Microwave oven with a fan and a mode stirrer mounted on a common driveshaft

Description

Field of the Invention

[0001] This invention relates to microwave ovens, and in particular to such ovens having magnetrons for delivering microwave power to the oven cavities and forced hot air systems for re-circulating hot air through the cavities. An example of such an oven is disclosed in the inventor's US Patent 5,434,391.

Summary of the Invention

[0002] According to the invention a microwave oven comprises a magnetron for delivering microwave power to a cavity of the oven, a rotatable phase modulator for preventing the establishment of dominant standing waves in the cavity, a forced hot air system comprising electrical resistance heating means and a rotatable fan for forcing air over the resistance heating means and through the cavity, characterised in that the phase modulator and the fan are mounted on a common driveshaft which extends through a launch box by means of which microwave power is delivered from the magnetron to the cavity, wherein the phase modulator has blades extending from one or both sides of a disc-like plate disposed in a plane perpendicular to the rotational axis of the shaft and wherein the disc-like plate is spaced from the internal surface of an outer wall of the launch box by a distance corresponding to a quarter wavelength at the operating frequency of the magnetron, wherein the fan also has blades extending from one or both sides of a disc-like plate disposed in a plane perpendicular to the rotational axis of the shaft, and wherein the respective planes of the disc-like plates of the modulator and the fan are spaced by a distance corresponding to a quarter wavelength, the fan being further away from the outer wall of the launch box than the phase modulator.

[0003] The spatial distance corresponding to a quarter wavelength is not invariable at a given operating frequency, because the length of a complete wave will depend on the surrounding structure. Thus, the spacing in millimetres between the disc-like plate of the phase modulator and the internal surface of the rear wall of the launch box is not necessarily identical to the spacing in millimetres between the plates of the phase modulator and the fan. Indeed, in the preferred embodiment these dimensions are respectively 36mm and 44mm at the magnetron operating frequency of 2455 plus or minus 20Mhz.

[0004] It was found by experiment that the position of the phase modulator and the position of the fan were crucial to the achievement of even microwave power distribution within the cavity.

[0005] In a preferred embodiment the common driveshaft is rotatably driven by an electric motor attached to an external surface of an outer wall of the launch box. The rotational speed of the driveshaft is preferably between 1500 and 2500rpm, most preferably about 2000rpm.

[0006] The electrical resistance heating means conveniently comprises an electrical resistance heating element disposed radially outwardly of the fan, and may also include further electrical resistance heating elements in the cavity.

[0007] In the embodiment to be described, a metal plate is fixed in the cavity and serves not only as a match plate for transferring microwave power from the launch box to the cavity but also serves as a baffle plate for the forced hot air convection system, in the manner disclosed in the inventor's US Patent 5434391.

[0008] The metal plate may be supported in the cavity so that a front surface of the plate faces into the cavity and a rear surface of the plate faces a launch area, there being a spacing between the peripheral edge of the plate and the walls of the cavity, the spacing being defined partly by an air gap and partly by blanking material which allows the passage of microwave power but blocks or impedes the passage of forced hot air. Preferably, microwave power reaches the cavity around the complete periphery of the plate but forced hot air reaches the cavity through said air gap but not through the blanking material. The blanking material has low thermal conductivity but allows the transmission of microwave energy, a suitable material being glass or mica.

[0009] Preferably, the metal plate is apertured by virtue of having a series of holes therein and the fan is operative to draw air from the cavity through the holes and thence to direct the hot air through the air gap so as to re-circulate the air to the cavity.

[0010] The metal plate is preferably rectangular, with a first pair of opposed sides of the peripheral edge being spaced by the air gap and the other pair of opposed sides of the peripheral edge adjoining strips of the blanking material.

[0011] Preferably the first pair of opposed sides are formed by upper and lower side edges of the metal plate, meaning that all or substantially all the hot air reaches the cavity by passing the upper and lower edges of the metal plate, and in this case the cavity advantageously has upper and lower electrical resistance heating elements constituting grill and baking elements respectively.

Brief Description of the Drawings

[0012] 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 an isometric view of the oven with parts in exploded view, and

Figure 2 is a sectional view of the oven on a central vertical plane.

Detailed Description of the Drawings

[0013] The oven has a cavity defined by two side walls 1 and 2, a rear wall 3, a base wall 4 and a top wall 5. The front of the cavity is closable by a hinged door (not shown).

[0014] Within the cavity and adjacent to the top wall 5 is a top electrical resistance heating element 6, of sinuous shape, serving as a grill element. A bottom electrical resistance heating element 7 of similar shape, spaced a short distance above the base wall 4, serves as a baking element.

[0015] The rear wall 3 has a rectangular aperture 8 therein. A vertical metal plate 9 is mounted in the cavity a short distance in front of the rear wall 3, the plate 9 overlapping the aperture 8. A central circular area of the plate 9 has a series of holes 10 therein. The metal matchplate 9 has dimensions chosen to create edge coupling to all the resonant modes that exist within the oven cavity over the frequency range of 2455 ± 20Mhz. An arcuate electrical resistance heating element 12 is positioned behind the plate 9 and lies outwardly of the aperture 8.

[0016] To the outside of the rear wall 3 and covering the aperture 8 is attached a launch box 13 having a rear wall 14 parallel with the rear wall 3 of the cavity and side walls terminating in out-turned flanges by which the launch box 13 is secured (e.g. by bolts or welding) to the rear wall 3. A wave guide 15 is attached to the lower side of the launch box 13 and directs microwave energy from a magnetron (not shown) into the launch box 13. An electric motor 16 and choke assembly 17 are attached to the external side of the rear wall 14 of the launch box 13, a drive shaft 18 of the electric motor extending horizontally through the launch box and also through the aperture 8. The shaft 18 carries, within the launch box, a rotatable phase modulator 19 and the extremity of the shaft carries a rotatable fan 20 which is positioned radially inwardly of the element 12. The phase modulator 19 prevents the establishment of dominant standing waves in the cavity.

[0017] The phase modulator 19 has a central disc-like plate 22 with blades extending on both sides therefrom and the plate 22 is accurately positioned to be a distance of 36mm from the launch box rear wall 14, which corresponds to a quarter wavelength in that situation. The fan 20 has a disc-like plate 24 with blades extending forwardly thereof and the plate 24 is accurately positioned to be a distance of 44mm from the plane of the plate 22, this distance of 44mm corresponding to the quarter wavelength in the situation illustrated. It will be seen from Figure 2 that the wall 14, the plates 22 and 24, the wall 3 and the plate 9 occupy parallel vertical planes.

[0018] As a consequence of the plate 9 being spaced a short distance in front of the rear wall 3, the four edges of the plate 9 are separated from the walls of the cavity, and along the two side edges of the plate 9 this separation is spanned by strips 26 of mica or like material which allows the propagation of microwave energy but impedes the passage of air flow. Along the upper and lower edges of the plate 9, the spacing between the plate 9 and the cavity wall 5 forms open slots 23 through which hot air reaches the cavity.

[0019] In use, the magnetron delivers microwave power to the cavity, via the wave guide 15, the launch area defined by the launch box 13 and around all four edges of the plate 9. Simultaneously, the electric motor 16 drives the shaft 18 (typically at 2000rpm) and rotates the phase modulator 19 and convection fan 20 at this rotational speed. Air, driven by the fan 20, passes over the convection element 12 where it is heated and then passes into the cavity through the two slots 23 along the upper and lower edges of the plate 9. This air passes across the upper and lower heating elements 6 and 7 before being drawn through the central holes 10 in the plate 9 which thus serves not only as a baffle plate for the convection air but also as a match plate for the microwave system. The arrows in Figure 2 show the air circulation within the cavity.

[0020] The components shown in the drawings comprise a modularised system in that the components can be fitted to the back wall of any size of oven cavity, providing that the plate 9 is sized effectively to couple microwave power to the propagating modes that exist within the oven cavity. In the example described and illustrated in the drawings, the cavity has a depth of 406mm, a height of 350mm and a width of 472mm. The use of a single motor 16 to drive the phase modulator 19 and the fan 20 has advantages of simplicity and effectiveness, providing that the phase modulator 19 and fan 20 are accurately positioned in the manner described.

Claims

1. A microwave oven comprising a magnetron for delivering microwave power to a cavity of the oven, a rotatable phase modulator (19) for preventing the establishment of dominant standing waves in the cavity, a forced hot air system comprising electrical resistance heating means (12) and a rotatable fan (20) for forcing air over the resistance heating means (12) and through the cavity, characterised in that the phase modulator (19) and the fan (20) are mounted on a common driveshaft (18) which extends through a launch box (13) by means of which microwave power is delivered from the magnetron to the cavity, wherein the phase modulator (19) has blades extending from one or both sides of a disc-like plate (22) disposed in a plane perpendicular to the rotational axis of the shaft (18), and wherein the disc-like plate (22) is spaced from the internal surface of an outer wall (14) of the launch box (13) by a distance corresponding to a quarter wavelength at the operating frequency of the magnetron, wherein the fan (20) also has blades extending from one or both sides of a disc-like plate (24) disposed in a plane perpendicular to the rotational axis of the shaft (18), and wherein the respective planes of the disc-like plates (22,24) of the modulator and the fan are spaced by a distance corresponding to a quarter wavelength, the fan (20) being further away from the outer wall (14) of the launch box (13) than the phase modulator (19).

2. A microwave oven according to claim 1, characterised in that the common drive shaft (18) is rotatably driven by an electric motor (16) attached to an external surface of the outer wall (14) of the launch box (13).

3. A microwave oven according to claim 1 or 2, characterised in that the rotational speed of the driveshaft (18) is between 1500 and 2500rpm.

4. A microwave oven according to any of the preceding claims, characterised in that the electrical resistance heating means comprises an electrical resistance heating element (12) disposed radially outwardly of the fan (20).

5. A microwave oven according to any of the preceding claims, characterised in that a metal plate (9) is fixed in the cavity and serves not only as a match plate for transferring microwave power to the cavity but also serves as a baffle plate for the forced hot air convection system.

6. A microwave oven according to Claim 5, characterised in that the metal plate (9) is supported in the cavity so that a front surface of the metal plate faces into the cavity and a rear surface of the metal plate faces a launch area, there being a spacing between the peripheral edge of the metal plate and the walls of the cavity, the spacing being defined partly by an air gap and partly by blanking material (26) which allows the passage of microwave power but blocks or impedes the passage of forced hot air.

7. A microwave oven according to claim 6, characterised in that microwave power reaches the cavity around the complete periphery of the metal plate (9) but forced hot air reaches the cavity through said air gap but not through the blanking material (26).

8. A microwave oven according to claim 6 or 7, characterised in that the blanking material (26) is glass or mica.

9. A microwave oven according to any of claims 6 to 8, characterised in that the metal plate (9) is apertured by virtue of having a series of holes (10) therein and the fan (20) is operative to draw air from the cavity through the holes (10) and thence to direct the hot air through the air gap so as to re-circulate the air to the cavity.

10. A microwave oven according to any of claims 6 to 9, characterised in that the metal plate (9) is rectangular, with a first pair of opposed sides of the peripheral edge being spaced by the air gap and the other pair of opposed sides of the peripheral edge adjoining strips of the blanking material (26).

11. A microwave oven according to claim 10, characterised in that the first pair of opposed sides are formed by upper and lower side edges of the metal plate (9), meaning that all or substantially all the hot air reaches the cavity by passing the upper and lower edges of the metal plate (9).

12. A microwave oven according to claim 11, characterised in that the cavity has upper and lower electrical resistance heating elements (6, 7) constituting grill and baking elements respectively.

Ansprüche

1. Mikrowellenofen umfassend ein Magnetron zur Zufuhr von Mikrowellenleistung an einen Garraum des Ofens, einen drehbaren Phasenmodulator (19) zur Verhinderung der Ausbildung von vorherrschenden stehenden Wellen in dem Garraum, eine Einrichtung zur Erzeugung einer erzwungenen Heißluftströmung umfassend elektrische Widerstandsheizmittel (12) und ein drehbares Gebläse (20) zur Zwangsführung von Luft über die elektrischen Widerstandsheizmittel (12) und durch den Garraum, dadurch gekennzeichnet, dass der Phasenmodulator (19) und das Gebläse (20) auf einer gemeinsamen Antriebswelle (18) montiert sind, welche sich durch eine Startbox (13) erstreckt, mittels welcher Mikrowellenleistung von dem Magnetron dem Garraum zugeführt wird, wobei der Phasenmodulator (19) Schaufeln aufweist, die sich ausgehend von einer oder von beiden Seiten einer scheibenförmigen Platte (22) erstrecken, welche in einer im rechten Winkel zur Drehachse der Welle (18) stehenden Ebene angeordnet ist, und wobei die scheibenförmige Platte (22) in einem Abstand zur Innenfläche einer Außenwand (14) der Startbox (13) steht, der einer Viertelwellenlänge bei der Betriebsfrequenz des Magnetrons entspricht, wobei das Gebläse (20) ebenfalls Schaufeln aufweist, die sich ausgehend von einer oder von beiden Seiten einer scheibenförmigen Platte (24) erstrecken, welche in einer im rechten Winkel zur Drehachse der Welle (18) stehenden Ebene angeordnet ist, und wobei die jeweiligen Ebenen der scheibenförmigen Platten (22, 24) des Modulators und des Gebläses in einem Abstand zueinander stehen, der einer Viertelwellenlänge entspricht, wobei das Gebläse (20) von der Außenwand (14) der Startbox (13) weiter entfernt ist als der Phasenmodulator (19).

2. Mikrowellenofen nach Anspruch 1, dadurch gekennzeichnet, dass die gemeinsame Antriebswelle (18) von einem an einer Außenfläche der Außenwand (14) der Startbox (13) angebrachten Elektromotor (16) drehbar angetrieben ist.

3. Mikrowellenofen nach Anspruch 1 oder 2, dadurch gekennzeichnet, dass die Drehzahl der Antriebswelle (18) zwischen 1500 und 2500 U/min beträgt.

4. Mikrowellenofen nach einem der vorstehenden Ansprüche, dadurch gekennzeichnet, dass die elektrischen Widerstandsheizmittel ein elektrisches Widerstandsheizelement (12) umfassen, welches außen am Gebläse (20) radial angeordnet ist.

5. Mikrowellenofen nach einem der vorstehenden Ansprüche, dadurch gekennzeichnet, dass eine Metallplatte (9) in dem Garraum befestigt ist und nicht nur als Adapterplatte zur Übertragung von Mikrowellenleistung an den Garraum sondern auch als Ablenkplatte für das Heißluftgebläse mit erzwungener Konvektion dient.

6. Mikrowellenofen nach Anspruch 5, dadurch gekennzeichnet, dass die Metallplatte (9) in dem Garraum derart abgestützt ist, dass eine Stirnfläche der Metallplatte dem Garraum zugewandt ist und eine Rückseite der Metallplatte einer Startfläche zugewandt ist, wobei zwischen der umlaufenden Kante der Metallplatte und den Wänden des Garraums ein Abstand vorgesehen ist, wobei der Abstand teilweise durch einen Luftspalt und teilweise durch die Mikrowellenleistung durchlassendes, den Durchtritt des erzwungenen Heißluftstroms jedoch blockierendes bzw. behinderndes Abdichtungsmaterial (26) definiert wird.

7. Mikrowellenofen nach Anspruch 6, dadurch gekennzeichnet, dass die Mikrowellenleistung den Garraum über den gesamten Umfang der Metallplatte (9) erreicht, die erzwungene Heißluft aber den Garraum zwar über den Luftspalt, nicht jedoch über das Abdichtungsmaterial (26) erreicht.

8. Mikrowellenofen nach Anspruch 6 oder 7, dadurch gekennzeichnet, dass das Abdichtungsmaterial (26) Glas oder Glimmer ist.

9. Mikrowellenofen nach einem der Ansprüche 6 bis 8, dadurch gekennzeichnet, dass die Metallplatte (9) dadurch gelocht ist, dass eine Reihe von Löchern (10) darin vorgesehen sind, und das Gebläse (20) wirksam ist, um Luft aus dem Garraum durch die Löcher (10) anzusaugen und daher die Heißluft durch den Luftspalt zu leiten, um die Luft wieder in den Garraum zurückzuführen.

10. Mikrowellenofen nach einem der Ansprüche 6 bis 9, dadurch gekennzeichnet, dass die Metallplatte (9) rechteckig ist, wobei zwei erste einander gegenüberliegende Seiten der umlaufenden Kante durch den Luftspalt voneinander beabstandet sind und die beiden anderen einander gegenüberliegenden Seiten der umlaufenden Kante sich an Streifen des Abdichtungsmaterials (26) anschließen.

11. Mikrowellenofen nach Anspruch 10, dadurch gekennzeichnet, dass die ersten beiden einander gegenüberliegenden Seiten durch eine obere und eine untere Seitenkante der Metallplatte (9) gebildet sind, das heißt, dass die gesamte bzw. im Wesentlichen die gesamte Heißluft den Garraum dadurch erreicht, dass sie an der oberen und der unteren Kante der Metallplatte (9) entlang strömt.

12. Mikrowellenofen nach Anspruch 11, dadurch gekennzeichnet, dass der Garraum obere und untere elektrische Widerstandsheizelemente (6, 7) aufweist, welche Grill- bzw. Backelemente darstellen.

Revendications

1. Four à micro-ondes, comprenant un magnétron pour délivrer un flux de micro-ondes vers une cavité du four, un modulateur de phase rotatif (19) pour empêcher l'établissement d'ondes stationnaires dominantes dans la cavité, un système d'air chaud soufflé comprenant des moyens de chauffage à résistance électrique (12) et un ventilateur rotatif (20) pour souffler de l'air sur les moyens de chauffage à résistance électrique (12) et à travers la cavité, caractérisé en ce que le modulateur de phase (19) et le ventilateur (20) sont montés sur un arbre d'entraînement commun (18) qui s'étend à travers un carter de lancement (13) au moyen duquel un flux de micro-ondes est délivré du magnétron dans la cavité, dans lequel le modulateur de phase (19) comprend des pales s'étendant depuis un ou les deux côtés d'une plaque en forme de disque (22) disposée dans un plan perpendiculaire à l'axe de rotation de l'arbre (18), et dans lequel la plaque en forme de disque (22) est espacée de la surface interne d'une paroi extérieure (14) du carter de lancement (13) d'une distance correspondant à un quart de la longueur d'onde à la fréquence de fonctionnement du magnétron, dans lequel le ventilateur (20) comprend également des pales s'étendant depuis un ou les deux côtés d'une plaque en forme de disque (24) disposée dans un plan perpendiculaire à l'axe de rotation de l'arbre (18), et dans lequel les plans respectifs des plaques en forme de disque (22, 24) du modulateur et du ventilateur sont espacés d'une distance correspondant à un quart de longueur d'onde, le ventilateur (20) étant plus éloigné de la paroi extérieure (14) du carter de lancement (13) que le modulateur de phase (19).

2. Four à micro-ondes selon la revendication 1, caractérisé en ce que l'arbre d'entraînement commun (18) est entraîné de façon rotative par un moteur électrique (16) fixé à une surface externe de la paroi extérieure (14) du carter de lancement (13).

3. Four à micro-ondes selon la revendication 1 ou 2, caractérisé en ce que la vitesse de rotation de l'arbre d'entraînement (18) est comprise entre 1500 et 2500 tpm.

4. Four à micro-ondes selon l'une quelconque des revendications précédentes, caractérisé en ce que les moyens de chauffage à résistance électrique comprennent un élément de chauffage à résistance électrique (12) disposé radialement à l'extérieur du ventilateur (20).

5. Four à micro-ondes selon l'une quelconque des revendications précédentes, caractérisé en ce qu'une plaque de métal (9) est fixée dans la cavité et sert non seulement de plaque de raccordement pour transférer le flux de micro-ondes vers la cavité, mais sert aussi de déflecteur pour le système de convection à air chaud soufflé.

6. Four à micro-ondes selon la revendication 5, caractérisé en ce que la plaque de métal (9) est supportée dans la cavité de telle sorte qu'une surface avant de la plaque de métal soit tournée vers l'intérieur de la cavité, et qu'une surface arrière de la plaque de métal soit tournée vers une zone de lancement, un espace étant laissé entre le bord périphérique de la plaque de métal et les parois de la cavité, l'espace étant défini en partie par un entrefer et en partie par une matière de remplissage (26) qui permet le passage du flux de micro-ondes mais bloque ou empêche le passage d'air chaud soufflé.

7. Four à micro-ondes selon la revendication 6, caractérisé en ce que le flux de micro-ondes atteint la cavité autour de la périphérie totale de la plaque de métal (9), mais l'air chaud soufflé atteint la cavité à travers ledit entrefer mais pas à travers la matière de remplissage (26).

8. Four à micro-ondes selon la revendication 6 ou 7, caractérisé en ce que la matière de remplissage (26) est le verre ou le mica.

9. Four à micro-ondes selon l'une quelconque des revendications 6 à 8, caractérisé en ce que la plaque de métal (9) est ajourée dans la mesure où elle comporte une série de trous (10), et le ventilateur (20) sert à aspirer de l'air hors de la cavité à travers les trous (10) et par conséquent à diriger l'air chaud à travers l'entrefer de manière à remettre l'air en circulation vers la cavité.

10. Four à micro-ondes selon l'une quelconque des revendications 6 à 9, caractérisé en ce que la plaque de métal (9) est rectangulaire, où une première paire de côtés opposés du bord périphérique est espacée par l'entrefer, et l'autre paire de côtés opposés du bord périphérique est attenante aux bandes de la matière de remplissage (26).

11. Four à micro-ondes selon la revendication 10, caractérisé en ce que la première paire de côtés opposés est formée par des bords latéraux supérieur et inférieur de la plaque de métal (9), avec comme conséquence que la totalité, ou sensiblement la totalité de l'air chaud atteint la cavité en franchissant les bords supérieur et inférieur de la plaque de métal (9).

12. Four à micro-ondes selon la revendication 11, caractérisé en ce que la cavité comprend des éléments de chauffage à résistance électrique supérieur et inférieur (6, 7) constituant des éléments de grill et de cuisson, respectivement.

Drawing