Induction Heating System Primarily Intended For Cooking Use

Abstract

An induction heating system is disclosed which is primarily intended for use in cooking, such as in cooking foods for human consumption. The structure of such a system preferably includes a flat ceramic stove top. Preferably a flat induction coil which is only one conductor thick is located on the lower surface of this top and is connected to a power supply means for operating the coil so as to cause the coil to produce a continuously varying magnetic field. The system includes a cooking utensil such as a pan having at least a ferromagnetic bottom adapted to be movably located on the top. The coil used is preferably of an unsymmetrical character so that the amount of such heating can be varied depending upon the location of the utensil relative to the coil.

Parent Case Text

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of the co-pending Cunningham U.S. Pat. application Ser. No. 257,203 filed May 26, 1972, entitled "Temperature Controlled Induction Heating" and now abandoned. This application is also a continuation-in-part of the co-pending U.S. Pat. application Ser. No. 294,218 filed Oct. 2, 1972, entitled "Load Controlled Induction Heating." The entire disclosures of both of these co-pending applications are incorporated herein by reference for the purpose of amplifying the disclosure set forth herein.

Description

BACKGROUND OF THE INVENTION

Since the 1800's efforts have been made to develop useable, satisfactory induction heating systems for use in cooking operations such as domestic cooking operations. Although the desirability of using induction heating for cooking food and the like has long been recognized, up until fairly recent times induction heating systems are considered to have been employed for this type of purpose only very infrequently. The reasons for this are considered to be complex and to an extent unknown.

However, during the period when it has been known to use induction heating for cooking processes the art of induction heating has significantly developed and induction heating processes and systems have been increasingly used for various industrial and related heating purposes. It has been conventional practice to use alternating currents having frequencies varying in the range of from about 400 to about 500,000 cps for various different industrial and related induction heating applications depending upon the nature of such applications. It will be recognized that heating within this frequency range involves heating utilizing what may be referred to as ultrasonic frequencies.

It has long been recognized that the circuits used in supplying currents at frequencies as indicated for the purpose of industrial induction heating were of a general utility nature in that they could be utilized not only for industrial or commercial induction heating, but also for heating such as is involved in domestic cooking operations. The comparatively recent advent of solid state electronic components at comparatively nominal costs has lead to the development of quite a number of commercial or industrial induction solid state circuits which are to varying degrees applicable for use in various known induction heating apparatuses capable of being employed for cooking or similar operations.

It is considered that the inherent disadvantages of the use of common domestic cooking appliances such as gas fired cooking stoves, cooking stoves employing resistors to create heat for cooking operations and the like has motivated a renewed interest in the use of induction heating in connection with various types of domestic cooking operations. It is believed that advancements in electronic circuits and in particular the development of solid state electronic circuits now makes it practical from a utilitarian and from an economic standpoint to use induction heating for various types of cooking operations as indicated.

A review of the various types of actual cooking stuctures previously employed for such induction heating--cooking operations--has indicated that many of such structures are in effect unacceptable from either a utilitarian or a commercial standpoint for use in the present-day society. It is not considered necessary to go into the reasons for this in connection with an explanation of the present invention. It is considered that such a review and a consideration of the commercial stoves being marketed clearly indicates that there is a need for improvement in connection with the field of induction heating systems which are primarily intended for domestic type cooking purposes.

SUMMARY OF THE INVENTION

An objective of the present invention is to fulfill this need. More specifically, an objective of the present invention is to provide a new and improved induction heating system which is primarily intended for domestic and similar cooking uses. A further objective of the present invention is to provide a system as described which may be manufactured at a comparatively nominal cost, which is capable of giving prolonged reliable performance at a comparatively economic cost and which may be easily and conveniently used efficiently with a minimum of difficulty.

Such a system employs a number of individual components such as a stove top as herein described, an induction coil as herein described (which is preferably in effect a part of the stove top) and a power supply means for use in operating the coil so as to cause the production of a constantly varying magnetic field which will in turn cause heating of another component of such a system--a utensil--used with the system. These various individual components herein described are capable of being separately utilized with other components than are indicated in this specification. It is, however, considered that most satisfactory results are achieved when all of the components used together are as indicated in this specification.

Because of these factors other more specific objectives of the present invention are to provide various new and improved parts or components of an induction heating system as indicated by the preceding discussion. The invention is especially concerned with new and improved stove tops and new and improved induction coils for use with such stove tops. Other objectives of the present invention are to provide extremely utilitarian, relatively inexpensive stove tops and coils as indicated which may be easily and conveniently manufactured and which are capable of giving prolonged, reliable performance. The invention also has as one of its objectives as providing such stove tops and coils as integral units which are to be shipped and handled as staple articles of commerce.

In accordance with this invention certain of the foregoing objectives are achieved by providing induction heating system of a type primarily intended for cooking purposes which includes a generally flat electrically non-conductive, non-ferromagnetic stove top, at least one cooking utensil having at least a ferromagnetic bottom located on the stove top so as to be supported by it, an induction coil located along the lower surface of the stove top adjacent to this lower surface and a power supply means for operating the coil so as to cause the coil to produce a continuously varying or changing magnetic field which will extend through the stove top so as to cause the utensil to become heated when it is located on a stove top.

Such a stove top is preferably a ceramic stove top having an imporous upper surface formed so as to visually indicate the configuration and location of the coil used. Such a coil is preferably a flat coil of an electrical conductor which is only as thick as the conductor. Such a coil may be located so as to fit within a groove separating its turns formed in the lower surface of such a stove top and may be secured to such a top as through the use of an appropriate cement. Preferably the power supply means employed differs from a conventional induction heating power supply in that it does not supply an alternating current to the coil, but instead supplies DC current pulses to a parallel, tuned resonant tank circuit into which the coil is connected as a part of the tank circuit. The DC power used will normally be a full-wave rectified AC current and for safety reasons the entire power supply means will be grounded.

BRIEF DESCRIPTION OF THE DRAWING

Further details of the present invention are best indicated with reference to the accompanying drawing in which:

FIG. 1 is a top plan view of a presently preferred embodiment of an induction heating system in accordance with this invention installed on a countertop;

FIG. 2 is an enlarged diagrammatic partial cross-sectional view taken at line 2--2 of FIG. 1;

FIG. 3 is a diagrammatic bottom plan view of the stove top and coil employed in this heating system; and

FIG. 4 is a schematic view indicating the principal elements or parts of a circuit preferably employed with the illustrated heating system.

A consideration of the accompanying drawing and of this specification will indicate that the invention set forth herein involves a number of essentially intangible concepts. Such concepts are set forth and defined in the appended claims forming a part of this disclosure. They may be used in various structures and systems which differ from the precise structures indicated in the drawing in various ways through the use of routine engineering skill or ability.

DETAILED DESCRIPTION

In the drawing there is shown a complete induction heating system of the present invention. This system employs a generally flat stove top 10 which is preferably formed out of an inert electrically non-conductive, non-ferromagnetic ceramic. A large number of different grades and types of ceramics can be used. It is considered preferable to utilize heat resistant ceramics such as are commonly utilized in various kitchen utensils. This term "ceramic" is used here in a broad sense so as to include glasses.

This top 10 should be sufficiently thick so that there is substantially no danger of it becoming broken as it is used when it is supported by an edge lip 12 so as to fit within an opening 14 in a conventional countertop 16. If desired, such a countertop 16 may be integral with a conventional frame for supporting a common stove. Preferably the top 10 includes a slightly raised peripheral lip 18 which will serve to prevent any liquid which may be spilled on the top 10 from draining off of it.

Preferably the top 10 is provided with a glazed ceramic upper surface 20 which serves to render the entire top 10 waterproof in character. Such a surface 20 is preferably both electrically non-conductive and non-ferromagnetic in character. Both the surface 20 and the top 10 should, of course, be free from cracks or other imperfections which might weaken the described top 10 and/or make it possible for liquids to permeat this top 10.

From a commercial point of view one of the advantages of the top 10 is that the surface 20 can be formed in various known manners to have virtually any type of desired appearance. This is considered important from an aesthetic point of view. One of the objections to conventional prior stoves relates to the appearance of such stoves and the fact that they cannot be readily cleaned. Because of the flat character of the surface 20 cleaning it is substantially no significant problem.

The ability of this surface 20 to be decorated in any desired manner may be effectively utilized by providing on the surface 20 in a known manner a decorative design consisting of an unsymmetrical arrangement of rectilinear or other shapes 24 as shown in FIG. 1 which corresponds to the location and configuration of an induction coil 26 used with the top 10 so as to in effect form a part of this top 10. If desired, the shapes may be of various colors of gradually increasing intensity terminating in a centermost shape having color which is more distinctive and intense than the remainder of the surface 20 so as to indicate the relative degree of heating which may be achieved by locating a utensil 28 upon the top 10 so that the top 10 serves as a support plate.

The coil 26 employed preferably consists of more turns than shown of a single conductor 30 which has a copper or aluminum conductor roughly 0.0100 inch .times. 0.0125 inch in cross-sectional configuration. This conductor 30 in the coil 26 extends in what may be considered or termed an "unsymmetrical" path in which the turns of the coil 26 to the left of FIG. 3 of the drawing are closer together than the turns of the coil 26 towards the right of FIG. 3 of the drawing. In the embodiment of the invention shown the turns of the coil 26 are illustrated with the conductor 30 extending in essentially a rectilinear path roughly corresponding to the outlines of the shapes 24.

This type of unsymmetrical structure is intended so that a utensil 28 such as a pan will be heated to varying extents depending upon its location upon the stove top 10 during the operation of the herein described system. In general, when such a utensil 28 is located where the various turns of the coil 26 are closely spaced together the heating action will be more pronounced than when such a utensil 28 is located adjacent to where the turns of the coil 26 are relatively far apart.

The broad principals of the invention are applicable to the use of any coil of a symmetrical category. However, with such a symmetrical coil an individual does not have the option of picking an area of the stove top 10 in locating a utensil such as the utensil 28 where the heating action is more or less pronounced than in other areas of the top 10. This feature of the selectivity of the heating action achieved can be accomplished through the use of a coil corresponding to the coil 26 in which the conductor 30 employed extends in a wide variety of different types of paths.

Thus, it is possible to form tops such as the top 10 using a coil 26 in which the individual turns are essentially of an elliptical or oval type of shape and are located so that the conductor 30 passes closely adjacent to itself in various turns in some regions of the stove top 10 and not in others. It is possible to attain this type of feature with a coil 26 having an essentially spiral like configuration in which the outer turns of such a coil 26 are spaced further from one another than the innermost turns. It is possible to have such a spiral which is in effect "flatened out" so that at two opposed sides of the spiral the turns are relatively far from one another while at the other two sides of such a spiral the turns are relatively close to one another.

Various composite configurations of this general type may be used in forming the coil 26 as may be desired in promoting the utility of the system herein indicated. For this reason the term "unsymmetrical" is utilized in this specification (and the accompanying claims) in a broad sense to designate any structure of a coil such as the coil 26 in which the conductor 30 is not uniformly spaced in successive turns in the coil 26.

An important aspect of the coil 26 is the fact that this coil is of a flat configuration and is only of one thickness of the conductor 30 in thickness. Because of the flat configuration of the coil 26 the entire structure of the top 10 can be rather thin for shipping and related purposes. Further, the shape of this coil 26 corresponds in shape to the flat surface 20 of the top 10. This enables it to be utilized effectively in heating as herein described. With this configuration all of the conductor 30 in the coil 26 can be located in an effective position closely adjacent to the surface 20 so that effective heating can be obtained.

Preferably the coil 26 is located so as to extend in a groove 32 formed in the lower surface 34 of the stove top 10. This coil 26 may be directly formed by diecasting or similar techniques in this location. Preferably, however, it is secured in place in this location by a conventional non-conductive cement (not shown) located between the conductor 30 and the bottom of the groove 32. Although this use of the groove 32 is not mandatory, it is considered desirable since it effectively isolates the turns of the coil 26 and since it serves to a degree to physically protect the coil 26 against damage. Preferably further protection is achieved by covering the lower surface 34, the groove 32 and the coil 26 by an adherent, inert, electrically non-conductive, non-ferromagnetic coating 36. Such a coating 36 may consist of a known type of inorganic cement or the like.

The ends of the conductor 30 in the coil 26 are preferably attached as by welding to conventional terminals 38 which extend out through the coating 36. These terminals 38 are preferably connected to a power supply 39 as indicated in FIG. 2 which is capable of operating the coil 26 in a known manner so as to cause this coil 26 to create a continuously varying or changing magnetic field. Such a field will extend upwardly through the stove top 10 where it will be received by the utensil 28 so as to cause the generation of heat by interacting with a ferromagnetic material in a known manner.

Although all of the operating parts of this utensil 28 can be formed of a ferromagnetic material such as common cast iron or various grades of steel, it is preferred to form the utensil 28 as shown so that it has a ferromagnetic bottom 40 bonded to a body 42 of a heat conductive metal which will conduct heat away from the bottom 40 efficiently so that substantially all portions of the interior of the utensil 28 are substantially uniformly heated as the described system is operated. It is also preferable to form the utensil 28 with a non-heat conductive, non-ferromagnetic handle 44 so as to facilitate the use of this utensil 28.

It will be recognized that a number of materials can be utilized in the bottom 40 or in the entire utensil 28. No point is seen in encumbering this specification with a long listing of the various iron and steel compositions which are of a ferromagnetic character inasmuch as these materials are well known. The bottom 40 or all of the utensil 28 except the handle 44 may be conveniently covered with a conventional electrically non-conductive, non-ferromagnetic protective coating such as a decorative enamel coating 46 as shown. When such a coating 46 is not utilized, it is considered that for appearance and similar reasons it is preferable to utilize a ferromagnetic grade of stainless steel such as a stainless of a Martensite type as the bottom 40.

If desired, the material in the bottom 40 can be extended completely around all operative portions of the utensil 28 so that the body 42 of the utensil 28 is completely encased. When this is done the body 42 will function substantially as indicated in the preceding as a sort of a heat distributor. For this body 42 to function effectively in this regard in conducting heat it should have a relatively high thermal conductivity. For purely economic reasons expensive metals of this type such as silver should not be used.

Normally economics will dictate the use of a metal such as aluminum or copper or an alloy of either of these metals in forming the body 42 of a utensil 28 as described. Since such metals and alloys are somewhat susceptible to corrosion, it is considered preferable to use a coating 46 with them as indicated. It is possible to obtain effective results by forming the body 42 of a stainless steel of a non-ferromagnetic character since this material will effectively resist corrosion, but will still be relatively effective in conducting heat.

This matter of heat distribution will in general make it disadvantageous to utilize the utensil 28 with the body 42 formed completely of a known ceramic, such as a glass or the like. It is, however, possible to form either the bottom 40 or the entire utensil 28 out of a ferromagnetic ceramic, such as a known ferrite composition. When the utensil 28 is formed out of such a ferromagnetic ceramic it operates in substantially the same manner as if it were formed of a magnetic iron composition or the like. Any ceramic used may, of course, be covered with a coating such as the coating 46.

It is presently preferred that any utensil 28 used be constructed in a manner as indicated in the aforenoted co-pending U.S. Pat. application Ser. No. 257,203 using a ferromagnetic material which has a Curie point approximately corresponding to the maximum temperature to which it is desired to heat anything in the utensil 28. This is considered important in preventing the burning of foods and the like. Various manners of constructing a utensil 28 to utilize this feature are indicated in this co-pending application.

One important feature of the present invention is that a single coil such as the coil 26 can be effectively utilized in simultaneously heating a number of utensils such as the utensil 28. To this end the stove top 10 is, of course, made large enough so that it will accommodate at least one and preferably a large number of utensils corresponding to the number of utensils which a housewife or cook may normally desire to use simultaneously. Such utensils may differ from one another as to size and as to construction. Each utensil used should be constructed as indicated in the preceding discussion. The particular configuration of the coil 26 described is considered to be advantageous for such multiutensil use inasmuch as it permits different such utensils to be heated to varying extents at the same time.

The coil 26 may be powered using a wide variety of different general utility induction heating power circuits. Generally when conventional induction heating circuits are used it will be preferable to utilize a more conventional type of coil for reasons pertaining to the efficiency of the device. It will be recognized that such general utility circuits have been designed for use with both two and three wire AC current sources. Many of such circuits do not specify the source of DC power used in them since this is essentially a matter of choice.

In practically all cases such prior induction heating power supply circuits rectify AC power in a conventional manner so as to obtain full-wave current rectification. Frequently, such rectified full-wave rectified power is passed to a conventional filter serving to smooth out the wave form of the DC current resulting from the rectification. No point is seen in specifically indicating conventional AC power rectification and/or filter circuits in this specification since they are well known and are shown by a number of text authorities. Such circuits may conveniently be solid state circuits.

It is commonplace in such conventional induction heating power systems to take the DC current obtained as the result of such rectification and to pass such current to an inverter so as to obtain an AC current of a frequency as may be desired for a particular induction heating application. Solid state circuits can easily be used for this purpose. The AC current developed by such an inverter is of course used to develop an alternating magnetic field in a work coil. Various frequencies as have been used for such applications are indicated in the background of the invention section of this specification. Ultrasonic frequencies can be used for cooking operations such as are contemplated in accordance with this invention when an inverter is employed.

Such circuits are preferably grounded in the interests of safety and so as to meet requirements of various organizations such as the requirements of the Underwriter's Laboratory Organization. Such grounding can be in accordance with conventional practice. Thus, for example, a circuit used in induction heating can have three input terminals constituting a grounded power distribution system having a neutral terminal. Various operative parts of the inverter circuit and the heating coil in such a circuit may be directly connected to the grounded neutral terminal used.

In such a circuit it is possible to utilize a plurality of inverters so that each will operate a separate coil so as to create an alternating magnetic field. The field of such a series of inverters may be coupled together through a transformer to a completely separate work coil used for induction heating purposes. Such a coil would correspond to the coil 26 described although technically the coils powered by each of the inverters in such a system would serve as a work coil.

Although a wide variety of power sources may be used as the power source 39 in accordance with this invention it is considered highly preferable to utilize as a power source a power supply means 39 which employs a parallel, tuned, resonant tank circuit into which the coil 26 is connected as a part of the tank circuit. Such a circuit is indicated in FIG. 4 of the drawing. The effectiveness of such a circuit in driving a flat coil 26 is considered extremely important for the herein described system to operate effectively and economically.

This circuit is designed to receive power from a conventional AC power source as shown. Then this circuit utilizes a conventional full-wave rectifier 48 which is used to supply a rectified DC current to an SCR 50. At periodic intervals this SCR 50 releases DC pulses to a parallel, tuned, resonant tank circuit 52 through a conventional filter 51. This particular tank circuit 52 illustrated includes an inductor 54, a capacitor 56 and the work coil 26 located as shown. Some rearrangements of these components within the tank circuit 52 is possible provided this tank circuit operates after such rearrangement so that the coil 26 produces a consistently varying magnetic field. The tank circuit 52 is grounded at 58. Such a ground may be connected to a grounded input line used to supply the AC power employed.

In the power supply 39 a means must be used to govern the release of DC pulses to the tank circuit 52. Such a means is a control means 60 as shown in FIG. 4. This control means 60 is connected to the tank circuit 52 and to the gate of the SCR 50. The particular control means 60 may comprise a structure as set forth in the co-pending U.S. Pat. application Ser. No. 294,218. Such a structure involves a voltage detector for detecting the voltage within the tank circuit 52 and a triggering oscillator for triggering the SCR 50 in accordance with the voltage detected by the voltage detector so that the triggering oscillator will cause the release of current pulses by the SCR 50 in accordance with voltage within the tank circuit 52.

Preferably a control means as set forth in this co-pending application includes a timer and a sampling oscillator as described in this co-pending application. This co-pending application also indicates the use of a thermocouple or a related condition sensing device such as an optical device which will indicate the degree of heating in accordance with a color change. It is possible to utilize such condition sensing means to detect the temperature of a utensil 28 or its contents in accordance with this invention although this will not normally be necessary.

Thus, for example, a condition sensing means 62 may be connected to the control means illustrated for this purpose. If such a condition sensing means is a device such as a thermocopule, a thermister or even a common bi-metal thermostat, a temperature sensing device operating on a Curie point principal or the like, such a device may be mounted on a utensil 28 in a known conventional manner or located within such a utensil or even if desired mounted directly upon the surface 20 so that it will be contacted with such a utensil.

Such expedients are not considered desirable in the system herein described because of the nature of this system and because of the ability of a user to use utensils which control the degree of heating achieved through a Curie point type of action serving to cut off further induction heating. In the event such utensils are not available or for one reason or another are not to be used, it may be desirable to use such condition sensing means.

No effort is made in this specification to completely describe the control means 60 since such a control means is indicated in detail in the co-pending U.S. Pat. application Ser. No. 294,218 which is incorporated herein by reference. Other control means which may be effectively utilized are indicated in the Cunningham U.S. Pat. No. 3,637,970 issued Jan. 25, 1972 entitled "Induction Heating Apparatus." The control means specified in this patent are not set forth herein in the interest of brevity. The entire disclosure of this patent is incorporated herein by reference. This disclosure is incorporated herein since it shows a very effective, simple way of controlling the operation of the SCR 50.

This disclosure of this U.S. patent is also incorporated herein inasmuch as the patent indicates how it is possible to utilize two tank circuits corresponding to the tank circuits 52 together off of single rectified power source which operates alternatively on a cyclic basis without any significant delay between the intervals of their operation. This will provide more effective rapid heating than where a single tank circuit is utilized. If desired, the output from two work coils used in two tank circuits employed together as indicated in FIG. 7 of said Cunningham patent may be coupled together through a transformer so that only a single work coil is used. Such coupling is considered to be indicated to be old in the induction heating art as is shown by the French Pat. specification No. 1,245,149.

It will be realized from the aforegoing that the invention constitutes a very simple, effective surface heating unit system in which the coil 26 is operated so as to function as the primary of the transformer so as to cause all or part of a utensil such as the utensil 28 to function as a secondary of the transformer. In order to minimize losses in the changing or varying magnetic field utilized it is considered that a utensil 28 should have a flat bottom which will fit closely up against the surface 20 so as to minimize magnetic losses as the result of the presence of any significant air gap.

Other conforming shapes of the coil 26, the surface 20 and the utensil 28, or all three of them than a flat shape can be used in specialized circumstances. Similarly in order to minimize losses there should be no magnetic or electrically conductive material between the coil 26 and any utensil 28 used, and this utensil should not have any electrical conductor located on it generally between the portions of it coupled to the coil 26.

Claims

I claim:

1. A stove top to be used in induction heating of a utensil employed in domestic cooking which is at least partially ferromagnetic in character which comprises:

a flat, self-supporting, electrically non-conductive and non-ferromagnetic member serving as a stove top, said member having an imporous upper surface and having a lower surface,

a flat induction coil located along and adjacent to said lower surface, said induction coil consisting of a single conductor, the thickness of said coil being the thickness of said conductor,

said lower surface including a groove corresponding to the configuration of said conductor of said induction coil, said conductor of said induction coil being located within said groove.

2. A stove top as claimed in claim 1 wherein:

said coil is an unsymmetrical coil in which successive turns of said coil are not equally spaced from one another.

3. A stove top as claimed in claim 1 wherein:

said stove top is sufficiently large so that several utensils can be used on said upper surface,

said upper surface is decorated so as to correspond to the configurations of said coil so as to indicate the extent to which a utensil will be heated depending upon its location on said upper surface.

4. A stove top as claimed in claim 1 wherein:

said coil is an unsymmetrical coil in which successive turns of said coil are not equally spaced from one another,

said stove top is sufficiently large so that several utensils can be used on said upper surface,

said upper surface is decorated so as to correspond to the configurations of said coil so as to indicate the extent to which a utensil will be heated depending upon its location on said upper surface,

said coil is secured to said member so that said member and said coil are an integral unit.

5. In the combination of an induction heating coil, power supply means for operating said coil so as to cause it to produce a continuously changing magnetic field and a utensil located so as to be magnetically coupled to said coil by said field, the improvement which comprises:

an imporous, electrically non-conductive, non-ferromagnetic support for said utensil located between said coil and said utensil,

said coil being an unsymmetrical, flat coil in which successive turns of said coil are unsymmetrical in that successive turns on part of said coil are closer together than successive turns in another part of said coil so that different heating effects can be achieved by placing a load adjacent to different portions of said coil.

6. The combination as claimed in claim 5 wherein: said coil is a flat induction coil consisting of a single conductor, the thickness of said coil being the thickness of said conductor.

7. The combination as claimed in claim 5 wherein:

said support is of such dimension as to be capable of holding several utensils in various locations relative to said coil and above said coil,

the surface of said support is decorated so as to correspond to the configuration of said coil and to the location of the turns of said coil relative to said support so as to indicate the extent to which said utensil will be heated depending on its location on said support.

8. The combination as claimed in claim 5 wherein:

said coil is a flat induction coil consisting of a single conductor, the thickness of said coil being the thickness of said conductor,

said support is of such dimension as to be capable of holding several utensils in various locations relative to said coil and above said coil,

the surface of said support is decorated so as to correspond to the configuration of said coil and to the location of the turns of said coil relative to said support so as to indicate the extent to which said utensil will be heated depending on its location on said support, and

said coil is secured to said support so as to form a part of said support.