Note: Descriptions are shown in the official language in which they were submitted.
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The present invention relates to a high frequency
heating apparatus and more particularly to a microwave oven
or electronic oven. In one example, the oven is equipped
with a rotary table or turntable for receiving an object to
be heated.
A microwave oven which is now widely used essentially
includes an oven-defining structure preferably of double wall
construction provided therein with a heating cavity or
heating chamber having a hingedly supported door which is adapted
to selectively open and close an access opening formed at one
side of the oven defining structure. A magnetron assembly
is provided for generating high fre~uency energy to heat an
object ox food material placed in the heating cavity based `~ -
on the principle of dielectric heating. Some microwave ovens
are further provided with rotary tables or turntables within
the heating chambers for rotation together with the objects
or food material mounted thereon, so that the degree of heating
of such objects is more uniform.
Conventionally, in a microwave oven equipped with
a rotary table of the above described type, the rotational
force of the motor employed as the driving source is mechanically
transmitted via reduction gears to a rotary shaft which extends
into the heating chamber through its bottom plate and is
rotatably journalled to be fitted, for example, into a corres-
; ponding opening formed in the rotary table for engagement
therebetween. A plurality of rollers are mounted between the ;~
lower surface of the rotary table and the bottom plate of the
heating chamber for smooth rotation of the rotary table.
The conventional microwave ovens of the above ~
30 described type, however, have various diciadvantages in that; -
(i) Since the rota~y shaft extends into the -
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heating chamber through the bottom pla-te thereof, excessive
heat generation, electric wave leakage and the like tend to
take place during operation of the microwave oven, at the
portion oE the rotary shaft pro~ecting into the heating
chamber. Prevention of such inconveniences causes the construc-
tion of the microwave oven to become undesirably complicated,
with consequent high cost.
(ii) The projection of the rotary shaft into the
heating cavity and provision of the rollers undetachably
disposed within the heating chamber for smooth rotation of
the rotary table make it difficult to clean the interior of
the heating cavity in an efficient manner.
(iii) Oil drops, juice and the like produced by
the food material during cooking in the heating chamber,
or water used for cleaning the interior of the heating chamber,
tend to leak out of the heating chamber through a clearance
between the shaft and the bottom plate of the heating
chamber to find their way toward the motor, reduction gears
and the like, ~hus not only giving rise to various troubles,
~0 but emitting odor in the course of time.
(iv) Since the rotary table is mechanically
; coupled to the shaft, it is difficult to readily remove
the rotary table when necessary.
(v) Although non-magnetic metallic material is
most suitable for the walls of the heating chamber from
various aspects, such non-magnetic metallic material is
expensive, resulting in a high cost for the oven. On the other ~ -
hand, if a magnetic metallic material which is less expensive
is employed, corrosion of such material comes into question,
and
(vi) When a vessel or container of glass, ceramic
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material and the like is mounted on the rotary table, with the
food material placed therein, such a vessel of hard material
tends to damage the walls of the heating chamber upon coming
into contact therewith during rotation of the rotary table
or insertion and withdrawal of the vessel into and from the
heating chamber.
Accordingly, an essential object of the present
i invention is to provide a microwave oven equipped with a
rotary table in which a magnetic driving system is employed
to eliminate the rotary shaft extending into the heating
chamber for efficient use of the microwave oven, with
interior of the heating cavity coated to impart a clean
appearance and also an economical and durable structure to the
heating chamber.
Another important feature of an embodiment of the
present invention is to provide a microwave oven of the above
described type in which a non-magnetic metallic material of
high magnetic permeability is employed for a particular wall
of the heating chamber to allow magnetic flux to pass there-
20 through for the magnetic driving, while the other walls of -
the same heating chamber are formed with a less expensive
metallic material other than the non-magnetic metallic material,
whereby to reduce the cost for the oven.
A further feature of an embodiment of the present
invention is to provide a microwave oven of the above
described type in which the interior of the heating chamber
is coated to prevent corrosion, especially at the junctions
between the walls of the non-magnetic metallic material and the
the metallic material other than the non-magnetic metallic
30 material, due to battery action or the like. ~ -
A still further feature of an embodiment of the -
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present invention is to provide a microwave oven of the above
described type in which a vessel or container to be mounted
on the rotary table for placing food material to be heated
therein is unlikely to come into contact with or rub against
the coated walls of the heating cavity to avoid damage to the
coating and consequent corrosion.
Another feature of an embodiment of the present
invention is to provide a microwave oven of the above described
type in which an upper surface of the rotary table is arranged
. 10 to be higher than that of the heating chamber bottom wall
or base plate for improved processability during manufacture
and efficient cleaning in use, with simultaneous facilitation
of insertion and withdrawal of the object to be heated into and
from the heating chamber.
Having regard to the foregoing the invention can :
be defined as a microwave oven comprising: an oven .
defining structure; a heating cavîty within said oven ~;
defining structurer said heating cavity being defined by a .
base plate formed oE a non-magnetic metallic material and
by a top wall, a front wall having an access opening
therein, a pair of side walls, and a rear wall, said walls
: being formed of magnetic metallic material, said base
; plate being joined àt junctions to said front wall, said : -:
side walls and said rear wall; door means for selectively
~' opening and closing said access opening in said front
; wall; means for supplying microwave energy into said
heating cavity: rotary table means, selectively and ::
readily removably positioned to rotate in contact with a ~.
top surface of said base plate within said heating cavity,
for receiving an object to be heated, said rotary table
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means being free of any fixed attachment to said base
plate; magnetic means, mounted below said base plate and
exterior of said heating cavity, for rotating said rotary
table means within said heating cavity; and coating means,
covering said junctions and said top surface of said base
plate, except a portion thereof contacted by
said rotary table means, for preventing corrosion of said
heating cavity.
These and other features of the present invention
will become apparent from the following description taken
in conjunction with the preferred embodiment thereof with
reference to the attached drawings in which; -
Fig. 1 is a perspective view of a microwave oven
to which the present invention is applicable, with a door
thereof in an opened state,
Fig. 2 is a similar view to Fig. 1, but an outer
casting and the door thereof are removed for clarity,
Fig~ 3 is a schematic sectional view of the
microwave oven of Fig. 1,
Fig. 4 is an exploded view showing construction
of the heating chamber employed in the microwave oven of
Fig. 1, and
Fig. 5 is a similar view to Fig. 3, but showing a
modification thereof.
Referring now to the drawings, there is shown in
Figs. 1 and 2 a m:icrowave oven M to which the present inven~
tion is applicable. The microwave oven M heat-treats objects -
or food material based on the principle of dielectric heating
by utilizing high frequency energy, for example, of the order
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of about 2450 MHz~ and generally includes an outer casing 1
of cubical box-like shape open at the front side thereof.
The outer casing 1 forms a double wall structure together with
inner walls 2 and defines a heating cavity or heating chamber
H. The inner walls 2 include a horizontal bottom wall or
base plate 2a, vertical side walls 2b, a top wall 2c and a
rear wall 2d and also a front wall 2e which defines an access
opening O at the front of the oven M. In the heating chamber
. H immediately above the horizontal base plate 2a, there is
rotatably disposed a rotary table 8 in a manner described
later, on which table 3, a vessel or dish for accommodating
an object or food material to be heated is detachably mountable.
Outer surfaces of the walls 2a, 2b, 2c and 2d are spaced
from the corresponding walls of the outer casing 1 for providing
spaces therebetween. The outer casing 1 further includes an
outside front wall portion la immediately above the opening
O. On the front wall portion la, there is mounted a control
panel 7 for controlling the functioning of a high frequency
oscillator. The control panel 7 carries a rotatable drum D
having a scale Ds of heating times for a plurality of foods
imprinted on its peripheral surface in positions correspon- .
ding to a timer scale ts, a pointer needle Da for the drum D, .:
a drum rotating ring Db, a timer operating knob t, a function '
indication lamp Q to be illuminated during emission of high
frequency energy and a cooking start button c for initiating
the high frequency heating. For setting the heating time for
: optimum cooking, the drum D is rotated by the ring Db to find
the heating time required for a particular food to be cooked,
to which the pointed needle Da is aligned by the timer operating
knob t.
Further included in the microwave oven M is a ~ ~
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door 4 provided with a handle 6 adjacent one edge thereof
remote from a hinge through which the door 4 is supported at
the lower edge thereof to the lower front edge of the casing
1 in a position corresponding to the access openiny 0 for pivotal
upward and downward movements about the hinge to open and
close the opening O. The door 4 has a rectangular observation
window 5 to allow the object in the heating cavity ~ to be
observed and also for preventing the high frequency energy from
leaking out of the heating chamber H during operation. The
outer casing l further includes an interior portion which
provides sliding accommodation for a pair of door arms 3,
each of which is pivotally connected to a lower side portion
of the door 4 and passes through a portion between the ~ide
walls of the casing 1 and the corresponding side walls 2h of
the heating cavity H.
Referring particularly to Fig. 2, on the top
wall 2c of the heating chamber H in the space defined by said
top wall 2c and the corresponding top wall of the outer
casing 1, there are mounted the high frequency oscillator
or magnetron assembly 10 for radiating high frequency energy
into the heating chamber H, a fan motor 11 for cooling the
magnetron assembly 10, an air guide 12 for leading the
air after cooling the magnetron assembly 10 into the heating
chamber H, a high voltage transformer 13 for supplying high
voltage to the magnetron assembly 10, and worm gears 15 for ~-
transmitting the rotation of the fan motor 11 toward the
rotary table 8 in a manner described in detail later, while
a stirrer fan 14 is disposed to protrude into the upper portion `
of the heating chamber H for stirring the high frequency energy
therein.
Referring also to Fig. 3, the mechanism for driving
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the rotary table 8 in the heating chamber H will be described
hereinbelow.
A first pulley lla secured to a rota$irlg shaft of
the fan motor 11 is connected to a second pulley 15a fixed
to a worm shaft of the worm gears 15 through a belt 16,
while a wheel shaft of the worm gears 15 extends downwardly
to a bearing (not shown) secured to the bottom wall of the
outer casing 1 to form a conducting rod 17, with a third
pulley 18 being secured adjacent to the lower end of the
rod 17. The pulley 18 is connected through a belt 20 to a
pulley 19 which is rotatably disposed at approximately the ~:
same height as the pulley 18 under the lower surface of a
circular recess 9 formed in a generally central portion of
the base plate 2a of t.he heating chamber H for accommodating :
therein the rotary table 8. The pulley 19 is mounted on a shaft
l9a, being secured, at one end thereof, in approximately the :
central portion of the lower surface of the recess 9, for
example, by welding and with the support of a pin l9b. On
the upper surface of the pulley 19 there are disposed a
plurality of magnets 21, and rollers 22, for example, of
synthetic resinous material, which rotatingly contact the
lower surface of the circular recess 9. The rotary table 8
rotatably supported within the recess 9 through rollers 23,
for example, of similar synthetic resinous material, is also
provided with a plurality of magnets 24 fixed at its lower
surface in positions corresponding to the magnets 21 of the
pulley 19. Accordingly, upon rotation of the pulley 19,
the magnets 24 of the rotary table 8 are attracted by the
magnets of the pulley 19, with consequent rotation of the rotary
30 table 8. The rollers 23 for the rotary table 8 are more
than three in number and are hPld in place, for example, by
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a coupling ring 25. The vessel or container V in which the
object or food material F is to be placed is detachably mounted
on the rotary table 8 for insertion in-to or withdrawal
from the heating chamber H as required.
Referring also to Fig. 4, the side walls 2b and
the rear wall 2d are preferably integrally formed into a
single wall of approximately U-shaped cross section. The
top wall 2c, the main portions of the front wall 2e, side
walls 2b, rear wall 2d and top wall 2c are made of a metallic
material other than a non-magnetic metallic material, while
the base plate 2a is constituted by a non-magnetic metallic
material having high permeability to magnetic flux, for example,
aluminum, aluminum alloy, stainless steel of the austenite
group or the like.
In general, although a non-magnetic metallic material
having a large elongation and readily subject to a drawing
process during manufacture while retaining sufficient strength
is suitable for magnetic driving of the rotary table 8 due to
its good permeability to magnetic flux, such material is
very expensive, resulting in an extremely high cost for the
microwave oven, if the heating chamber H is constructed
totally from non-magnetic metallic material. Accordingly, in a
microwave oven M according to the present invention, use of
this expensive material is limited to the portion directly
utilized for the magnetic driving, while other portions of
; the heating chamber H can be made of any other suitable
material, such as steel, iron or plastic. This enables a
marked reduction of manufacturing cost which is achieved with-
out sacrificing the performance of the microwave oven.
Furthermore, after the walls 2a to 2e have been
assembled, for example, by welding to form the heating chamber
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- H, at least junctions iY (Fig. 3) between the walls of the
non-magnetic material and the other material are covered with
a suitable coating, for example, of a synthetic resinous
material for protection against air, water, juice or the like
from the food. The ~unctions are thus free from wet corrosion,
and the heating chamber has high durability. The formation
of the coating at the junctions j is also advantageous to
avoid accumulation of crumbs or chips of food material
thereat for providing a clean heating chamber free from an
unsanitary appearance.! Additionally, a sufficiently durable
coating may also be formed on the portion on the upper surface
of the circular recess 9 contacted by the rollers 23 suppor-
ting the rotary table 8. Such coating should resist soiling
by pigments or the like from crumbs or chips of food
material depressed by the rollers 23 to which all the weight
of the rotary table 8, the vessel V and the food material F
is applied. The coating should also resist being peeled off.
By this arrangement, not only is the coating kept clean for a
long period of time, but the slippage loss in the rotation
of the rollers 23 can be reduced, because the frictional
resistance upon contact between synthetic resinous materials
is generally larger than in the case where metallic material -
contacts synthetic resinous material.
The vessel V for accommodating the food material
F is made of glass or ceramic material so that the food
material F is heated from all sides by the high frequency
` energy, while preventing damage to the vessel V even if the
; food material F is scorched or burnt. Therefore, if a
coating or paint is applied to the surface of the rotary
table 8, such coating may readily be spoiled and give rise
to corrosion, if the vessel V is repeatedly placed on or
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removed from the surface of the rotary table 8. Accordingly,
a corrosion resistant metallic plate, for example, of stainless
steel or synthetic resin such as a polyester, is employed
for the rotary table 8. A microwave oven having a clean
appearance and free from corrosion is thus obtained with
improved durability. Moreover, since the surface of the
rotary table 8 is arranged to be higher than the surface of
the base plate 2a by a distance h (Fig. 3), it is possible to
increase the diameter of the rollers 23 to reduce the frictional
resistance during their rotation, and also to make it easy
for the rollers 23 to override minor obstacles such as crumbs
or chips of food material which may be present. The arrange-
ment described above is also effective for reducing the depth
of the circular recess 9 which tends to be large in conven-
tional microwave ovens wherein the distance h is zero or
negative. This facilitates the drawing process in he manu-
facture of the ovens, with simultaneous elimination of the
development of cracks at the corners during such process, and
an improvement of yield. Also the mounting and detaching of
the rotary table 8 in the heating chamber H are facilitated,
with improved accessibility to the recess 9 for cleaning.
Furthermore, according to the arrangement of the microwave
oven described above, the bottom rim Va of the vessel V is
advantageously prevented from touching the surface of the base
plate 2a during insertion and withdrawal of the vessel V into
and from the heating chamber H, so that even when this
surface of the base plate 2a is coated, such coating is free
from damage whereby to provide the heating chamber with
improved durability. Additionally, when the microwave oven
is provided with the door 4 which can be opened toward the
user as shown in Fig. 3, the height of the inner surface c-c'
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of the door 4 when fully opened is arranged to be approximately
the same as that of the upper surface of the rotary table 8.
Therefore the vessel V can be held horizontal at all times
during insertion or withdrawal of such vessels for preventing
any food materlal from spilling therefrom, thus making
the microwave oven very convenient to use.
Referring to Fig. 5, there is shown a modification
of the microwave oven of Figs. 1 to 4. In this modification,
the insides of the walls 2b, 2c, 2d and 2e of the heating
chamber H are coated with a coating film P, except for the
inner surface of the circular recess 9 of the base plate 2a,
by which arrangement the corrosion resistance of the heating
chamber H is further increased, with improved appearance and
cleanliness. Since the other functions and construction
of the microwave oven of Fig. 5 are similar to that of Figs.
1 to 4, the detailed description thereof is abbreviated
for brevity.
It is to be noted here that, although the present
invention is mainly described with reference to a microwave
oven equipped with the rotary table, the concept of the
present invention is not limited to such microwave ovens
alone~ but may readily be applicable to other heating
apparatus without rotary tables which utilize high frequency
energy for dielectric heating.
Although the present invention has been fully
described by way of example with reference to the attached
drawings, it is to be noted that various changes and
modifications are apparent to those skilled in the art. For
example, the rear wall 2d described as integrally formed -
with the side walls ~b to form the single wall of U-shaped
cross section may be modified to be separately formed for
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rigid connection with said side walls 2b. Therefore, unless
such changes and modifications depart from the scope of the
present invention, they should be construed as lncluded
therein.
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