The present invention relates to a microwave oven and feeding of the same with microwaves.
Microwave ovens for domestic use and the like are usually made with an essentially rectangularly parallelepipedal cavity. This is due to both manufacturing considerations and a conventional view of what microwave ovens should look like. Although it is possible to a certain extent to calculate which so-called cavity modes are obtained in a rectangularly parallelepipedal cavity, it has proved difficult to achieve a reasonably symmetrical heating pattern in such a cavity, in particular when it is fed sideways. So-called field stirrers or rotating bottom plates for the load are often employed with a view to improving the heating pattern.
In similar contexts, such as tunnel furnaces for industrial microwave heating, vertical cross-section geometries (across the direction of transportation through the furnace) other than rectangular ones have been used. In this connection, it has been considered that the action of the walls can be interpreted on the analogy of optical reflection, that is, one conceives of being able to obtain optical-geometrical control of the distribution of the microwaves.
An example of such a configuration of microwave ovens where "microwave reflection" is considered to be employed is disclosed in EP 0 268 379 B1. A microwave oven having sloping corners surfaces between the side walls and the top wall is disclosed by US-A-2814708. DE-A-1515 149 discloses a microwave oven cavity having vertical side walls that do not form corners of 90° between each other.
An object of the present invention is to make possible an improved heating pattern in a rectangularly parallelepipedal cavity in a microwave oven.
Another object of the invention is to make possible such an improved heating pattern without employing moving parts, such as field stirrers or rotating bottom plates.
A further object of the invention is to provide a method which allows improved configuration of a rectangularly parallelepipedal cavity with respect to the resulting heating pattern.
The above-mentione objects are achieved by a microwave oven according to claim 1 and a method for microwave feeding of a rectangular parallelepipedal microwave oven cavity according to claim 8 which exhibit the features stated in the appended claims.
The basis of the invention is thus that, surprisingly, it has been found that the heating pattern in a rectangularly parallelepipedal oven cavity can be advantageously affected by the arrangement of a non-constant horizontal cross-section in the cavity. In this connection, the cross-section changes concerned do not change the general rectangularly parallelepipedal shape and cannot be compared to wall embossing, rounding of corners, etc. which are conventionally done in order to adjust the rectangularly parallelepipedal dimensions for manufacturing reasons and generally to facilitate the manufacture of the cavity and improve mechanical stability.
We have found that the approach to reflection described above lacks relevance in the case of smaller cavities of the kind which exist in microwave ovens for domestic use.
The invention is thus based on the insight that the relative phase and amplitude of the modes in the cavity can be modified favourably in and for heating pattern levelling-out or equalisation by means of well-judged, relatively modest vertical changes to the horizontal cross-section of the cavity. Put another way, by making these kinds of cross-section changes, one can control the mode balance with respect to both relative amplitudes and phase of the modes involved so that a more even heating pattern is achieved.
We have found that if the cross-section modifications according to the invention are kept relatively small and preferably with a slow vertical change, characterisable cavity volume modes are maintained while the heating pattern is changed. The changes can be observed experimentally, which provides an opportunity for adjustments in order to achieve the best heating pattern.
In experimental tests, when the cross-section change is varied in small steps, it has been found to be possible to follow the heating pattern changes continuously, which, as will be appreciated, provides very good optimisation conditions.
According to the invention it is preferable to employ a horizontal cross-section change vertically, which means that the horizontal cross-section decreases upwards at least in the upper part of the cavity. Advantageously, the cross-section change can be achieved by a cavity side wall being made at least partly inward sloping.
Thus, according to a first aspect of the invention, a method is provided for microwave feeding of a rectangularly parallelepipedal microwave oven cavity, which means that in and for equalisation of the effect of the microwaves in the cavity, the mode balance of the microwave field is affected by the cavity being given, in relation to the horizontal cavity bottom cross-section, at least in the upper part of the cavity an upwardly decreasing horizontal cross-section. In this connection, it is preferred to achieve the decreasing horizontal cross-section by sloping inwards at least the upper part of a cavity side wall. The feeding of microwaves is advantageously done through an opening in a cavity side wall opposing the inward sloping wall, the microwave feeding preferably taking place along an area closely adjacent to the cavity ceiling.
Alternatively, it is also possible to feed microwaves through an opening in the cavity ceiling, also in this case suitably extended in a direction parallel to the sloping side wall.
According to a second aspect of the invention, a method is provided for acting on the heating pattern, which is obtained by means of microwaves fed into an rectangularly parallelepipedal microwave oven cavity, changes to the heating pattern being achieved by a cavity side wall being caused to slope inwards at least in its upper part, so that the cavity has an upwardly decreasing horizontal cross-section in relation to the available bottom cross-section of the cavity. In this connection, it is possible on the basis of a chosen cavity width to vary the horizontal cross-section change by changing the degree and/or extent of the inward slope of the cavity wall and observing the resulting changes to the heating pattern. It will be appreciated that by carrying out such changes of the inward slope of the side wall for a number of different cavity widths, it is possible to determine the cavity width and side wall inward slope combination which provides the best heating pattern.
In this connection, it should be mentioned that it has been found that the inventive effect on the heating pattern is particularly pronounced for certain cavity widths.
According to a third aspect of the invention, a microwave oven is provided, comprising a rectangularly parallelepipedal cavity and a microwave source connected thereto for feeding microwaves into the cavity, the cavity, in relation to its available bottom cross-section, having an upwardly decreasing horizontal cross-section at least in the upper part of the cavity. In this connection, the cavity preferably has a side wall, which at least partly slopes inwards in order to provide the desired horizontal cross-section decrease. Advantageously, the inward slope is in the upper part of the side wall, while the lower part of the side wall is vertical and preferably at least 50 mm high. The inward slope of the upper part of the side wall does not need to extend all the way up to the cavity ceiling, rather the side wall can have a short vertical ceiling connection part terminating at the top. Although, preferably, the sloping side wall part is plane, other configurations are also possible. The slope of the side wall is preferably over essentially the whole actual wall width and advantageously over at least about half the wall height.
For manufacturing reasons, the slope of the side wall can be made within a deep-drawn portion of the side wall, so that the joining with the back wall and the front of the oven can be done with vertical joints. In this case, the slope can advantageously be over about 85% of the total depth of the cavity.
A microwave oven according to the invention suitably has a horizontal cross-section which is approximately square. The "depth" is preferably between about 85% and about 120% of the "width". For a microwave oven for conventional domestic use (with a microwave frequency of 2450 MHz) is has been found that the following two dimensionings are particularly advantageous:
In both cases, the height of the cavity from the bottom plate or the cavity bottom to the cavity colling is from about 160 mm to about 230 mm.
The invention affords a number of advantages :
The invention will be described in more detail below by way of embodiments with reference to the accompanying drawings.
The embodiment of a microwave oven according to the invention illustrated in Figs 1 and 2 comprises an outer casing 1 which is outlined only and an oven cavity 2, which is closed by means of a front door 3. The cavity 2 is formed by a horizontal cavity bottom 4, a horizontal cavity ceiling 5, a vertical back cavity wall 6, a front vertical wall on the inside of the front door 3 and two cavity side walls 7 and 8. The right side wall 7 in Figs 1 and 2 is vertical and provided with a microwave unit for feeding microwaves into the oven cavity through a slot-shaped, rectangular side wall opening 10, such as will be described in more detail below. The left side wall 8 in Figs 1 and 2 slopes inward somewhat and thus forms an angle of about 3° to vertical. Consequently, the cavity is rectangularly parallelepipedal except for the inward slope of the side wall 8. However, the inward slope is so small that it cannot be considered that there is any essential deviation from the rectangularly parallelepipedal shape, except for the resulting microwave characteristics.
All the cavity defining wall surfaces in this embodiment are generally plane, although the sloping side wall 8 joins the cavity ceiling at the top with a vertical wall part 11 of a small height.
It will be appreciated that in the example shown the horizontal cross-section of the oven cavity will gradually decrease upwards by the horizontal cross-section width decreasing upwards from the available bottom width B1 to the ceiling width B2. It will be appreciated that the horizontal cross-section in depth is constant.
The microwave unit 9 comprises in a manner known per se a magnetron 13 and a waveguide device 14 connected thereto for feeding microwaves into the cavity through the feeder slot 10. As was mentioned above, this slot is usually elongated and rectangular and arranged at the top and in the midpoint on the side wall 7 in the connection area to the cavity ceiling 5.
As will seen from Fig. 2, the oven can comprise a microwave-transparent bottom plate 16 for a foodstuff load 17. If desired, this plate can be rotatable. However, it has been found that the oven constructed according to the invention despite the sideways feeding of microwaves gives a heating pattern which is so even that a rotating bottom plate cannot be considered necessary.
Examples of typical dimensions for an oven for domestic use according to Figs 1 and 2 are B1 - about 333 mm, B2 - about 325 mm; cavity height (over the bottom plate) - about 185 mm; and cavity depth - about 370 mm.
Fig. 3 shows a schematic vertical cross-section of the same kind as in Fig. 2, which illustrates another embodiment of an oven according to t.he invention. The latter differs from the oven according Lo Figs 1 and 2 with respect to the design of the sloping side wall and the microwave feeding.
In this example, the sloping side wall 8' in Fig. 2 has a lower part 21, which is vertical, and an upper inward-sloping part 22. The inward slope begins about halfway up the side wall and amounts to about 6°. The horizontal cross-section decrease adjacent to the cavity ceiling is here somewhat larger than in the oven according to Figs 1 and 2, typically about 10 mm.
In Fig. 3 the microwave feeding is done through a rectangular ceiling slot 10', which is centrally located in the cavity ceiling 5 and whose long side is parallel to the cavity side walls 7 and 8' and which is fed by the intermediary of a waveguide device 14' arranged on the cavity ceiling.
As the skilled person will appreciate, the invention is not limited to the examples shown, and modifications are possible within the scope of the appended claims. For example, it is possible to divide up the side wall slope between the two opposing side walls.