US3781503A

US3781503A - Solid state induction cooking appliances and circuits

Info

Publication number: US3781503A
Application number: US00200424A
Authority: US
Inventors: J Harnden; W Kornrumpf
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1971-11-19
Filing date: 1971-11-19
Publication date: 1973-12-25
Anticipated expiration: 1990-12-25
Also published as: NL7208372A; JPS4859440A; FR2160362A1; IT956803B; DE2231339A1; CA956703A

Abstract

An economical smooth-top cooking appliance for inductively heating cooking utensils preferably comprises a flat air-core induction heating coil driven at an ultrasonic frequency by a simplified one-thyristor, one-transistor, or two-transistor resonant inverter. The series and parallel resonant circuits in the inverters are formed by a capacitor and the induction heating coil. The one-thyristor series resonant circuit is desirable because power control to adjust the cooking temperature is obtained by the combined effect of increasing the frequency and amplitude of the sinusoidal current pulses supplied to the coil. The coil is also movable to change the gap spacing between coil and utensil to vary the coupled power. Flux beneath the coil is used for warming purposes.

Description

United States Patent [1 Harnden, Jr. et a1.

[ SOLID STATE INDUCTION COOKING APPLIANCES AND CIRCUITS [75] Inventors: John D. Harnden, Jr.; William P. Kornrumpf, both of Schenectady, NY.

[73] Assignee: General Electric Company,

Schenectady, NY. [22] Filed: Nov. 19, 1971 [21] Appl. No.: 200,424

[52] US. Cl 219/10.49, 219/1077, 219/1079, 321/45 [51] Int. Cl. 1105b 5/04 [58] Field of Search 219/1049, 10.75, 219/1077, 10.79; 321/27, 45, 4, 44

[56] References Cited UNITED STATES PATENTS 3,530,499 9/1970 Schroeder 219/1049 1,992,515 2/1935 Uhlmann 219/1049 3,710,062 l/l973 Peters 219/1049 400,978 1889 Watts 219/1049 3,187,151 6/1965 Baermann 219/1049 1,052,119 2/1913 Anderson 219/1049 3,637,970 l/l972 Cunningham; 219/1075 3,129,366 4/1964 Fry 219/1077 UX 3,368,164 2/1968 Shapiro 321/45 X 3,256,417 6/1966 Merrett 219/1079 1 Dec. 25, 1973 FOREIGN PATENTS OR APPLICATIONS 1,157,711 7/1969 Great Britain 219110.49

OTHER PUBLICATIONS Maphaim, A Low Cost Ultrasonic Frequency Invention Using a Single SCR," Application Note 200.49, General Electric Co., Syracuse, NY.

Primary Examiner-Bruce A. Reynolds Attorney-John F. Ahern et a1.

[ 5 7] ABSTRACT An economical smooth-top cooking appliance for inductively heating cooking utensils preferably comprises a flat air-core induction heating coil driven at an ultrasonic frequency by a simplifiedone-thyristor, one-transistor, or two-transistor resonant inverter. The series and parallel resonant circuits in the inverters are formed by a capacitor and the induction heating coil. The one-thyristor series resonant circuit is desirable because power control to adjust the cooking temperature is obtained by the combined effect of increasing the frequency and amplitude of the sinusoidal current pulses supplied to the coil. The coil is also movable to change the gap spacing between coil and utensil to vary the coupled power. Flux beneath the coil is used for warming purposes.

23 Claims, 17 Drawing Figures RICH/7L? /a 801/0 57A TE Mm? m2 WA TPASOMC F/Pmumcy) PATENTED M51915 3.781.503

' sum 3 or 5 In rent on! John Q Harnden, Jr." M/flam P lrornrumpf The/r AW SOLID STATE INDUCTION COOKING APPLIANCES AND CIRCUITS CROSS-REFERENCE TO RELATED APPLICATIONS Induction cooking appliances with other inverter power circuits and multicylinder power circuits are respectively disclosed and claimed more broadly in the following concurrently filed applications assigned to the same assignee as this invention: US. Pat. Ser. No. (200,526) by David L. Bowers, Donald S. Heidtmann, and John D. Hamden, Jr.; and U.S. Pat. Ser. No. (200,528) by William P. Kornru'mpf and John D. Harnden, Jr.

BACKGROUND OF THE INVENTION This invention relates to cooking appliances based on induction heating and to solid state circuits therefor, and more particularly to improved, economical power circuits for induction cooking appliances operating in the ultrasonic frequency range.

The use of induction heating to heat a food containing cooking utensil is a theoretically efficient process since heat is generated only in the metallic utensil where it is wanted. The ordinary gas range and electric range bycomparison have greater losses due to poor coupling of heat to the utensil and heating the surrounding atmosphere. Although known in principle for a number of years, prior food cooking appliances based on induction heating have been unsatisfactory. Equipment using available line frequencies of 50 Hz or 60 Hz was inadequate and cumbersome, and eddy current cookers employing a circular arrangement of alternately poled permanent magnets rotated by an electric motor were bulky, expensive, and for other reasons not suitable for wide usage.

Induction cooking appliances employing solid state power conversion circuits to drive the induction heating coil at the higher frequencies that are desirable have made possible a significant reduction in cost and size of the equipment. By operating at ultrasonic frequencies of -18 kHz and above, the appliance is inaudible-to most humans, and solid state power conditioning at'these frequencies is presently reliable. The aboveidentified application of Bowers et al discloses a power converter including a rectifier and a two-thyristor series sine wave inverter that drives a pancake ferromagnetic-core induction heating coil. To be truly competitive for commercial introduction, however, improved and more economical appliances and circuitry are needed.

I SUMMARY OF THE INVENTION Thesolid state induction cooking appliance to which the invention applies comprises a substantially nonmetallic, plate-like support having a substantially unbroken utensil supporting surface. A flat or nominally flat induction heating coil is mounted adjacent the support and generates an alternating magnetic field that couples with metallic portions of a utensil placed on the support. A static power conversion circuit typically comprising a rectifier and an inverter generates an ultrasonic output frequency wave for driving the induction heating coil. To obtain an economical appliance, such as a cooktop unit or a portable counter-top warming or cooking appliance, the improvement is made of using a relatively simple inverter comprising a capacitor and the induction heating coil connected as a series or parallel resonant circuit that is controlled by a single solid state switching device and control circuit therefor. While it is desirable to use an inexpensive air-coil core, the simplified inverters are usable with the ferrite ferromagnetic-core coils also.

In the preferred embodiments, the inverter is a series resonant sine wave inverter in which a thyristor (SCR) and an inverse-parallel connected diode are connected in series with the capacitor and coil, and reset inductor means is provided to reset the capacitor during nonconducting intervals of the thyristor-diode combination. Power control to adjust the cooking temperature or utensil heating level is obtained by the combined effect of both increasing the frequency and amplitude of the sinusoidal current pulses supplied to the coil. Other embodiments are one-thyristor and one-transistor parallel resonant circuits comprising a capacitor and the coil connected in parallel. The inverter power output and thus the power coupled to the utensil is adjusted by respectively changing the amplitude or frequency of the sinusoidal current pulses.

Other low cost power conversion circuits disclosed are a multi-cylinder inverter based on the one-thyristor series resonant inverter and, two-transistor inverters using parallel and series resonant circuits. Output power control in any of these power conversion circuits is also achieved by using a phase controlled rectifier to vary the d-c supply to the inverter. Another technique is to mount the coil adjustably relative to the utensil support to change the gap spacing between coil and utensil. These power control techniques can be used in any selected combination to obtain the range of cooking temperatures and settings desired. An additional feature of an appliance with a flat air-core induction heating coil is a non-metallic warming tray beneath the coil for supporting covered utensils heated by magnetic flux emanating from the bottom of the coil.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram of a solid state converter for supplying ultrasonic frequency power to a flat spiral induction heating coil, shown in plan view, in an induction cooking appliance constructed according to the invention;

FIG. 2 is a diagrammatic cross-sectional view illustrating the relation of the induction heating coil, support plate, cooking utensil, and optional warming tray in a cooking appliance;

FIG. 3 shows several modifications of FIG. 2 including a coil made with braided ribbon, an improved ferromagnetic core for the coil, and adjustment of the cooking temperature by changing the coil position;

FIG. 4 is a perspective view of the upper portion only of an electric range with a food warming facility;

FIG. 5 is a detailed schematic circuit diagram of one embodiment of a static power conversion circuit comprising an economical series resonant inverter using only one thyristor and the induction heating coil in a dual function as the commutating inductance;

FIG. 6 is the equivalent circuit diagram of the induction heating coil and cooking utensil load;

FIGS. 7a and 7b are waveform diagrams of the induction coil current and commutating capacitor voltage for the circuit of FIG. 5, showing in each diagram the waveforms at two different inverter output frequencies;

FIG. 8 is a modification of the FIG. converter employing a phase controlled rectifier and a modified gating and power circuit;

FIG. 9 shows another modification of the inverter of FIG. 5;

FIG. 10 is' a schematic circuit diagram of a low cost multi-cylinder power converter (less the rectifier) based on the inverter configuration of FIG. 5;

FIG. 11 is an induction coil current waveform diagram usefulin explaining the operation of FIG. 10;

FIGS. 12 and 13 are schematic circuit diagrams of parallel resonant inverters respectively using a single thyristor. and a transistor;

FIG. 146 shows a two-transistor inverter circuit;

FIG. 14b is a plan view in schematic formof the series aiding, equal-tum induction heating coil used in the FIG. 14a circuit; and

FIG. 15 is a detailed circuit diagram of another twotransistor resonant inverter.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The induction cooking appliance shown in FIGS. 14 will be described with regard to an induction surface unit in an electric range, but essentially the same mechanical structure and circuitry in a lower power version isalso suitable for a portable counter-top food cooking or warming appliance. The static power conversion circuitindicatedgenerally at 12 is preferably energized by a single phase commercially available 60 Hz, I volt or 240 volt source of alternating current potential, however in appropriate cases this equipment can be designed for use with other low frequency, low voltage or d-csources. Static power converter 12 comprises generally a rectifier 13 and a solid state inverter l4for converting the unidirectional rectifier output to an ultrasonic frequency wave for driving an induction heating coillS. Induction heating .coil 15 is a single layer,.annular, flat spiral, air-core coil wound with solid flat strip conductors or braided ribbon with a rectangular cross section. To generate sufficient magnetic flux toheat theutensil to the'desired cooking temperature, coil 15 is tightly wound with the short cross sectional dimension facing upwards and adjacent turns separated by a flat insulating strip 20. While coil 15 is usually perfectly flat, it is within the scope of the invention to use a nominally flat-coil such as one that is slightly dished to improve the magnetic field distribution.

In the cooking appliance (FIG. 2), induction heating coil 15 is appropriately mounted in a horizontal position immediately below a non-metallic support 16 typically made ofa thin sheet of glas, ceramic, or plastic. Support plate 16 is commonly referred to as the cooking surface and supports the metallic cooking utensil 17 to be heated. If required, support 16 can have some metallic content forelectrostatic shielding or decorative purposes, but this is necessarily limited to a small amount to permit nearly full power to be coupled to the utensil. Cooking utensil 17 is more particularly an ordinary cooking pot or pan, a frying pan, or some other available metallic utensil used in food preparation. The utensil can be made of a magnetic material such as magnetic stainless steel, enameled steel, or cast iron;a non-magnetic material such as aluminum; or a laminated product such as copper-clad stainless steel or triple-clad (stainless-cast iron-stainless steel). Special cooking utensils are not required, although the best and most efficient results are obtained by optimizing the size, shape, and material of the cooking utensil. Because of their relatively thin skin depth at ultrasonic frequencies which results in a high equivalent impedance and also because of the high magnetic losses,

magnetic steel utensils couple well to coil 15 and are heated most efficiently. Some copper-clad utensils and' thick aluminum utensils do not couple well to the coil due to their low equivalent impedance, while the laminated and cast iron utensils are inbetween. Any of these may be used, however, in an induction cooking appliance when the coil 15, static power conversion circuit 12, and the gap between the coil and utensil are properly designed. Ordinarily a gap of at least one-eighth inch is required between the top of coil 15 and the bottom of utensil 17 to allow space for non-metallic support 16, and the gap is no greater than about one-half inch at full power rating in order to couple sufficient power into the utensil bottomto produce adequate heating for cooking purposes. It is also important to impedancematch the utensil to the power circuit for best overall efficiency.

Operation of solid state inverter l4 to impress an ultrasonic frequency wave on induction heating coil 15 results in the generation of an alternating magnetic field. The magnetic flux, in particular flux emanating from the top of coil 15, is coupled across the air ,gap through nonmetallic support 16 to utensil 17. An ultrasonic frequency above 18 kHz or so is normally considered to be the upper range of human hearing and is selected to make the cooking appliance inaudible to most people. The induction heatingcoil 15 shown in FIG. 2 is constructed with solid copper flat strip with a rectangular cross section. Conveniently, the coil is wound using alternating flat strips of copper conductor and a suitable insulating material. To reduce the high frequency losses due to the skin effect, it is preferable to use a coil 15 made with braided ribbon as is illustrated schematically in FIG. 3. A still further reduction in losses is'obtained by using ribbon litz wire. Because of the usual physical constraints, only a limited number of ampere-turns can be includedin a practical induction heating coil. Since the power coupled to the utensil is theoretically proportional to the square of the ampere turns, the reduction of losses is of consequence in increasing the efficiency.

Some of the other options that are possible as to the mechanical and electrical arrangement of induction heating coils 15 and 15 are also illustrated in FIG. 3. One of these is to use-a ferromagnetic-core coil rather than air-core coil. To this end, a ferromagnetic core 18 for coil 15' is mounted horizontally beneath the coil spaced from it by a predetermined air gap. The core serves as a high permeability path for the return magnetic flux, one such path being illustrated at 19, and is preferably a thin plate of ferrite dispersed for instance in an extruded rubber matrix. The ferrite plate runs cool and has superior high frequency performance as compared to a laminated steel plate, and for a given induction heating coilthe power output requirements of static power conversion circuit 12 are lower as compared to the air-core coil situation. Usually, coils l5 and 15' are mounted stationary beneath utensil support 16 with a fixed air gap spacing by means of an appropriate mechanical supportstructure not-here shown. Increasing or decreasing the gap between the coil and utensil 17, however, changes theamount of power coupled to the utensil and therefore the temperature to which the utensil is heated. Other waysof adjusting the cooking temperature by control of the power output of static power conversion circuit 12 will be discussed later. Rather than moving utensil support 16 relative to coil 15 (FIG. 3), a preferred arrangement in view of the fact that coil 15 is relatively light is to change the gap' spacing by mounting coil 15' for vertical movement. As illustrated here schematically, coil support 21 underlies coil 15' and has a vertical extension that extends above the surface of support 16 so as to be available to the person doing the cooking to make a manual adjustment. Another suitable arrangement employs a wire wound on the shaft of a control knob on the range control panel.

FIG. 4 shows an induction cooking appliance in the form of an electric range with four induction surface unit positions indicated in dotted lines on the surface of utensil support 16. An important feature of the invention isthat support 16 is made of a non-metallic material such as plastic or glass, and preferably has a smooth and unbroken utensil supporting surface. This is not to preclude the inclusion of sensors or indicators in support 16. At ultrasonic frequencies there are insignificant reaction forces which at lower frequencies would cause utensil 17 to move horizontally when placed. on the support plate approximately centered with respect to one of the induction surface unit positions. The transfer of energy to the utensil to heat it is relatively efficient since heat is generated only in the utensil and'none is lost because of mismatch in size between the coil andutensil. Although induction heating coil 15 may require some cooling, the surface of support 16 is relatively 0001 since the highest temperatures involved are about 450 F, the approximate maximum temperature to which the bottom of utensil 17 is heated to'cook' food as for example in frying operations. Be-

cause of the cool cooking surface, spilled foods do not burn and char and both support 16 and utensil 17 are easy. to clean. The cool, smooth support 16 also makes it possible to use this surface before cooking, or even immediately after cooking, for other functions related to'foodpreparation such as opening cans, trimming and cutting: vegetables, transferring cooked food from the cooking. utensil to a. serving dish, etc. Another advantageof induction cooking is that induction heating results in'a low thermal mass system. Since there is a relatively low storage of heat in utensil 17 itself, the temperature to which the utensil is heated can be changed rapidly, as from boiling to simmering to warming tem- -.peratures.

A new feature in an electric range or a portable counter-top warming or cooking appliance made possible by the inductioncooking principle is the inclusion in the applianceof a food warmingopening and tray beneath the:induction h'eatingcoil. Referring to FIG. 2, an alternatingmagnetic field is also produced at the bottom of air-core induction heating coil 15 as well as the top,

and aportion of this magnetic flux is available for warming or heating food in a closed metallic container ;.22(shown in dotted lines) supported on a non-metallic tray 23*. Frozen foods in a closed aluminum foil containercan also be'defrosted in this manner. To provide a the food warming. facility in an electric range, the upper portion of the range (FIG. 4) has a food warming cavity 24 that extends beneath all four coils of the four induc- .tion surface units. Tray 23 is preferably made of plastic l and is slidable in and out of the food warmingcavity.

A variety of thermoplastic and thermosetting plastics, with decorative surface designs if desired, can be used to make tray 23 and utensil support 16. As previously mentioned, this is because no point in the system needs to be in excess of a maximum temperature of about 450 F. Suitable plastics that can be used are Textolite (trademark of the General Electric Company). epoxy. silicone, polyimides, and others. These plastic materials can also be used in the fabrication of induction heating coils 15 and 15.

The embodiment of power converter circuit 12 illustrated in FIG. 5 is characterized by a relatively simple and inexpensive inverter that uses only one thyristor and gating circuit, and employs induction heating coil 15 in a dual function to couple power to the utensil (load) and to provide commutating inductance in the thyristor commutation circuit. To gain commercial acceptance, the ultrasonic frequency converter must be relatively low cost and reliable under both load and noload conditions. The power

converter input terminals

27 and 28 are adapted to be connected across a volt, 60 Hz source of a-c supply voltage. The power supply section of the converter circuit includes a full wave diode bridge rectifier 13 and a filter capacitor 29 connected between the rectifier output terminals, thereby providing a pair of direct

current supply terminals

30 and 31 for inverter 14. lnverter 14 is a onethyristor series capacitor commutated or series resonant inverter that generates damped sinusoidal pulses. The power circuit comprises essentially induction heating coil 15 connected in series circuit relationship with a commutating capacitor 32 and a unidirectional conducting thyristor 33 between the d-c

power supply 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 33' is 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 power circuit also includes a reset inductor 35 connected directly across commutating capacitor 32. As is explained in detail later, the function of reset inductor 35 is to recharge commutating capacitor 32 between complete cycles of operation when both thyristor 33 and diode 34 are nonconductive.

Thyristor 33 is more particularly a silicon controlled rectifier, but other solid state switching devices including a pair of transistors connected to operate in thyristor fashion can also be used. Power diode 34 can also be replaced by a thyristor controlled to be conductive for a complete half cycle. In this low cost circuit, however, the combination of the inverse-parallel connected silicon controlled rectifier and diode are clearly preferred. Only one gating circuit is required since diode 34 becomes forward biased and conducts when the. current in the series resonant circuit reverses in the negative polarity half of the cycle. Gating circuit 36 for thyristor 33 includes a series RC timing circuit comprising a fixed resistor 37, an adjustable resistor 38, and a timing capacitor 39 connected between the anode of the power device and negative d-c supply terminal 31. A voltage sensitive signal level semiconductor such as Shockley diode 40 and a series resistor 41 are connected between the junction of resistor 38 and timing capacitor 39 and the gate of thyristor 33. Timing begins when both

power devices

33 and 34 have ceased conducting, and the next gating pulse is generated by the charging of timing capacitor 39 to a predetermined voltage which casues Shockley diode 40 to break over and conduct.

Although utensil 17 is referred to loosely as the inverter load, it is more accurate to say that the inverter load is the electrical loss in the utensil. With respect to the utensil load, induction heating coil 15 functions as the primary winding of an air-core transformer. Utensil 17 functions as a single-turn secondary winding with a series resistance l7r representing the resistive part of the FR. or eddy current losses, and hysteresis losses where applicable. The currents and voltage induced in utensil 17 when the induction surface unit is in operation are determined essentially by the transformer laws. The physical equivalent for utensil 17 in the form or a single turn winding and resistive losses l7r is given in FIG. 5. FIG. 6 shows the equivalent electricl circuit for coil 15 and utensil 17. Coil 15 is represented by a series connected inductance 15i and resistance 15r, and these are in turn in series with the parallel combination of an inductance 17i and resistance l7r representing the utensil. This electrical equivalent circuit is based on conventional transformer equivalent circuit analysis and has been found to reasonably agree with experimental results. 7 4

With the utensil load in place, the commutating inductance for the series resonant circuit comprising coil 15 and commutating capacitor 32 is composed of both the coil inductance li and the reflected utensil inductance 17i. Under no-load conditions, with the utensil removed from the induction surface unit, the amount of commutating inductance increases. This causes a change in the resonant frequency of the series resonant circuit, so that there is a slight decrease in the inverter output frequency. With an average or selected utensil load in place, the series resonant circuit is tuned to resonance at a resonant frequency higher than the highest desired output frequency. The ultrasonic output frequency range of interest is between approximately 18 kHz and 40 kHz. The upper limit of this frequency range is determined largely by economic considerations, in conjunction with the high frequency limitations of available thyristor devices.

The operation of an induction cooking appliance employing the static power conversion circuit illustrated in FIG. 5 is as follows. The appliance is turned on by means of an on-off switch 42 shown only in FIG. 4 on the rear control panel, at the back of the electric range. Switch 42 is preferably combined with adjustable resistor 38 in gating circuit 36 to control the gating signal repetition rate and therefore the cooking temperature. Whenthe circuit is initially energized thyristor 33 is nonconducting and diode 34 is reverse-biased, but a circuit is completed through coil 15, commutating capacitor 32, and reset inductor 35 to the series RC timing circuit in gating circuit 36. Adjusting resistor 38 changes the time constant of this timing circuit. After the selected time delay, timing capacitor 39 charges to the predetermined voltage at which Shockley diode 40 breaks over and conducts. A gating pulse is supplied to the gate-cathode circuit of power thyristor 33, turning it on, and at the same time timing capacitor 39 is reset. A positive polarity half-sinusoidal current pulse flows through induction heating coil and theoretically charges commutating capacitor 32 to a value approaching twice the supply voltage V Because of the ducts and aids the turn-off of thyristor 33 byapplying reverse voltage to the device. At the end of the negative polarity half cycle (see FIG. 7a) the current through induction heating coil15 remains at zero since the next gating pulse is not applied to thyristor 33 at this time.

While

power devices

33 and 34 were conducting, the

current in reset inductor 35 on a steady state basis was increasing due to the net positive d-c voltage oncommutating capacitor 32 during the conduction cycle. During the circuit off-time reset inductor 35 discharges, thereby leaving commutating capacitor 32 negatively charged at the end of the circuit off-time. Voltage is supplied to gating circuit 36 during'the circuit off-time, and at the end of the predetermined time delay determined by the setting of adjustable resistor 38, a gating pulse is produced to render conductive thyristor 33 and begin the next cycle of operation.

FIG. 7a shows in full lines the sinusoidal induction coil current for two complete cycles of operation separated by a time delay 43. The corresponding commutating capacitor voltage is shown in full lines in FIG. 7b. At the end of the first conduction cycle, capacitor 32 has a net positive d-c voltage. The action of reset inductor 35 during the time delay period 43 is to change the capacitor voltage almost linearly as indicated at 44, leaving the capacitor negatively charged at the end of time delay interval 43. Due to this extra charge on commutating capacitor 32, the peak capacitor voltage 45 during the next cycle of operation is higher than in the dampened oscillatory case since the system energy is replenished. The effect of shortening time delay 43 by adjusting resistor 38 is to increase the inverter output frequency and therefore the current and power sup plied to utensil 17. Increasing the inverter output frequency also has the beneficial result of increasing the amplitude of the sinusoidal current pulses. This is illustrated in FIG. 7a by the second cycle dashed line current waveform. By advancing the thyristor gating pulse, the ratio of conduction time to capacitor recharge time increases, thereby on a steady state basis increasing the average current in reset inductor 35. The result is that, referring to the dashed line capacitor voltage characteristic in FIG. 7b, commutating capacitor 32 is charged negatively to a higher negative voltage during the circuit off-time as indicated by 44 so that the peak capacitor voltage 45' during the next conduction cycle is higher than the peak voltage 45 for the lower inverter frequency case. A limit on inverter frequency is reached when the value of the current in reset inductor 35 becomes significant in comparison to,the high frequency reverse current pulse in coil 15 and commutating capacitor 32,.since this in effect reduces the commutation time available to the thyristor. In summary, there are two effects that increase the power in watts supplied to utensil 17 when the inverter frequency is increased. There are larger, more frequently applied current pulses in induction heating coil 15.

The static power conversion circuit illustrated in FIG. 8 shows several modifications of the FIG. 5 con verter. The full wave'diode rectifier 13 is replaced by a phase controlled rectifier simply by substituting thyristors for two of the diodes in FIG. 5 to thereby provthesephase controlled gating circuits can be connected to be adjustable by rotation of on-off switch 42 (FIG. 4). Thefqur rnechanisms that have been described for achieving yariable'power control of the static conversion circuit can be .used in any desired combination. These are, briefly, the mechanical method of raising and lowering the induction heating coil (FIG. 3) to change the gap spacing, the power circuit technique of varying the inverter output frequency to change the frequency and'magnitude of the current pulses supplied to the coil, and the control circuit technique of using :a-p'hase controlled rectifier to adjust the d-c supply voltage. Still another mechanism disclosed in the first cross-"reference application is to step change the commutating circuit capacitance and inductance parameters. To-obtain a'large range of power output, such as a 3051 ratio, combinations of these techniques undoubtedly are required.

"Gatingcircuit 36' in FIG. 8 is a modified form of the .gating circuit disclosed in FIG. 5, which times the delay interval-43 between the end of one cycle and the beginning of the next. The disadvantage of this arrangement is that it is load dependent. The preferred timing arrange-ment obtained by gating circuit 36 is to time from the beginning of one cycle to the beginning of the next cycle. To this end,resistor 37 in the timing circuit is connected through an on-off switch 42, which can be identical to the switch 42 in FIG. 4 or operated concurrently'with it, to the positive d-c supply terminal 30. .A1so,'a constant source of voltage for adjustable resistor .38zand timing capacitor '39 is provided by connectingaZener diode 46 inparallel with both these composnents. Once gatin circuit 36" is ener ized b closin g g y g switch-42', gating pulses for thyristor 33 are produced attan. approximately constant repetition rate depending onthe-setting of adjustable resistor 38. One possible shortcoming of the FIG. 5 power circuit is that the return path for high frequency current pulses is through 'filter capacitor 29, which is ordinarily an electrolytic capacitor normally having a high impedance at ultrasonic'frequencies. The power circuit modification of FIG. 8 eliminates this problem and has improved performance. In this power circuit, the resonant circuit comprising coil and commutating capacitor 32 is connected between the anode of thyristor 33 and negativey'd-c supply terminal 31. Reset inductor 35 is now connected in series with thyristor 33 between d-c ter- -

minals

30 and 31, the latternormally at ground potential The impedance of reset inductor 35 is sufficiently largeto supply relatively constant current to the resonant circuit.

ith thyristor 33 or diode 34 conducting, the ultrasonic frequency return path .is to ground rather than through filter capacitor 29.'The operation of this modifiedzpower circuitis believed to be obvious from the previous discussion of FIG. '5. Reset inductor 35 discharges during the circuit off-time through coil 15 to charge commutating capacitor 32 positively. As the inverter output frequency increases, the average current in reset inductor 35 also increases, thereby resetting capacitor 32 to a more positive level in each cycle.

A second modification of the one-thyristor series resonant power circuit is given in FIG. 9. This is similar to FIG. 5 with the exception that reset inductor 35 is connected in parallel with both coil 15 and commutating capacitor 32. This allows the discharge reset current of reset inductor 35 to circulate through inductionheating coil 15 (as in FIG. 8), thereby increasing the power for a given set of circuit parameters. For a given ultrasonic output frequency, the FIG. 8 configuration gives the highest output power and accordingly is preferred.

By way of example of a specific induction cooking appliance suitable for use as an induction surface heating unit in an electric range, the required maximum power output is 1,500 watts and the ultrasonic frequency operating range is about 18 kHz to 25 kHz. Using a volt, 60 Hz a-c source, the peak input d-c voltage is volts. Induction heating coil 15 has an outside diameter of seven and one-half inches and an inside diameter of one and one-half inches. Coil 15 more specifically has 43 turns of No. I l braided ribbon conductor having a cross section of about fivesixteenths inches by less than one-sixteenth inch with a turn-to-turn spacing of at least 10 mils. The combined equivalent coil inductance 15i and utensil conductance 17i (see FIG. 6) is typically 150 microhenries, and the combined equivalent series coil resistance l5r and utensil resistance 17r is 15 ohms. For a counter-top warming or cooking appliance a lower maximum output power is satisfactory, such as 200-400 watts.

The outstanding advantage of the cooking appliance power conversion circuits shown in FIGS. 5 and 8, and of the modification of FIG. 9, is that these circuits are relatively simple and economical though yet being satisfactory for a full range of cooking tasks. A minimum of power circuit components are employed, due particularly to the use of the thyristor-diode combination, and to the dual function of induction heating coil 15 as a substantial part of the commutating inductance and as a transformer to couple power to the utensil load. Furthermore, only one relatively simple gating circuit for thyristor 33 is required. This one-thyristor series resonant power circuit operates in a full satisfactory manner under both load and no-load conditions. Although not optimized for every conceivable type of available pot and pan, a variety of ordinary utensils that couple well or reasonably well to the coil can be used. This inverter is appropriate for operation by nontechnical persons since a minimum number of user controls are needed. There is no failure due to shootthrough when adjustable resistor 38 is slewed.

The low cost multi-cylinder cooking appliance power converter shown in FIG. 10 is based on the simple onethyristor inverter of FIG. 5. This is a two-cylinder converter using two such inverters 14a and 14b assembled in mirror-image fashion between d-c

power supply terminals

50 and 51 and common point 54. A centertapped type power supply is used providing +V (such as +150 volts) at positive d-c terminal 50 and V (-150 volts) at negative terminal 51. Between these terminals are a pair of

voltage dividing capacitors

52 and 53, with their junction 54 connected to ground. Induction heating coil 15 is common to both inverters and is connected in the bidirectional conducting branch of the converter between point 54 and the junction 55 between the remaining components of the two FIG. 5 type inverfers.

Referring also to FIG. 11, the two converters 14a and 14b can be .operated alternately with or without a time delay between conduction cycles, or can be operated alternately in overlapping fashion. In the first mentioned mode, thyristor 33a and diode 34a conduct consecutively, and then thyristor 33b and diode 34b are rendered conductive as in the usual half bridge inverter configuration. The induction coil currents are shown in FIG. 11. Variable output power in watts is obtained with this circuit in the same manner as previously discussed, by control of the frequency of the current pulses through coil 15 by varying the repetition rate of supplying gating pulses alternately to thyristors 33a and 33b. The advantage of this circuit is that the high frequency requirements of thyristors 33a and 33b are not as stringent, since in the usual mode of operation each has more turn-off time than in the FIG. inverter. In the same manner the other components have a lower duty cycle and are lower cost components.

FIGS. 12 and 13 show low cost inverters suitable for induction cooking appliances characterized as onethyristor and one-transistor parallel resonant circuits. In these circuits induction coil 15 and capacitor 32 are connected parallel to one another in a tank circuit rather than in series as in the previous inverters. In FIG. 12, thyristor 58 is connected in series with a commutating inductor 59 and parallel-connected coil 15 andresonating capacitor 32', this entire circuit being connected between

d-c supply terminals

30 and 31. A solid state firing circuit 61 is connected between the gate of thyristor 58 and negative terminal 31, and is controlled by a sense voltage derived from the voltage across capacitor 32', thus closing the oscillatory loop. Waveform 62 represents the capacitor voltage. Power control is achieved by varying the voltage at which thyristor'58 fires within the thresh-old limits indicated by arrow 63. The theory of operation is similar to that for conventional Class C tube oscillators. The interval of conduction of thyristor 58 is limited to a small part of the oscillatory cycle. When the threshold capacitor voltage at which thyristor 58 is fired is between zero volts and the upper positive limit (the voltage on terminal30), the amplitude of the sinusoidal oscillations decreases, and thus the power output decreases as the threshold voltage becomes larger. With increasing negative threshold voltages, the power output increases. The parallel resonant circuit comprising coil 15 and capacitor 32' is, of course, tuned to resonate at a selected ultrasonic frequency. Commutating inductor 59 provides a means for turning off thyristor 58 by reverse biasing it during a part of each cycle when the capacitor voltage resonates above the supply voltage. Suitable firing circuits that can be used are described in the General Electric SCR Manual.

' The advantage of both the FIG. 12 and FIG. 13 parallel resonant circuits is that the high frequency circulating currents do not pass through the solid state power device. If the circuit has had appreciable operating Q, the power device losses'can be considerable compared to the useful power delivered to the load. A circuit which removes the thyristor or transistor from the oscillating tank circuit theoretically exhibits a higher efficiency and'less of a heat disposal problem. FIG. 13 is the transistor version of the one-thyristor parallel resonant circuit. In this power circuit the capacitor 32'.-coil l5 tank circuit is in series with a blocking diode 64 and apower transistor 65. A commutating inductor is not required since the transistor is turned off by base drive circuit 66. Suitable solid state base drive circuits that can be employed are described in the General Electric TransistorManual, seventh edition, copyrightl964. A feedback connection 67 senses the transistor collector voltage and is operative to turn on transistor drive circuit 66 to supply base current to power transistor when the collector voltage is at its minimum. Referring to capacitor voltage waveform 68, during the conduction time of transistor 65 the capacitor voltage falls to zero, after which coil 15 is charged with current until a timing circuit shuts off the transistor. The resulting transistor dissipation is the minimum possible with this circuit. Control of the conducting interval of transistor 65, indicated by arrow 69,,accomplishes power output control of the circuit. Increasing the conduction interval of transistor 65 increases the current in coil15. Decreasing the interval conversely decreases the current and therefore the power delivered to the utensil. An alternative technique for controlling power output is by varying the peak transistor current.

FIGS. 14a and 15 show low cost two-transistor ultrasonic frequency inverters for solid state cooking appliances. The induction heating coil for the FIG. 14a parallel resonant circuit comprises two coil sections 15a and 15b with the center-tap 15c'connected to the positive d-c supply terminal 30. Coils 15a and 15b are series aiding, equal-turn coils wound two-in-hand as shown in FIG. 14b. The free terminals of the two coils are connected to negative d-c supply terminal 31 through the respective power transistors -72 and 73 controlled by

base drive circuits

74 and 75. A tuning capacitor 76 is connected across a selected number of turns of coils 15a and 15b. ln'operation,

power transistors

72 and 73 are rendered conductive alternately for desired intervals of conduction. When transistor 72 conducts current flow through coil 15a produces a pulse of one polarity, and a pulse of the other polarity is generated when transistor 73 conducts. Variable power control is achieved by input voltage control, or

- by varying the conduction interval of the two transistors to change the amplitude of the ultrasonic wave.

In the low cost two-transistor series resonant sine wave inverter shown in FIG. 15, the two

power transistors

72 and 73 are connected in series between

d-c supply terminals

30 and 31, and diodes and 81 are connected across the emitter-collector terminals of the respective devices. The series resonant circuit comprising commutating capacitor 32 and coil 15 is connected to the junction 77 between the two power transistors and to the junction between a pair of

voltage divider capacitors

78 and 79. In operation, transistor72 is rendered conductive during generation of the positive polarity half sinusoidal current pulse, and diode 80 conducts when the current reverses and the series resonant circuit generates a negative polarity half sinu'soidalpulse. Transistor 73 and diode 81 conduct, during the next conduction cycle. The output power isagain controlled by modulating the inverter output frequency, or by input voltage control.

In summary, improved economical solid state induction cooking appliances are characterized by relatively simple power conversion circuits for use with inexpensive flat air-coreinduction heating coils as well as improved ferroma'gnetic-core coils. The ultrasonic frequency inverters preferably employ a single controlled switching device, and therefore onlyone control circuit, and use the induction heating coil in a dual function to couple power to the utensil and as a commutating inductor or resonating inductor. One-thyristor series resonant circuits, one-thyristor and one-transistor parallelvresonant circuits, two-transistor circuits, and multicylinder series resonant circuits have been discussed. The first is advantageous because power control to adjust the temperature to which the utensil is heated is by the combined effect of increasing both the amplitude and frequency of the sinusoidal current pulses supplied to the coil. Other power control techniques usable in different combinations are to employ a phase controlled rectifier and to move the coil relative to the utensil support to change the gap spacing. An additional feature of the air-core coil is the provision of a warming cavity and tray beneath the coil.

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.

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

1. A solid state cooking appliance for inductively heating cooking utensils comprising a substantially non-metallic, plate-like support having a substantially unbroken utensil supporting surface, a nominally flat induction heating coil mounted adjacent said support for generating an alternating magnetic field that extends across a gap including said support'and beyond said utensil supporting surface, and a static power conversion circuit including a unidirectional voltage supply and a solid state inverter for generating an ultrasonic frequency wave that drives said induction heating coil,

said inverter comprising a capacitor and said induction heating coil connected as the only resonating components of a single resonant circuit that is operated by a single controlled solid state switching device and control circuit therefor. 2. A cooking appliance according to claim 1 wherein said induction heating coil is an air-core coil, and the appliance further includes a non-metallic warming tray mounted beneath said air-core coil for supporting a utensil to be warmed.

3lA cooking appliance according to claim 1 wherein said induction heating coil is adjustably mounted relative to said support to change said gap and therefore the power coupled to the utensil.

4. A cooking appliance according to claim 1 wherein said induction heating coil is a ferromagnetic-core coil and further includes a ferrite plate mounted approximately parallel to said coil to serve as a high permeability path for return magnetic flux.

5."A cooking appliance according to claim 1 wherein said induction heating coil is a single layer coil made with braided ribbon conductor. 7

6. A cooking appliance according to claim 1 wherein said induction heating coil is a single-layer coil made with ribbon litz wire.

7'. A cooking appliance according to claim 1 wherein said source of unidirectional voltage supply is a source of variable input unidirectional voltage to thereby modulate the power output of said inverter and thus the power coupled to the utensil.

8. A cooking appliance according to claim 1 wherein said inverter resonant circuit is a series resonant circuit in which said capacitor and induction heating coil are effectively connected in series circuit relationship with one another and said controlled solid state switching device,

a diode connected in inverse-parallel with said controlled switching device, and

reset inductor means operative during nonconducting intervals of said controlled switching device and diode for resetting said capacitor.

9. A solid state cooking appliance for inductively heating a cooking utensil comprising a substantially non-metallic, plate-like support hav ing a substantially unbroken utensil supporting surface, I

a nominally flat air-core induction heating coil mounted adjacent said support for generating an alternating magnetic field that extends across a gap including said support and beyond said utensil supporting surface, and

a static power conversion circuit including full wave rectifying means and a solid state inverter for generating an ultrasonic output frequency wave that drives said induction heating coil,

said inverter comprising a capacitor and said induction heating coil connected as the only resonating components of a single resonant circuit that is operated by a single controlled solid state switching device and control circuit therefor.

10. A cooking appliance according to claim 9 wherein said inverter resonant circuit is a series resonant circuit in which said capacitor and induction heating coil are effectively connected in series circuit relationship with one another and said controlled solid state switching device, said controlled switching device being a thyristor,

a diode connected directly in inverse-parallel with said thyristor to form a thyristor-diode combination, and

reset inductive means for resetting said capacitor during nonconducting intervals of said thyristor and diode.

11. A cooking appliance according to claim 10 wherein said series connected capacitor, induction heating coil and thyristor-diode combination are effectively coupled between the output terminals of said rectifying means, and

said reset inductive means is connected across said capacitor.

12. A cooking appliance according to claim 10 wherein said reset inductive means and thyristor-diode combination are effectively coupled in series circuit relationship between the output terminals of said rectifying means, and

said series connected capacitor and induction heating coil are connected across said thyristor-diode combination.

13. A cooking appliance according to claim 10 wherein said series connected capacitor, induction heating coil, and thyristor-diode combination are effectively coupled between the output terminals of said rectifying means, and

said reset inductive means is connected'across said series connected capacitor and induction heating coil.

14. A cooking appliance according to claim 10 wherein said control circuit for said thyristor controlled switching device operates at a variable repetition rate to change both the amplitude and frequency of said output frequency wave and therefore the power coupled to the utensil.

15. A cooking appliance according to claim 10 wherein said rectifying means comprises a phase controlled rectifier which supplies a variable input voltage to thereby change the amplitude of said output frequency wave and thus the power coupled to the utensil.

16. A cooking appliance according to claim 9 wherein said induction heating coil is adjustably mounted relative to said support to change said gap and therefore the power coupled to the utensil.

17. A solid state induction cooking appliance comprising I a substantially non-metallic, plate-like support having a planar, substantially unbroken utensil supporting surface, I

a nominally flat air-core induction heating coil mounted beneath said support for generating an alternating magnetic field that extends across a gap 7 including said support and is coupled with metallic portions of a cooking utensil placed thereon, and

a static power conversion circuit including unidirectional voltage supply and a solid state inverter for generating an ultrasonic frequency output wave that drives said induction heating coil,

said inverter comprising a capacitor and said coupled induction heating coil and utensil connected as a resonant circuit that is operated by a single controlled solid state switching device and control circult.

18. A cooking appliance according to claim 17 further including a non-metallic warming tray movably mounted beneath said induction heating coil for supporting a utensil to be warmed.

19. A cooking appliance according to claim 17 wherein said induction heating coil is adjustably mounted relative to said support to change said gap and therefore the power coupled to said utensil.

20. A solid state cooking appliance for inductively heating a cooking utensil comprising a substantially non-metallic, plate-like support having a substantially unbroken utensil supporting surface,

a nominally flat air-core induction heating coil mounted adjustably beneath said support for generating an alternating magnetic field that extends across a gap including said support and beyond said utensil supporting surface,

a static power conversion circuit including a unidirectional voltage supply and a .solid state inverter for generating an ultrasonic frequency wave that drives said induction heating coil,

means for modulating the power output of said inverter to thereby control the power coupled to the utensil, and

means for moving said induction heating coil relative to said support to independently control the power coupled to the utensil by changing the gap between said coil and support.

21. A solid state cooking appliance for inductively heating a cooking utensil comprising an induction heating coil mounted adjacent a substantially non-metallic unbroken utensil support and generating an alternating magnetic field,

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

said inverter being a one-thyristor series resonant inverter comprising reset inductive means connected in series circuit relationship with said thyristor and an inverse-parallel connected diode between said unidirectional voltage terminals, and only a commutating capacitor and said induction heating coil connected in series circuit relationship directly across said thyristor and inverse-paralleldiode, and

a single control circuit for turning on said thyristor at a variable repetition rate to adjust both the amplitude and frequency of said output frequency wave and therefore the power coupled to the cooking utensil.

22. A cooking appliance according to claim 21 wherein said reset inductive means is a reset inductor that operates to recharge said commutating capacitor during nonconducting intervals of said thyristor and inverse-parallel diode.

23. A cooking appliance according to claim 21 wherein said static power conversion circuit further includes unidirectional voltage power supply connected to said pair of unidirectional voltage terminals that converts an alternating supply voltage to a variable unidirectional voltage to further control the power coupled to the cooking utensil.

Claims

1. A solid state cooking appliance for inductively heating cooking utensils comprising a substantially non-metallic, plate-like support having a substantially unbroken utensil supporting surface, a nominally flat induction heating coil mounted adjacent said support for generating an alternating magnetic field that extends across a gap including said support and beyond said utensil supporting surface, and a static power conversion circuit including a unidirectional voltage supply and a solid state inverter for generating an ultrasonic frequency wave that drives said induction heating coil, said inverter comprising a capacitor and said induction heating coil connected as the only resonating components of a single resonant circuit that is operated by a single controlled solid state switching device and control circuit therefor.

2. A cooking appliance according to claim 1 wherein said induction heating coil is an air-core coil, and the appliance further includes a non-metallic warming tray mounted beneath said air-core coil for supporting a utensil to be warmed.

3. A cooking appliance according to claim 1 wherein said induction heating coil is adjustably mounted relative to said support to change said gap and therefore the power coupled to the utensil.

5. A cooking appliance according to claim 1 wherein said induction heating coil is a single layer coil made with braided ribbon conductor.

7. A cooking appliance according to claim 1 wherein said source of unidirectional voltage supply is a source of variable input unidirectional voltage to thereby modulate the power output of said inverter and thus the power coupled to the utensil.

8. A cooking appLiance according to claim 1 wherein said inverter resonant circuit is a series resonant circuit in which said capacitor and induction heating coil are effectively connected in series circuit relationship with one another and said controlled solid state switching device, a diode connected in inverse-parallel with said controlled switching device, and reset inductor means operative during nonconducting intervals of said controlled switching device and diode for resetting said capacitor.

9. A solid state cooking appliance for inductively heating a cooking utensil comprising a substantially non-metallic, plate-like support having a substantially unbroken utensil supporting surface, a nominally flat air-core induction heating coil mounted adjacent said support for generating an alternating magnetic field that extends across a gap including said support and beyond said utensil supporting surface, and a static power conversion circuit including full wave rectifying means and a solid state inverter for generating an ultrasonic output frequency wave that drives said induction heating coil, said inverter comprising a capacitor and said induction heating coil connected as the only resonating components of a single resonant circuit that is operated by a single controlled solid state switching device and control circuit therefor.

10. A cooking appliance according to claim 9 wherein said inverter resonant circuit is a series resonant circuit in which said capacitor and induction heating coil are effectively connected in series circuit relationship with one another and said controlled solid state switching device, said controlled switching device being a thyristor, a diode connected directly in inverse-parallel with said thyristor to form a thyristor-diode combination, and reset inductive means for resetting said capacitor during nonconducting intervals of said thyristor and diode.

11. A cooking appliance according to claim 10 wherein said series connected capacitor, induction heating coil and thyristor-diode combination are effectively coupled between the output terminals of said rectifying means, and said reset inductive means is connected across said capacitor.

12. A cooking appliance according to claim 10 wherein said reset inductive means and thyristor-diode combination are effectively coupled in series circuit relationship between the output terminals of said rectifying means, and said series connected capacitor and induction heating coil are connected across said thyristor-diode combination.

13. A cooking appliance according to claim 10 wherein said series connected capacitor, induction heating coil, and thyristor-diode combination are effectively coupled between the output terminals of said rectifying means, and said reset inductive means is connected across said series connected capacitor and induction heating coil.

17. A solid state induction cooking appliance comprising a substantially non-metallic, plate-like support having a planar, substantially unbroken utensil supporting surface, a nominally flat air-core induction heating coil mounted beneath said support for generating an alternating magnetic field that extends across a gap including said support and Is coupled with metallic portions of a cooking utensil placed thereon, and a static power conversion circuit including unidirectional voltage supply and a solid state inverter for generating an ultrasonic frequency output wave that drives said induction heating coil, said inverter comprising a capacitor and said coupled induction heating coil and utensil connected as a resonant circuit that is operated by a single controlled solid state switching device and control circuit.

20. A solid state cooking appliance for inductively heating a cooking utensil comprising a substantially non-metallic, plate-like support having a substantially unbroken utensil supporting surface, a nominally flat air-core induction heating coil mounted adjustably beneath said support for generating an alternating magnetic field that extends across a gap including said support and beyond said utensil supporting surface, a static power conversion circuit including a unidirectional voltage supply and a solid state inverter for generating an ultrasonic frequency wave that drives said induction heating coil, means for modulating the power output of said inverter to thereby control the power coupled to the utensil, and means for moving said induction heating coil relative to said support to independently control the power coupled to the utensil by changing the gap between said coil and support.

21. A solid state cooking appliance for inductively heating a cooking utensil comprising an induction heating coil mounted adjacent a substantially non-metallic unbroken utensil support and generating an alternating magnetic field, a static power conversion circuit having a pair of unidirectional voltage terminals and including a solid state inverter for converting the unidirectional voltage to an ultrasonic output frequency wave that drives said induction heating coil, said inverter being a one-thyristor series resonant inverter comprising reset inductive means connected in series circuit relationship with said thyristor and an inverse-parallel connected diode between said unidirectional voltage terminals, and only a commutating capacitor and said induction heating coil connected in series circuit relationship directly across said thyristor and inverse-parallel diode, and a single control circuit for turning on said thyristor at a variable repetition rate to adjust both the amplitude and frequency of said output frequency wave and therefore the power coupled to the cooking utensil.