U.S. patent number 3,786,222 [Application Number 05/245,570] was granted by the patent office on 1974-01-15 for metallic foil induction cooking.
This patent grant is currently assigned to General Electric Company. Invention is credited to John D. Harnden, Jr., William P. Kornrumpf.
United States Patent |
3,786,222 |
Harnden, Jr. , et
al. |
January 15, 1974 |
**Please see images for:
( Certificate of Correction ) ** |
METALLIC FOIL INDUCTION COOKING
Abstract
An ultrasonic frequency induction surface cooking unit heats
aluminum foil and other thin metal utensils placed on the cool
cooking surface. For optimum heating the aluminum foil has a
thickness of 0.5 mils. A uniform heating distribution is obtained
by preferably using a rectangular induction heating coil with
several series-connected elongated coil sections, or by varying the
metal thickness to graduate the energy acceptance. Frozen
convenience foods can be defrosted in this manner, and the aluminum
foil can be wrapped about the food and shaped by the user into
disposable utensils using a set of molds. Disposable foil cooking
obviates the clean-up and storage problems of pots and pans.
Inventors: |
Harnden, Jr.; John D.
(Schenectady, NY), Kornrumpf; William P. (Schenectady,
NY) |
Assignee: |
General Electric Company
(Schenectady, NY)
|
Family
ID: |
22927199 |
Appl.
No.: |
05/245,570 |
Filed: |
April 19, 1972 |
Current U.S.
Class: |
219/622; 219/624;
219/621; 219/675; 426/107 |
Current CPC
Class: |
H05B
6/06 (20130101) |
Current International
Class: |
H05B
6/06 (20060101); H05B 6/12 (20060101); H05b
005/04 () |
Field of
Search: |
;219/10.49,10.75,10.79,201,385,430 ;99/1,171H |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Other References
Mapham, "A Low Cost, Ultrasonic Frequency Inverter Using a Single
SCR," Application Note 200.49, General Electric, Semiconductor
Products Dept., Feb., 1967. .
"Aluminum Foil Packs," Packaging, Dec., 1961, pp. 35-37.
|
Primary Examiner: Reynolds; Bruce A.
Attorney, Agent or Firm: John F. Ahern et al.
Claims
What we claim as new and desire to secure by Letters Patent of the
United States is:
1. A metallic foil induction surface cooking unit comprising
a substantially unbroken, relatively smooth, non-metallic cooking
surface,
an induction heating coil mounted adjacent to and beneath said
cooking surface, and
a solid state power conversion circuit for producing an ultrasonic
frequency wave that drives said induction heating coil and
generates an alternating magnetic field distribution which produces
relatively uniform heating in a variety of sizes and shapes of food
containing metallic foil utensils and containers placed on top of
said cooking surface.
2. A surface cooking unit according to claim 1 in which said
induction heating coil is a nominally flat even-heating induction
coil comprising a plurality of elongated coil sections connected
together and arranged side-by-side with adjacent coil sides close
to one another and aiding.
3. A surface cooking unit according to claim 1 in which said
induction heating coil is a nominally flat even-heating induction
heating coil comprising an odd number of series connected elongated
coil sections arranged side-by-side and close together such that
current in adjacent coil sides flows in the same direction.
4. A surface cooking unit according to claim 3 further including a
peripheral coil section closely surrounding said elongated coil
sections and also connected in series therewith such that current
in adjacent coil sides flows in the same direction.
5. A surface cooking unit according to claim 1 in which said
induction heating coil is an even-heating induction coil, and said
surface cooking unit further includes
a removable cover enclosing the top of the cooking surface above
said induction heating coil.
6. A surface cooking unit according to claim 5 in which said
even-heating induction coil has a rectangular configuration and
includes at least three substantially identical elongated central
coil sections arranged close together side-by-side and closely
surrounded by a peripheral coil section, the current in the center
one of said central coil sections flowing in the opposite direction
to the current in the other central coil sections and the
peripheral coil section.
7. A surface cooking unit according to claim 5 in which said
even-heating induction coil has an oval configuration and includes
at least three elongated central coil sections arranged close
together side-by-side and closely surrounded by a peripheral coil
section, the current in the center one of said central coil
sections flowing in the opposite direction to the current in the
other central coil sections and the peripheral coil section.
8. A surface cooking unit according to claim 5 further including an
inductively heated aluminum foil utensil on said cooking surface
underneath said removable cover which is comprised by a sheet of
aluminum foil having a thickness of approximately 0.5 mils for
optimum energy acceptance that is user-shaped as a cavity for
receiving the food.
9. A surface cooking unit according to claim 5 further including an
inductively heated aluminum foil container on said cooking surface
underneath said removable cover with a contoured bottom wall having
a variable thickness to graduate the energy acceptance and thereby
obtain a more uniform heating distribution, the optimum energy
acceptance thickness being approximately 0.5 mils.
10. A surface cooking unit according to claim 5 further including
an inductively heated aluminum foil container on said cooking
surface underneath said removable cover having a contoured bottom
wall with at least two food cavities at different heights from said
cooking surface to obtain differential heating.
11. A surface cooking unit according to claim 1 further including
an inductively heated aluminum foil disk disposed in the bottom of
a non-metallic container placed on said cooking surface, said
aluminum foil disk having a thickness of approximately 0.5 mils for
optimum energy acceptance.
12. An aluminum foil induction cooking unit comprising
a nominally flat even-heating induction heating coil mounted
adjacent a substantially unbroken non-metaliic plate to thereby
provide a relatively cool cooking surface,
said induction heating coil comprising a plurality of coupled
elongated coil sections electrically and physically arranged to
produce relatively uniform heating in disposable aluminum foil
utensils and containers placed on said cooking surface,
a static power conversion circuit comprising a solid state inverter
for converting a unidirectional voltage to an ultrasonic frequency
wave that drives said induction heating coil, and for adjusting the
power output of said inverter, and
a removable cover enclosing the cooking surface above said
induction heating coil.
13. A cooking unit according to claim 12 wherein said even-heating
induction heating coil has a rectangular configuration and wherein
said elongated coil sections are series-connected and arranged
side-by-side so that adjacent coil section sides are aiding.
14. A cooking unit according to claim 12 wherein said even-heating
induction heating coil includes an odd number of said elongated
coil sections arranged side-by-side and surrounded by a peripheral
coil section, said coil sections being series-connected with the
current in the center one of said central coil sections flowing in
the opposite direction to the current in the other central coil
sections and the peripheral coil section.
15. An induction surface cooking unit comprising
a nominally flat induction heating coil mounted adjacent a
substantially unbroken non-metallic cooking surface and generating
an alternating magnetic field,
a static power conversion circuit including a solid state inverter
for converting a unidirectional voltage to an ultrasonic frequency
wave that drives said induction heating coil, said inverter having
an operating frequency between 18 and 40 kilohertz, and
an inductively heated aluminum foil food container placed on said
cooking surface, said aluminum foil container having a variable
metal thickness to graduate the energy acceptance and thereby
obtain a relatively uniform heating distribution, the optimum
energy acceptance thickness being approximately 0.5 mils.
Description
BACKGROUND OF THE INVENTION
This invention relates to metallic foil cooking on cool-top
induction surface cooking units. More particularly, the invention
relates to induction cooking appliances suitable for the cooking of
food in thin metal containers and wrappings usually made of
disposable aluminum foil, and to the various ways that metallic
foil and thin metals can be fashioned as disposable and reusable
inductively heated utensils.
The heating in an oven of precooked frozen convenience foods
packaged in thin metal containers and the baking of potatoes
wrapped in aluminum foil are both well known. In an oven, however,
the efficiency of heat transfer to the metallic foil is relatively
low due in part to the reflection of heat by the shiny metal. It is
necessary to use the full oven cavity and the heating source must
obtain a high temperature before the warm-up process begins with
allowance for loss of heat to the room. Consequently, an extended
period of time is required to heat the food to the serving
temperature. It is recognized generally in U.S. Pat. No. 3,294,946
that the induction heating of thin metal frozen food containers is
inherently more efficient, but the eddy current cooker disclosed is
specially designed for this purpose and is not suitable for general
cooking or for use with the widely different sizes and shapes of
containers now available. It utilizes pairs of motor-driven
circular disks with alternately poled permanent magnets with the
frozen food container inserted between so as to pass magnetic flux
perpendicularly through the container. The reaction forces produced
by the low frequency rotating magnetic fields are used to propel
the fully heated food container from the cooker. These eddy current
cookers are bulky, expensive, not suitable for flat-top cooking
units and portable convenience food warmers, and further give
unsatisfactory cooking results because of the uneven heating
distribution.
The recently developed cool-top induction surface cooking units
used in the practice of this invention have been described
heretofore mainly as to heating conventional pots and pans and
other cooking utensils, both magnetic and non-magnetic. A complete
induction range or cooktop unit for domestic appliances is
disclosed in copending application Ser. No. 212,351, filed on Dec.
27, 1971 by the present inventors, and in other copending
applications referenced therein. The use of aluminum foil for
cooktop cooking is described briefly and claimed more broadly.
These appliances have such desirable user features as a cool,
counter-top, clean wire cooking surface; fast utensil warm-up and
responsive heating with lower power requirements; noiseless
operation; and complete freedom of movement of the utensil.
SUMMARY OF THE INVENTION
In accordance with the invention, the range of usefulness of
cool-top induction surface cooking units is extended by the ability
to heat and defrost foods in aluminum foil containers and wrappings
on the cooking surface rather than only in the oven as previously.
At ultrasonic operating frequencies the reaction forces are
insignificant and do not cause movement of the lightweight foil.
The invention is applicable to thin metals and metallic foils
generally, but aluminum foil with an optimum thickness of 0.5 mils
is heated efficiently and is the most common suitable material that
is widely available to the user. In addition to defrosting and
heating frozen convenience foods packaged in aluminum foil
containers, a sheet of foil can be wrapped about the food to be
heated (such as soft-boiled egg or a plastic frozen food pouch) to
achieve an oven effect, or can be fashioned into user-made
disposable utensils with a food receiving cavity (as for a fried
egg or hamburger) using a set of standard molds. The use of a
two-part mold set is advantageous because one part of the mold can
be employed both to shape the aluminum foil utensil and provide
support while eating from it. An inductively heated two-part cooker
with a plurality of appropriately shaped cavities is appropriate
for party and commercial cooking, for example, to heat a batch of
hot dogs.
Good cooking results are obtained in foil cooking on a cool-top
cooking surface by selecting the static power conversion circuit
and induction heating coil to generate an alternating magnetic
field distribution that produces relatively uniform heating in the
foil or other thin metal. Aluminum foil in particular has such a
thin cross section that it has poor lateral heat spreading
qualities. Accordingly, to achieve uniform heating it is necessary
to inductively heat all areas of the foil directly, or to reduce
the extent and location of areas not directly inductively heated so
that these latter areas can be heated by the limited lateral heat
spreading mechanism. The preferred arrangement here disclosed is to
employ a nominally flat even-heating induction coil comprising a
plurality of elongated coil sections mounted side-by-side such that
adjacent coil sides are aiding. The even-heating coil desirably has
a rectangular configuration and a surrounding peripheral coil
section, all series-connected. An alternative technique for use
with foil cookers that heat non-uniformly such as those with a flat
spiral coil, is to employ a disposable container or utensil with a
variable metal thickness bottom wall to graduate the energy
acceptance and thereby achieve uniform heating. Differential
heating for different types of foods can be achieved by contouring
the container bottom wall to have different heights so as to vary
the spacing to the coil.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a plan view of an even-heating multi-section induction
heating coil and a block representation of a solid state power
conversion circuit for supplying ultrasonic power to the coil;
FIG. 2 is a diagrammatic cross-sectional view showing the relation
of the induction heating coil, cooking surface, optional warming
tray, and disposable aluminum foil utensils;
FIG. 3 is a perspective view of a portable induction cooking unit
especially suitable for metallic foil cooking but useful also for
cooking with conventional utensils;
FIG. 4 is a schematic circuit diagram of a preferred form of
inverter for the induction cooking unit;
FIG. 5 illustrates temperature distribution characteristics for the
rectangular induction heating coil of FIG. 1 (A and B) and the flat
spiral coil of FIG. 8 (C);
FIG. 6 shows the power-thickness characteristic for aluminum
foil;
FIG. 7 is a modification of the rectangular even-heating induction
heating coil of FIG. 1 in a circular or oval configuration;
FIG. 8 is a diagrammatic cross section similar to FIG. 2 showing a
flat spiral induction heating coil and a circular thin metal
container with a varying bottom thickness to effect
even-heating;
FIG. 9 is a cross section of a plastic frozen food pouch wrapped in
inductively heated aluminum foil;
FIGS. 10 and 11 respectively are perspective views of a user-made
disposable utensil formed by pressing aluminum foil about the
circular mold of FIG. 11;
FIG. 12 is a perspective view of a plasticized thin metal cooker
for a plurality of hot dogs;
FIG. 13 is a cross section through the loaded and closed hot dog
cooker of FIG. 12;
FIG. 14 is a cross section through a glass container with an
immersed metallic foil heating element;
FIG. 15 is a cross-sectional view showing a metal foil container
with a contoured bottom for differential heating of foods by
varying the spacing to the cooking surface or coil; and
FIG. 16 is a diagrammatic side view partly in cross section of a
two-part plastic mold used both to shape a disposable utensil and
provide support while using it to eat from.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The induction cooking unit shown in FIGS. 1-3 is described with
regard to a portable, single coil induction surface unit for
metallic foil cooking and warming as well as some general cooking.
It is understood that essentially the same mechanical structure and
electronic circuitry with power ranges as required can be included
at one or more coil positions as an induction surface cooking unit
for an electric range or cooktop. The invention is further
described primarily with regard to the use of aluminum foil, but
the same principles are applicable where appropriate to composites
of metal foil with paper, cardboard, and plastic, and to disposable
and reusable thin metallic utensils made of stamped tin plate or
other suitable thin metals. The static power conversion circuit
indicated generally at 12 is preferably energized by a single phase
commercially available 60 Hz, 120 or 240 volt source of alternating
voltage. Static power converter 12 comprises a solid state a-c to
d-c power supply 11 including a full wave rectifier and a filter
network for producing a d-c supply voltage that is converted by a
solid state inverter 14 to an ultrasonic frequency voltage wave for
driving the induction heating coil 15.
Induction heating coil 15 is a multi-section, single layer,
air-core or ferromagnetic-core coil, especially built with a
rectangular configuration for the even heating of foods packaged in
rectangular and square thin metal containers. Precooked frozen
convenience foods such as the popular "TV dinner" and various
precooked meats, fish, vegetables, and potatoes are commonly
packaged in such containers. Coil 15 is comprised by four
series-connected coil sections including three elongated, identical
central coil sections 15a, 15b, and 15c, all surrounded by a
generally rectangular peripheral coil section 15d. Each of the coil
sections is wound continuously in spiral fashion, with braided
ribbon conductors or solid flat strip conductors. To generate
sufficient magnetic flux to heat the metallic foil container or
wrapping to the desired level, each coil section is tightly wound
with the short cross-sectional dimension of the conductors facing
upwards and adjacent turns separated by a strip 20 of insulating
material. The current direction in each coil section is indicated
by the arrows. Along line B--B in the width direction, the currents
in any two adjacent parallel legs flow in the same direction, and
thus the magnetic fields reinforce one another. The magnetic field
changes polarity as the line B--B is scanned from one side to the
other. To obtain this magnetic field distribution, it is evident
that there must be an odd number of central coil sections. The
magnetic field produced in the longitudinal direction along the
line A--A, on the other hand, is approximately equal except at the
ends of the coil. This coil arrangement produces relatively
even-heating in inductively heated metallic foils as is explained
later with regard to FIG. 5.
In the cooking unit (FIG. 2), induction heating coil 15 is
appropriately mounted in a horizontal position immediately below a
non-metallic or substantially non-metallic support plate 16,
typically made of a thin sheet of glass or plastic. Induction
heating coil 15 is preferably a flat coil, but can deviate slightly
from flatness so as to effect a better field distribution. Plate 16
supports the magnetic or non-magnetic cooking utensil to be heated,
and is hereafter referred to as the cooking surface. The cooking
utensil can be a pot or pan or other conventional cooking vessel. A
novel feature of the invention, however, is that cooking surface 16
is used to support a disposable or reusable utensil made of thin
metallic foil, such as the aluminum foil sheet 17a illustrated in
FIG. 2 that is slightly dished by the user for the cooking of a
hamburger, a fried egg, or other appropriate food. The aluminum
foil 17a can also be wrapped about an appropriate food to produce
the oven effect, such as to cook a soft boiled egg or warm a piece
of pie. Precooked frozen convenience foods in thin metal containers
can also be heated by placing them on top of the cooking surface
16. Operation of static power converter 12 to impress an ultrasonic
frequency voltage wave on induction heating coil 15 results in the
generation of an alternating magnetic field. The magnetic flux is
coupled across the air gap through non-metallic cooking surface 16
to disposable aluminum foil utensil 17a. At ultrasonic operating
frequencies in the range of 18 to 40 kHz, the cooking unit is
inaudible to most people. An even more important consequence from
the standpoint of the present invention of the use of this
ultrasonic frequency range is that there are insignificant
pondermotive forces and other forces of either attraction or
replusion which at lower frequencies would cause light weight
aluminum foil sheets, wrappings, and containers to move when placed
on the cooking surface. A sheet of aluminum foil, without food, can
be placed on cooking surface 16 and remains in place while being
inductively heated without the fear of it being thrown to the side
by the action of pondermotive forces to thereby cause possible
injury to the user or nearby persons.
An optional feature in an induction cooking unit is the inclusion
in the appliance of a food warming opening and tray beneath the
induction heating coil 15. An alternating magnetic field is also
produced at the bottom of air-coil induction heating coil 15 as
well as at the top, and a portion of this magnetic flux is
available for warming and heating food enclosed in an aluminum foil
wrapping 17b supported on a non-metallic tray 23. Other articles
such as dishes can be warmed by wrapping them in foil. Tray 23 is
preferably made of plastic and is slidable in and out of the
cooking unit as taught in copending application Ser. No. 200,424,
filed Nov. 19, 1971, by the present inventors. The warming of the
piece of pie wrapped in aluminum foil is illustrated. Frozen foods
in closed aluminum foil containers can also be defrosted in this
manner, although not as efficiently when placed on the cooking
surface 16.
The single-coil portable metallic foil induction cooker shown in
FIG. 3 is intended to stand on a kitchen counter surface and is
energized by a 120 volt source. A relatively small box-like housing
19 contains the electronic circuitry and induction heating coil 15,
and the unit is controlled by an on-off knob 21 and a power level
knob 21'. A pair of combination leg and handle units 22 are
attached to each side of housing 19 to facilitate easy handling and
storage. As was previously mentioned, the rectangular configuration
of induction heating coil 15 is particularly suitable for
defrosting and heating the "TV dinner" dinner illustrated at 17c.
For defrosting frozen foods prepackaged in metallic foil containers
and wrappings, it has been found that faster and more efficient
results are produced by enclosing the cooking surface 16 with a
cover 24, which conveniently is made of transparent plastic. Of
course, this induction cooker can also be used for general purpose
cooking using conventional cooking utensils, depending on the power
range of the unit. At least a 10:1 power range and a maximum power
of 1 to 1.5 kilowatts is required for general purpose cooking,
whereas an induction cooker solely for metallic foil cooking can
have a maximum power as low as 200-400 watts.
An important feature of the induction cooking equipment is that the
cooking surface 16 is relatively smooth and substantially
continuous and unbroken. Because of the lack of reaction forces of
either attraction or replusion, the user has complete freedom to
move the disposable foil utensil or conventional utensil on the
cooking surface. Among the other advantages of induction cooking
are, briefly, that the cooking surface 16 remains relatively cool
since the highest temperatures involved are the temperature of the
foils and utensils themselves, and consequently spilled foods do
not char and the cooking surface is easy to wipe clean. The
transfer of energy to the foil or utensil is relatively efficient
and consistent, not degrading as the heating coil ages, since heat
is generated only in the foil or utensil where it is wanted. Since
this is a low thermal mass system and there is thus a relatively
low storage of heat in the foil or utensil, the heating level of
cooking temperature can be changed rapidly. The cool, smooth
cooking surface 16 is available for other food preparation and
cooking tasks, such as cutting and trimming vegetables, opening
cans, etc. This feature may be of more utility in a portable
induction cooker for a camp or cottage, particularly since inverter
14 can be powered by a battery rather than by alternating voltage.
Another advantage of induction cooking that is particularly
important for metallic foil cooking is that the foil is heated
relatively uniformly. The importance of this feature particularly
for aluminum foil cooking in view of the poor lateral heat
spreading in the foil will be developed later.
To understand the electrical aspects of the uniform, fast,
efficient induction heating of thin aluminum foil and other thin
metallic foils, it is of benefit to discuss inverter 14 and the way
in which power is coupled to the foil. FIG. 4 shows a preferred
form of inverter 14 used in the practice of ultrasonic frequency
induction cooking. This one-thyristor series resonant inverter
requires a small number of components, only one gating circuit, and
produces a wide range of output power levels and corresponding
utensil heating levels. Only a brief description of the
construction and operation is given in view of the discussion in
the aforementioned application as well as in Kornrumpf application,
Ser. No. 200,530, filed on Nov. 19, 1971, now U.S. Pat. No.
3,697,716 dated Oct. 10, 1972, which shows the same inverter
configuration. Inverter 14 comprises a unidirectional conducting
power thyristor 33 connected in series circuit relationship with a
reset inductor 35 between d-c input terminals 30 and 31. A constant
or variable direct voltage E.sub.dc is supplied to d-c terminals 30
and 31. A diode 34 to conduct power current in the reverse
direction is connected across the load terminals of thyristor 33. A
series RC circuit is also usually connected across the load
terminals of thyristor 33 for dv/dt protection to limit the rate of
reapplication of forward voltage to the device. The basic power
circuit is completed by a commutating capacitor 32 and induction
heating coil 15 connected in series with one another and coupled
directly across the terminals of the inverse-parallel combination
of thyristor 33 and diode 34. The four coil sections 15a-15d are
series-connected. When either of the power devices is conducting,
capacitor 32 and induction heating coil 15 form a series resonant
circuit for generating damped sinusoidal current pulses that flow
through induction heating coil 15, which has the dual function of
providing commutating inductance as well as coupling power to the
load. Reset inductor 35 functions to reset the commutating
capacitor by charging commutating capacitor 32 positively during
the non-conducting intervals of the thyristor-diode combination.
Each cycle of current flow is initiated by a gating pulse applied
to thyristor 33 by a variable repetition rate gating circuit 36. A
user control 37, for example, an adjustable potentiometer actuated
by control knob 21' on the cooker control panel (FIG. 3), sets the
repetition rate of gating circuit 36.
The application of a gating pulse to thyristor 33 by main gating
control circuit 36 causes it to turn on, energizing the series
resonant circuit essentially comprising commutating capacitor 32
and induction heating coil 15. A damped sinusoidal current pulse
flows through induction heating coil 15 and charges commutating
capacitor 32 negatively. At this point the current in the series
resonant circuit reverses and a damped sinusoidal current pulse of
the opposite polarity flows through induction heating coil 15 and
diode 34. During the time that feedback diode 34 is conducting,
thyristor 33 is reverse biased by the voltage across diode 34 and
turns off. When the current in the series resonant circuit again
attempts to reverse, thyristor 33 does not conduct since it has
regained its forward voltage blocking capabilities, and a gating
pulse is not applied to the thyristor at this time. Because of the
losses in the electrical circuit due to the heating of the utensil,
commutating capacitor 32 at the end of the complete conduction
cycle on a steady state basis is left charged to a lower voltage
than it had at the beginning of the oscillation.
During the circuit off-time when both of the power devices 33 and
34 are non-conducting, the energy stored in reset inductor 35 is
discharged and transferred primarily to commutating capacitor 32,
thereby leaving the commutating capacitor 32 with a net positive
charge at the end of the circuit off-time or energy transfer
period. Waveform 38 in FIG. 4 shows the asymmetrical sinusoidal
induction coil current for two complete cycles of operation
separated by a time delay interval corresponding to the energy
transfer period. A small current circulates in coil 15 during the
energy transfer period due to the recharge current of capacitor 32.
With practical component choices, the circuit transfers more energy
from reset inductor 35 to commutating capacitor 32 as the transfer
period is made shorter, relative to the high frequency oscillation
period. There are thus two effects that increase the power of watts
supplied to the load when the inverter operating frequency or
repetition rate is increased. There are larger as well as more
frequently applied current pulses in induction heating coil 15.
The load for inverter 14 is provided by the electrical losses in
the utensil. With respect to the utensil load, induction heating
coil 15 functions as the primary winding of an air-core
transformer. In a physical equivalent circuit for the utensil,
identified generally at 17, the utensil functions as a single turn
secondary winding with a series resistance 17r connected between
the ends of the single turn representing the I.sup.2 R or eddy
current losses, and hysteresis losses where applicable. The
currents and voltages induced in the utensil are determined
essentially by transformer laws. An important consideration in
induction cooking is the source-to-load impedance matching to
enhance efficiency of power transfer. The situation in this case
using a commercially available a-c source is that the input circuit
impedance is fixed if minimum cost is to result. Ordinary utensils
have a fixed range of impedances. Since the utensil functions as a
single turn secondary winding, and the primary side voltage is
fixed at 120 volts (or 240 volts), 60 Hz, it follows that the
primary side impedance is also prescribed. This dictates certain
impedance matching characteristics in static power converter 12. It
is possible for the primary side circuit to include an impedance
matching transformer, however none is used in FIG. 4 in order to
reduce the number of components and the circuit cost.
The even-heating, multi-section, rectangular induction heating coil
15 shown in FIG. 1 heats all sections of uniform thickness aluminum
foil containers and disposable aluminum foil utensils to a
relatively uniform temperature. The temperature distributions A and
B shown in FIG. 5 are produced respectively along the length
dimension and along the width dimension such as A--A and B--B in
FIG. 1. In the length direction the magnetic field distribution and
the temperature to which the foil is heated is approximately
constant except at the two ends where the amount of heating is
reduced. In the width direction, the magnetic field varies
cylically, as does the heating of the foil. There is actually no
induction heating at the center of any of the coil sections, and it
is necessary to rely on lateral heat spreading in the foil in order
to heat those portions of the foil over the coil centers. Aluminum
is of course a non-magnetic metal and is heated only by the I.sup.2
R losses resulting from the circulation of eddy currents. Another
characteristic of aluminum foil due to its thin cross section is
poor lateral heat spreading. Conventional thick aluminum cookware
is known for its good thermal conductivity, however any foil
suffers in its ability to spread or conduct heat laterally due to
the thin cross section and large surface area. These considerations
underlie the selection of the elongated, relatively narrow central
coil sections. Each portion of the aluminum foil is inductively
heated directly, such as areas above a coil side or between
adjacent coil sections, or if not heated inductively such as those
portions overlying the centers of the coil sections, are a
relatively short distance (about 1 inch or less) from an area that
is heated. At these short distances, the lateral heat spreading
mechanism is effective to obtain the over-all result of relatively
uniform heating in all sections of the aluminum foil.
Uniform heating of the foil container or disposable utensil is
particularly needed in the case of defrosting frozen foods because
cool spots develop if the heat is not distributed uniformly. The
oven defrosting of frozen foods, it is noted, is an even-heat
environment. For other foods such as fried bacon and scrambled
eggs, uniform heating of the pan or other cooking utensil is needed
for good cooking results, and it is a matter of common knowledge
that foods of this type are often turned and moved about as the
cooking proceeds to prevent localized burning.
Another important aspect of aluminum foil induction cooking is that
the thickness of aluminum foils, contrary to what might be
expected, is favorable to the efficient coupling of power to the
metal by induction cooking equipment. The conventional approach to
obtaining efficient induction heating is based on skin effect
considerations, in that the usual rule of the thumb is that the
thickness of the inductively heated metal should be at least as
great as the skin depth and preferably three to four times the skin
depth. The depth of induced current penetration is generally given
as being proportional to the square root of resistivity divided by
the relative permeability times the frequency. Thus, the depth of
penetration varies with the square root of frequency and is
dependent both on resistivity and the magnetic property
characteristics of the material. Since aluminum is non-magnetic,
the latter is not a consideration. For 52S aluminum the depth of
penetration at 25 kHz is approximately 30 mils. Based purely on
skin effect considerations, then, for most efficient induction
heating an aluminum cooking utensil should be at least 30 mils and
up to 120 mils in thickness. However, it is found that due to the
impedance matching considerations mentioned, club aluminum ware is
not heated efficiently by induction surface cooking units operating
in the range of 18-40 kHz. Instead, it is found that the amount of
power coupled to aluminum utensils is a function of thickness and
has an optimum value as shown in FIG. 6. For the frequency range of
interest and practical physical dimensions, the power is most
efficiently coupled to the utensil when it has a thickness of about
0.5 mils, and there is very poor coupling at thicknesses of less
than 0.1 mil and greater than 2 mils. The thickness of heavy duty
household aluminum foil is approximately 0.5 mils, and thus it can
be concluded that aluminum foil in thicknesses already commonly
available to the user is heated efficiently by ultrasonic induction
cooking equipment.
In view of the fact that the depth of current penetration at the
representative frequency of 25 kHz is 30 mils, it is evident that
in aluminum foil induction cooking most of the magnetic flux
penetrates completely through the thickness of the metal. The
explanation for the power-thickness characteristic shown in FIG. 6
is that at 0.5 mils the resistance of aluminum foil is sufficiently
high that there is a good impedance match between source and load
(assuming a 120 volt or 240 volt supply). The reader is referred
back to the previous discussion on impedance matching in which it
was developed that the one turn secondary winding provided by the
utensil, and the fixed primary supply voltage, means that the
primary impedance characteristic of the induction cooking equipment
is also fixed. With this type of analysis, it is determined that at
30 mils the resistance of aluminum utensils is too low to obtain a
good impedance match. Efficiency of power coupling is improved by
reducing the thickness of the aluminum utensil to obtain a higher
effective resistance and therefore a better impedance match. Most
efficient heating, therefore, occurs when the thickness is about
0.5 mils, with poorer power coupling as the thickness decreases
below this value or appreciably above this value. The explanation
of the increased resistance of aluminum as the thickness is reduced
is based on the well-known formula that the resistance of the
conductor depends on its specific resistivity .rho. and is directly
proportional to its length and inversely proportional to its area.
It can be shown by known induction surface heating analysis that
the area heated by each coil section can be divided up into a
number of concentric elongated annuluses having the same shape as a
coil section turn. As used in the formula, the length is the
circumferential length at the center of each elongated annulus and
the area is the product of the width of the elongated annulus and
the thickness of the utensil. The circumferential length and the
width of the elongated annulus are both fixed by the dimensions of
the induction heating coil section. Varying the thickness of the
aluminum utensil, then, varies the conductor resistance.
Using the metallic foil induction cooker shown in FIG. 3, precooked
frozen convenience foods packaged in aluminum foil containers are
defrosted and heated with good results generally in less time than
is required for defrosting in an oven. Furthermore, there is less
room-heating and no oven preheating time is required because of the
general feature of induction cooking of fast utensil warm-up. The
power control knob 21' is set to an appropriate position, easily
determined, to obtain rapid defrosting and heating without
scorching and burning the food. By way of example, a well-known TV
dinner bears the instructions to heat in a preheated oven for 25
minutes at 450.degree.F. The corresponding time for the induction
cooker is 15 minutes. In general, it can be said that the
defrosting and heating time for induction cooking is less than that
for oven heating, and in addition there is no preheating
requirement. As was previously mentioned, the induction cooker can
be used for the warming of foods such as rolls, a piece of pie, and
grapefruit by wrapping the food in aluminum foil and placing it on
the cooking surface for heating. Temperature feedback control is
probably desirable in order to provide the optimum results with
minimum care on the part of the user. In either case, the aluminum
foil disposable utensil is heated efficiently and uniformly with
good cooking results. If desired, the metallic foil induction
cooker of FIG. 3 or range with a similar induction cooking surface
unit can be provided with built-in storage for aluminum foil,
either in the form of rolls, or square pull-out sheets. Thus a
complete cooking appliance is provided without the need for pots
and pans with their subsequent washing and clean-up problem.
Referring to FIG. 7, the even-heating, multi-section induction
heating coil 15' is a modification of the FIG. 1 coil in a round or
oval configuration. To conform to the peripheral shape of coil
section 15d', coil sections 15a' and 15c' are identical to one
another but considerably shorter than the other central coil
section 15b'. The oval or round configuration is especially
suitable as one or more of the heating coils in a conventional
four-coil electric range or cooktop. The even-heating coil 15' is
used to advantage, of course, for the uniform heating of
conventional pots and pans and other cooking utensils.
Another technique for achieving uniform distribution of heat in the
utensil is to contour the thickness of the utensil to graduate the
energy acceptance. The induction heating coil 15" shown in cross
section in FIG. 8 is an annular, flat spiral, single layer,
air-core coil of the type illustrated in the aforementioned Harnden
and Kornrumpf application, Ser. No. 212,351. As is shown in FIG. 5,
the aluminum foil temperature characteristic (C) is relatively
non-uniform due to the corresponding flux distribution produced by
such a coil. The composite paper-and-aluminum foil container 17d in
FIG. 8 by way of example is a circular frozen pie container and has
an inner foil layer with a variable thickness to obtain a more
uniform temperature distribution. The thickness of the aluminum
foil layer is approximately 0.5 mils for optimum coupling of the
power with the exception of a thicker circular area 40 at the
center of the container and a thicker annular ring 41 about midway
between the center and the edges of the container. The thicker ring
41 couples power less efficiently than the optimum thickness of 0.5
mils (see FIG. 6) while at the center of the container there is no
coupled magnetic flux and the thicker central area 40 is provided
for better thermal conduction of the heat through the metal from
the surrounding heated areas. The two techniques for obtaining a
uniform heating distribution that have been described can be
combined together, that is, designing the induction heating coil to
have a suitable magnetic flux distribution and by contouring the
thickness of the metallic foil to achieve a graduated energy
acceptance.
It is realized that when considering the general subject of
defrosting and heating frozen convenience foods, there are many
pre-cooked convenience food packages with different types of foods
that benefit from heating at different temperatures of different
heating rates. The heating considerations for a homogeneous food
are not the same as those for "TV dinners" with several different
types of food. In an even-heat environment such as an oven, it may
be necessary to arrange the food or the foil coversheet to obtain
differential heating. An alternative approach made possible by the
induction heating of thin metal convenience food containers is
shown in FIG. 15. The thin metal container 17c' is made of uniform
thickness aluminum foil and is divided into several food cavities
55-58 by raised dividers 59. The innovation illustrated is that the
bottom wall of containers 17c' is contoured so that selected food
cavities are at different heights from cooking surface 16 and
induction heating coil 15. Since the spacing of the food cavities
from coil 15 is different, the amount of induction heating is
variable according to the food being cooked. Thus, cavities 56 and
57 are at intermediate and high levels as compared to cavities 55
and 58, and are suitable for foods that require a reduced heating
level such as applesauce and butter. Pastries are also a problem
area with unique heating requirements that make the contoured
bottom wall container approach desirable.
Some of the other aspects of metallic foil induction cooking are
illustrated in FIGS. 9-14. In FIG. 9, a frozen food pouch-type
plastic package 42 is enclosed in an aluminum foil wrapping 17e for
thawing and heating by placing it on the cooking surface 16 of the
induction cooker shown in FIG. 3. This eliminates the need for
boiling water to prepare these foods. FIG. 10 illustrates the
forming of a user-made disposable aluminum foil utensil 17f with a
functional cavity tailored for the particular food being cooked. A
circular cavity is suitable for pancakes, fried eggs, hamburgers,
and the like. A simple way to fashion such a disposable aluminum
foil utensil 17f is by use of the circular mold or pre-form 43
shown in FIG. 11. A sheet of aluminum foil is easily pressed about
mold part 43 which can be made of plastic or wood. Additional
standard molds in other shapes can be provided for other types of
foods, such as a square mold, a rectangular mold, a mold for hot
dogs, etc.
The two-part mold set shown in FIG. 16 has the dual function of
shaping the disposable utensil and serving as a support for the
vessel while its contents are being consumed by the user. The
heat-and-serve disposable utensil 17f' is made slightly deeper so
as to be suitable for heating soup and thereafter serving as a
disposable soup bowl. The two-part mold 60, 61 is used to stamp out
disposable utensil 17f' by pressing the foil into conformity with
the cavity on part 60, and is preferably made of plastic. Since
there are a number of plastic materials that withstand temperatures
of 450.degree.-500.degree.F, the highest temperatures encountered
in induction cooking, the female mold section 60 can also be used
to support disposable utensil 17f' while the food is being heated.
Since heat-and-serve mold section 60 is not directly heated while
on the metal foil cooker, it can be picked up by the user and moved
to a table where it supports the vessel, which now functions as a
soup bowl. Desirably there is a thin bottom wall section on mold 60
to provide support for the food or liquid as it is being cooked and
transported.
The reusable stamped metal cooker 45 shown in FIGS. 12 and 13 is
used for the simultaneous cooking of a plurality of food items with
a repeatable shape such as hamburgers and hot dogs. The two
identical halves 46 and 47 of the cooker are made, for instance, of
a stamped thin tin plate or steel sheet 48 coated with a layer of
plastic 49 to improve its appearance and handling. The thickness of
the steel layer 48 can be uniform or can be non-uniform for better
heat distribution. As here illustrated, the identical food cavities
in each of the cooker halves 46 and 47 are shaped to receive hot
dogs. Upon being loaded and closed, the hot dogs 50 are almost
completely surrounded as shown in FIG. 13 and are cooked uniformly.
This utensil can be washed after use, and is suitable for party
cooking in the home or for commercial cooking.
To complete the presentation of the utility of aluminum foil in
induction cooking, FIG. 14 illustrates the heating of water in a
glass container 51 by means of an inductively heated foil disk 52
dropped into the bottom of the container. Ordinary glass containers
can be used without danger of breakage with the only limitation
being that the glass be capable of withstanding boiling water
without cracking. Pre-cut aluminum disks can be provided, or can be
cut from a sheet of heavy duty aluminum foil by the user.
Similarly, plastic vessels can be employed effectively as utensils
in induction cooking systems.
In summary, the range of usefulness of ultrasonic induction surface
cooking units is extended by the ability to cook foods in thin
metallic foil utensils on the cooking surface rather than only in
the oven as previously known. Aluminum foil with an optimum
thickness of 0.5 mils is efficiently heated inductively, and it is
desirable for good cooking results to obtain a uniform heating
distribution as by proper selection of the induction heating coil
or by varying the thickness of the foil to achieve graduated energy
acceptance. In addition to defrosting and heating frozen
convenience foods packaged in thin metal containers, aluminum foil
can be wrapped about food to be heated or fashioned into user-made
disposable utensils using a set of standard molds. Stamped-out thin
metal cookers with appropriately shaped cavities are suitable for
party and commercial cooking.
While the invention has been particularly shown and described with
reference to several preferred embodiments thereof, it will be
understood by those skilled in the art that various changes in form
and details may be made therein without departing from the spirit
and scope of the invention.
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