Microwave Oven Provided With Turntable

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 are 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 turntable on the rotary disc while rotating, firmly support 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 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.

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 at 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 the turntable on which a foodstuff to be cooked is mounted, the arms and said peripheral edge defining openings in order for the microwave energy reflecting from the walls of the heating chamber to pass therethrough, a protruded member made of microwave reflecting material and arranged at the central portion of the rotary support member, and an electric motor for rotating the rotary support member.

In the accompanying drawings:

FIG. 1 is a schematic elevational sectional view showing a conventional 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; and

FIG. 6 is a diagrammatic sectional view showing a heated condition of a sponge cake having a central height h.

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 1a 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 16 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 to form projections on the flat rotary disc 10 and grooves engaging with the projections on the rear surface of the turntable 12, but this construction 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, 1977 (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 14. The dish-shaped support member 14 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 rotary 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 16 fixed to the bottom wall 4b. A box defined by the bottom wall 4b of the heating chamber 2 and the bracket 13 function as a (.lambda./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 14a is formed at the central portion of the rotary support member 14. In shape the protruded member 14a 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 14a is illustrated in FIG. 3 as one typical example. The protruded member 14a fixed to the support member 14 is made of microwave reflecting material such as metal. Therefore, the support member 14 and the protruded member 14a 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 14b and a plurality of arms 14c which define openings 15 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 food-stuff 11 is heated.

A plurality of projections 12a and 12b 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 600 W 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.

Type C was further experimented with changing diameter D.sub.0 of the protruded cylindrical member 14a and the result is shown in the following Table 2.

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 (.lambda./2) and (.lambda./4) in diameter.

In our further experiment, a dish shaped supporting member 14 provided with a protruded member 14a 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 14a and an obtained sponge cake showed that it was heated uniformly. In this case, the better result was obtained from constructing the frustoconical member 14a having a top diameter D.sub.1 of about (.lambda./4) and a bottom diameter D.sub.2 of about (.lambda./2) while slightly rounding the upper edge portion of the frustoconical member 14a 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 14a, shown in FIG. 4b, having a diameter D.sub.2 of about (.lambda./2) and a height H of about (.lambda./8) 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 (.lambda./8).

Although in FIG. 3, the top surface of the cylindrical protruded member 14a 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 the top surface of the cylindrical member 14a from the turntable in case that heat is excessively transmitted to the member 14a 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.