EP2738474B1

EP2738474B1 - Microwave heating device

Info

Publication number: EP2738474B1
Application number: EP12817529.6A
Authority: EP
Inventors: Masaki Shibuya; Hiroshi Kawai; Ryuta Kondo; Makoto Nishimura
Original assignee: Panasonic Corp
Current assignee: Panasonic Corp
Priority date: 2011-07-25
Filing date: 2012-02-06
Publication date: 2016-12-14
Anticipated expiration: 2032-02-06
Also published as: JP5891453B2; WO2013014820A1; EP2738474A4; JPWO2013014820A1; EP2738474A1

Description

Technical Field

The present invention relates to microwave heating devices for performing microwave heating.

Background Art

Microwave heating devices include heating cookers for performing cooking through microwave heating on objects to be heated, for example. Such heating cookers have been adapted to use a roasting grid constituted by a combination of metal rod members welded to each other, in order to float objects to be heated from the bottom surface of the heating chamber (refer to Patent Literature 1, for example).
US3798404A discloses a device according to the preamble of claim 1.

Citation List

Patent Literatures

Patent Literature 1: Japanese Unexamined Utility Model Application Publication No. 55-059209

Summary of Invention

Technical Problem

Fig. 6 is a plan view illustrating a metal roasting grid 101 in a conventional heating cooker, illustrating a state where the roasting grid 101 is placed on a rail 104 provided on wall surfaces of a heating chamber 100. As illustrated in Fig. 6, the metal roasting grid 101 in the conventional heating cooker is provided with a plurality of bridge members 103 constituted by rod members, which are coupled to a rectangular-shaped outer frame 102 in parallel with each other.
When the roasting grid 101 in Fig. 6 is structured to have first sides 123 with a length of 300 mm, each of which connects the two points at which a bridge member 103 is electrically connected to the outer frame 102 in the roasting grid 101, since the wavelength of microwaves used in the heating cooker is about 120 mm, the first sides 123 correspond to 5/2 the wavelength. This is an integer multiple of 1/2 the wavelength, which induces resonance of microwaves, thereby causing resonance of microwaves in the bridge members 103 in the roasting grid 101, in a microwave heating mode. This results in larger electric currents flowing through the bridge members 103, which causes heat generation and energy losses. This has induced the problem of reduction of the heating efficiency of the heating cooker.
Further, if the roasting grid 101 is continuously in a state where there is heat generation from the bridge members 103 in the microwave heating mode, this induces deformations of the outer frame 102. This has caused the risk of serious accidents of fractures of the roasting grid 101.
The conventional roasting grid 101 illustrated in Fig. 6 is supported by a rail 104 formed integrally with the wall surfaces of the heating chamber 100 made of a metal, which is grounded. The roasting grid 101 illustrated in Fig. 6 has the shortest paths between a point at which the bridge member 103 is electrically connected to the outer frame 102 and a point at which the outer frame 102 is in contact with the rail 104, namely the intervals between the point at which the rail 104 grounded is in contact with the outer frame 102 and the point at which the bridge member 103 closest to the rail 104 is coupled to the outer frame 102, wherein the shortest paths are referred to as second sides 124. When the second sides 124 have a length of 30 mm, since the wavelength of microwaves is about 120 mm, the second sides 124 correspond to 1/4 the wavelength. This is an integer multiple of 1/4 the wavelength, which induces resonance of microwaves, when the second sides 124 are grounded at their one ends. This induces resonance of microwaves in the outer frame 102, which causes larger electric currents flowing therethrough, thereby causing heat generation and energy losses, in the microwave heating mode. This has degraded the heating efficiency of the heating cooker. Accordingly, if such a heat-generating state is further continued, this causes deformations of the outer frame 102, thereby causing fractures of the roasting grid 101. This has been a problem.
Some conventional heating cookers have been structured such that, in order to prevent electric contact between the roasting grid and the wall surfaces of the heating chamber, coatings are applied to the contact portions or spacers made of insulating materials are provided between the roasting grid and the wall surfaces of the heating chamber. However, these structures have had the problems of increased costs and increased weights of the heating cookers.
The present invention was made in order to solve the aforementioned problems of conventional microwave heating devices and aims at providing a microwave heating device having inexpensiveness, a light weight, excellent durability and higher efficiency and being capable of reducing energy losses.

Solution to Problem

To solve the problems of the conventional microwave heating devices, a microwave heating device of the present invention includes:

a heating chamber being formed from a grounded wall surface and being adapted to heat an object to be heated; a microwave generating portion adapted to supply a microwave to inside the heating chamber; and a roasting grid (4B) for placing the object to be heated, the roasting grid being formed from metal rod members which are in contact with the opposite wall surfaces of the heating chamber to be electrically connected, wherein the roasting grid includes an outer frame, and
a plurality of bridge members formed from metal rod members across an area encircled by the outer frame, the outer frame is formed to have a substantially-rectangular shape from a linear rod member extending in a longitudinal direction and a linear rod member extending in a lateral direction, each of the bridge members is provided in parallel with the rod member in the longitudinal direction of the outer frame or with the rod member in the lateral direction of the outer frame, respectively, the bridge members are intersected with each other and contacted with each other to be in a state where the bridge members are electrically connected to each other; and
in a closed loop having a single opening which is formed by the bridge members and the outer frame, a single side formed by one of the bridge members, of which one end is contacted with the wall surface of the heating chamber and of which the other end is placed within the heating chamber, is made to have a length of an odd number multiple of about 1/8 the wavelength of the microwave supplied from the microwave generating portion.

The microwave heating device having the aforementioned structure according to the present invention is capable of preventing larger electric currents from flowing through the bridge members and/or the outer frame which constitute the metal roasting grid, thereby suppressing heat generation in the roasting grid, in the microwave heating mode. Therefore, the microwave heating device forms a heating device which is capable of heating with higher energy efficiency and with less energy losses and, thus, has inexpensiveness, a light weight and excellent durability.

Advantageous Effects of Invention

According to the present invention, it is possible to provide a microwave heating device which is capable of suppressing heat generation in the roasting grid within the heating chamber in the microwave heating mode, thus is capable of performing microwave heating with less energy losses and with higher energy efficiency and, therefore, has inexpensiveness, a light weight and excellent durability.

Brief Description of Drawings

Fig. 1 is a front cross-sectional view schematically illustrating the structure of a heating cooker according to a first embodiment, as an example of a microwave heating device according to the present invention.
Fig; 2a is a plan view illustrating a roasting grid which is not an embodiment of the invention.
Fig. 2(b) is a plan view illustrating a roasting grid for use in the heating cooker according to the first embodiment of the present invention.
Fig. 3 is a cross-sectional view illustrating the roasting grid and the like in the heating cooker according to the first embodiment illustrated in Fig. 2.
Fig. 4 is a characteristic view illustrating results of experiments conducted using the roasting grid in the heating cooker according to the first embodiment.
Fig. 5 is a characteristic view illustrating results of experiments conducted using the roasting grid in the heating cooker according to the first embodiment.
Fig. 6 is a plan view illustrating a metal roasting grid in a conventional heating cooker.

Description of Embodiments

In an aspect of the present invention, in at least a single closed loop having a single opening which is formed by the bridge members, a single side is made to have a length of an odd number multiple of about 1/4 the wavelength of the microwave supplied from the microwave generating portion. With the microwave heating device having this structure in the fourth aspect, it is possible to cause microwaves to cancel each other, which inhibits electric currents from flowing through the bridge members or the outer frame, thereby suppressing heat generation in the roasting grid in the microwave heating mode. Therefore, the microwave heating device in the fourth aspect forms a microwave heating device which is capable of performing microwave heating with less energy losses and with higher energy efficiency and, thus, has inexpensiveness, a light weight and excellent durability.
In an aspect of the present invention, in a closed loop having a single opening which is formed by the bridge members, or by the bridge members and the outer frame, a largest linear length in the opening in its inner dimension may be made to be equal to or more than 1/2 the wavelength of the microwave supplied from the microwave generating portion. The microwave heating device having this structure in the sixth aspect is adapted to allow microwaves to easily pass through the opening of the closed loop in the roasting grid, thereby allowing microwaves to come around to the back side of the object to be heated with respect to the microwave radiating position. Therefore, the microwave heating device in the sixth aspect is enabled to heat the object to be heated at its upper and lower sides, with higher efficiency and without heating unevenness.
Hereinafter, preferred embodiments of a microwave heating device according to the present invention will be described, with reference to the accompanying drawings. Further, while the microwave heating devices according to the following embodiments will be described with respect to microwave ovens having oven functions as heating cookers, these microwave ovens having such oven functions are merely illustrative, and the microwave heating device according to the present invention is not limited to such microwave ovens and also include microwave ovens, oven-type microwave ovens and electric ovens as heating cookers which utilize microwave heating, commercial microwave heating of various types, heating devices in industrial fields, such as thawing devices and drying devices, and other microwave heating devices for ceramic-art heating, sintering, biochemical reaction, and the like. Further, the present invention is not limited to the concrete structures according to the following embodiments and also includes structures based on technical concepts similar thereto.

(First Embodiment)

Fig. 1 is a front cross-sectional view schematically illustrating the structure of a heating cooker according to a first embodiment, as an example of a microwave heating device according to the present invention.
The heating cooker according to the first embodiment illustrated in Fig. 1 is provided with a heater provided in an upper side (hereinafter, abbreviated as an upper heater) 2, and a heater provided in a lower side (hereinafter, abbreviated as a lower heater) 3, within a heating chamber 1 having surfaces formed from vitreous-enamel-coated steel plates. Further, within the heating chamber 1, there is provided a roasting grid 4 constituted by a combination of metal rod members such as stainless-steel rod members which are welded to each other, for example. The roasting grid 4 is slidably supported by being sandwiched, at its upper and lower sides, by a lower rail 12 and an upper rail 13 which are provided on the opposite side wall surfaces of the heating chamber 1.
The heating cooker according to the first embodiment is adapted such that food 5 as an object to be heated is placed on the roasting grid 4, and this food 5 is subjected to radiation heating from thereabove and therebelow by the upper heater 2 and the lower heater 3.
The heating chamber 1 is formed by wall surfaces (a ceiling surface, a back surface, opposite side surfaces, and a bottom surface) which have corners formed from curved surfaces, and the bottom surface of the heating chamber 1 is formed to have a large arc shape and, thus, has a curved shape protruding downwardly. Further, the walls of the heating chamber 1 are grounded by being connected to an earth cord (not illustrated). Further, the lower rail 12 and the upper rail 13 for supporting the metal roasting grid 4 at a predetermined position within the heating chamber 1 are formed integrally with the walls of the heating chamber 1 and are electrically grounded.
Further, although the heating cooker according to the first embodiment will be described with respect to an example of a structure where vitreous-enamel coating has been applied to the wall surfaces of the heating chamber 1, other thermal-resistant coating may be also applied to the wall surfaces of the heating chamber 1. Further, as the metal material of the walls of the heating chamber 1, it is also possible to employ stainless-steel members or PCM (pre-coated metal) steel plates. Further, as the metal material of the roasting grid 4, it is also possible to employ steel members having been subjected to plating, and the like.
As illustrated in Fig. 1, above the heating chamber 1, to the right thereof, a magnetron 6 is provided, as a microwave generating portion. The magnetron 6 is coupled, at its output portion, to a waveguide 14 such that it protrudes in the horizontal direction.
The heating cooker according to the first embodiment is adapted to perform heating processing on the food 5 as the object to be heated, through at least any one heating method, out of microwave heating with microwaves from the magnetron 6, and heater heating with radiant heat and convection heat using the upper heater 2 and the lower heater 3.
In the heating cooker according to the first embodiment, an upper-heater thermocouple 7 is provided in such a way as to be in contact with the surface of the upper heater 2. The upper-heater thermocouple 7 is covered with a metal pipe, in order to be prevented from being influenced by microwaves from the magnetron 6. The temperature of the upper heater 2 is detected by the upper-heater thermocouple 7 having this structure.
Further, similarly to the upper-heater thermocouple 7, a lower-heater thermocouple 8 is provided in such a way as to be in contact with the surface of the lower heater 3, and the lower-heater thermocouple 8 is covered with a metal pipe, in order to be prevented from being influenced by microwaves from the magnetron 6. The temperature of the lower heater 3 is detected by the lower-heater thermocouple 8 having this structure.
A thermistor 9 for detecting the temperature inside the heating chamber 1 is secured to a wall surface of the heating chamber 1. The upper-heater thermocouple 7, the lower-heater thermocouple 8 and the thermistor 9 are electrically connected to a control portion 10, and respective detection signals therefrom are inputted to the control portion 10. The control portion 10 is enabled to control the supply of electricity and the like to the upper heater 2 and the lower heater 3 based on the respective detection signals for performing control for increasing and decreasing the amount of heating supplied to the inside of the heating chamber 1.
Referring to Fig. 1, the magnetron 6, which has a bore diameter of about 70 mm when viewed at its right side, is coupled to the waveguide 14 such that its output portion protrudes in the horizontal direction. The waveguide 14 has an interior path which is formed to have an L shape and is constituted by a vertical portion to which the output portion of the magnetron 6 is connected, and a horizontal portion extended horizontally from one end portion of the vertical portion. Near the center of the horizontal portion of the waveguide 14, a rotational antenna 11 as a radio-wave stirring portion is provided and is connected to a motor 18. The rotational antenna 11 is constituted by a rotating portion 11a for stirring microwaves, and a shaft portion 11b connected to the motor 18. The horizontal portion of the waveguide 14 is provided with a feeding port 17, in its surface facing the heating chamber 1. The shaft portion 11b of the rotational antenna 11 penetrates through the feeding port 17. Further, the waveguide 14 is provided with a conical dome 15 such that it surrounds the feeding portion 17. The antenna portion 11a of the rotational antenna 11 is placed inside the dome 15, and the antenna portion 11a is adapted to rotate inside the dome 15.
The antenna portion 11a of the rotational antenna 11 is formed from a metal plate with a substantially-disk shape and with a thickness of 1 mm and a diameter of about φ62. The shaft portion 11b is coupled to the disk plate which forms the antenna portion 11 a at a position deviated from its center by about 12 mm.
The shaft portion 11b of the rotational antenna 11 has a shaft portion formed from a fluorocarbon resin, which is closer to the motor 18, and, further, has a shaft portion formed from a metal, which is closer to the antenna portion 11a. The shaft portion 11b formed from the metal is protruded at its one side, by about 11 mm, into the waveguide 14. The other side of the shaft portion 11b formed from the metal penetrates through the feeding portion 17 in the dome 15 and, further, protrudes toward the heating chamber 1 by about 15 mm. There is secured a gap of 5 mm or more, between the shaft portion 11b and the feeding portion 17.
Further, the heating cooker according to the first embodiment has been described with respect to an example where the microwave supply portion constituted by the magnetron 6, the rotational antenna 11, the waveguide 14, the dome 15, the feeding portion 17 and the like is provided above the heating chamber 1. However, the present invention is not limited to this structure. For example, the microwave supply portion can be provided near the bottom portion of the heating chamber or near the side-surface portions thereof. Further, the magnetron and the waveguide can be installed in any orientations.
The heating cooker according to the first embodiment is provided with a cover 16 made of a mica, under the dome 15, in order to prevent contaminations from adhering to the rotational antenna 11. The cover 16 is structured such that it is detachable through a hook 19 made of an insulating member, which is secured to the ceiling surface of the heating chamber 1.
In the heating cooker according to the first embodiment, the upper heater 2 is placed such that it evades the position beneath the lower opening portion in the dome 15, in order that the upper heater 2 is not directly influenced by microwaves. Further, although the cover 16 has been described with respect to an example where the cover 16 is made of mica, which is a low-loss dielectric material, it can be similarly made of ceramics or glasses.
Fig. 2 is plan views illustrating respective roasting grids 4 (4A and 4B) of two types for use in the heating cooker. Fig. 2 illustrates a state where each of the roasting grid 4 (4A and 4B) is supported on the lower rail 12 in the hearing room 1.
The roasting grid 4A illustrated in (a) of Fig. 2 is constituted by a rectangular-shaped annular outer frame 21A formed from a rod member with a diameter of φ6 which is made of a stainless-steel, and a plurality of bridge members 20 formed from rod members with a diameter of φ 3 which are made of a stainless-steel, similarly. The plurality of the bridge members 20 are linear rod members having the same length. The roasting grid 4A is provided, in its center portion, with the plurality of the bridge members 20 in parallel with each other at predetermined intervals, such that they connect the front-surface side and the rear-surface side of the rectangular-shaped outer frame 21A which are opposed to each other. The bridge members 20 and the outer frame 21A are mechanically coupled and electrically connected to each other, by being welded to each other at the positions where the outer frame 21A intersects with the opposite end portions of the bridge members 20. The roasting grid 4A having this structure is placed on and contacted with the lower rail 12 formed integrally with the heating chamber 1 which is grounded and, thus, the roasting grid 4A is provided slidably with respect to the lower rail 12.
As illustrated in (a) of Fig. 2, the roasting grid 4A is provided with nine bridge members 20 which are placed in parallel with each other at even intervals. The roasting grid 4A according to the first embodiment has first paths (sides) 22 with a length of 30 mm, each of which connects the two coupling points at which bridge members 20 adjacent to each other are electrically connected to a single side of the outer frame 21A. The length of the first paths (sides) 22 is equal to 1/4 times the wavelength of microwaves used in the heating cooker, which is 120 mm.
There are second paths (sides) 23 having a length of 330 mm, each of which connects the two coupling points at which the front-surface side and the rear-surface side of the outer frame 21A, which are opposed to each other, are electrically connected to a single bridge member 20. The length of the second paths (sides) 23 is equal to 11/4 times the wavelength of microwaves used in the heating cooker, which is 120 mm.
Further, there are shortest paths (sides) having a length of 45 mm, each of which connects the coupling point at which the outer frame 21A is electrically connected to the bridge member 20 closest to a side wall surface of the heating chamber 1 to a point at which the outer frame 21A is in contact with the lower rail 12. Namely, the shortest paths (sides) are third paths (sides) 24 each connecting the point at which the outer frame 21A is in contact with the lower rail 12 to be grounded, to the coupling point closest to the side wall surface of the heating chamber 1. The length of the third paths 24 is equal to 3/8 times the wavelength of microwaves, which is 120 mm.
In the roasting grid 4A the outer frame 21A and bridge members 20 adjacent to each other illustrated in (a) of Fig. 2 form a closed loop which has an opening having a largest linear length 25 of 324 mm, at the inner side thereof. This largest linear length 25 is the largest linear length between the opposing sides of the opening of the closed loop in its inner dimension and is equal to or more than 1/2 times the wavelength of microwaves, which is 120 mm.
Further, in the roasting grid 4A illustrated in (a) of Fig. 2, the plurality of the bridge members 20 are placed in such a way as to be put on the outer frame 21A and, further, the bridge members 20 and the outer frame 21A are mechanically and electrically coupled to each other, and the bridge members 20 are protruded, at their opposite ends, outwardly from the outer frame 21A. However, the roasting grid 4 can be also structured by coupling the opposite end surfaces of the rod-shaped bridge members 20 to the inner surfaces of the outer frame 21A, such that the opposite ends of the bridge members 20 are not protruded outwardly from the outer frame 21A. With this structure, it is possible to prevent concentrations of electric fields at the opposite ends of the bridge members 20.
On the other hand, the roasting grid 4B illustrated in (b) of Fig. 2 is constituted by a rectangular-shaped annular outer frame 21B formed from a rod member with a diameter of φ6 which is made of a stainless-steel, a plurality of longitudinal bridge members 26 formed from rod members with a diameter of φ 3 which are made of a stainless-steel, similarly, and a plurality of lateral bridge members 27 formed from rod members with a diameter of φ 3 which are made of a stainless-steel, similarly. The plurality of the longitudinal bridge members 26 and the plurality of the lateral bridge members 27 are linear rod members having respective constant lengths. The roasting grid 4B is provided, in its center portion, with the plurality of lateral bridge members 27 in parallel with each other at predetermined intervals, such that they connect the opposite side-surface sides of the rectangular-shaped outer frame 21B which are opposed to each other. The roasting grid 4B is provided with three lateral bridge members 27 (27a, 27b and 27c).
In the roasting grid 4B, the rod-shaped lateral bridge members 27 are mechanically and electrically coupled, at their opposite end surfaces, to the inner surfaces of the outer frame 21B. The lateral bridge members 27 are structured such that their opposite ends are not protruded outwardly from the outer frame 21B. Further, the plurality of the longitudinal bridge members 26 are provided in parallel with each other at predetermined intervals, such that they are orthogonal to the lateral bridge members 27. The lateral bridge members 27 and the longitudinal bridge members 26 are mechanically and electrically coupled to each other, by being welded to each other at the positions where they intersect with each other. The roasting grid 4B having this structure is placed on and is contacted with the lower rail 12 formed integrally with the heating chamber 1, which is grounded, and, thus, the roasting grid 4B is provided slidably with respect to the lower rail 12.
As illustrated in (b) of Fig. 2, the roasting grid 4B is provided with nine longitudinal bridge members 26 which are placed in parallel with each other at even intervals. The roasting grid 4B according to the first embodiment has first paths (sides) 22 with a length of 30 mm, each of which connects the two points at which longitudinal bridge members 26 adjacent to each other are coupled to a single lateral bridge member 27. The length of the first paths (sides) 22 is equal to 1/4 times the wavelength of microwaves used in the heating cooker, which is 120 mm.
The nine longitudinal bridge members 26 are placed in parallel with each other at predetermined intervals in such a way as to connect the lateral bridge members 27a and 27c positioned at the opposite ends (in the front-surface side and the rear-surface side), out of the three lateral bridge members 27 placed in such a way as to intersect with the longitudinal bridge members 26 orthogonally thereto, and these longitudinal bridge members 26 and the lateral bridge members 27a and 27c are electrically and mechanically coupled to each other. Further, the longitudinal bridge members 26 are similarly coupled electrically and mechanically to the middle lateral bridge member 27b between the lateral bridge members 27a and 27c in the front-surface side and the rear-surface side. This middle lateral bridge member 27b is placed to divide each longitudinal bridge members 26 into two parts, which are a second path (side) 23a in the rear-surface side and a second path 23b in the front-surface side. The second path (side) 23a in the rear-surface side has a length of 90 mm, wherein the second path (side) 23a in the rear-surface side connects the two coupling points at which the rear-surface-side lateral bridge member 27c and the middle lateral bridge member 27b are electrically connected to the longitudinal bridge member 26. The length of the second path (side) 23a is equal to 3/4 times the wavelength of microwaves, which is 120 mm.
The second path (side) 23b in the front-surface side has a length of 210 mm, wherein the second path (side) 23b in the front-surface side connects the two coupling points at which the middle lateral bridge member 27b and the front-surface-side lateral bridge member 27a are electrically coupled to the longitudinal bridge member 26 existing on the straight line on which the second path (side) 23a exists. The length of the second path (side) 23b is equal to 7/4 times the wavelength of microwaves, which is 120 mm.
Further, there are shortest paths (sides) having a length of 45 mm, each of which connects the coupling point at which a lateral bridge member 27 is electrically connected to the longitudinal bridge member 26 closest to a side wall surface of the heating chamber 1 to a point at which the outer frame 21B, to which the lateral bridge members 27 are electrically connected, is in contact with the lower rail 12. Namely, the shortest paths (sides) are third paths (sides) 24 each connecting the point at which the outer frame 21B is in contact with the lower rail 12 to be grounded, to the coupling point closest to the side wall surface of the heating chamber 1. The length of the third paths (side) 24 is equal to 3/8 times the wavelength of microwaves, which is 120 mm.
The roasting grid 4B used in the heating cooker according to the first embodiment has fourth paths (sides) 28 with a length of 330 mm, each of which connects the two points at which the lower rail 12 is in contact, at its opposite sides, with the outer frame 21B, namely the two points closest to the grounded points in the opposite side surfaces of the heating chamber 1. The length of the fourth paths (sides) 28 is equal to 11/4 times the wavelength of microwaves, which is 120 mm.
Longitudinal bridge members 26 adjacent to each other, the middle lateral bridge member 27b and the rear-surface-side lateral bridge member 27c form a rear-surface-side closed loop which has an opening having a largest linear length 25 of 88 mm, at the inner side thereof. This largest linear length 25 is the largest linear length between the opposing sides of the opening of the rear-surface-side closed loop in its inner dimension and is equal to or more than 1/2 times the wavelength of microwaves, which is 120 mm. Further, longitudinal bridge members 26 adjacent to each other, the front-surface-side lateral bridge member 27a and the middle lateral bridge member 27b form a front-surface-side closed loop having an opening which is larger than the opening of the aforementioned rear-surface-side closed loop and, therefore, is equal to or more than 1/2 times the wavelength of microwaves, which is 120 mm, as a matter of course.
Further, in the roasting grid 4B, the plurality of the longitudinal bridge members 26 are placed in such a way as to be put on the lateral bridge members 27, further the lateral bridge members 27 and the longitudinal bridge members 26 are coupled to each other and, further, the longitudinal bridge members 26 are structured such that their opposite ends are not protruded rearwardly or forwardly from the lateral bridge members 27.
Fig. 3 is a cross-sectional view illustrating the roasting grid 4A and the like in the heating cooker according to the first embodiment illustrated in (a) of Fig. 2 and is a cross-sectional view of the same taken along the line III-III in (a) of Fig. 2.
As illustrated in Fig. 3, the roasting grid 4A mounted within the heating chamber 1 at a predetermined position is placed such that the outer frame 21A is sandwiched between the lower rail 12 and the upper rail 13. The upper rail 13 is provided, in order to prevent the roasting grid 4A from being inclined halfway through being pulled outwardly, when the roasting grid 4A is pulled outwardly from the heating chamber 1.
Hereinafter, the heating cooker having the aforementioned structure according to the first embodiment will be described, with respect to operations and effects thereof. Further, the heating cooker according to the first embodiment is adapted to have a microwave heating mode for performing heating with microwaves, and a heater heating mode for performing heating with the upper and lower heaters 2 and 3.
During a heating operation with the heating cooker according to the first embodiment, if a user selects the microwave heating mode and, further, turns on a heating switch, the magnetron 6 generates microwaves. The microwaves from the magnetron 6 are propagated through the waveguide 14 and are directed to the rotational antenna 11. The rotational antenna 11 being rotated by the motor 18 supplies microwaves to the inside of the heating chamber 1 while stirring the microwaves. The microwaves having been supplied to the inside of the heating chamber 1 are partly absorbed directly by the food 5 as the object to be heated and, also, are partly absorbed by the food 5 while being reflected by the wall surfaces of the heating chamber 1. Further, some portions of the microwaves having been supplied to the inside of the heating chamber 1 can easily pass through the roasting grid 4, then can be reflected by the bottom surface of the heating chamber 1 and can come around to below the food 5 to be absorbed by this food 5, since the openings in the roasting grid 4 have sizes equal to or more than 1/2 the wavelength of microwaves. Thus, the microwaves supplied to the inside of the heating chamber 1 are absorbed by the food 5, so that the food 5 is heated thereby.
On the other hand, during a heating operation with the heating cooker according to the first embodiment, if the user selects the heater heating mode and, further, turns on the heating switch, the upper heater 2 and the lower heater 3 are supplied with electricity. Since the upper heater 2 and the lower heater 3 are supplied with electricity, the upper heater 2 and the lower heater 3 generate heat, so that radiant heat is transmitted to the inside of the heating chamber 1. The food 5 placed between the upper heater 2 and the lower heater 3 is heated by radiant heat and convection heat from the upper heater 2 and the lower heater 3.
Fig. 4 is a characteristic view illustrating results of experiments conducted using the roasting grid 4A having the structure illustrated in (a) of Fig. 2. In Fig. 4, the vertical axis represents a highest temperature [°C] of heating generation in the roasting grid 4A, while the horizontal axis represents the length [mm] of the second paths (sides) 23. In these experiments, the highest temperature of heating generation in the roasting grid 4A was detected, while the length of each second path (side) 23 was varied, wherein each second path (side) 23 connects the two coupling points in a bridge member 20 which electrically connects the front-surface side and the rear-surface side of the outer frame 21A which are faced to each other.
As can be clearly seen from the characteristic view illustrated in Fig. 4, when the length of the second paths (sides) 23 in the roasting grid 4A was 60 mm, 120 mm, ..., namely when it was an integer multiple of 1/2 the wavelength of microwaves, which was 120 mm, the temperature of heating generation was equal to or higher than 60°C, and the temperature of heating generation was higher than in cases of other lengths. On the contrary, when the length of the second sides 23 in the roasting grid 4A was 30 mm, 90 mm, ..., namely when it was an odd number multiple of 1/4 the wavelength of microwaves, which was 120 mm, the temperature of heating generation was lower than temperatures of heating generation therearound.
This is because, if the length of the second paths (sides) 23 in the roasting grid 4A is a length close to an integer multiple of 1/2 the wavelength of microwaves, this induces resonance to cause larger electric currents to flow therethrough, thereby causing heat generation. On the contrary, if the length of the second paths (sides) 23 is an odd number multiple of 1/4 the wavelength of microwaves, microwaves cancel each other, which inhibits electric currents from flowing therethrough, thereby suppressing heat generation.
Fig. 5 is a characteristic view illustrating results of experiments conducted using the roasting grid 4A having the structure illustrated in (a) of Fig. 2. In Fig. 5, the vertical axis represents a highest temperature [°C] of heating generation in the roasting grid 4A, while the horizontal axis represents the length [mm] of the third paths (sides) 24. These experiments were conducted by varying the length of each shortest path (side) connecting a coupling point at which the outer frame 21A was electrically connected to the bridge member 20 closest to a side wall surface of the heating chamber 1 to a point at which the outer frame 21A was in contact with the lower rail 12, namely each third path (side) 24 connecting the point at which the outer frame 21A was in contact with the lower rail 12 to be grounded to the coupling point closest to the side wall surface of the heating chamber 1. The highest temperature of heating generation in the roasting grid 4A was detected, while the length of the third paths (sides) 24 was varied.
As can be clearly seen from the characteristic view illustrated in Fig. 5, when the length of the third paths (sides) 24 which were grounded at their one ends was 30 mm and 90 mm, namely when it was an odd number multiple of 1/4 the wavelength of microwaves, which was 120 mm, the temperature of heating generation was equal to or higher than 60°C, and the temperature of heating generation was higher than in cases of other lengths. On the other hand, when the length of the third paths 24 in the roasting grid 4A was 15 mm, 45 mm and 75 mm, namely when it was an odd number multiple of 1/8 the wavelength of microwaves, which was 120 mm, the temperature of heating generation was lower than temperatures of the heating generation therearound.
From the aforementioned description, it can be seen that even the same rod members exhibit different temperature of heating generation characteristics with respect to the length, depending on whether or not they are grounded at their one ends. This is considered to be caused by the following fact. That is, when the rod members forming the roasting grid 4 are grounded at their one ends, these one ends play a role as electric mirrors. It can be considered as if a length of an odd number multiple of 1/4 the wavelength of microwaves were a length of twice an odd number multiple of 1/4 the wavelength of microwaves, namely a length equal to an integer multiple of 1/2 the wavelength of microwaves. Therefore, if the rod members grounded at their one ends have a length equal to an odd number multiple of 1/4 the wavelength of microwaves, this induces resonance to generate larger electric currents flowing therethrough, thereby causing heat generation. On the other hand, when the rod members grounded at their one ends have a length of an odd number multiple of 1/8 the wavelength of microwaves (for example, 15 mm), it is considered as if they had a length of twice an odd number multiple of 1/8 the wavelength of microwaves, namely a length equal to an odd number multiple of 1/4 the wavelength of microwaves (for example, 30 mm). Therefore, if the rod members grounded at their one ends have a length equal to an odd number multiple of 1/8 the wavelength of microwaves (for example, 15 mm), microwaves cancel each other, which inhibits electric currents from flowing therethrough, thereby suppressing heat generation.
As described above, in the heating cooker, the lengths of the first paths (sides) 22 and the second paths (sides) 23, 23a and 23b in the roasting grid 4 (4A, 4B) are deviated from an integer multiple of 1/2 the wavelength of microwaves and, thus, are made to be respective odd number multiples of 1/4 the wavelength of microwaves. Further, the length of the third paths (sides) 24 grounded at their one ends in the roasting grid 4 is deviated from an integer multiple of 1/4 the wavelength of microwaves and is made to be an odd number multiple of 1/8 the wavelength of microwaves. By using the roasting grid 4 having the aforementioned structure, it is possible to prevent larger electric currents from flowing through the bridge members 20, 26 and 27 and the outer frame 21A or 21B in the microwave heating mode, which suppresses heat generation and reduces energy losses, thereby enabling microwave heating with higher energy efficiency. As a result thereof, the heating cooker according to the first embodiment forms an inexpensive and lightweight heating device having excellent durability.
Further, in the heating cooker according to the first embodiment, in the roasting grid 4B illustrated in (b) of Fig. 2, the fourth paths (sides) 28 are in contact, at their opposite ends, with the lower rail 12 provided on the opposite side surfaces of the heating chamber 1. Therefore, if the length of the fourth paths (sides) 28 is set to be a length close to an integer multiple of 1/2 the wavelength of microwaves, this induces resonance, which causes larger electric currents to flow therethrough, thereby causing heat generation from the fourth paths (sides) 28. On the contrary, if the length of the fourth paths (sides) 28 is set to be an odd number multiple of 1/4 the wavelength of microwaves, microwaves cancel with each other, which inhibits electric currents from flowing therethrough, thereby suppressing heat generation.
Accordingly, in the roasting grid 4B in the heating cooker according to the first embodiment, the length of the fourth paths (sides) 28 is deviated from an integer multiple of 1/2 the wavelength of microwaves and, thus, is set to be an odd number multiple of 1/4 the wavelength of microwaves (330 mm = 11/4 thereof). Therefore, through the fourth paths (sides) 28 in the roasting grid 4B, it is possible to prevent larger electric currents from flowing through the bridge members 26 and 27 and the outer frame 21B, thereby suppressing heat generation therein, in the microwave heating mode. Therefore, the heating cooker using the roasting grid 4B is capable of performing cooking through microwave heating with higher efficiency and with less energy losses and, thus, has inexpensiveness, a light weight and excellent durability.
Further, as illustrated in (b) of Fig. 2, the roasting grid 4B is structured such that the longitudinal bridge members 26 and the lateral bridge members 27 (27a, 27b and 27c) are orthogonally intersected and contacted with each other. This enables dividing each second path 23 into two parts at the position of intersection, which enables easily adjusting both the length of the second path (side) 23a in the rear-surface side and the length of the second path (side) 23b in the front-surface side to be respective lengths deviated from an integer multiple of 1/2 the wavelength of microwaves. Further, since the roasting grid 4B is structured such that the plurality of the bridge members 26 and 27 (27a, 27b, 27c) intersect with each other, it is possible to make the roasting grid 4B have enhanced rigidity, thereby offering a heating device with excellent durability.
If the roasting grid 4B is structured to be provided with no middle lateral bridge member 27b, the longitudinal bridge members 26 are electrically connected, at their opposite ends, to the lateral bridge members 27. The length of the path connecting these two coupling points to each other is the sum of the length of the second path (side) 23a in the rear-surface side (90 mm) and the length of the second path (side) 23b in the front-surface side (210 mm) and, thus, is 300 mm. This total length is an integer multiple of 1/2 the wavelength of microwaves, which is 120 mm, which causes larger electric currents to flow through these longitudinal bridge members 26, thereby causing heat generation. However, as illustrated in (b) of Fig. 2, the roasting grid 4B is structured to be provided with the middle lateral bridge member 27 with respect to the longitudinal bridge members 26, which enables easily setting the lengths of the second sides 23a and 23b to be respective odd number multiples of 1/4 the wavelength of microwaves.
The roasting grid 4B is structured such that the longitudinal bridge members 26 parallel with the side-surface sides of the outer frame 21B and the lateral bridge members 27 parallel with the front-surface/rear-surface sides of the outer frame 21B are coupled to the rectangular-shaped outer frame 21B. Further, the longitudinal bridge members 26 and the lateral bridge members 27 are constituted by rod members having respective linear shapes and respective constant lengths, which facilitates the manufacture of the roasting grid 4B, thereby enabling manufacture thereof with lower costs. This enables provision of a heating device with inexpensiveness and with a simple structure.
Further, in the heating cooker, the respective largest linear lengths 25 in the openings in the roasting grids 4A and 4B are made to be equal to or more than 1/2 the wavelength of microwaves. This structure allows microwaves to easily pass through the roasting grids 4A and 4B, thereby allowing microwaves to come around to the back side of the object to be heated. This can provide a heating cooker capable of heating the object to be heated at its upper and lower sides, with higher efficiency, and without inducing heating unevenness.
Further, in the structure of the roasting grid 4 (4A, 4B), the lengths of the first paths (sides) 22, the second paths (side) 23, 23a and 23b, the third paths (sides) 24, and the fourth paths (sides) 28 may be varied by about 5 mm at a maximum, due to fluctuations such as deviations of the placement of the roasting grid 4 within the heating chamber 1, deviations of the welding thereof during the manufacture, and the like. However, even if they have such a degree of variations, the heating cooker using the roasting grid is capable of offering substantially the same effects.
Further, in the structure of the roasting grid 4 (4A, 4B), it is desirable that the lengths of the first paths (sides) 22, the second paths (sides) 23, 23a and 23b, the third paths (sides) 24, and the fourth paths (sides) 28 are equal to or more than 5 mm, in consideration of processing variations, provided that they satisfy the condition that the largest linear lengths 25 be equal to or more than 1/2 the wavelength of microwaves. Further, it is appropriate that the size of the roasting grid 4A or 4B is about 450 mm at a maximum, although it is determined by the size of the heating chamber 1.
The microwave heating device according to the first embodiment has been described with respect to a heating cooker by exemplifying a roasting grid for placing food 5 thereon. However, the present invention is not limited to roasting grids for such an application and is intended to cover roasting grids (placement tables) for placing objects to be heated for performing microwave heating thereon.
The microwave heating device according to the present invention is adapted to use, within the heating chamber, the metal roasting grid for placing an object to be heated thereon, wherein the structure of the metal roasting grid is determined in consideration of the wavelength of microwaves to be used, as described with respect to the heating cooker in the first embodiment. This can provide a heating device having inexpensiveness, a light weight and excellent durability and being capable of microwave heating with higher efficiency and with less energy losses.

Industrial Applicability

The microwave heating device according to the present invention is capable of heating with higher efficiency and with less energy losses and, therefore, is applicable to microwave ovens, oven-type microwave ovens and electric ovens as cooking appliances which utilize microwave functions and, also, is applicable to commercial microwave heating of various types, heating devices in industrial fields, such as thawing devices and drying devices, and other various types of applications such as ceramic-art heating, sintering, biochemical reaction, and the like.

Reference Signs List

1: Heating chamber
2: Upper heater
3: Lower heater
4: Roasting grid
5: Food
6: Magnetron
11: Rotational antenna
12: Lower rail
13: Upper rail
14: Waveguide
20: Bridge member
21: Outer frame
22: First path (side)
23: Second path (side)
23a: Rear-surface side second path (side)
23b: Front-surface side second path (side)
24: Third path (side)
25: Largest linear length
26: Longitudinal bridge member
27: Lateral bridge member

Claims

A microwave heating device comprising:
a heating chamber (1) being formed from a grounded wall surface and being adapted to heat an object to be heated;

a microwave generating portion (6) adapted to supply a microwave to inside the heating chamber (1); and

a roasting grid (4B) for placing the object to be heated, the roasting grid (4B) being formed from metal rod members which are in contact with the opposite wall surfaces of the heating chamber (1) to be electrically connected,
wherein the roasting grid (4B) includes an outer frame (21B), and
a plurality of bridge members (26, 27) formed from metal rod members across an area encircled by the outer frame (21B),
the outer frame (21 B) is formed to have a substantially-rectangular shape from a linear rod member extending in a longitudinal direction and a linear rod member extending in a lateral direction, and
each of the bridge members (26, 27) is provided in parallel with the rod member in the longitudinal direction of the outer frame or with the rod member in the lateral direction of the outer frame (21B), respectively; the microwave heating device being characterised in that
the bridge members (26, 27) are intersected with each other and contacted with each other to be in a state where the bridge members (26, 27) are electrically connected to each other,
in a closed loop having a single opening which is formed by the bridge members (26, 27) and the outer frame (21B); and in that
a single side (24) formed by one of the bridge members (26, 27), of which one end is contacted with the wall surface of the heating chamber (1) and of which the other end is placed within the heating chamber (1), is made to have a length of an odd number multiple of about 1/8 the wavelength of the microwave supplied from the microwave generating portion (6).
The microwave heating device according to claim 1, wherein
in at least a single closed loop having a single opening which is formed by the bridge members (26, 27), a single side is made to have a length of an odd number multiple of about 1/4 the wavelength of the microwave supplied from the microwave generating portion.
The microwave heating device according to claim 1 or 2, wherein
in a closed loop having a single opening which is formed by the bridge members (26, 27),
a largest linear length in the opening in its inner dimension is made to be equal to or more than 1/2 the wavelength of the microwave supplied from the microwave generating portion (6).