Note: Descriptions are shown in the official language in which they were submitted.
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DESCRIPTION C200/M
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Title: Microwave ovens and rnethods of cookinq food
Field of invention
This invention relates to microwave ovens and to methods of
cooking food.
Back round to the invention
Microwave ovens are capable of cooking, defrosting and
reheating food very rapidly but because the heat energy is
generated within the food itself by agitation of the food
molecules, the exterior of the food does not reach the
temperature required for browning. This disadvantage is met
in known microwave ovens by providing some form of thermal
heating means in addition to the microwave generator. For
example, British Specifica~ions Nos. 1172918 and 1332122
disclose ovens which have thermal hea~ing elements in
addition to microwave generators.
Known microwave ovens adopt different ways of using the two
forms of energy 9 i.e. the microwave energy and thermal
energy. One known oven produces thermal power only during a
first cooking period followed by microwave power only during
a second cooking period, the changeover from thermal power
to mlcrowave power occurring automatically. ~uring the
first period, the thermal power tends to crisp and brown
food items without cooking them in the middle. During the
second period when microwave energy only is supplied to the
oven, steam is generated as a result of the middle of the
food becoming heated, and this moisture must find its way
out of the already crisp and brown outside of the food. The
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result is that the moisture tends to break down the outside
of the ~ood layer which absorbs the moisture and becomes
soft and unappetising. To disperse the moisture, the oven
is vented during the second period and the temperature drops
within the cavity causing deterioration in certain food
products r particularly pastry items.
A second known microwave oven operates in the reverse
sequence, by ~aving a first cooking period during which
microwave energy only is produced followed by a second
period during which thermal energy only is produced. During
the first period the oven cavity is vented by a forced flow
of air to dissipate steam. On commencement of the second
period, venting ceases and the thermal power is
lS applied with the object of browning the food item.
Roth of these known microwave ovens fail to deal with the
problem of dissipating the moisture as it is produced during
the period of production of microwave energy. Excess
moisture causes condensa~ion on cool oven suraces as well
as makiny the food soft and pappy. Too much venting to
dissipate moisture reduces the temperature of the oven
whilst the application of too much thermal energy tends to
burn the outside of the food before the food i5 cooked in
the middle.
The invention aims to provide a microwave oven and a method
of cooking food in which the thermal heat input into the
oven cavity is sufficient to cope with the moisture driven
off the food as the latter lS cooked by microwave energy.
To achieve this, it has been Eound that continuous microwave
power should be applied simultaneously with continuous
thermal power.
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Mîcrowave ovens are known in which microwave power is
applied simultaneously with thermal power. In general,
however, such ovens have been large commercial or industrial
ovens with high power ratings. This has meant that the
power needed for the microwave generator and for the thermal
heating elements for any cooking operation has hardly been
limited by a power input level to the oven. By contrast,
the invention is concerned with portable, plug-in appliances
which are designed for domes~ic use and which can be plugged
into a domestic socket having a specified power limit which
differs from coun~ry to country. The present invention is
concerned with providing an oven designed to operate with a
maximum power input of 1800 wa~ts (to suit the USA), and an
oven designed to operate with a maximum power input of 3000
watts (to suit the UK).
It is Xnown to provide microwave ovens with a fan which
ca~ses a flow of hot air (heated by thermal heating means)
to pass over the food being cooked. Successful cooking of
food in such microwave ovens depends on the correct choice
of a number of factors, one of which is the pa~tern of air
flow within the oven cavity. One commercially available
oven draws heated aîr into the cavity from the mid region of
the back wall of the cavity, the air being drawn forwardly
before splitting up into two streams which are turned to
flow backwards along the sides of the oven cavity, before
leaving the cavity at two locations in the back wall of the
cavity. It has been found that this air flow pattern does
not always satisEactorily dissipate moisture generated Erom
the food as it is being cooked by the microwave power. This
is a particular problem with pastry items which become soft
and pappy if the moisture resulting from the cooking is not
dissipated by the hot air stream.
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Summary of the invent.~on
According to one aspect of the invention a portable
microwave oven designed to be powered from a domestic power
socket with a rating not exceeding 1800 watts comprises a
microwave generator for supplying microwave power to a
cavity of the oven, a rotatable turntable on the base of the
cavity, thermal heating means capable of heating the air
wi~hin the oven cavity, and control means operative to
supply continuous microwave power to the cavity
simultaneously with the supply of thermal power to the
cavity, whereby the microwave power heats the inside of the
food and the thermal power dissipates the resulting moisture
and browns the external surface of the food, the thermal
heating means co~prising an electrical heating element and
fan means for recirculating air over the element and through
the cavity, the fan means causing a forced flow of air,
heated by the thermal heating means, to enter the cavity
from one side thereof, to pass over the turntable and leave
the cavity from the other side thereof.
Preferably, the oven cavity has a rear wall behind which is
a compartment accommodating the electrical heating element,
the rear wall having on said one side an aperture allowing
hot air to be forced by the fan to enter the cavity over a
2S substantial height thereof, and the rear wall having
adjacent the bottom corner at the other side a fan
constituting said fan means.
Preferably, the power rating of the oven does not exceed
1620 watts, which allows a margin for US power limits.
According to another aspect of the invention a portable
microwave oven designed to be powered ~rom a domestic power
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socket with d ratlng not excee~l~ng 3()00 watts compr~ses a
microwave gerlerator ~or supplying microwave power to a
cavity of the oven, a rotatable turntable at the base o~ the
cavity, thermal heating means capable of heating the air
~ithin the oven cavity, and control means operative to
supply continuous microwave power to the oven cavity,
simultaneously with the supply of thermal power to the
cavity, whereby the microwave power heats the inside of the
food and the thermal power dissipates the resulting moisture
and browns the external sur~ace oE the food, the thermal
heating means comprising an electrical heating element and
fan means for recirculating air over the element and through
the cavity, the fan means causing a forced flow of air,
heated by the thermal heating means, to enter the cavity
from one side thereof, to pass over the turntable and leave
the cavity from the other side thereof.
Preferably the power rating of the oven of said another
aspect does not exceed 2650 or 2700 watts, which allows a
margin for UK power limits.
According to a fur~her aspect of the invention a method of
cooking food in a portable microwave oven designed to be
powered from a domestic power socket with a rating either
not exceeding 1800 watts, or not exceeding 3000 watts,
comprises supplying continuous microwave power to a cavity
oE the oven accommodating the food and simultaneously
supplying thermal power to the cavity, whereby the microwave
power heats the inside of the food and the thermal power
dissipates the resulting moisture and browns the external
sur~ace o~ the food, the thermal power being applied by a
recirculating flow of air which is forced by a fan to pass
over an electrical heating element, to enter the oven cavity
~rom one side thereo~, to pass over the ~ood whilst the
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latter is rotate~ on a turntable and to leave the oven
cavity from the other side thereof.
With the lower power oven limited to 1620 watts total input
power for the US market, it was ~ound that thermal power of
1~50 watts and microwave into the cavity 3f 200 watts
produced good results. Since a typical source of microwave
power is about 50% efficient, or slightly better, a power
input into the cavity of 200 watts requires a power input to
the microwave source o~ about 360 watts. Preferably, the
oven has a single convection or thermal element producing
the thermal power, this being sufficient not to require pre-
heating of the oven. The microwave source (i.e. the
magnetron) may be selected to operate at a higher power to
deliver more po~er into the cavity, an interlock ensuring
that the magnetron is on low power settiny (i~e. 200 watts
into the cavity) whenever the thermal element is energised.
With the higher power oven limited to 2650 watts for the UK
market, more power is available and there are less
constraints. There may be low, medium and high power levels
for the magnetron, e.g. corresponding to 200, 300 and 600
watts into the cavity, and the single thermal heating
element may have a power rating of about 1450 watts. For
each of the lower and higher ovens, cooking preferably
commences ~rom a cold oven.
Two microwave ovens according to the invention will now be
described, by way of example, with reference to the
accompanyiny drawings, în which:-
30Figure 1 is a perspective view o~ the first oven,
Figure 2 is an electrical circuit diagram of the circuitry
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of the first oven,
Figure 3 is an electri.cal circuit diagram of the circuitry
of the second oven, and
S
Figure 4 shows an alternative control panel for the oven of
Figures 2 and 3, and
Figure 5 is a diagrammatic plan view of either embodiment of
oven, showing the flow pattern of hot air within the oven.
Figures 1 and 2 show an oven designed for use in the USA
where domestic appliances should not have a rating more that
about 1650 watts~ Referring to Figure 1 the microwave oven
has a casing 1 fitted with a hinged door 2 enclosing a
cavity 3. The base is fitted with a turntable 5 for
rotating the food during cooking. The sides of the cavity 3
have shelf guides 6 and the rear wall of the cavity 3 is
provided with a fan 7 for circulating air over a convection
element 8 (Figure 2) and through an apertured plate 9.
A control panel 10 has control knobs and dials for the
cooker.
Figure 2 shows the components of the electrical circuit of
the oven. Power Erom the mains is led through a 15 amp fuse
12, a magnetron cut-out 13, and a mains on/of~ switch 14.
When the switch 14 is closed, power at 115v at 60Hz is
supplied to a cavity lamp 15 and a blower motor 16 for the
cooker magnetron 17. A convection timer switch has ganged
contacts 18, 18a. The contact 18 is connected to a motor 19
for the fan 7, a cavity thermostat 20 in series with the
convection element 8 of about 1250 watts, a first relay coil
22, and a second relay coil 33.
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A door switch 24, a microwave timer swi~ch 25 and a cook
start switch 26 are connecte~ in series in the live power
supply line 27 which leads power to the magnetron 17 for
supplying microwave power to the cavity. The contacts 18a
and a convection timer motor 36 are connec~ed across the
live and neutral lines 27 and 32. A convection 'on' lamp 30
is connected in parallel acro~s the timer motor 36. A
microwave timer motor 37 and a microwave lonl lamp 38 are
connected across the llve and neutral lines 27 and 32 in the
position shown. A turntable motor 39 for the turntable 5 is
connected across a central contact of relay contacts 34 and
the neutral ]ine 32. The relay coil 33 controls normally
closed relay contacts 40 connected, together with a
microwave power change swi~ch 42 and a relay coil 35, across
the live and neutral lines 27 and 32. A transformer 44
takes power into the magnetron 17. A switch 45 isolates one
of two parallel-connected capacitors 46 so that the
magnetron delivers low power. When the relay switch 45 is
closed, both ca~acitors 46 are in circuit and the magnetron
17 delivers high power. On low power the magnetron delivers
about 200 watts into the cavity 3, and on high power the
magnetron delivers about 650 watts in~o the cavity 3. Since
the magnetron is about 50~ efficient, the power taken by the
magnetron is about twice that delivered to the cavity, in
2~5 each of the low and high power settings. For an output
power into the cavity of 200 watts, the magnetron requires
an input of about 360 watts.
Revertillg to Figure 1, the control panel 10 has the
Eollowing controls- the switch 42 for selecting high or low
microwave power; a rotary timer 50 which controls the motor
37 and which therefore sets the time up to a maximum of 60
; minutes for which the magnetron is operated; the microwave
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'on' lamp 38; the convection 'on' lamp 30; the mains switch
14; a rotary knob 52 which alters the settîng of the
thermostat 20 between 50 and 250C; a ~urther rotary timer
53 which controls the motor 36 and therefore sets the time
up to a maximu~ o~ 120 minutes for which the thermal power
is produced; and a push-button 54 controlling the switch 26
The oven is used from a cold start in a 'mixed facility'
mode, i.e. with thermal and microwave power, as follows:
a) the microwave timer 50 is set for the desired cooking
time;
b) the knob 52 is rotated to set the temperature of the
thermostat 20;
e) the eonvection timer 53 is set for the desired cooking
time;
d) the door 2 is closed and the eook button 54 is depressed
to close the switch 26.
The turntable S will revolve and both timers will run
simultaneously, causing microwave energy and thermal energy
~5 to be produced simultaneously.
So long at the relay coil 33 is energised (which oceurs so
long as the contact 18 is closed) the contaets 40 are open,
which has the ef~eet of ensuring that the magnetron produces
low power, regardless of the position of the switeh 42.
When the eonvection timer 53 reaches zero, the switeh
contacts 18, 18a open, whieh de-energises the eonveetion
heating element 8 and marks the end of the cooking period.
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Also, the coils 22 and 33 are de-energised. De-energisation
of the coil 33 causes the contacts 34 to change over. This
ensures that the turntable motor can be energised either
through the switches 25 and 26 or through the contact 18a.
The lamp 38 will glow (preferably green) as long as
microwave power is being produced, and the lamp 30 will glow
(preferably orange) as long as thermal power is being
produced. When the microwave timer 50 reaches zero, the
switch 25 opens, thereby isolating the magnetron 17. When
the oven door 2 is opened, switch 24 opens, thereby cutting
off the supply of power to the magnetron. The usual short
switch 47 is connected across the lines 27 and 32 to short
out the rnagnetron 17.
The oven of Figures 1 and 2'may be operated in a microwave
only mode or in a convection only mode, but best results
have been achieved in the mixed facility mode described.
The relay coil 22 also operates a shutter (or 1ap) 84 for
venting the oven cavity 3 under certain conditionsO The
shutter 84 is positioned near the top front corner of the
right-hand side wall of the cavity 3, as shown in Figure 5.
When convection heat is being supplied to the cavity 3, the
relay coil 22 is energised and this closes the shutter 34 to
prevent the flow of air delivered by the blower motor 16
from reaching the cavity 3. When convection heat is not
being supplied to the cavity 3, the coil 22 is de-energisedt
thereby opening the shutter 84 and allowing venting of the
cavity. During venting, air delivered by the motor 16 is
~orced into the cavity and thence to atmosphere to remove
moisture ~rom the cavity.
The oven in Figure 3 has been designed to suit the UK
market, ~here power up to almos~ 3 KW may be drawn ~rom a
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domestic plug/socket.
In Figure 3~ parts corresponding to those of Figure 2 have
been given the same reference numerals. The microwave
switch 42 has (in addition to an "off" position) low, medium
and high power settings, bringing in one, two or three
capacitors 46 respectively, and in any of the three settings
the switch 42 closes to conduct power to the transformer
44.
In series with the fuse 12 ~rated at 13A) is an oven
thermostat 60 which cuts out if the cavity temperature rises
too high. A magnetron thermostat 62 performs the same
function for the magnetron 17. The two switches 63 and 64
are linked to one another, and the switch 64 is linked to a
timer motor 65 in the followîng manner. The switch 63 is an
on/off switch corresponding to the switch 14 of Figure 2,
and the switch 64 opens when the timer motor 65 reaches
zero, the two switches 63 and 64 being controlled by a
single rotary timer knob on the control panel lO.
A convection switch 66 enables the element 8 to be
energised independently o~ the automatic timing facility
~ a~forded by the timer motor 65. Thus, in a manual mode,
closure of the switch 66 will energise the element 8,
causing thermal power to be produced without time limit and
without microwave power. For an automatic or timed mode,
the cook start switch 26 is closed, and the timer motor set
to run from a preset time. When this time has elapsed the
switch 64 will open, causing de-energisation of the element
8,
When the microwave power is required, the switch 42 is set
to the required microwave power level, the timer switch
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controlling the motor 65 is set to the desired time, and the
switch 26 is closed. When the timer motor 65 reaches zero,
the switch 64 will open, causing de-energisation of the
magnetron 17.
The switch 66 is linked to a thermostat switch 67 in the
oven cavity, and the monitor switch 47 is mechanically
linked to a turntable "ofE" switch 68.
As for the first embodiment, the oven is normally operated
with thermal and microwave power being produced
simultaneously, the end of the cooking period being marked
by opening of the switch 64 which ceases the production of
thermal and microwave power.
The relay coil 33 controls the contacts 34, as in the
previous embodiment, and the coil 33 is also used to control
the shutter 84. When the coil 33 is de-energised, air from
the blower 16 is directed into the cavity, but when the coil
33 is energised the shutter 84 closes an air entrance port
to the cavity, directing the air to atmosphere.
The invention enables a microwave oven to operate very
~ effectively on a maximum power intake suitable for US or UK
requirements. The simultaneous delivery of thermal and
microwave power enables the moisture to be dissipated by the
thermal power as the moisture is produced from the inside of
the food being cooked by microwave power. This has been
Eound to be particularly advantageous for pastry items,
~hich are particu1arly sensitive to any excess of moisture
during cooking.
Instead of the descri~ed control panel 10, the oven oE
Figure 2 or 3 may be equipped with the panel 70 shown in
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Figure 4. The panel 70 has touch-sensltive pads, and a
display 72. The pads comprise a COMBINATION pad 73, a
MICROWAVE pad 74, a CONVECTION pad 75, two TIME pads 76, a
TEMP pad 77, a RESET/OFF pad 78 and a START pad 79. The
panel 70 is operated as follows in three possible modes:
l) Combination cooking mode (ie microwave and thermal
power)
0 Touch COMBINATION pad 73 - once for low microwave power
twice or medium low micro-
wave power (Figure 3 only)
three times for high micro-
wave power ~Figure 3 only);
Touch TEMP pad 77 until desired temperature is reached, as
indicated by figures 80 illuminated in display 72;
Touch TIME pad(s) 76 until desired time is reached as
indicated by figures 82 in display 72,
Touch START pad 79.
During cooking, the display 72 shows the cooking time
counting down, the temperature selected and the COMB mode.
: For the UK model o~ Figure 31 it will also show which level
of combination.
25 ~
2) Microwave only cookin~ mode
Touch MICROWAVE pad 74 - once for high power
twice for medium low
three times for low power;
Touch TIME pad~s) 76 until desired time is reached;
Touch START pad 79.
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During cooking, the d1splay 72 shows the cooking time
counting down, MW mocle and microwave power levelO
3) Convection only cookin~ mode
Touch CONVECTION pad 75;
Touch TEMP pad 77 until desired temperature is reached;
Touch TIME pad(s) 76 until desired time is reached;
~ouch START pad 79.
If preheat is required, ~he oven is set up as described and
heated ~or a period of time or until the temperature is
reached.
To indicate that the temperature is reached, the temperature
indicator on the display could flash;
During cooking, the display 72 shows the cooking time
counting down, the temperature selected and CO~V mode.
At the end of the cooking time, a continuous bleep will
sound and the heat will be automatically switched off.
To switch off bleep sound, touch pad 78 and time of day will
be displayed. The pad 78 may alternatively be a depressible
switch.
The pad 70 may be linked to a microprocessor providing the
necessary control functions to the thermal heating elements,
~0 the microwave generator and other operative components o~
the oven.
The apertured plate 9 is positioned at the right-hand side
~g~p~
of the rear wall 3a and is vertically elongated so as to
allow hot air to enter the cavity 3 over a substantial
height thereof. The fan 7 is positioned at the leEt-hand
side of the rear wall 3a, near the lower corner, and is
electrically driven to force a ~low of hot air to pass over
the Eood whilst the latter is supported on the rotary
turntable 5.
Figure 5 shows the air flow pattern within the oven. Hot
air passing through the apertured plate 9 enters the cavity
3 (arrows 86), passes over the turntable 5 (arrows 88) and
is drawn through the fan 7 (arrows 90) into a a compartment
92 at the rear of the cavity 3. Within the compartment 92 is
arranged the thermal heating element 8 (shown
diagrammatically in Figure 5) which heats the air before the
latter enters or re-enters the cavity 3O The turntable 5
rotates counter-clockwise as shown in Figure S to maximise
the relative velocity between the hot air entering the
cavity through the aperture plate 9 and the food rotating on
the turntable 5.
~he described air flow pattern is efficient in heating the
ood, but leaves the walls of the oven cavity cool in
comparison with known air flow patterns where hot air is
blown alongside the cavity walls.
Above the Ean 7, the rear wall 3a has vent holes (not shown)
Eor venting oE moisture to the oven surroundings.
31) It will be seen from the circuit diagra~s of Figures 2 and 3
that iE the door is opened during cooking, resulting in
opening of the switch 24, the magnetron is de-energised but
power continues to the heating element 3.
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