Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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FIELD OF THE INVENTION
This invention pertains generally to induction heating and
cooking apparatus, and in particular to a new and improved cooking
assembly of the ceramic cooking support plate, the inductive heating
coil and the associated trim, which improves cooking performance and
reduces magnetic flux leakage.
BACKGROUND OF THE INVENTION
Apparatus for magnetically coup1ing an induction heating coil
with a ferrous cooking utensil to thereby electromagnetically heat the
contents of the utensil have been widely known and used for ~any years.
In such apparatus, the induction coil is usually located below a non-
magnetic cooking surface and an alternating current through the coil
causes a continuously changing magnetic field to be generated. The
~agnetic field extends through the cooking surface to link with the
cooking utensil to cause eddy currents in the utensil and allow it to
heat up.
Commercial versions of induction cooking apparatus provide for
a plurality of cooking areas on a smooth-top cooking surface made from
a single continuous rectangular piece of ceramic material. Each designated
cooking area on the cooking surface has an induction cooking coil located
thereunder so that cooking utens11s placed on the designated cooking
areas will be linked by the magnetic field generated by the cooking
coil.
One problem associated with this construction is cost; the
ceramic cooktop is more expensive than a comparably sized sheet steel
cooktop typically used in conventional electric or gas ranges.
Another problem is electromagnetic leakage. The electro-
magnetic leakage problem is aggravated by a variety of factors. One
factor is the misalignment of a cooking utensil with the magnetic field
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~enerated by the cooking coil. This problem is addressed in the context
of a conventional induction cooking construction by providing utensil
presence and position detection apparatus which insure that the induction
coil is not energized unless the cooking utensil is both present on the
cooktop and centered over the induction heating coil.
These sensing arrangements are designed to insure that the
high intensity electromagnetic fields which emanate from the induction
heating coil are generated only when a utensil is in position and centered
over the induction heating coil, thereby limiting the undesirable trans-
mission or leakage of electromagnetic flux into the free space surround-
ing the cooking appliance. Neither o~ these approaches, however, addresses
the problem of electromagnetic leakage resulting from the high reluctance
gap present in the flux path between the edges of the cooking utensil
and the flux-shaping coil support structure. This latter situation
creates an undesirable condition which results in the leakage of exces-
sive magnetic flux into the space surrounding the cooking surface, which
leakage may cause interference with television and radio signals and
other communication systems. For this reason, among others, governmental
regulating agencies have set limits on the magnetic field leakage of
this type attendant to the use of induction heating appliances. Since
the intensity of flux leaking into surrounding space increases as a
result of operation of an induction heating unit with such h~gh reluc-
tance gaps, it is desirable to provide an arrangement for operation of
the unit without such gaps or with a reduced number of them.
OBJECTS AND SUMMARY OF THE INVENTION
A primary object of the invention, therefore, is the provision
of an induction cooking arrangement which is simple in design, and inex-
pensive in implementation.
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A further obiect Ot the invention is the provision of an
induction cooking apparatus which employs small individual ceramic
plates as cooking surfaces for each cooking area, each plate being
supported in a main horizontal sheet metal cooktop surface.
A further object is the provision of an induction cooking
apparatus having a plurality of cooking units, each unit including a
ceramic plate supported in an opening in an otherwise continuous sheet
metal support surface.
A still further object of the invention is to provide an im-
T;0 proved arrangement for limiting the intensity of the magnetic field
leaked into the space surrounding an operating induction heating/cooking
apparatus by confining the field by means of substantially closed, high
relative penmeability, flux shaping means.
Yet another object is the provision of an induction cooking
arrangement having a relatively low reluctance path for flux linking the
cooking utensil to the induction cooking coil.
A further object is the proYision of a metal trim ~rame for
supporting a ceramic induction cooking plate in an opening in a horizon-
tal support surface, the cooking plate being provided with a metallic
layer on its surface to form a low reluctance flux path between a cook-
ing utensil supported on the plate and the frame to thereby eliminate a
main source of leakage flux from the unit.
These and other objects are accomplished according to the
principle of the invention by provision of an induction heating appara-
tus having a cooktop including a plurality of induction surface heating
units. The cooktop comprises a horizontally disposed planar metal
support surface having a plurality of openings therein. A ceramic
smooth-top plate is supported in each of the openings and adapted to
support a cooklng utensil thereon. An induction heating coil is sup-
ported subjacent the ceramic plate in a position to generate a magnetic
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field which passes through the plate to link the cooking utensil.
Each plate is supported in the openings by a metallic trim frame
and a conductive layer is provided on the plate, the frame and
layer combining to provide a low reluctance flux path for flux
generated by the coil and linking the utensil, the low reluctance
path operating to reduce the magnetic flux leaked into the space
surrounding the heating apparatus during operation.
BRIEF DESCRIPTION OF THE DRAWINGS
E~urther details of the present invention and many
additional advantages of this invention will be apparent from a
detailed consideration of the remainder of this specification
and the accompanying drawings in which:
FIG. 1 is a generalized perspective view of an
induction heating/cooking apparatus embodying the principles
of the invention; and
FIG. 2 is an illustrative vertical cross section
showing the relationship in an induction heating/cooking unit
between the cooking utensil, the ceramic insert which provides
cooking support surface, the induction heating coil, and the
metal cooktop, in accordance with the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring now to the drawings and initially to FIG. 1,
the induction heating/cooking apparatus generally designated
10 includes an upstanding substantially box-like metal body
12 having a substantially horizontal cooktop 14. The
cooktop 14 includes four surface cooking units 20 located at
the right rear, left rear, right front and left front positions.
An upstanding control and display panel 16 is located at the rear
of the cooktop 14. The control and display panel in a conventional
manner provides a means whereby a user of the cooking apparatus
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may control the cooking process on any or all of the surfac~ cooking
units 20 by utilizing suitable manually-actuatable controls associated
with the various cooking units. Suitable display devices may also be
included on the panel 16 to indicate to the user the current operational
state of these cooking units, such as temperature, cooking time, etc. A
hinged oven door 18 having a suitable handle 19 for opening and closing
thereof provides access to an oven cooking area (not shown).
The cooking units 20 are generally of the induction heating
type, and FI6. 2 shows a cross sectional view of a preferred embodiment
10; of one of the units 20 utilizing the principles of the invention, with
a cooking utensi1 22 resting thereon, the other cooking units being
substantially identical in construction to the one shown.
Referring to FIG. 2, it is seen that each cooking unit 20 com-
prises a support plate 21 on which a suitable cooking utensil 22 is
adapted to rest. The utensil 22 is preferably, but not necessarily,
made of a ferromagnetic material, such as iron or stainless steel, so
as to heat up more efficiently when subjected to a changing magnetic
field of the type used in induction heating units.
Plate 21 is màde of a material which is electrically insulat-
ing and thermally transmissive as well as being highly wear and thermalshock resistant, and resistant to the physical and chem~cal attacks of
foods and liquids which may come in contact with the plate during the
cooking process. Such materials are usually milk-white or black 1n
color, opaque, and sold under the trademarks "PYROCERAM", I'CER-VIT", and
"HERCUVIT". The plate 21 is also non-ferromagnetic in nature so as to
allow the flux generated by the induction heating coil to pass there-
through into linking relationship with the cooking utensil 22. While
the tenm glass-ceramic or crystalline glass will be used throughout in
referring to the material which comprises plate 21, it should be under-
stood that the invention encompasses other materials with similarcharacteristics, such as quartz, high silica glass, high temperature
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glasses and different ceramic materials. While the plate 21 is prefer-
ably circular in shape, other configurations may be used satisfactorily.
The plate 21 has deposited thereon a thin, ring-shaped, con-
ductive layer 27 covering the outermost peripheral portions of the plate
21. The layer 27 is preferably ferromagnetic and its purpose and opera-
tion will be addressed in greater detail hereinafter.
The circular plate 21 is supported by a circular metal trim
frame 23 of thin sheet stock of stainless steel or like material. The
trim frame 23 includes a flat annular rim 24 adjacent its outer peri-
phery which is adapted to overlie and rest on a generally flat horizon-
tal sheet metal surface 25 fo ming the major portion of the cooktop 14.
The trim frame 23 also includes a bifurcated formation on its inner
periphery facing the plate 21 which has inwardly extending anms 31 and
32. The top arm 31 contacts the top surface of the conductive layer 27,
and the bottom arm 32 in turn supports an annular lip 33 of a pan-like
member 34 in which is supported an induction heating coil 35. While the
trim frame 23 is preferably circular, it would, of course, confonm to
whatever shape is selected for the plate 21. The trim frame 23 is thus
seen to suppor~ both the cooking plate 21 and the induction cooking coil
35 from the horizontal surface 25.
Another pan-like member 40 which is generally U-shaped in
cross section serves to provide additional support for the cooking unit.
Specifically, the member 40 is provided with an outer rim 41 which ex-
tends radially a sufficient distance to rest on an inwardly extending
flange 42 of the support surface 25 located below the cooktop surface.
The upper surface of the rim 41 in turn abuts the bottom wall of the
pan 35 and is attached thereto, as by welding.
The lnduction coil 35 preferably has a flat pancake-like shape
and is mounted such that the central axis 36 of the coil, if extended
upwardly through the cooking plate 21, passes through the approximate
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geometric center of the cooking area on which the pan 22 is to be located.
The surface cooking unit also includes an inverter circuit (not shown)
well known in the art, which is coupled to the coil 35 for producing an
ultrasonic magne~ic field linking ~he cooking utensil 22. The utensil
22 acts as a single turn, shorted secondary to be heated by the energy
contained in the field. In a known manner, the field is produced by
causing bi-directional current pulses in the coil 35.
The cooking unit 20 is thus adapted, by virtue of the for-
mation of the pan members 40, 34 and the trim frame 23, to be inserted
lO~ as a unit from above a circular opening in the support surface 25.-
Suitable connections are, of course, provided to couple the electrical
power supply to drive the cooking unit to the induction coil 35 when in
place.
In addition to providing physical support for the cooking unit
20, the pan 34, trim frame 23, and layer 27 cooperate with the utensil
22 to fonm a flux shaping means for the magnetic field generated by coil
35. More specifically, the high relative permeability materials from
which these elements are made serve to shape and confine the flux generated
by the coil 35 during the induction heating process and to thereby
reduce undesirable leakage of magnetic flux into the surrounding space.
In contrast to this arrangement, prior art induction systems
are much less effective in preventing flux leakage because of the high
reluctance gaps in the flux-shaping paths, notably a gap 51 between the
utensil and the flux shaping means which supports the induction coil.
More specifically, in the prior art, since the cooking plate on which
the utensil rests is a large continuous glass surface which extends far
beyond the cooking area, a high reluctance gap 51 is presented between~
the utensil 22 and the flux shaping conductive support for the induction
coil, which is located below the cooking surface. Thus, the magnetic
circuit for flux linking the cooking utensil with the flux shaping means
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adjacent the heating coil includes a high reluctance gap 51, causing
fringing or 1eakage of flux into the surrounding space.
By the addition of the conductive layer 27, as shown in FIG.
2, a uniform low reluctance flux path is provided between the utensil
22 and the flux shaping members which surround the inductive coil. Thus,
with a utensil 22 in place on the plate 27 and positioned as shown~ the
flux generated by the coil 35 is confined in a substantlally closed area
bounded by the members 34, 23, layer 27 and the bottom wall of the uten-
sil 22. This substantially closed low reluctance flux shaping system
thereby more effectively confines the magnetic field and reduces un-
wanted leakage emission.
While the above-noted closed su~face flux confining arrange-
ment has been illustrated as being accomplished by means of a metal trim
frame 23 which couples the field shaping means below the plate (pan 34)
with the field shaping means above the plate (layer 27) it is possible,
though less practical, to utilize a large continuous ceramic sheet which
extends beyond a given cooking unit area, but which incorporates a low
reluctance conductor passing through the sheet to serve as a link to
magnetically couple the utensil to the flux shaping means below the
shee~.
Additionally, while the layer 27 in FIG. 2 as shown is higher
than the top surface of the plate 21, it is equally possible to provide
a recess in the glass surface in the area which carries this layer.
With this arrangement, the layer 27 would be flush or level with the
top surface of the cooking plate 21.
Other modifications of this lnvention will occur to those
skilled in the art; therefore, it is to be understood that this inven-
tion is not limited to the particular embodiments disclosed but that it
is intended to cover all modifications which are within the spirit and
scope of the invention as expressed in the accompanying claims.
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