EP0174732B1

EP0174732B1 - Microwave oven

Info

Publication number: EP0174732B1
Application number: EP19850305591
Authority: EP
Inventors: Kenneth Ian Eke
Original assignee: Microwave Ovens Ltd
Current assignee: Microwave Ovens Ltd
Priority date: 1984-08-14
Filing date: 1985-08-06
Publication date: 1989-11-08
Also published as: EP0174732A1; DE3574220D1

Description

Field of the invention

This invention relates to a microwave oven with means for launching microwave power into a cavity of the oven from a launch area in the base of the cavity.
In a microwave oven microwave power is transferred from a magnetron to the oven cavity in dependence upon the effectiveness of the coupling between the load of the oven cavity and the magnetron. Hitherto, microwave ovens have been designed to achieve optimum coupling for a wide range of loads corresponding to differing sizes and densities of food items placed in the cavity. This optimisation of coupling means that for a given input power to the magnetron the power into the cavity is optmised over the range of loads placed in the oven cavity. The invention adopts an entirely different approach by aiming to provide a microwave oven having a cavity which, when devoid of food, is a poor power match with the magnetron, with the result that the amount of power transferred from the magnetron to the food being cooked is dependent almost entirely on the load of the food item.
DE-B-1151611 discloses a microwave oven comprising a magnetron for producing microwave power to be delivered into the cavity of the oven, means for launching the microwave power into the cavity from a launch area, and a metal member supported in the cavity in front of the launch area so that the oven when devoid of food provides an inefficient power match with the magnetron.
FR A 2310058 discloses a microwave oven with means for launching microwave power into the oven cavity through the base of the cavity, but does not disclose a metal tray supported above the launch area, which is essential to the present invention.
Attention is drawn to the applicant's European Patent application No. 85305588.7 (Publication No. 0173491) which discloses similar subject matter.

Summary of the invention

According to the invention a microwave oven comprises a magnetron for producing microwave power to be delivered into the cavity of the oven, means for launching the microwave power into the cavity- from α- launch area, a metal member supported in the cavity facing the launch area so that the oven when devoid of food provides an inefficient power match with the magnetron, characterized in that the oven has a power rating within the power limitations of a domestic plug/ socket, the launch area is in the base of the cavity, the member is a tray made of sheet metal with the peripheral edge of the tray spaced from the cavity walls and the metal tray is above the launch area, so that when food items are placed on said tray the resulting dielectric load determines the power coupled to the loaded oven from the magnetron.
Accordingly, in the invented microwave oven the amount of microwave power coupled into a loaded oven is substantially proportional to the dielectric load. The result of this is that the microwave oven need not have selectable microwave power settings which the user must first preset, because the load of the food item itself determines the amount of power delivered by the magnetron to the loaded cavity.
The tray may be circular and may form a rotatable turntable on which the food items are supported during cooking.
The turntable is preferably driven by a rotatable drive member extending upwardly through the base of the cavity, and this drive member may be arranged concentrically with a further drive member which rotates a mode stirrer in the base.
The positioning and size of the tray in the cavity are important factors in ensuring that the tray presents a load which is a poor match for the magnetron in terms of effectiveness of power transfer from the magnetron to the oven cavity. In a particular example it has been found that good results are obtained if the tray is between 18 and 25 mm (preferably about 20 mm) above the base, and is between 350 and 380 mm in diameter, preferably about 360 mm. The tray will normally be made of sheet metal, which may be stove enamelled.
The oven preferably has thermal heating means in addition to the magnetron, the thermal heating means providing a forced flow of hot air through the cavity, as a result of air being blown over an electrical resistance heating element by means of a fan. The air flow pattern is preferably such that hot air is forced into the cavity through an inlet in a rear wall of the cavity, and leaves the cavity through an outlet in the rear wall. Alternatively, the back wall may have two inlets for the entry of forced hot air and two outlets for the exit of forced hot air.
The oven may be a small, portable microwave oven.
A microwave oven according to the invention will now be described, by way of example, with reference to the accompanying drawings, in which:

Figure 1 is a perspective view of the oven cavity with a door and surrounding structure removed,
Figure 2 is an elevation of a rear wall of the oven cavity, showing inlet and outlet apertures for a forced flow of hot air.

Referring to Figure 1, the oven is a small table top oven powered from a domestic plug/socket. The oven is generally rectangular in shape and the cavity is defined by two side walls 2, 4, a back wall 6, a top panel 8 and a base panel 10. Microwave power is launched into the cavity through a rectangular aperture 12 in the base panel 10. A mode stirrer (not shown) is mounted in the aperture 12 and is rotatably driven about a vertical axis.
The cavity accommodates a removable turntable in the form of a metal tray 14. The tray 14 has a circular base 360 mm in diameter, surrounded by an upstanding wall or rim 25 mm high. The tray 14 is formed of sheet metal which may be stove enamelled. The cavity may have a height of 230 mm, a width of 400 mm and a depth of 390 mm. The tray 14 is spaced 20 mm above the base panel 10, and the underside of the tray 14 is engaged by rollers 16 which are mounted on the base panel 10.
Drive means for rotating the turntable extend upwardly through the aperture 12 and are shown diagrammatically at 18. Such drive means are coaxially arranged with the drive to the mode stirrer, for example by the turntable being rotatably driven by a central vertical shaft surrounded by a drive sleeve driving the mode stirrer. The drive shaft and drive sleeve are driven at their appropriate speeds, eg by belt drives from a motor. It will be appreciated that all this structure, together with a magnetron for delivering the microwave power, will be positioned below the cavity but within the oven outer casing which is not shown in the drawings.
A forced flow of hot air is passed through the cavity simultaneously with the application of microwave power, so that food items placed on the tray 14, are subjected both to hot air and microwave power. Figure 2 shows the hot air inlet and outlet in the back wall 6, as the latter is viewed from the front of the oven. The back wall 6 has a vertically elongated inlet 20 through which hot air is forced by a fan to enter the cavity. Having passed over the food items, the air leaves the cavity through the circular air outlet 22. The air is then forced over an electric resistance heating element (disposed in a compartment behind the rear wall 6) before being recirculated through the inlet 20 and the cavity. The arrows in Figure 2 depict the air flow diagrammatically: it will be appreciated that the hot air is forced forwardly into the cavity from the inlet 20 before being drawn back to the outlet 22. It will also be appreciated that the cavity has a moisture vent, for example in the back wall 6.
In use the turntable rotates about a central vertical axis, the underside of the tray 14 engaging the rollers 16.
When the oven is powered solely by microwave energy, (ie without any forced hot air), the oven walls may be made of a single thickness of metal, so reducing construction costs and weight.

Claims

1. A microwave oven comprising a magnetron for producing microwave power to be delivered into the cavity of the oven, means for launching the microwave power into the cavity from a launch area, a metal member supported in the cavity facing the launch area so that the oven when devoid of food provides an inefficient power match with the magnetron, characterized in that the oven has a power rating within the power limitations of a domestic plug/socket, the launch area (12) is in the base of the cavity, the member is a tray (14) made of sheet metal with the peripheral edge of the tray spaced from the cavity walls and the metal tray (14) is above the launch area (12), so that when food items are placed on said tray the resulting dielectric load determines the power coupled to the loaded oven from the magnetron.

2. A microwave oven according to claim 1, characterised in that the tray (14) is circular and forms a rotatable turntable on which the food items are supported and rotated during cooking.

3. A microwave oven according to claim 2, characterised in that the turntable is driven by a rotatable drive member (18) extending upwardly through the base of the cavity.

4. A microwave oven according to claim 3, characterised in that said drive member is arranged concentrically with a further drive member which rotates a mode stirrer in the base of the cavity.

5. A microwave oven according to any of claims 2 to 4, characterised in that the tray is between 18 and 25 mm above the base, and is between 350 and 380 mm in diameter.

6. A microwave oven according to any one of the preceding claims, characterised in that the tray is of sheet metal the outer surface of which is stove enamelled.

7. A microwave oven according to any of the preceding claims, characterised in that the oven has thermal heating means in addition to the magnetron, the thermal heating means providing a forced flow of hot air through the cavity, as a result of air being blown over an electrical resistance heating element by means of a fan.

8. A microwave oven according to Claim 7, characterised in that the hot air is forced into the cavity through an inlet (20) in the rear wall of the cavity, and leaves the cavity through an outlet (22) in the rear wall of the cavity.

9. A microwave oven according to claim 7, characterised in that the air flow pattern is such that hot air is forced into the cavity through two inlets in a rear wall of the cavity, and leaves the cavity through two outlets in the rear wall.