GB2089630A

GB2089630A - An electric oven

Info

Publication number: GB2089630A
Application number: GB8136403A
Authority: GB
Original assignee: G Bauknecht GmbH
Current assignee: G Bauknecht GmbH
Priority date: 1980-12-13
Filing date: 1981-12-02
Publication date: 1982-06-23
Also published as: GB2089630B; DE3047112C2; IT8125522A0; SE8107228L; SE447599B; IT1140106B; DE3047112A1

Abstract

An electric oven incorporates microwave heating means (12, 13, 14) and secondary electric heating means (18, 19) provided with a reflector (17). The secondary electric heating means (18, 19) and the reflector (17) are formed as an integral unit which can be releasably mounted on a wall of the oven, high frequency screening means (29) being provided to prevent microwave leakage through the wall in the vicinity where the secondary electric heating means and reflector unit is mounted. Securing means is provided to secure the secondary electric heating means and reflector unit on the wall, in a releasable manner. <IMAGE>

Description

SPECIFICATION Electric oven The invention relates to an electric oven. The invention is particularly concerned with electric ovens incorporating microwave heating means together with a complementary electrical heating means.

In orderto accelerate the speed of cooking and also to obtain a surface on the food such as is poss isle with purely conventional heating and preparation, electric ovens are frequentiy employed which, in addition to a microwave heating means, also have further complementary electric heating means, for example an additional electric grill heater. In the case of exclusively microwave ovens it is possible to have components mounted on the walls of the food compartment which are produced, for example, from synthetic plastics material, but in the case of electric ovens with microwave heating means and an additional heating means, this is not possible because, due to the great heat produced by the additional electric heating provision, the components of plastic material might eventually be destroyed.For this reason, the walls of the food compartment of such combination ovens are of metal. If such an electric oven is equipped with a microwave heating means and a grill heater, due to the microwave energy irradiated into the food compartment, electric sparking is liable to occur between the heating coils of the grill heater and the metal constituents of the food compartment.

The employment of metal walls in electric ovens with microwave heating means and a further conventional heater is also necessary in order to prevent the escape of microwave energy from the interior of the food compartment. In a previously proposed electric oven with microwave heating means and a grill heater comprising only heating coils, said heating coils are constructed so asto be exchangeable, i.e. they can be plugged in. In the lateral walls of the grill, electric socket elements are inserted which provide the necessary supply of energy for the heater. These socket elements are also provided with expensive means for screening off and destroying the microwave energy escaping from the food compartment in the vicinity of the plugs.These measures, as disclosed in German OS 10 231, can be extensive and costly.

Due to the high temperature of the grill and to the vigorous sputtering from food during preparation, electric cookers with microwave heating provision and a grill heater rapidly collect accumulations of dirt. These form coatings on the walls of the food compartment which in turn are heated by the microwave energy irradiated into the food compartment, thus producing a highertemperature in the food compartment, whilst at the same time adversely affecting the efficiency of the microwave heating means.In the case of electric cookers with a microwave heating means and an additional grill heater, which are mass produced for the endconsumer, it is therefore desirable for the user of the oven himself, and not only a technician, to be in a position to rectify this deterioration in the efficiency of the microwave heating means and also the increase in temperature in the food compartment.

The microwave energy is preferably irradiated from a top side of the food compartment because, in the case of irradiation from the base, a supporting grid for the food, or food container, might lead to microwave energy losses. In addition, in the immediate vicinity of microwave energy irradiation from beneath, the minima and maxima of the microwave energy waves are very pronounced whereby the food in preparation is not uniformly heated. This is obviated if the microwave energy is irradiated from the top side of the food compartment because then the food to be prepared is further removed from the microwave source and there is better dispersion of the microwaves by multiple reflection of the microwaves from the walls. Consequently, the maxima and minima are no longer so strongly marked and, in addition, losses due to the grid or container supporting the food are now negligible.If, however, the microwave energy source is located on the top side of the food compartment, in the case of a combination oven, the microwave energy must be irradiated through the grill which is also located on the top side of the food compartment. If a fully efficient grill is required which extends practially over the entire top side of the food compartment, and if at the same time a saving of energy is required in the heating of the grill, a reflector must be fitted above the heating coils of the grill. With irradiation from the top side of the food compartment however, this reflector again produces losses of microwave energy.

An object of the present invention is to provide an improved form of electric oven which is simple and inexpensive to manufacture in quantities and which incorporates both microwave heating means as well as additional complementary electric heating means.

According to the present invention there is provided an electric oven comprising microwave heating means and secondary electric heating means for simultaneously irradiating and cooking food in a food compartment formed by metal walls, said secondary electric heating means being adapted to be located adjacent a wall of the compartment and being provided with a reflector having apertures therein characterised in that the secondary electric heating means and its reflector are formed as an integral unit which can be releasably mounted in said compartment by means of a plug and socket connection; high frequency screening means is provided on the reflector in the vicinity of the plug and socket connection, said screening means being adapted to engage positively the metal wall of the food compartment into which the secondary electric heating means is mounted; and means being provided in said high frequency screening means for enabling the secondary electric heating means and reflector unit to be releasably secured within the oven.

The problem underlying the present invention is therefore to provide a simple, inexpensive electric oven suitable for mass production, which has a microwave heating means together with further com plementary electric heating means, e.g., including a grill heater, in which it is possible to heat simultaneously with the microwave heating device and with other different kinds of heating, and which in particu lar prevents the escape of microwave energy from the food compartment.According to the invention it is also possible for the oven user to prevent a loss of efficiency in the microwave heating due to dirt accumulations in the food compartment, whilst at the same time minimising the losses of microwave energy irradiated into the food compartment where the source of microwave energy is situated on the top side of the food compartment and where a reflector for the grill heater extending almost over the entire surface of the top side of the food compartment is located in the immediate vicinity of said top side. Furthermore, heat protection is provided for electric equipment of the oven accommodated above the top side of the food compartment and a saving of energy is made possible when heating with the grill heater.Finally, the possibiiity of electrical sparking of microwave energy occurring at the metal parts within the food compartment is prevented.

The advantages of the invention are to be found especially in the fact that every end-consumer, i.e., oven user, can himself rectify the loss of efficiency of the microwave heating arrangement due to accumulated deposits in the food compartment and especially between the top side of the food compartment and the reflector. All he requires to do is to remove the entire rigid unitary grill heater comprising heating coils and reflector from the food compartment, which is made possible by the special construction of the grill heater with special measures to simplify its removal. As the same time this arrangement ensures an intensive electrical contact between the metal reflector and the top side of the food compartment, or as the case may be, at the other points where it is secured to the food compartment.A high frequency (HF) screening arrangement is in the form of a flat angle piece disposed parallel to the wall, into which the socket connections for the plug pins of the heating coils are inserted, which arrangement provides in a simple manner HF screening of the plug and socket elements for the microwave energy. No expensive measures are required to prevent the escape of microwave energy in the vicinity of the plugs of the heating coils since the HF screening provided is simple of manufacture and is also reliable and easy to secure in position.

The arrangement of a reflector above the heating coils also enables the energy supply to the heating coils to be reduced whilst obtaining the same roasting effect as when grilling without a reflector. The reflector also ensuresthatthere is less heat-radiation towards the top side of the food compartment so that heat protection is provided for electric equipment of the electric oven accommodated above the top side of the food compartment.

A special form of reflector having slits in its upper surface ensures the reduction to a minimum of losses in the irradiation of microwave energy through the grill which extends practically over the entire top side of the food compartment.

An embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings in which: Fig. lisa general view of an electric oven with microwave heating arrangement and further electric heating arrangements, shown in section; Fig. 2 is a detachable grill heater comprising reflector and heating coils; Fig. 3 is a detail view showing high frequency (HF) screening fitted to the reflector of the grill heater; Figs. 4 and 5 are further detail views showing the operation of an "on" switch by a rod provided on the HF screening, and Fig. 6 illustrates a spring clip for securing the reflector to the top side of the food compartment.

Referring to the drawings, Fig. 1 is a general view of an electric oven incorporating a microwave heating unit and a supplementary electric heating assembiy including a grill heater for irradiation and cooking of food in a food compartment 1. The electric oven is provided with a microwave heating unit and supplementary electric heaters such as a grill heater, a hot-air unit and/or any other suitable conventional heating arrangement. The illustration in Fig. 1 shows only part of the microwave heating unit and a grill heater, and also, diagrammatically, a hot-air unit with fan. All other electrical heating arrangements have been omitted for the sake of clarity. The electric oven shown in Fig. 1 comprises a frame 2 which is shown only in section in Fig. 1.The frame 2 also carries a panel 3, shown only diagrammatically, to receive operating controls, indicators and the like which are also not illustrated. Behind the panel 3 is a recess 4 for electrical wiring and other equipment secured to a mounting plate5.Atemper- ature control device and a time switch (also not shown in Fig. 1) are provided for operating the heating arrangements of the electric oven.

The food compartment 1 has four side walls, one of which is in the form of a door 6. Of the otherthree side walls, only the rear wall 7 is shown in section in Fig. 1 and the two other side walls 8 and 9 in Fig. 2.

The food compartment 1 is also bounded by a top wall 10 and a base wall 11. The side walls 7,8 and 9, the top wall 10 and base wall 11 are each of metal since the electric oven is intended for simultaneous heating with a microwave device and with further electrical heaters such as a grill heater. The metal walls resist the high temperatures thus produced.

They also serve as screening for the microwave energy irradiated into the food compartment.

The microwave heating unit comprises a microwave generator 12 with magnetron, an antenna 13 for directing the microwave energy into the food compartment 1, a wave conductor 14 of.hollow con- struction, and an aperture leading to the food compartment 1 in the form of a detachable rectangular window 15. The microwave generator is supplied from a high tension transformer and all other fittings for the microwave heating unit have been omitted from Fig. 1 for the sake of clarity. The detachable rectangular window 15 is closed by a plate 16 having low dielectric losses in order to prevent ingress of foreign matter and excessive heat to the microwave generator.

The detachable rectangular window 15 through which the microwave energy enters the food compartment 1 is located on the top wall 10 of the food compartment centrally between the two lateral walls 8 and 9 and directly above the surface of a reflector 17 of a grill heater. The grill heater comprises the reflector 17 and heating elements mounted beneath the reflector surface. These heating elements may, for example, be in the form of one or more U-shaped heating coils 18, 19. Longitudinal limbs 20 of the U-shaped heating coils 18, 19 are parallel to each other.

The reflector 17 of the grill heater extends practi callyoverthe entire width of the food compartment between the lateral walls 8 and 9 and extends as far as the rear wall 7. Alongside the lateral walls 7,8, 9 of the oven three gaps 21, 22, 23 respectively are provided which, in addition to the main supply of microwave energy through slits in the reflector as will be described hereinafter, contribute to a better and more uniform supply of microwave energy to the food compartment 1. At the same time, the microwave energy is reflected many times between the top wall 10 of the food compartment and the rear face of the reflector 17.It has been found advantageous that the distance between the top wall 10 of the food compartment 1 and the rear face of the reflector 17 should be equal to or greater than A/4 of the radiation microwave length.

In the case of simultaneous heating with microwave energy and another electrical heating device, for example a grill heater, the food compartment becomes heavily soiled by sputtering of the food whereby there is a loss of efficiency of the microwave heating arrangement. It is consequently necessary for the endconsumer, i.e. the oven user, to be in a position always to ensure maximum efficiency (i.e. heating effect) from his electric oven. In the case of the electric oven according to the present invention, this is effected by constructing the reflector 17 and heating coils 18, 19 of the grill heater as an integral unit. To this end, the heating coils are connected to the reflector, for example by welding, or by any other suitable connecting method and are thus solidly connected to each other.Furthermore, this rigid unit of reflector and heating coils is constructed so asto be removable from the food compartment 1.

This is made possible by supplying electric current to the heating coils 18, 19 by way of contact pins 24 to 27, whilst in addition the grill heater is secured to the walls of the food compartment in such a manner as to be easily detachable by the oven user as described hereinafter.

In orderto prevent sparking of irradiated microwave energy between the heating coils 18, 19 and the metal reflector 17, the U-shaped heating coils 18, 19 are secured to the reflector 17, with the exception of the area of the contact pins 24 to 27, by means of distance pieces 28. Said distance pieces are of a material which has low dielectric loss, for example ceramic or any other suitable material.

The electric oven according to the invention is intended for mass production. In the area of the contact pins 24 to 27 of the rigid, unitary grill heater, apertures are provided in the rear wall 7 of the food compartment 1 for socket elements 31 and 32, of which only the socket element 31 is to be seen in the sectional drawing in Fig. 1. Through these apertures in the rear wall 7, and also by way of the contact pins of the heating coils themselves, microwave energy escapes from the food compartment, which escape must be prevented. Thus, on the reflector 17 of the grill heater at least one high-frequency screening element is fitted in the vicinity of the connections for the contact pins 24 to 27 of the heating coils 18, 19 of the rigid unitary grill heater.HF screening elements 29 and 30 are in the form of flanges disposed at an angle of 90 to the reflector surface. The surfaces of these flanges are parallel to the food compartment wall, that is to say in the embodiment of Fig. 1, to the rear wall 7 into which the socket connections for the contact pins of the heating coils 18, 19 of the grill heater have been inserted. The HF screening is consequently effected by positive contact of HF screening elements 29,30 of the rigid unitary grill heater with the metal wall of the food compartment 1 enclosing the contact pins 24 to 27 for the heating coils 18,19. The HFscreening elements 29,30 and the reflector 17 of the grill heater are, in this case, integral with each other.Due to the flat construction of the flanges of the HF screening elements 29, 30 there is therefore produced over the whole surface of these elements an electrical contact with the metal rear wall 7 of the food compartment 1, which contact reliably prevents the HF energy contained in the food compartment 1 from escaping from the food compartment in the region of the contact pins. These HF screening elements can be simply and inexpensively mass-produced together with the reflector 17.

In addition to their screening function, the HF screening elements 29,30 have, at the same time, a second function as means for fastening the rigid unitary grill heater to one of the oven walls of the food compartment 1, as will be observed from Figs.

1 and 3. As fastening means, apertures 33 are provided in the flat flanges of the HF screening elements 29,20. The rigid unitary grill heater may be secured by means of the apertures 33 in the screening plates 29, 30, for example employing screws 34. A corresponding threaded hole is then provided in the rear wall 7. Alternatively, the HF screening can also be secured by providing in the rear wall 7 of the food compartment 1 a projecting threaded bolt over which the HF screening elements 29,30 are passed by means of their apertures 33 and are then held firmly attached thereto by nuts. For this purpose, for example crufiform nuts, knurled nuts or any other suitable nuts or screws can be employed. It is possible of course, to employ any other suitable fastening method for the HF screening elements, i.e. for the rigid unitary grill heater, provided that the particular connection between the HF screening elements and the wall of the food compartment to which they are fastened is intensively and electrically conductive whilst at the same time the connection can be easily released by an unskilled person, i.e. the oven user.

In order to prevent sparking of microwave energy between the metal parts located in the food com partment 1, it is generally necessary to ensure a per fectly close contact between the metal parts. To ensure electrical contact between the reflector 17 and the top wall 10 of the food compartment, at least one spring clip 35 is fitted to the forward part of the reflector 17 which is disposed towards the door 6.

Terminal portions 37 of the clip are concave when viewed from above and define a slot 36, the opening of which is disposed towards a correspondingly formed pin 38 (see Fig. 6). When the rigid unitary grill heater is pushed into the food compartment, the strong spring arms 37 are clamped between the pins 38 and the top wall 10 of the food compartment, thus automatically ensuring a firm, reliable electrical contact between the top wall 10 of the food compartment and the reflector 17 so that electrical sparking is reliably prevented at this point.

There is also provided, on the reflector 17 at the side of the grill heater disposed towards the door 6 of the food compartment, a baffle 39 (see Figs. 1,2 and 6), the purpose of which is, in particular, to screen the microwave field at the side towards the door. The baffle 39 is produced integrally with the reflector 17 of the grill heater. It is of approximately V-shape in cross-section, the limb of the V-shaped baffle 39 adjacent the door 6 of the food compartment 1 being extended upwardly almost as far as the top wall 10 of the food compartment and being at the same disposed parallel to the lateral face of the door.

Since the detachable window 15 for the microwave energy to be irradiated into the food compartment 1 is located centrally between the two lateral walls 8 and 9 directly above the surface 17 of the reflector, the main radiation path of the microwave energy must pass through the reflector 17. For this purpose, slits 40 and 41 are provided in the reflector surface through which the microwave are for the most part irradiated into the food compartment 1.

The heating coils 18, 19 may not project into the apertures provided in the reflector 17 bytheslits40, 41 because otherwise they would absorb microwave energy and would lead to excessive losses in the microwave heating arrangement. The longitudinal sides 42 of the slits 40,41 are arranged at an angle of 90C to the direction of diffusion of the microwaves and are disposed parallel to the longitudinal limbs 20 of the U shaped heating coils 18, 19. Moreover, the longitudinal limbs 20 of the U-shaped heating coils 18, 19 are arranged at a mutual distance apart of A/2 of the microwave length irradiated through the slits.

The slits 40, 41 are advantageously positioned approximately atthe centre of the reflecting surface of the reflector 17. It is also of advantage if the slits 40, 41 in the reflector 17 are positioned directly opposite the detachable window 15. The longitudi nal sides 42 of the slits 40, 41 have a length of at least A12 or of an nth multiple of X/2 of the microwave length irradiated through the slits. The shorter sides 43 of the slits 40,41 are in each case less than AJ2 of the microwave length irradiated through the slits.All features and measures which have just been described concerning the arrangement and execu sion of the slits 40,41 in the reflector 17 are intended in particular to minimise the losses of microwave energy irradiated into the food compartment where the microwave energy is directed in through the top wall 10 of the food compartment 1. It is also assumed that a fully equipped grill is contained in the electric oven, the reflector of said grill extending over almost the entire surface of the top wall 10 of the food compartment whereby a reasonably large article of food can be completely grilled in the food compartment The arrangement of a reflector 17 above the heating coils 18, 19 also offers a saving of energy in the supply of current to the heating coils.Due to the effect of deflection of the heat radiation of the heating coils bythe reflector, it is possible to obtain with less energy consumption, the same heating effect as would be obtained without a reflector and with a greater quantity of energy. The reflector 17 also serves as an additional heat shield for the top wall 10 of the food compartment, that is to say for the recess 4 located above it in which the electric wiring and equipment for operating the electric oven is accommodated.

When the rigid unitary grill heater comprising reflector 17 and heating coils 18, 19 is removed from the food compartment, a change occurs in the microwave field in the food compartment. When the grill heater is removed, there is therefore mismatching of the microwave generator in the food compartment 1.

In orderto prevent this, on the face of the HF screening 29 remote from the interior of the food compartment, a rod 44 is provided to operate an "on" switch 45 for the control circuit of the microwave generator (see Figs. 4 and 5). When the grill heater is removed the "on" switch 45 is opened. Thus, no microwave energy can be irradiated into the food compartment when the grill heater is not properly inserted. When the grill heater is secured in the food compartment, the rod 44, the length of which is suitably dimensioned, closes the "on" switch by way of an operating lever 46 and a pin 47. Immediately the control circuit of the microwave generator receives current so that the microwave heating arrangement can now be operated. In a further development, the rod 44 is constructed to serve likewise as a protective conductor for the grill heater, i.e. for the food compartment 1. The contracting of the rod 44 as protective conductor occurs in the socket element 31 which is mounted on a bracket 48. It extends by means of a guide piece for the contact pins through an intermediate wall 49 to the apertures for the plug connections in the rear wall 7 of the food compartment 1.

Claims

1. An electric oven comprising microwave heating means and secondary electric heating means for simultaneously irradiating and cooking food in a food compartment formed by metal walls, said secondary electric heating means being adapted to be located adjacent a wall of the compartment and being provided with a reflector having apertures therein characterised in that the secondary electric heating means and its reflector are formed as an integral unit which can be releasably mounted in said compartment by means of a plug and socket connection; high frequency screening means is provided on the reflector in the vicinity of the plug and socket connection, said screening means being adapted to engage positively the metal wall of the food compartment into which the secondary electric heating means is mounted; and means being provided in said high frequency screening means for enabling the secondary electric heating means and reflector unit to be releasably secured within the oven.

2. An electric oven as claimed in claim 1, in which the secondary electric heating means is a grill heater.

3. An electric oven according to claim 1 or 2, in which the microwave heating means comprises a microwave generator with magnetron, a wave conductor and a detachable window through which microwave energy can enterthe food compartment.

4. An electric oven as claimed in any of claims 1 to 3, in which the microwave heating means is arranged to introduce microwave energy through a top wall of the food compartment and the grill heater and reflector is located directly below said top wall.

5. An electric oven as claimed in claim 4 in which the reflector extends over a major portion of the top wall of the food compartment.

6. An electric oven as claimed in any of claims 1 to 5, in which the secondary electric heating means comprises one or more substantially U-shaped heating coils, the longitudinal limbs of which are disposed parallel to each other.

7. An electric oven according to any of claims 1 to 6, in which the high-frequency screening means is in the form of a flange or flanges disposed at an angle of 90" to the reflector surface and in that the surfaces of each flange are parallel to the food compartmentwall into which secondary electric heating means is mounted by means of the plug and socket connection.

8. An electric oven according to any preceding claim, in which the high-frequency screening means and the reflector of the secondary electric heating means are formed as one piece.

9. An electric oven according to claims 7 or 8, in which the securing means in the high-frequency screening means are in the form of an aperture or apertures in each flange.

10. An electric oven according to any preceding claim, in which the high-frequency screening means is fastened to the oven wall by means of screws.

11. An electric oven according to any preceding claim, in which electrical contacting of the reflector with an adjacent wall of the food compartment is effected by means of at least one spring clip.

12. An electric oven according to any preceding claim, in which the element of the secondary electric heating means is secured to the reflector by means of distance pieces of a material low in dielectric loss, exceptforthe area where the plug and socket connection passes through the high-frequency screening means.

13. An electric oven according to any preceding claim, in which a baffle is fitted to the reflector of the secondary electric heating means at that side thereof which is disposed towards a door of the food compartment.

14. An electric oven according to claim 13, in which the baffle is formed integrally with the reflector of the secondary electric heating means.

15. An electric oven according to claim 13 or 14, in which the baffle is of approximately V-shape in cross-section, the limb of the V-shaped baffle adjacent the door of the food compartment being extended close to the top wall of the food compartment and is disposed substantially parallel to the lateral face of the door.

16. An electric oven according to any preceding claim, in which on that face of the high-frequency screening means remote from the interior of the food compartment, a rod is provided to operate a switch for the control circuit of the microwave heating means.

17. An electric oven according to claim 16, in which said rod is constructed to serve likewise as a protective conductor for the grill heater, i.e. for the food compartment.

18. An electric oven according to any of claims 6 to 17, in which a window for the microwave energy entering the food compartment is located centrally between two lateral walls of the food compartment above the surface of the reflector; at least one slit is provided in the reflector surface and the heating coils are disposed so that they do not project into the apertures provided in the reflector by said slit or slits, the longitudinal sides of each slit being arranged at an angle of 90" to the direction of diffusion of the microwaves and being disposed parallel to the longitudinal limbs of the U-shaped heating coils, said longitudinal limbs of the heating coils being arranged at a mutual distance apart of A/2 of the microwave length irradiated through said slit or slits.

19. An electric oven according to claim 18in which each slit is situated approximately at the centre of the reflector surface.

20. An electric oven according to claim 18 or 19, in which each slit in the reflector lies directly opposite the window.

21. An electric oven according to any of claims 18 to 20, in which the longitudinal sides of each slit have a length of at least A/2 or of an nth multiple of A/2 of the microwave length irradiation through each slit.

22. An electric oven according to any of claims 18 to 21, in which the shorter sides of each slit is smaller than X/2 of the microwave length irradiated through said slit.

23. An electric oven according to any preceding claim, in which gaps are provided between lateral walls of the oven and the reflector.

24. An electric oven according to any preceding claim, in which the distance between the rear face of the reflector and the adjacent wall of the food compartment is not less than At4 of the radiation microwave length.

25. An electric oven substantially as hereinbefore described with reference to any of the accompanying drawings.