GB2098838A - Microwave oven - Google Patents

Abstract

A microwave oven has a mode stirrer 1 including blades 2 mounted to rotate about axis A-A in an oven cavity. The mode stirrer 1 is supported on a plastics shaft 3 which extends through opposing walls 8, 9 of a waveguide 4. An electrically conductive rod 10 in electrically conductive contact with the blades 2 extends along the axis A-A into the waveguide to improve temperature uniformity. <IMAGE>

Description

SPECIFICATION Microwave oven This invention relates to a microwave oven and is more particularly concerned with improving the uniformity of temperature within the oven to enable food to be cooked more uniformly.

In microwave ovens electric field patterns, called modes, are set up in an oven cavity where food is cooked. The modes represent the variations of electric field intensity with position in the cavity. These variations in intensity tend to give rise to different heating effects on food placed at different locations in the cavity, or to non-uniformity of heating of different parts of a food item. Therefore, some microwave ovens have a mode stirrer to change the modes with time, to try and produce a heating effect on a food item which is more uniform independently of the position of the food item within the cavity. A mode stirrer usually comprises a set of blades (like a fan for example) which are rotated to "stir" the modes in the oven cavity. Even with a mode stirrer, however, the heating effect is not wholly uniform over the oven cavity.

The problem of not being able to cook certain food in a microwave oven to a degree of uniformity which is acceptable is well recognised and attempts to alleviate this problem, for example, by varying the size of the mode stirrer, have been made with varying degrees of success. One proposal shown in U.K. Patent Specification No. 1328548 which attempts to solve the aforementioned problem provides two waveguides at right angles to one another, one of the waveguides being constituted by a specially shaped conductor, connected to the other waveguide, and by an elongated conductor attached to the mode stirrer and which also extends into the second waveguide. Providing extensions on the specially shaped conductor or altering its shape changes the uniformity of temperature in the oven cavity, and the elongate member co-operates with the specially shaped conductor.Another proposal involves rotating the food on a turntable within the oven cavity.

It is an object of the present invention to alleviate the aforementioned problem.

According to the present invention, there is provided a microwave oven comprising a microwave generating means and an oven cavity, a waveguide coupling the microwave generating means to the cavity so that microwaves can pass from the microwave generating means through the wave guide and into the cavity, at least one mode stirrer adapted to rotate on an axis to stir microwaves within the oven cavity, and an electrically conductive member connected to said at least one mode stirrer and which extends into the waveguide, the arrangement being such that the electrically conductive member is of such material and dimensions and so positioned relative to the mode stirrer, waveguide and oven cavity, that, in use of the microwave oven, the uniformity of temperature within the oven cavity determined by the nine beaker test as herein defined gives a percentage standard deviation lying within the range 18 to 13%.

The nine beaker test is a standard test used to measure the uniformity of temperature within a microwave cavity, and is herein defined as being the standard deviation expressed as a percentage of the mean temperature rise of water in nine 100 ml beakers, each beaker containing 100 ml of water and being heated simultaneously from ambient temperature (i.e. 20 C) in a microwave oven for a time period of 50 secs. This test will be referred to later.

In a particular embodiment of a microwave oven the electrically conductive member may be elongated and in electrically conductive connection with the mode stirrer but electrically isolated from the waveguide. Preferably, the elongated member comprises a shaft rotatable on the same axis as the mode stirrer.

Conveniently the electrically conductive member may be connected to the mode stirrer by a screw thread on the member; the member may be screwed to a hub supporting the mode stirrer.

It has been found that where the electrically conductive member is elongated as aforesaid good results occur when the member extends substantially all the way across the waveguide. In this case the waveguide has two opposing walls defining a width of the waveguide and the member extends into the waveguide through one of the opposing walls but finishes short of the other of the opposing walls.

The elongated member may be sheathed for substantially all of that part of its length which extends into the waveguide with electrically insulating material, or for substantially its entire length.

Good results have been found in a particular oven having a waveguide of width 3.1 cm, an elongated member of length 5 cm with the member finishing short of said other of the opposing walls by 0.2 cm.

An embodiment of a microwave oven in accordance with the present invention will now be described, by way of example only, with reference to the accompanying diagrammatic drawings in which: Figure 1 is a end-elevational view, partly in section and not to scale, of a mode stirrer and waveguide of the embodiment; Figure 2 shows a perspective view of the waveguide and mode stirrer, and Figs. 3a and 3b show a comparison between a food item cooked in a known microwave oven and an identical food item cooked in the embodiment.

Figs. 1 and 2 show a mode stirrer 1 comprising, in this example three flat aluminium blades 2 equi-angularly spaced around an axis of rotation A-A and mounted obliquely to it.

The mode stirrer 1 is supported by an enlarged disc shaped end or hub 4 of a shaft 3 which extends from the stirrer 1, through the waveguide 4, to a pulley 5. The shaft is journalled for rotation in plain bearings 6 and 7 where it passes through opposing top and bottom walls 8, 9 of the waveguide 4. In this example the disc 4, shaft 3, and bearings 6 and 7 are of plastics material. In practice the pulley 5 is connected by a belt to a motor for rotating the mode stirrer at 1 r.p.s. for example.

In order to prevent leakage of microwave radiation through the top wall 8 of the waveguide 4 where the shaft 3 passes through that wall 8 in the bearing 7, a known leakage prevention device 9 is provided.

Although not shown, apertures are provided in the bottom wall 9 of the waveguide 4 adjacent the mode stirrer 1 to couple the waveguide to the oven cavity (not shown, but in which the mode stirrer is located).

An elongated electrically conductive member in the form of a metal rod 10 is in electrically conductive contact with the mode stirrer 1 and extends along the axis A-A within the shaft 3 and into the waveguide, being electrically isolated from the waveguide by the shaft 3.

As shown in this particular embodiment, the rod 10 is inserted into a central bore in the shaft 3, and is held in place by a screw threaded portion 11 having a screw-head 14 contacting metal supports 1 3 integral with the blades 1. The rod extends into the waveguide with its end 1 2 close to, but spaced from, the upper wall 8.

Only one waveguide 4 is provided and is coupled to a magnetron (not shown) which may be operated at 2.45GHz + 50MHz. The waveguide 4 has a width 3.1 cm (as measured between the top and bottom walls 8 and 9), the rod is 5 cm long (from the screw head to the end 12) and 0.3 cm in diameter in the waveguide 4, and the end 1 2 spaced from the top wall by 0.2 cm. The rod 10 is electrically connected to the blades 2 of the mode stirrer which is arranged to be rotated at 1 r.p.s.

Experiments were conducted on a microwave oven of a known kind and on the described embodiment of a microwave oven in accordance with the present invention. The known oven was not provided with an electrically conductive member extending from the mode stirrer into the waveguide, but was in other respects identical. Both the known oven and the embodiment were provided with a part spherical, stub-like, electrically conductive projection (not shown), projecting into the waveguide frorn the top wall of the waveguide. Such a projection is to tune the magnetron to the microwave oven cavity. Each projection was of radius 2 cm and located at a distance of 20 cm from the antenna of the magnetron.

Nine 100 ml beakers were placed in the known oven each containing 100 ml of water and heated for 50 secs. The temperature rise of each beaker was plotted on a graphy about the mean temperature rise, the deviations of the temperature of each beaker from the mean were measured in each case and the standard deviation calculated from the formula

S represents the standard deviation and d-dg represent the deviations of temperature from the mean temperature rise in each case. S was then represented as a percentage of the mean temperature rise (i.e.

S x 100 T where T = mean temperature rise). The percentage standard deviation in the known microwave oven was not lower than 18%, while the percentage standard deviation given by the described embodiment was typically in the range 1 3 to 15%. It is envisaged that it should be possible with refinements to achieve a percentage standard deviation of 10%. The lower the percentage standard deviation the greater the uniformity of temperature within the oven cavity.

Further experiments were conducted on different kinds of foods where the percentage standard deviation was calculated in a similar manner to that described above. The results are tabulated below.

%S of %S of embodiment known oven Flat sponge cake 7.8% 17% Poached egg 5.0% 31% Egg custard 29% 52% As can be seen from the table there is a significant improvement in cooking in each case, and this improvement can mean the difference between a food item being cooked satisfactorily (i.e. having large satisfactorily cooked areas with insubstantial areas which are undercooked, overcooked or both) or unsatisfactorily (i.e. having substantial areas which are undercooked, overcooked or both).

Fig. 3a shows a flat sponge cake cooked in the known oven for a time period of eight minutes forty-eight seconds, and Fig. 3b shows an identical sponge cake cooked in the described embodiment at an identical location in the oven cavity for the same time period.

Blank areas indicate satisfactorily cooked areas; hatched areas indicate undercooked areas and pebbled-dash areas indicate overcooked areas. As seen an overall satisfactory result is achieved with the described embodiment in contrast to the unsatisfactory result achieved with the known oven.

Various modifications may be made to the embodiment described. For example, some heating of the plastics shaft 3 surrounding the rod 10 takes place and in order to reduce the heating, a suitably dimensioned metal rod insulated from the waveguide by a plastics bearing, but not sheathed in a plastics shaft, could be used.

In a further modification the rod 3, may have a diameter and length in association with the bore diameter that bearing 6 fits into the waveguide face 9, such that it forms a coaxial output connector. The dimensions so chosen would relate to the waveguide dimensions for good impedance matching and thereby elimate local heating of rod, plastic bearings, and such.

Furthermore, instead of using aluminium blades, plastics blades coated with aluminium or other conductive material could be used.

Claims (11)

1. A microwave oven comprising a microwave generating means and an oven cavity, a waveguide coupling the microwave generating means to the cavity so that microwaves can pass from the microwave generating means through the wave guide and into the cavity, at least one mode stirrer adapted to rotate on an axis to stir microwaves within the oven cavity, and an electrically conductive member connected to said at least one mode stirrer and which extends into the waveguide, the arrangement being such that the electrically conductive member is of such material and dimensions and so positioned relative to the mode stirrer, waveguide and oven cavity, that, in use of the microwave oven, the uniformity of temperature within the oven cavity determined by the nine beaker test as herein defined gives a percentage standard deviation of less than 18%.

2. An oven as claimed in Claim 2 in which the electrically conductive member is elongated and in electrically conductive connection with the mode stirrer but electrically isolated from the waveguide.

3. An oven as claimed in Claim 2 in which the elongated member comprises a shaft rotatable on the same axis as the mode stirrer.

4. An oven as claimed in any one of the preceding claims in which the electrically conductive member is connected to the mode stirrer by a screw thread on the member.

5. An oven as claimed in any one of Claims 2 to 4 in which waveguide has two opposing walls defining a width of the waveguide and the elongated member extends into the waveguide through one of opposing walls, and substantially all the way across the waveguide but finishes short of the other of the opposing walls.

6. An oven as claimed in Claim 5 in which the length of the elongated member is 5 cm and its width 0.3 cm, the width of the waveguide being 3.1 cm and the member finishing short of said other of the opposing walls by 0.2 cm.

7. An oven as claimed in any one of claims 2 to 6 in which the elongated member is sheathed for substantially all of that part of its length projecting in the waveguide in a sheath of electrically insulating material.

8. An oven as claimed in any one of the preceding claims in which only one waveguide is provided in association with said at least one mode stirrer.

9. An oven as claimed in any one of the preceding claims in which a part-spherical, electrically conductive projection extends into the waveguide from a wall thereof.

10. An oven as claimed in any one of the preceding claims in which the mode stirrer comprises 3 blades equi-angularly spaced about said axis of rotation.

11. An oven as claimed in Claim 10 in which the mode stirrer comprises plastics blades coated with electrically conductive material such as aluminium.

1 2. A microwave oven substantially as herein described with reference to the accompanying drawings.