Abstrict
A microwave oven (1) comprises an oven cavity (3) containing a
rotatable turntable (4) for supporting one or more items to be heated.
A radiant electric heater (5) is arranged for location above the
turntable (4) and comprises at least one first heating element (6)
and at least one second heating element (7). The at least one first
heating element (6) overlies only a proportion of the total upper
surface area of the turntable (4) such that, in operation, for each
complete rotation of the turntable substantially all parts of the
upper surface of the turntable are heated for substantially the
same duration of time, the at least one first heating element (6)
providing a first power density. The at least one second heating
element (7), or a combination with the at least first heating element
(6), overlies substantially the total upper surface area of the
turntable (4), with the at least one second heating element operated
alone, or an operating combination of the at least one second heating
element (7) with the at least one first heating element (6), providing
a second power density lower than the first power density.
Claims
What is claimed is:
1. A microwave oven comprising an oven cavity having therein a
rotatable turntable, for supporting one or more items to be heated,
and a radiant electric heater located above the turntable and comprising
at least one first heating element and at least one second heating
element, the at least one first heating element being adapted and
arranged to overlie only a portion of the total upper surface area
of the turntable to the extent that in operation alone, for each
complete rotation of the turntable substantially all parts of the
upper surface of the turntable are heated for substantially the
same duration of time, the at least one first heating element providing
a first power density, and the at least one second heating element
being adapted and arranged such that it, or a combination thereof
with the at least one first heating element, overlies substantially
the total upper surface area of the turntable, and such that the
at least one second heating element operated alone, or an operating
combination of the at least one second heating element with the
at least one first heating element, provides a second power density
lower than the first power density.
2. A microwave oven according to claim 1, wherein the at least
one first heating element is located within the confines of the
at least one second heating element.
3. A microwave oven according to claim 1, wherein the at least
one first heating element is provided to occupy a region selected
from a substantially rectangular, a part-annular, and an arcuate,
region of the heater.
4. A microwave oven according to claim 3, wherein the region of
the heater subtends an angle of about 180 degrees.
5. A microwave oven according to claim 1, wherein a single first
heating element and a single second heating element are provided.
6. A microwave oven according to claim 1, wherein a single first
heating element and two further heating elements are provided, one
of the further heating elements being arranged substantially complementary
to the first heating element, a plurality of second heating elements
being provided in operation comprising the other of the further
heating elements connected in series with a parallel combination
of the first heating element and the one of the further heating
elements.
7. A microwave oven according to claim 1, wherein at least one
first heating element and first and second further heating elements
are provided, a plurality of second heating elements being provided
in operation comprising the first of the further heating elements
connected in parallel with a series combination of the at least
one first heating element and the second of the further heating
elements.
8. A microwave oven according to claim 1, wherein the at least
one first heating element is located in the heater such as to be
as remote as possible from a door of the microwave oven.
9. A microwave oven according to claim 1, wherein the first power
density is such as to provide at least one of griling and browning
of at least one food item located in the oven cavity.
10. A microwave oven according to claim 1, wherein the second power
density is such as to provide baking of at least one food item located
in the oven cavity.
11. A microwave oven according to claim 1, wherein the at least
one first heating element and the at least one second heating element
are selected from coiled wire, metal ribbon, metal foil, and lamp,
forms, and combinations thereof.
12. A microwave oven according to claim 1, wherein the at least
one first heating element and the at least one second heating element
are supported relative to a base of thermal insulation material.
13. A microwave oven according to claim 12, wherein the thermal
insulation material comprises microporous thermal insulation material.
14. A microwave oven according to claim 1, wherein a dish-like
supporting means is provided for the heater.
Description BACKGROUND TO THE INVENTION
It is known to provide microwave ovens with heating devices additional
to the microwave heating means. In particular, radiant electric
heating devices are known to be located at the top of the oven cavity
to provide high power density for grilling or browning food items
or moderate power density for baking food items.
At present, products are available which can achieve either baking
or browning cooking effects but cannot provide both from the same
heater. Either the baking element would operate at too low a power
density to grill or the grilling element would be too hot to bake
without burning. A grilling heater cannot be arranged to cover the
entire area of an underlying turntable in the oven as it would require
far too high power and smaller grilling heaters can result in uneven
heating of the underlying turntable.
It is also necessary to locate a heater used for high power grilling
as far away from the oven door as possible, to prevent damage to
the door and to reduce the temperature experienced by a user of
the oven.
OBJECT OF THE INVENTION
It is an object of the present invention to overcome or minimise
these problems.
SUMMARY OF THE INVENTION
According to the present invention there is provided a radiant
electric heater for a microwave oven, the oven comprising an oven
cavity having therein a rotatable turntable for supporting one or
more items to be heated, the radiant electric heater being arranged
for location above the turntable and comprising at least one first
heating element and at least one second heating element, the at
least one first heating element being adapted and arranged to overlie
only a proportion of the total upper surface area of the turntable
to the extent that in operation alone, for each complete rotation
of the turntable substantially all parts of the upper surface of
the turntable are heated for substantially the same duration of
time, the at least one first heating element providing a first power
density, and the at least one second heating element being adapted
and arranged such that it, or a combination thereof with the at
least first heating element, overlies substantially the total upper
surface area of the turntable, and such that the at least one second
heating element operated alone, or an operating combination of the
at least one second heating element with the at least one first
heating element, provides a second power density lower than the
first power density.
The at least one first heating element may be located within the
confines of the at least one second heating element.
The at least one first heating element may be provided to occupy
a substantially rectangular, or part-annular, or arcuate, region
of the heater. Such part-annular or arcuate region of the heater
may subtend an angle of about 180 degrees.
A single first heating element and a single second heating element
may be provided.
Alternatively a single first heating element and two further heating
elements may be provided, one of the further heating elements being
arranged substantially complementary to the first heating element,
a plurality of second heating elements being provided in operation
comprising the other of the further heating elements connected in
series with a parallel combination of the first heating element
and the one of the further heating elements.
As a further alternative, at least one first heating element and
first and second further heating elements may be provided, a plurality
of second heating elements being provided in operation comprising
the first of the further heating elements connected in parallel
with a series combination of the at least one first heating element
and the second of the further heating elements.
The at least one first heating element may be located in the heater
such as to be as remote as possible from a door of the microwave
oven.
The first power density may be such as to provide grilling and/or
browning of one or more food items located in the oven cavity.
The second power density may be such as to provide baking of one
or more food items located in the oven cavity. The at least one
first heating element and the at least one second heating element
may comprise any of the well known forms such as coiled wire, metal
ribbon, metal foil or lamp forms, or combinations thereof.
The at least one first heating element and the at least one second
heating element may be supported relative to a base of thermal insulation
material, such as microporous thermal insulation material.
A dish-like supporting means may be provided for the heater.
By means of the invention a single radiant heater is provided for
installation in a microwave oven and which will achieve quick, efficient
and even grilling and browning, when the oven turntable is used.
The same heater provides a baking facility of lower power density
and which provides an even baking effect over the entire oven cavity
at a reasonable baking position. Advantages are low cost with high
functionality and efficiency since the best cooking element for
the purpose is always available.
For a better understanding of the present invention and to show
more clearly how it may be carried into effect reference will now
be made, by way of example, to the accompanying drawings in which:
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic top plan view representation of a microwave
oven incorporating a radiant electric heater according to the present
invention;
FIG. 2 is a schematic top plan view representation of the microwave
oven of FIG. 1, illustrating the heating zones provided by a radiant
electric heater according to the present invention;
FIG. 3 is a plan view of the radiant electric heater of the invention
as incorporated in the microwave oven of FIGS. 1 and 2;
FIG. 4 is a plan view of an alternative arrangement of radiant
electric heater according to the present invention;
FIGS. 5A and 5B are circuit diagrams showing the electrical connections
of the heater of FIG. 4;
FIG. 6 is a plan view of a further arrangement of radiant electric
heater according to the present invention; and
FIGS. 7A and 7B are circuit diagrams showing the electrical connections
of the heater of FIG. 6.
DESCRIPTION OF PREFERRED EMBODIMENTS
Referring to FIG. 1, a microwave oven 1 has a door 2 and is provided
with an oven cavity 3. The basic construction of the oven is of
well known form and features such as a magnetron for the source
of microwave radiation are not shown since they are not essential
to an understanding of the invention.
A well-known form of turntable 4 is provided in the oven cavity
and arranged for rotation by well known means (not shown). The turntable
4 is intended for supporting one or more food items, or containers
thereof, to be heated.
A radiant electric heater 5, as shown in detail in FIG. 3, is located
at the top of the oven cavity overlying the turntable 4. The heater
5 comprises a first radiant heating element 6 and a second radiant
heating element 7 supported relative to a base 8 of microporous
thermal insulation material in a dish-like supporting means 9. The
heating elements are of well-known form, such as coiled wire, metal
ribbon, metal foil, or lamp forms or combinations thereof.
A terminal block arrangement 10 at the edge of the heater provides
for connection of the heating elements 6 and 7 to a power supply
(not shown) for independent energisation thereof.
The first heating element 6 is arranged to occupy a substantially
part-annular or arcuate region of the heater with such substantially
part-annular or arcuate region suitably subtending an angle of about
180 degrees. The first heating element 6 is located within the confines
of the second heating element 7, the second heating element 7 being
relatively large with quite widely spaced element turns.
Alternatively, the first heating element 6 may be arranged to occupy
a substantially rectangular region of the heater.
The first heating element 6 is arranged to provide a relatively
high power density, for example about 5 watts per square centimetre,
suitable for browning or grilling a food item on the underlying
turntable 4. The first heating element 6 is arranged in the heater
so as to be as remote as possible from the door 2 of the oven when
the heater is installed in the oven.
The first heating element 6 overlies only a proportion of total
upper surface area of the turntable so that it provides a heated
zone as denoted by reference numeral 11 in FIG. 2. When the heating
element 6 is in operation alone, it is arranged that for each complete
rotation of the turntable 4 substantially all parts of the upper
surface of the turntable are heated for substantially the same duration
of time. Thus uniform grilling or browning of food items on the
turntable is achieved.
The second heating element 7 is arranged to overlie at least substantially
the total upper surface area of the turntable 4 and if required
substantially the entire base of the oven cavity. It is arranged
for operation alone (that is without operation of the first heating
element 6) and provides a lower power density than the first heating
element 6. Such lower power density may be of the order of 2 watts
per square centimetre and provides an even baking effect for food
items located on the turntable 4 or on a shelf inserted into the
oven cavity. The heated zone resulting from the second heating element
is typically as denoted by reference numeral 12 in FIG. 2.
Instead of a heater as shown in FIG. 3, an alternative form of
heater as shown in FIG. 4 may be provided.
In FIG. 4, a first heating element 13 is provided which is adapted
and arranged to operate in exactly the same way as the first heating
element 6 of FIG. 3 and is energised alone as indicated in FIG.
5A to fulfil the same browning or grilling function as the heating
element 6 of FIG. 3.
Two further heating elements 14 and 15 are provided. One of the
further elements 14 is of substantially complementary form to the
first element 13 and of substantially the same electrical resistance.
The other of the further elements 15 is provided surrounding the
elements 13 and 14.
In effect a plurality of second heating elements is provided, connected
for operation as shown in FIG. 5B, and comprising the other of the
further heating elements 15 connected in series with a parallel
combination of the first heating element 13 and the one of the further
heating elements 14. This effective plurality of second heating
elements is arranged to provide a lower power density (for example
of the order of 2 watts per square centimeter) than the first heating
element 13 operating alone (for example of the order of 5 watts
per square centimeter).
The heater of FIG. 4 achieves effectively similar browning/grilling
performance from the first heating element 13 operated alone as
is achieved by the heating element 6 of FIG. 3, and very uniform
baking performance from the operating combination of the heating
elements 13, 14 and 15.
A further form of heater may be provided as shown in FIG. 6.
In FIG. 6 a first heating element 16 is provided of substantially
rectangular shape and which is adapted and arranged to operate in
the same way as the part-annular or arcuate first heating element
6 of FIG. 3 and is energised alone as indicated in FIG. 7A to fulfil
the same browning or grilling function as the heating element 6
of FIG. 3.
First and second further heating elements 17 and 18 are provided.
In effect a plurality of second heating elements are provided,
connected for operation as shown in FIG. 7B and comprising the first
further heating element 17 connected in parallel with a series combination
of the first heating element 16 and the second further heating element
18. This effective plurality of second heating elements is arranged
to provide a lower power density (for example of the order of 2
watts per square centimetre) than the first heating element 16 operating
alone (for example of the order of 5 watts per square centimetre)
and provides good baking performance.
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