in series therewith such that current in adjacent coil sides flows in the same direction.

5. A surface cooking unit according to claim 1 in which said induction heating coil is an even-heating induction coil, and said surface cooking unit further includes

a removable cover enclosing the top of the cooking surface above said induction heating coil.

6. A surface cooking unit according to claim 5 in which said even-heating induction coil has a rectangular configuration and includes at least three substantially identical elongated central coil sections arranged close together side-by-side and closely surrounded by a peripheral coil section, the current in the center one of said central coil sections flowing in the opposite direction to the current in the other central coil sections and the peripheral coil section.

7. A surface cooking unit according to claim 5 in which said even-heating induction coil has an oval configuration and includes at least three elongated central coil sections arranged close together side-by-side and closely surrounded by a peripheral coil section, the current in the center one of said central coil sections flowing in the opposite direction to the current in the other central coil sections and the peripheral coil section.

8. A surface cooking unit according to claim 5 further including an inductively heated aluminum foil utensil on said cooking surface underneath said removable cover which is comprised by a sheet of aluminum foil having a thickness of approximately 0.5 mils for optimum energy acceptance that is user-shaped as a cavity for receiving the food.

9. A surface cooking unit according to claim 5 further including an inductively heated aluminum foil container on said cooking surface underneath said removable cover with a contoured bottom wall having a variable thickness to graduate the energy acceptance and thereby obtain a more uniform heating distribution, the optimum energy acceptance thickness being approximately 0.5 mils.

10. A surface cooking unit according to claim 5 further including an inductively heated aluminum foil container on said cooking surface underneath said removable cover having a contoured bottom wall with at least two food cavities at different heights from said cooking surface to obtain differential heating.

11. A surface cooking unit according to claim 1 further including an inductively heated aluminum foil disk disposed in the bottom of a non-metallic container placed on said cooking surface, said aluminum foil disk having a thickness of approximately 0.5 mils for optimum energy acceptance.

12. An aluminum foil induction cooking unit comprising

a nominally flat even-heating induction heating coil mounted adjacent a substantially unbroken non-metaliic plate to thereby provide a relatively cool cooking surface,

said induction heating coil comprising a plurality of coupled elongated coil sections electrically and physically arranged to produce relatively uniform heating in disposable aluminum foil utensils and containers placed on said cooking surface,

a static power conversion circuit comprising a solid state inverter for converting a unidirectional voltage to an ultrasonic frequency wave that drives said induction heating coil, and for adjusting the power output of said inverter, and

a removable cover enclosing the cooking surface above said induction heating coil.

13. A cooking unit according to claim 12 wherein said even-heating induction heating coil has a rectangular configuration and wherein said elongated coil sections are series-connected and arranged side-by-side so that adjacent coil section sides are aiding.

14. A cooking unit according to claim 12 wherein said even-heating induction heating coil includes an odd number of said elongated coil sections arranged side-by-side and surrounded by a peripheral coil section, said coil sections being series-connected with the current in the center one of said central coil sections flowing in the opposite direction to the current in the other central coil sections and the peripheral coil section.

15. An induction surface cooking unit comprising

a nominally flat induction heating coil mounted adjacent a substantially unbroken non-metallic cooking surface and generating an alternating magnetic field,

a static power conversion circuit including a solid state inverter for converting a unidirectional voltage to an ultrasonic frequency wave that drives said induction heating coil, said inverter having an operating frequency between 18 and 40 kilohertz, and

an inductively heated aluminum foil food container placed on said cooking surface, said aluminum foil container having a variable metal thickness to graduate the energy acceptance and thereby obtain a relatively uniform heating distribution, the optimum energy acceptance thickness being approximately 0.5 mils.