Classifications

• • H—ELECTRICITY
  • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
  • H05B—ELECTRIC HEATING; ELECTRIC LIGHT SOURCES NOT OTHERWISE PROVIDED FOR; CIRCUIT ARRANGEMENTS FOR ELECTRIC LIGHT SOURCES, IN GENERAL
  • H05B6/00—Heating by electric, magnetic or electromagnetic fields
  • H05B6/64—Heating using microwaves
  • H05B6/70—Feed lines
  • H05B6/707—Feed lines using waveguides

Definitions

• the present invention relates to heating of a load in a microwave oven, as well as to a microwave oven for use therefor.
• a substantial problem associated with known microwave ovens is a tendency to uneven heating of the load.
• One reason for this tendency is the varying relative effective permittivity, herein designated ⁇ , of the load.
• Another reason is the existence of what is usually called cold and hot spots.
• Yet another reason is the existence of an edge-heating effect.
• the object of the present invention is to provide improved microwave heating in a microwave oven, whereby, in particular, the need to use rotating load carriers or field stirrers is eliminated.
• the above-mentioned object is achieved by a microwave oven, a heating method and a microwave usage as defined in the appended claims.
• the invention is thus based on an understanding of the advantages of providing a resonant quarterwave mode, preferably of the TM type (in the load) , in a preferably rectangularly parallelepipedal cavity of a microwave oven. It has been found that such a resonant quarterwave mode provides good heating throughout the surface of an extended load, when the load is placed somewhat spaced from the bottom of the cavity in the usual manner. Moreover, surprisingly, it has been found that there is no pronounced edge-heating effect.
• the field seen vertically, ends in a quarter- wave in the lower part of the cavity.
• the oven cavity in order for the reso- nant quarterwave mode to occur, the oven cavity must have suitable dimensions therefor, and, in addition, the wave impedance of the mode inside the load must be higher than it is outside the load.
• the latter means that the resonant quarterwave is generated when the ⁇ of the load is low, typically between about 4 and about 40. Since a load which is deep-frozen has low ⁇ , this, in turn, means that the resonant quarterwave is particularly suitable for use in connection with defrosting. In connection with "drying" heating or “expanding” baking, the load will have increasingly lower ⁇ , which means that in these cases, too, it can be particularly advantageous to utilise a resonant quarterwave mode.
• the resonant quarterwave mode can, in the case of a load with higher ⁇ , i.e. typically greater than 40, be made to gradually become a Brewster mode, i.e. essentially reflectionless at the load because of impedance similarity.
• ⁇ i.e. typically greater than 40
• a Brewster mode i.e. essentially reflectionless at the load because of impedance similarity.
• the quarterwave resonant mode/Brewster mode with an additional mode which is a halfwave resonant mode or a Brewster mode, with the surprising result that the combination provides to an essential degree quadrature field patterns at the load.
• quadrature refers to the fact that the field patterns are such that they are added arithmetically rather than vectorial- ly. It has been found that this leads to the heating effect being considerably more even than is normally the case, i.e. the problem of cold and hot spots can to an essential degree be eliminated without any requirement for the load to rotate in the usual manner.
• the additional mode a mode which for a load with low ⁇ is a Brewster mode and for a load with high ⁇ is a normally resonant halfwave mode, but with the same horizontal mode index.
• the additional mode will then automatically switch between said two modes when the ⁇ of the load increases and decreases respectively.
• the heat- ing effect becomes less sensitive than usual to the load height, i.e. the thickness of the load.
• a mode combination according to the invention when utilised, it is preferable that it supplies a substantial part, preferably the main part, of the power to the load, but it will be appreciated that other modes could also exist in the oven cavity, in particular where small loads are concerned.
• the two modes of the mode combination according to the invention with the aid of two separate cavity feed ports, which are located at the top and the bottom respectively of the oven cavity.
• the upper port is used as the main feed port and the lower port is used for transmitting power in ways which favour the modes excited by the upper port.
• This requires the desired modes generated by the respective port to be suitably phase-shifted in relation to each other.
• this can advantageously be achieved by the utilisation of a resonant area in the device feed- ing the two feed ports. In this way, the required phase difference between the mode fields excited by the ports can be obtained by means of a suitable difference in the feeding route from the resonant area feeding the two ports .
• the dimensioning of the oven cavity will affect the achievement of desired modes in the same. It has been found that the cavity dimensions are relatively sensitive when it comes to achieving the effects according to the invention, and the skilled person may be obliged to carry out a number of attempts with varying dimensions before obtaining modes with the desired relative strength.
• a microwave oven comprising a preferably at least essentially rectangularly parallelepipedal oven cavity with a cavity bottom for receiving a load which is to be heated, and with means for feeding microwaves into the oven cavity, which means comprise a feed port in the ceiling and/or side wall of the oven cavity, the oven cavity being dimensioned to provide a resonant quarterwave mode therein when the wave impedance of the mode inside the load is higher than it is outside the load.
• the oven cavity is preferably dimensioned to also provide a resonant halfwave mode or a Brewster mode in a quadrature field pattern in combination with the quarterwave mode at the load.
• the microwave oven has two feed ports arranged in a cavity side wall, of which feed ports a first one is arranged at the ceiling of the cavity and a second one is arranged adjacent to the bottom of the cavity, and a resonant waveguide device is arranged for feeding microwaves to said two feed ports, the waveguide device being dimensioned to provide such a phase difference between the microwave feeds from the two feed ports that the mode field excited from said second feed port favours the mode field excited by said first feed port, in order to provide the desired mode balance.
• a method for heating a load in the cavity of a microwave oven with the aid of microwaves comprising the generation in the cavity of a first mode which, in the case of a load with low ⁇ , typically lower than about 40, is a quarterwave resonant mode.
• a preferred embodiment of the method also comprises the generation in the cavity of an additional mode, which is a halfwave resonant mode or a Brewster mode, so that the combination of it with the quarterwave mode provides quadrature field patterns at the load.
• Said additional mode is preferably a Brewster mode in the case of a load with low ⁇ .
• the Brewster mode is advantageously generated so that in the case of a load with higher ⁇ , it will be a normal resonant halfwave mode .
• the excitation of the quarterwave resonant mode and the additional mode is effected with the aid of two separate cavity feed ports located at the top and the bottom respectively of the oven cavity, the upper feed port being utilised as the main feed port and microwaves being fed to the lower feed port with such a forced phase difference in relation to the upper feed port that modes excited by the upper feed port are favoured.
• a third aspect of the invention comprises the use of a resonant quarterwave mode in the cavity of a microwave oven for heating a load with low ⁇ , such as initial defrosting of a deep-frozen load or final heating of a load with decreasing ⁇ as a result of the heating. 7
• a fourth aspect of the invention comprises the use of a resonant quarterwave mode in combination with a resonant halfwave mode or a Brewster mode in phase quadrature in the cavity of a microwave oven for heating a load with at least temporarily low ⁇ .
• Fig. 1 is a schematic perspective view showing an outline of a microwave oven with an oven cavity and its associated microwave feeding system according to an embodiment of the present invention.
• Fig. 2 is a schematic vertical sectional view of the oven cavity with the associated microwave feeding system in Fig. 1.
• Fig. 3 is a schematic horizontal view of the ceiling area of the oven cavity illustrating an alternative arrangement of a cavity feed port and providing an outline of the H field patterns of a mode combination according to the invention at the top of the cavity.
• Fig. 4 is a schematic horizontal view of the bottom area of the oven cavity with an outline of the H field patterns of the same mode combination at the bottom of the cavity. In addition, the existence of cold spots is indicated.
• Fig. 5 is a simplified schematic vertical view of the same kind as in Fig. 2 with an outline of a momentary pattern of the vertical E field with areas of maximum and minimum field strength respectively indicated.
• Figs 1 and 2 schematically show a microwave oven and its associated microwave system according to an embodiment of the invention.
• the microwave system comprises an oven cavity 1, a waveguide device 2 applied to one side wall 10 of the oven cavity, on one side of which waveguide device there is a bulge 3 with a hole for inserting a coupling antenna 9 of a microwave source comprising a standard magnetron 8 with a frequency of 2.46 GHz.
• a bottom plate 5 transparent to microwaves on which the load 11, e.g. a foodstuff in a preparation container 13, is placed raised above the cavity bottom 15 during cooking.
• a microwave oven also comprises a power-supply unit driven by line voltage and generating high tension voltage to the magnetron, as well as control means for controlling the power-supply unit with respect to, inter alia, cooking time and power levels.
• the power-supply unit and said control means are of the standard type in microwave ovens and have been omitted for the sake of simplicity since they lie outside the scope of the invention.
• Fig. 2 thus shows a part-sectional side view of the cavity 1 with the waveguide device 2 upon which a magne- tron 8 with a coupling antenna 9 is mounted.
• the waveguide device 2 and the cavity are integrated, whereby the broad dimension of the waveguide facing the cavity is formed by a corresponding part of the side wall 10 of the cavity.
• the side wall 10 of the cavity there are a lower and an upper feed port 16 and 17 respectively, which are slot-shaped and communicate with the waveguide device 2, for feeding microwaves from the magnetron to the cavity 1.
• the waveguide device is dimensioned so that it is resonant for the purpose of providing advantageous field patterns in the cavity.
• the waveguide device is dimensioned so that there is a phase difference between the field which is excited by the feed port 17 and the feed port 16 respec- tively.
• the difference can be in the order of 45°.
• the waveguide device comprises a vertically arranged, rectangular waveguide 21 which feeds microwave power to the port 16.
• a controllable microwave-blocking member 23 is rotatably arranged spaced from the port 16.
• the member is of the type shown in Applicant's Swedish Patent Application 9703528-1, which is herewith incorporated by reference.
• the axial direction of the member 23 is thus perpendicular to the plane of the Figure and the member is indicated in the position in which it does not block the transportation of the microwave power through the waveguide 21.
• the member 23 is suitably rotated about its horizontal axis to the blocking position with the aid of a stepping motor or the like (not shown) , which suitably can be controlled by the usual control means of the microwave oven as desired for influencing the power balance in the cavity.
• a stepping motor or the like (not shown)
• any other suitable controllable blocking means could be utilised for controlling the feeding of microwaves from the feed port 16 into the oven cavity.
• the at least essentially rectangularly parallel- epipedal oven cavity 1 is dimensioned to provide a resonant quarterwave mode of the TM 43 type in the load in combination with an additional TM 5i mode.
• the oven cavity should have an effective height of about 180 - 200 mm.
• the effective height refers to the height between the load and the ceiling of the oven cavity. Said height dimension corre- sponds to the wavelength of the TM 43 mode, which is about 760 mm, i.e.
• the height measurement corresponds to a quarter wavelength for the TM 43 mode and half a wavelength for the TM 51 mode; i.e. the conditions are met for a 90° phase difference at the load.
• a width of about 330 mm and a depth of about 280 mm provide the desired conditions for the two modes.
• Said width dimension and depth dimension should be considered guiding values, which can be averages in the event that one cavity wall slopes somewhat in accordance with the description in Applicant's Swedish Patent Application 9700448-5, which is herewith incorporated by reference.
• the H fields associated with the modes of the TM 43 and TM 51 types are schematically illustrated in Figs 3 and 4 by means of the associated field pattern at the cavity ceiling (Fig. 3) and at the load (Fig. 4) respectively.
• the mode (4; 3) is represented by circles 31 and the mode (5;1) by ellipses 33.
• Fig. 4 also illustrates by means of shaded areas 35 the locations where cold spots are obtained. As can be seen, these spots are relatively faint and, in the central area of the oven cavity in particular, the heating pattern is exceedingly even. Moreover, in the case of very low ⁇ , it has been found that the vertical E field provides a heating effect which further reduces the occurrence of cold spots.
• Fig. 3 illustrates an alternative to the double feeding shown in Figs 1 and 2.
• a single feed port in the form of a rectangular feeding slot 41, which is centrally located in the ceiling of the cavity directly adjacent to the cavity side wall and has its longitudinal extent in the depth direction.
• the slot can be fed in the usual manner by the intermediary of a feeding waveguide located on the ceiling of the oven cavity, through which the two desired 11 modes have thus been excited simultaneously.
• the slot 41 can typically be about 70 mm long and 15 mm wide.
• Fig. 5 very schematically shows a pattern of the momentary vertical E field in a central vertical section in connection with double feeding of microwaves according to Figs 1 and 2 and under time-harmonic conditions. Areas 51 with maximum field strength and areas 53 with minimum field strength are indicated. The Figure clearly shows the phase difference between the field excited by the upper feed port 17 and the field excited by the lower feed port 16.
• the special double feeding of the oven cavity with a forced, specific phase dif- ference between the microwaves leaving the two feeding slots 16 and 17 also makes it possible to influence the balance between the excited modes.
• a reduced phase difference has been found to provide stronger excitation of the TM 43 mode, and an increased phase difference has been found to provide stronger excitation of the TM 5i mode.
• a suitable distribution of the intensities is that they be about equal.

Landscapes

• Physics & Mathematics (AREA)
• Electromagnetism (AREA)
• Constitution Of High-Frequency Heating (AREA)
• Electric Ovens (AREA)

Priority Applications (2)

Applications Claiming Priority (2)

Publications (1)

Family

ID=20410557

Family Applications (1)

Country Status (4)

Cited By (1)

Citations (4)

• 1998
  • 1998-03-16 SE SE9800856A patent/SE511668C2/sv not_active IP Right Cessation
• 1999
  • 1999-03-08 AU AU30324/99A patent/AU3032499A/en not_active Abandoned
  • 1999-03-08 WO PCT/EP1999/001489 patent/WO1999048336A1/en not_active Application Discontinuation
  • 1999-03-08 EP EP99911754A patent/EP1072170A1/en not_active Withdrawn

Patent Citations (5)

Also Published As

Similar Documents

Legal Events