GB2122060A

GB2122060A - Microwave oven provided with turntable

Info

Publication number: GB2122060A
Application number: GB08314970A
Authority: GB
Inventors: Kazufumi Matsushima; Yoshinari Arabori; Hisao Sato
Original assignee: Tokyo Shibaura Electric Co Ltd
Current assignee: Toshiba Corp
Priority date: 1982-05-31
Filing date: 1983-05-31
Publication date: 1984-01-04
Also published as: GB2122060B; AU540115B2; DE3319170C2; GB8314970D0; NL8301909A; NL186487C; NL186487B; DE3319170A1; AU1508083A

Abstract

A microwave oven provided with a turntable generally comprises a device for generating microwave energy, a waveguide for coupling the microwave energy to a heating chamber defined by metal walls, a conical rotary support member (14) disposed in the heating chamber for supporting the turntable (12) on which a foodstuff to be cooked is mounted, and an electric motor (9) for rotating the rotary support member. The conical rotary support member is provided with a plurality of arms and a peripheral edge (14b) to which the arms are connected and the arm and the peripheral edge define openings through which microwave energy reflected from the walls of the heating chamber. The conical rotary support member is further provided with a protruded member (14a) made of microwave reflecting material and arranged at the central portion of the rotary support member to sufficiently support the turntable and evenly heat the foodstuff on the turntable. <IMAGE>

Description

SPECIFICATION Microwave oven provided with turntable BACKGROUND OF THE INVENTION This invention relates to a high frequency electronic cooking device and more particularly to a microwave oven provided with an improved turntable support member: Generally, in a high frequency electronic cooking device, a foodstuff to be cooked is heated by microwave energy generated from a magnetron and/or radiant heat from electric heaters.

The electronic cooking device of this type generally comprises a box-shaped cabinet, a control panel and an oven door which are arranged on the front side of the cabinet, a heating chamber located inside the oven door and defined by an oven frame, a waveguide provided at its one end with a port opened at the top wall of the heating chamber, a magnetron coupled to the other end of the waveguide, and a flat rotary disc on which a turntable is removably mounted. A pair of electric heaters are horizontally located in the heating chamber to heat the foodstuff mounted on the turntable from the upper and lower sides. An electric motor for driving the rotary disc is disposed below the bottom wall of the heating chamber through a drive shaft which extends into the oven through bearings provided at the central portion of the bottom wall.

In a typical known cooking device, the turntable is supported by the flat rotary disc in frictional engagement or in engagement with projections formed on the rotary disc and grooves formed on the rear surface of the turntable. In addition, in some cases a plurality of rollers as disposed on the bottom wall of the heating chamber so as to rotatably support the rotary disc and prevent undesired wobbling motion thereof. However, various problems have been encountered in-these supporting structures when it is desired to exactly position the turntabie on the rotary disc while rotating, firmly support the same, and maintain the oven in a clean state after the heating operation, which disadvantages will be described in detail hereinafter with reference to accompanying drawings.Moreover, there is such a problem that the foodstuff is not sufficiently and evenly heated for the reason that the electromagnetic waves concentrate at the edge portion of the flat rotary disc.

In order to obviate these disadvantages, there has been proposed an improved rotary support member in which the rotary disc is formed as a dish shaped member provided with through holes.

However, this prior art design can also not positively support the turntable and uniformly heat the foodstuff on the turntable.

SUMMARY OF THE INVENTION An object of this invention is to provide an improved microwave oven capable of positively supporting a turntable on which a foodstuff to be heated is mounted and uniformly heating the foodstuff.

Another object of this invention is to provide a microwave oven having a dish-shaped rotary member provided with a protruded member as its central portion for supporting the turntable and uniformly heating the foodstuff mounted thereon.

According to this invention there is provided a microwave oven provided with a turntable comprising a device for generating microwave energy, a heating chamber defined by metal walls, an element for coupling the microwave energy to the heating chamber, a rotary support member provided in the heating chamber with a plurality of arms and a peripheral edge to which the arms are connected for supporting a turntabie on which a foodstuff to be cooked is mounted, the arms and the peripheral edge defining openings in order for the microwave energy reflecting from the walls of the heating chamber to pass therethrough, and an electric motor for rotating the rotary support member, and the microwave oven is characterized in that the rotary support member is provided with a protruded member made of microwave reflecting material and arranged at the central portion of the rotary support member, the protruded member is axially symmetrical in shape with respect to a rotation axis thereof and a top portion of the protruded member does not project into a plane defined by an upper surface of the peripheral edge of the rotary support member.

BRIEF DESCRIPTION OF THE DRAWINGS In the accompanying drawings: FIG. 1 is a schematic elevational sectional view showing a convention microwave oven with a turntable; FIG. 2 is a sectional view of one example of a conventional turntable supporting member disposed in a microwave oven; FIG. 3 is a sectional view of showing a turntable and a turntable support member according to this invention; FIGS. 4a and 4b are sectional views showing other examples of a rotary support member according to this invention; FIGS 5a and 5b are plan views showing the rotary support members shown in FIGS. 4a and 4b, respectively, in which a lower heater is disposed; FIG. 6 is a diagrammatic sectional view showing a heated condition of a sponge cake having a central height h;; FIG. 7 shows one example of a rotary support member and a driving mechanism for rotating the rotary support member according to this invention; and FIG. 8 shows a modification of the bottom wall shown in FIG. 7.

DESCRIPTION OF THE PREFERRED EMBODIMENTS To aid the full understanding of the present invention, general aspects of a high frequency electronic cooking device, particularly a microwave oven provided with heaters, and various problems encountered in known devices of this type will first be considered in conjunction with FIGS. 1 and 2.

Referring to FIG. 1, a microwave oven is provided with a housing 1 and a door 1 a inside of which a heating chamber 2 and a machinery space 3 are defined by a metal wall 4 and along a top wall 4a is located a waveguide 5 having one end opened to the oven 2 and the other end operatively connected to a magnetron oscillator 6. An electric motor 9 is disposed below the bottom wall 4b and a drive shaft 9a of the motor 9 extends into the heating chamber 2 through bearings 1 6 provided at the central portion of the bottom wall 4b. The upper portion of the drive shaft 9a is connected to a flat rotary disc 10 and a turntable 12, made of glass, for instance, on which a foodstuff is rested is frictionally mounted on the rotary disc 10 so that when the rotary disc 10 slowly rotates, the turntable 12 is also rotated.A pair of electric heaters 7 and 8 are located respectively at the upper and lower positions in the heating chamber 2 to heat the foodstuff from the upper and lower sides and the lower heater 8 is disposed between the bottom wall 4b and the rotary disc 10.

When the magnetron oscillator 6 is energized microwave energy generated propagates through the waveguide 5 to heat the foodstuff 11 on the turntable 12 in the heating chamber 2, and at the same time, the drive motor 9 is driven to slowly rotate the rotary disc 10 and the turntable 12 so that the foodstuff is evenly heated. However, since the turntable 12 is supported by the rotary disc 10 in only frictional engagement, it is relatively difficult to locate the turntable in a correct position on the rotary disc 10. Accordingly, the turntable 12 might slip or shift horizontally on the rotary disc 10 when it is rotated.

To obviate these defects, it has been proposed that projections are formed on the flat rotary disc 10 and grooves engaging with the projections are formed on the rear surface of the turntable 12, but with this construction, it is insufficient to prevent undesired wobbling motion of the turntable during rotation thereof.

A plurality of (at least three) rollers R, shown by dotted lines in FIG. 1, may be disposed on the bottom wall 4b to rotatably support the turntable 12 for preventing the undesired wobbling motion thereof. However, the location of rollers R on the bottom wall 4b makes it difficult to maintain the surface of the bottom wall 4b in a clean state after the heating operation of the microwave oven because the space between the bottom wall 4b and the rotary disc 10 is considerably narrow.

In order to obviate these disadvantages, there is disclosed in Japanese Utility Model Application (application No. 56665/47) published June 22, 1 977 (publication No. 27388/52) a conical dish shaped rotary support member for supporting the turntable 12 as shown in FIG. 2 by a reference letter 20. The dish-shaped support member 20 positively supports the turntable 12 at its peripheral edge portion.

However, with the dish-shaped rotary member 14, the heating effect at the central portion is left out of consideration and the foodstuff 11 on the turntable is not evenly heated because the microwave energy generated from the magnetron oscillator generally concentrates at the peripheral edge of the support member 14. Thus, the central portion of the foodstuff is not sufficiently heated and it remains with a certain height h uncooked as shown in FIG. 6 after a predetermined heating period of time.

Referring to FIG. 3, in which like reference numerals are applied to elements corresponding to those shown in FIG. 1 or 2, an electric motor 9 is disposed below a metal bracket 13 attached to the bottom wall 4b. A drive shaft 9a coupled to the motor 9 extends into the heating chamber 2 through bearings 1 6 fixed to the bottom wall 4b. A box defined by the bottom wall 4b of the heating chamber 2 and the bracket 1 3 function as a A/2 electromagnetic choke which prevents the microwave energy from leaking outside of the heating chamber 2. The top portion of the drive shaft 9a is connected to a conical dish shaped support member 14 on which a turntable 12 is supported and a lower heater 8 is horizontally disposed between the bottom wall 4b and the rotary support member 14.A protruded member 1 4a is formed at the central portion of the rotary support member 14. In shape the protruded member 1 4a is axially symmetrical with respect to the axis of the drive shaft 9a and, in height, is substantially equal to or slightly smaller than the height H of the rotary support member 14.

A cylindrical member 1 4a is illustrated in FIG. 3 as one typical example. The protruded member 1 4a fixed to the support member 14 is made of microwave reflecting material such as metal. Therefore, the support member 1 4 and the protruded member 1 4a may be integrally fabricated by treating a steel plate, for instance.

As shown in FIGS. 5a and 5b, the rotary support member 14 is provided with a peripheral edge 1 4b and a plurality of arms 1 4c which define openings 1 5 through which the microwave energy reflected from the walls 4, especially the bottom wall 4b, and radiant heat and hot air generated by the lower heater 8 (which is diagrammatically shown in FIG. 5a or 5b) passes when the foodstuff 11 is heated.

A plurality of projections 1 2a and 1 2b are formed on the rear surface of the turntable 12 to prevent it from accidentally shifting or slipping outwardly.

In our experimental evaluation for various types of rotary support members, a foodstuff on the turntable was heated under the same conditions of the microwave oven except for configuration of the rotary support members, and the results obtained are shown in the following Table 1.

The raw material was prepared to obtain a sponge cake by mixing a cake mix powder (375 g), water (200 cc), and one egg (50 g), and 300 g of the raw material thus prepared was heated by microwave output power 600W (frequency 2450 MHz) and electric heaters of resultant rated power consumptions 700 W (upper heater) and 500 W (lower heater) on a turntable having a diameter of 210 mm. After heating for a predetermined period of time the height h (FIG. 6) at the central portion of the material which is considered to be relatively insufficiently heated was measured to examine to what extent the material was evenly heated.

TABLE 1

Type A Type B Type C heating time (seconds) 330 330 330 Height h (mm) 24 20 18 Type A: A flat rotary supporting disc 10 as shown in FIG. 1 was used.

Type B: A dish shaped supporting member 20 as shown in FIG. 2 was used.

Type C: A dish shaped supporting member 14 provided with a cylindrical protruded member 1 4a as shown in FIG. 3 was used and the protruded member 1 4a was about -R in diameter (R is the wavelength of microwave energy).

Type C was further experimented with changing diameter Do of the protruded cylindrical member 1 4a and the result is shown in the following Table 2.

TABLE 2

- larger than A A 3 A A Diameter 8 4 8 2 2 Heating time (seconds) 330 300 270 300 330 height h (mm) 18 15 10 16 20-24 As can be understood from the Tables 1 and 2, a sponge cake heated effectively and uniformly is obtained in a shorter heating period of time when a dish shaped supporting member 14 is provided with a protruded cylindrical member 14a ranging between about andin diameter.

In ourfurther experiment, a dish shaped supporting member 14 provided with a protruded member 1 4a having a frustoconical configuration as shown in FIG. 4a, was used to effectively utilize the microwave energy reflected by an inclined surface of the frustoconical member 1 4a and an obtained sponge cake showed that it was heated uniformly.In this case, the better result was obtained from constructing the frustoconical member 1 4a having a top diameter D, of about A4 and a bottom diameter D2 of about while slightly rounding the upper edge portion of the frustoconical member 1 4a so as to prevent excessive concentration of the electric field at the upper edge portion, thereby avoiding excessive heating of the edge portion of the sponge cake.As one of the various alternations, a roundly protruded member 1 4a, shown in FIG. 4b, having a diameter D2 of about A and a height H of about A8 was formed on the rotary support member 14 and showed substantially the same result as in the case of the configuration illustrated in FIG. 4a. These experiments indicate that it is preferable to set the height H of the protruded member 14a to be about+ Although in FIG. 3, the top surface of the cylindrical protruded member 1 4a shown in FIG. 3 is in direct contact with the lower surface of the turntable 12 to more firmly support the same, it should be better to slightly separate to top surface of the cylindrical member 1 4a from the turntable in case that heat is excessively transmitted to the member 1 4a from the turntable 12 due to the strong concentration of the electric field.

The protruded member may be constructed to be removable from the dish shaped support member.

The number of arms of the support member 14 can be selected to a suitable number preferably 4 to 6 as shown in FIGS. 5a and 5b by taking into consideration the rigidity and the wavelength of the microwave.

FIG. 7 shows one example of the rotary support member and a driving mechanism for rotating the rotary support member according to this invention, in which a choke case 1 7 for preventing the wave energy from leaking outside of the heating chamber 2 is secured to the bottom wall 4b by screw means 1 8 and an attaching member 1 9 is secured to the choke case 17 by screw means 21 so as to define a gear box 20 between the choke case 17 and the attaching member 19. An electric motor 22 is located below the gear box 20 through bearings 23 and a drive shaft 22a of the motor 22 extends into the gear box 20.A main gear 24 made of a synthetic resin, for instance, Teflon (trade name), is mounted to the drive shaft 22a so as not to generate spark due to the wave energy and the main gear 24 is meshed with a follow-up gear 26 integrally formed with a rotary shaft 25 which is rotatably supported by the bearings 23 and bearings 27 disposed below the choke case 1 7.

The rotary shaft 25 extends into the heating chamber 2 through the bottom wall 4b and the upper portion of the shaft 25 is detachably fitted into a boss 29 of the rotary support member 28. A groove 30 is formed on the outer surface of the boss 29 to be engaged with pins attached to the rotary shaft 25.

The dish shaped rotary support member 28 is provided with protruded member 28a having frustoconical shape in this example at the central portion above the boss. In this example, there is shown a turntable 33, on which foodstuff is mounted, having a bottom plate provided with the rear surface slightly recessed at its central portion and the top portion of the frustoconical member 28a slightly projects over a plane defined by an upper surface of the peripheral edge 28b of the rotary support member 28a into the recess of the bottom plate of the turntable 33 so as to form a small gap 34 therebetween to avoid directly transferring heat from the turntable to the protruded member 28a when the turntable 33 is set on the rotary support member 28.

The lower heater 8 is disposed between the bottom wall 4b and the rotary support member 28 and positional relationship therebetween is shown in FIG. 5a or 5b. In this connection, in FIG. 8 grooves 32 are formed on the bottom wall 4b and the grooves 32 have shapes corresponding to the lower heater 8 so as to reduce thermal affection of radiant heat of the heater 8 as well as to increase rigidity of the bottom wall 4b.

It is to be understood by those skilled in the art that the foregoing description refers to some preferred embodiments of this invention and that various modification and changes may be made without departing from the true scope and spirit of the invention as defined in the appended claims.

Claims

1. A microwave oven provided with a turntable comprising a device for generating microwave energy, a heating chamber defined by metal walls, an element for coupling the microwave energy to the heating chamber, a rotary support member provided in the heating chamber with a plurality of arms and a peripheral edge to which the arms are connected for supporting a turntable on which a foodstuff to be cooked is mounted, said arms and said peripheral edge defining openings in order for the microwave energy reflecting from the walls of the heating chamber to pass therethrough, and an electric motor for rotating said rotary support member, characterized in that said rotary support member is provided with a protruded member made of microwave reflecting material and arranged at the central portion of said rotary support member, said protruded member being axially symmetrical in shape with respect to a rotation axis thereof and that a top portion of protruded member does not project into a plane defined by an upper surface of the peripheral edge of said rotary support member.

2. The microwave oven according to claim 1 wherein said top portion of said protruded member and the turntable make a gap to avoid directly transferring heat from the turntable to said protruded member when the turntable is set on said rotary support member.

3. The microwave oven according to claim 1 or 2 wherein said protruded member is a cylindrical member.

4. The microwave oven according to claim 3 wherein said cylindrical member has a diameter within a range between about A andaF where A is a wavelength of a microwave energy.

5. The microwave oven according to claim 1 or 2 wherein said protruded member is a frustoconical member.

6. The microwave oven according to claim 5 wherein said frustoconical member has an upper diameter of about and a lower diameter of about 12 wherein A is a wavelength of a microwave energy.

7. The microwave oven according to claim 1 or 2 wherein said protruded member is a round member.

8. The microwave oven according to claim 7 wherein said round member has a bottom diameter of about where A is a wavelength of a microwave energy.