AU684024B2 - A device for homogenizing the distribution of radiatons within the cooking space of a microwave oven - Google Patents

Description

WO 95131085 PCT/iT95/00068 -1- "A DEVICE FOR HOMOGENIZING THE DISTRIBUTION OF RADIATIONS WITHIN THE COOKING SPACE OF A MICROWAVE OVEN" The present invention relates to microwave ovens, and in particular to a device for homogenizing the distribution of the microwaves within the cooking space.

It is known that a microwave oven includes a microwave-emitting device (magnetron) which emits radiations at a specific frequency, e.g.

2450 MHz according to the rules in force in Italy. The magnetron antenna emitting said radiations may be placed directly inside the cooking space of the oven, but it is more common that it emits the radiations within a waveguide which conveys them up to the apertures for the inlet into the cooking space.

The propagation of the radiations along the waveguide takes place with the waves in phase (monomode) for conveying efficiency reasons, but a homogeneous distribution of the radiated energy is necessary in the cooking space to achieve a uniform heating of the food. In order to obtain this, it is necessary to dephase and "mix" the waves through multiple reflections, so that they reach the food from all directions in almost equal amounts. To this purpose, a relative motion is generated between the food and the waves impinging thereon. This motion may be obtained by means of a rotating dish on which the food is placed, or by means of a metallic "fan" (wave scrambler) which spreads the microwaves upon entrance in the cooking space, or finally by having the magnetron antenna rotate in case it is directly inserted inside the cooking space.

Whichever solution or combination of solutions is adopted, it implies the presence of a rotation mechanism essential for the proper operation of the oven. It is clear that said mechanism, besides involving a certain cost, is potentially subject to failure. Moreover, such a failure is not promptly detected by the user in the case of the wave scrambler and of the rotating antenna, which are usually not visible like the rotating dish. In this case, the oven operation becomes unsatisfactory and the magnetron may be damaged, for example by radiations impinging on the antenna which are reflected back by a blade of the wave scrambler stopped in front of the microwave inlet aperture.

Therefore, the object of the present invention is to provide a device I II WO 95/31085 PCT/IT95/00068 -2for homogenizing the distribution of the radiations within the cooking space, which device overcomes the above-mentioned drawbacks.

This object is achieved by means of a device having the characteristics cited in claim 1.

The first apparent advantage of the device according to the invention is to eliminate a mechanism potentially subject to failure by replacing it with a fixed structure which thus provides an absolute reliability.

A second advantage stemming from the first one is the longer life of the magnetron always operating in the same conditions, whereby it does not risk damages in case of failure of the wave scrambler.

A further advantage of the present device is the reduction in the oven cost thanks to the elimination of the rotation mechanism, whichever may be.

These and other advantages and characteristics of the device according to the present invention will be apparent to those skilled in the art from the following detailed description of a preferred embodiment thereof referring to the annexed drawings wherein: Fia. is a schematic, partially see-through, rear view of a microwave oven provided with a device according to the invention; Fi.2 is a side view of the upper part of the device from the side of entrance of the microwaves; Fig.3 is a bottom plan view of the upper part of the device; Fio. is a sectional side view of the upper part of the device along line A-A of fig.2; Fia.j is a top plan view of the lower part of the device; and Fig.6 is a sectional side view of the lower part of the device along line B-B of With reference to fig.1, there is seen that a device according to the present invention is positioned on top of the oven at the center of the cooking space roof. This device includes a substantially parallelepipedal upper part called "mixing prechamber" 1, and a substantially flat lower part consisting in a fixing flange 2. In the illustrated example the magnetron 3 is directly mounted on prechamber 1 with the antenna 4 inserted therein, but it is clear that the following description is suitable also for the case in which magnetron 3 is spaced from prechamber I and connected thereto through a conventional waveguide.

-i WO 95/31085 PCT/IT95/00068 -3- Referring now to figs.2, 3 and 4, the shape of prechamber I is illustrated in detail. In fig.2 there is seen that it has a circular opening 6 on one face 5 for inserting the antenna 4 of magnetron 3, as well as two ears 7 for securing the latter. Opening 6 is centered on the mid-line with a diameter approximately equal to half of the width of face 5, its top being aligned with the lower edge of a 450 bevelling 8 located at the inner top of the opposite face 9 (fig.4). The plan view of fig.3 and the dotted lines of fig.2 show that said bevelling 8 is also present on the other two faces aside opening 6, but it is absent above the latter at the inner top of face The plan view also clearly shows a lower peripheral lip 10 provided with a series of bores 11 for the coupling with the underlying fixing flange 2.

The inner cross-section of prechamber I is rectangular with the short sides, on faces 5 and 9, which have a difference from the long sides not greater than 10% of their length. The long sides also have a slight convergence towards face 9, with a conicity of about 20. The inner height of prechamber 1 is about 1.5 times the short side, and it has a slight downwards slope of about 1 when moving from face 5 to face 9. It may be considered, as a mere example, that the inner size of the illustrated prechamber 1 is 68 x 73 mm, with an height of 96 mm.

Finally referring to figs.5 and 6, there is seen that flange 2 has an inner rectangular section on the upper side having a size some millimeters larger than that of prechamber 1 (in the illustrated example 75 x 80 mm) so as to allow the insertion of the latter. Prechamber I is obviously fixed by means of screws passing through a series of bores 12 corresponding to bores 11 and formed in a peripheral lip 13 similar to lip Flange 2 has a central circular opening 14 for the passage of the microwaves, which has an upper diameter 15 equal to the short side of the inner section of flange 2 (75 mm in the example) and a lower diameter 16 equal to the short side of the inner section of prechamber 1 (68 mm in the example). Therefore, opening 14 for the passage of the microwaves into the cooking space has an area lower than that of the inner section of prechamber 1, namely equal to the area of the circle inscribed in said inner section. The tests carried out by the Applicant have shown that this choking of the passage section contributes to homogenize the distribution of the radiations within the cooking space.

Opening 14 is closed by a quartz glass which separates prechamber WO 95/31085 PCT/IT95/00068 -4- 1 from the cooking space, thus making the cleaning thereof easier, and prevents the cooking vapours from causing encrustations inside prechamber 1.

It is clear that the above-described and illustrated embodiment of the device according to the invention is just an example susceptible of several changes. In particular, the shape and size of prechamber 1 and of the relevant flange 2 may be varied somewhat, as long as the cross-section of prechamber 1 remains generally prismatic with a bevelling 8 at the inner top thereof and opening 14 has an area smaller than the relevant section of prechamber 1.

Claims (7)

1. A device for homogenizing within the cooking space of a microwave oven the distribution of the radiations emitted by the magnetron characterized in that it includes a mixing prechamber having an inner prismatic cross-section provided with an opening for the inlet of the radiations therein, and a fixing flange located under the prechamber and provided with an opening (14) for the inlet of the radiations into the cooking space, said opening (14) having an area smaller th n said inner section of the prechamber

2. A device according to claim 1, characterized in that the prechamber has an inner rectangular cross-section and a 450 bevelling at the inner top of each face but on the face on which the opening is formed, the flange has an inner rectangular section on the upper side, said section having a size some millimeters larger than that of the prechamber so as to allow the insertion of the latter, and the opening (14) has a circular shape.

3. A device according to claim 2, characterized in that a short side of the inner section of the prechamber is along the face on which the opening is formed and it has a difference from the long sides not greater than 10% of its length, the long sides being convergent towards the face opposite to said face with a conicity of about 20, and in that the inner height of the prechamber is about 1.5 times the short side and it has a downwards slope of about 1 when moving from said opening to the opposite face

4. A device according to claim 2 or 3, characterized in that the opening (14) of the flange has an upper diameter (15) equal to the short side of the inner section of the flange and a lower diameter (16) equal to the short side of the inner section of the prechamber

5. A microwave oven characterized in that it includes a device according to one of the preceding claims.

6. A microwave oven according to claim 5, characterized in that the magnetron is secured on the prechamber and its antenna is inserted in the prechamber through the opening -i i; O iOllKTCl40li3l 0 23,7 -6-

7. A microwave oven substantially as hereinbefore described with reference to the accompanying drawings. r o o os DATED this 25th day of August, 1997 SMEG S.P.A. By its Patent Attorneys DAVIES COLLISON CAVE r e i-2 r" K L. N L.