GB2284133A

GB2284133A - High-frequency heater and method of controlling same

Info

Publication number: GB2284133A
Application number: GB9423263A
Authority: GB
Inventors: Kaoru Ozawa; Tatsuya Nakagawa
Original assignee: Toshiba Corp; Toshiba AVE Co Ltd
Current assignee: Toshiba Corp; Toshiba AVE Co Ltd
Priority date: 1993-11-15
Filing date: 1994-11-15
Publication date: 1995-05-24
Anticipated expiration: 2014-11-15
Also published as: KR950016447A; CN1096218C; GB2284133B; CN1110463A; GB9423263D0

Abstract

The high-frequency heater has a magnetron for generating the microwaves, openings 9, 10 for emitting the microwaves into the heating chamber, waveguides 6, 7 for transmitting the microwaves from the magnetron to the openings, a changeover switch 12 for directing the microwaves from the magnetron alternately toward the openings during the heating of the object, and a controller for controlling at least the change-over switch such that the microwaves from the magnetron are alternately directed toward the openings according to a predetermined mode. An opening in an annular movable wave guide is also disclosed (figs 24 - 27). <IMAGE>

Description

TITLE OF THE INVENTION HIGH-FREQUENCY HEATER AND METhOD OF CONTROLLING SAME BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-frequency heater that emits microwaves toward an object placed in a heating chamber, to dielectrically heat the object, and a method of controlling the heater. In particular, the present invention relates to a high-frequency heater that is capable of uniformly and quickly heating an object and a method of controlling the heater.

2. Description of the Prior Art The applicant of the present invention has proposed, in Japanese Laid-open Patent Publication No. 5-266977, a high-frequency heater having a heating chamber and openings arranged at upper and lower parts of the heating chamber, to emit microwaves toward an object placed in the heating chamber. The openings are connected to waveguides, which are joined and connected to a microwave generator. A change-over switch is arranged at the joint of the waveguides, to deliver microwaves generated by the microwave generator to one of the openings. When heating a liquid such as Japanese sake, the heater uses only the lower opening to uniformly heat the liquid.

This high-frequency heater, i.e., microwave oven has room for improvement in shortening a cooking time and decreasing temperature difference on a heated object.

Namely, to shorten the cooking time and reducing the temperature difference, the microwave oven must heat a liquid, solid, or semisolid object by supplying microwaves from the microwave generator alternately to the openings according to heating patterns suitable for the object.

Japanese Laid-Open Patent Publication No. 5-54965 discloses another high-frequency heater. This heater has a heating chamber, an opening arranged at the center of the bottom of the heating chamber, a magnetron for generating microwaves, and a waveguide for transmitting the microwaves to the opening. A vertical rotary shaft is arranged in the opening, to support a roller stay consisting of a metal stay and rollers. The rollers support a nonmetal turntable. When the opening emits microwaves, the roller stay is turned around the shaft, to rotate the turntable.

At this time, the roller stay serves as a radiation antenna to radiate the microwaves. To efficiently heat a large object, a second opening maybe arranged at the ceiling or sidewall of the heating chamber.

This prior art employs the roller stay to widely radiate microwaves in the heating chamber, and therefore, is impossible to heat only the bottom of a thin, tall container that contains a liquid. The liquid is heated to have large temperature difference between upper and lower parts thereof in the container. The second opening of the prior art is incapable of selecting heating patterns according to an object, which may be tall, flat, or else.

Namely, the prior art employs only one heating pattern without regard to the kind and shape of an object to heat.

In addition, the roller stay hinders to incorporate a weight sensor.

SUMMARY OF THE INVENTION An object of the present invention is to provide a high-frequency heater that quickly and uniformly heats any one of solid, semisolid, and liquid objects, and a method of controlling the heater.

Another object of the present invention is to provide a high-frequency heater that is compact and capable of incorporating a weight sensor, determining the position of an opening according to the kind of an object to heat, and locally heating a liquid object contained in a tall container, to efficiently and uniformly heating the object.

In order to accomplish the objects, a first aspect of the present invention provides a high-frequency heater having a heating chamber, a microwave generator for generating microwaves, a plurality of openings arranged in the heating chamber, for emitting the microwaves toward an object, and a change-over controller for directing the microwaves alternately toward the openings.

The first aspect supplies microwaves from the microwave generator alternately to the openings, to uniformly heat an object, shorten a cooking time, and reduce temperature difference on the object.

A second aspect of the present invention provides a high-frequency heater having a heating chamber, a microwave generator for generating microwaves, a plurality of openings arranged in the heating chamber, for emitting the microwaves toward an object, a temperature detector for detecting the temperature of the object, and a change-over controller for directing the microwaves alternately toward the openings at switching frequencies and strength determined according to the output of the temperature detector.

The second aspect optimizes the change-over frequencies and output levels of the openings according to the temperature-of an object, to shorten a cooking time and reduce temperature difference on the object.

A third aspect of the present invention provides a high-frequency heater having a heating chamber, a microwave generator for generating microwaves, a plurality of openings arranged in the heating chamber, for emitting the microwaves toward an object, and a change-over controller for directing the microwaves alternately toward the openings and stopping the microwave generator during a change-over interval of the alternating operation.

The third aspect stops the microwave generator during transient intervals, to prevent the change-over controller and microwaves from adversely affecting the microwave generator during the change-over intervals to alternately provide the openings with microwaves.

A fourth aspect of the present invention provides a high-frequency heater having a heating chamber, a microwave generator for generating microwaves, a plurality of openings arranged in the heating chamber, for emitting the microwaves toward an object, and a change-over controller for directing the microwaves alternately toward the openings according to switch-over frequencies per unit time that increase as heating time passes.

The fourth aspect directs microwaves from the microwave generator alternately toward the openings according to switch-over frequencies per unit time that are increased as heating time passes, to thereby shorten a cooking time and uniformly thaw and heat, for example, frozen shaomais.

A fifth aspect of the present invention provides a high-frequency heater having a heating chamber, a microwave generator for generating microwaves, a plurality of openings arranged in the heating chamber, for emitting the microwaves toward an object, and a change-over controller for directing the microwaves alternately toward the openings while reducing the strength of the microwaves per unit time as heating time passes.

The fifth aspect directs microwaves from the microwave generator alternately toward the openings, and at the same time, reduces the strength of the microwaves per unit time as heating time passes, to thereby shorten a cooking time and uniformly thaw and heat, for example, a block of frozen tuna.

A sixth aspect of the present invention employs the arrangement of the fifth aspect, and when a required output level of each of the openings is below the lower limit of the variable output range of the microwave generator, combines ON and OFF intervals to form a unit operation period of each opening. to adjust the average output of each of the openings to the required output level.

The sixth aspect properly combines ON and OFF intervals to form a unit operation period of each of the openings, to adjust an average of the output of each of the openings to a required level when the required level is below the lower limit of the variable output range of the microwave generator.

A seventh aspect of the present invention provides a high-frequency heater having a heating chamber, a microwave generator for generating microwaves, a plurality of openings arranged in the heating chamber, for emitting the microwaves toward an object, and a change-over controller for directing the microwaves alternately toward the openings while changing the ratio of output energy per unit time between the openings as heating time passes.

The seventh aspect directs microwaves from the microwave generator alternately toward the openings while changing the ratio of output energy per unit time between the openings as heating time passes, to thereby shorten a cooking time and uniformly thaw, for example, frozen ground beef.

An eighth aspect of the present invention provides a high-frequency heater having a heating chamber, a microwave generator for generating microwaves, upper and lower openings arranged in the heating chamber, for emitting the microwaves toward an object, and a change-over controller for letting the lower opening emit microwaves at first and then the upper opening emit microwaves.

The eighth aspect lets the lower opening emit microwaves at first and then the upper opening emit microwaves, to uniformly thaw an object.

A ninth aspect of the present invention provides a high-frequency heater having a heating chamber, a microwave generator for generating microwaves, a plurality of openings arranged in the heating chamber, for emitting the microwaves toward an object, and a change-over controller for directing the microwaves alternately toward the openings while increasing the frequency of alternations per unit time, decreasing the strength of the microwaves, and changing the ratio of output energy between the openings as heating time passes.

The ninth aspect directs microwaves from the microwave generator alternately toward the openings while increasing the frequency of alternations per unit time, decreasing the strength of the microwaves, and changing the ratio of output energy between the openings as heating time passes, to thereby cook a solid, semisolid, or liquid object according to optimum heating patterns, shorten a cooking time, and reduce temperature difference on the object.

A 10th aspect of the present invention provides a high-frequency heater having a microwave generator, a waveguide for transmitting microwaves generated by the microwave generator, and an opening for emitting the microwaves from the waveguide into a heating chamber, to heat an object placed in the heating chamber. The waveguide has an annular part, which is circumferentially divided into an annular fixed waveguide fixed to the heating chamber and an annular movable waveguide. The microwave generator is connected to the fixed waveguide, and the opening is connected to the movable waveguide.

The 10th aspect properly turns the movable waveguide to freely position the opening according to the kind of an object to heat. The annular part may be arranged at the bottom of the heating chamber with the movable waveguide being inside the fixed waveguide. The opening is positioned at the center of the movable waveguide, to efficiently uniformly locally heat a liquid contained in a tall container. Even when the annular part is arranged at the bottom of the heating chamber, it is possible to arrange a weight sensor on a shaft of the annular part.

An 11th aspect of the present invention provides a high-frequency heater having a microwave generator, a waveguide for transmitting microwaves generated by the microwave generator, and an opening for emitting the microwaves from the waveguide into a heating chamber, to heat an object placed in the heating chamber. The waveguide is rectangular in cross section and has an annular part. An H-plane of the annular part is circumferentially divided to form an annular fixed wave guide fixed to the heating chamber and an annular movable waveguide. The microwave generator is connected to the fixed waveguide, and the opening is connected to the movable waveguide.

The 11th aspect circumferentially divides the H-plane of the annular part along a separation line, to form the annular fixed waveguide and the annular movable waveguide.

The separation line leaks substantially no microwaves, and therefore, almost all microwaves generated by the microwave generator are emitted from the opening. Accordingly, the 11th aspect properly locally heat an object according to the kind of the object.

A 12th aspect of the present invention provides a high-frequency heater having a microwave generator, a waveguide for transmitting microwaves generated by the microwave generator, and an opening for emitting the microwaves from the waveguide into a heating chamber, to heat an object placed in the heating chamber. The waveguide is rectangular in cross section and has an annular part. An H-plane of the annular part is orthogonal to the center axis thereof. The H-plane of the annular part is circumferentially divided, to form an annular fixed waveguide fixed to the heating chamber and an annular movable waveguide. The microwave generator is connected to the fixed waveguide, and the opening is connected to the movable waveguide.

The 12th aspect employs the annular part having a rectangular cross section and whose H-plane is orthogonal to the center axis thereof. The annular part is flat, and a top plate thereof serves as the bottom of the heating chamber. Namely, the annular part is easily assembled to the bottom of the heating chamber, to easily locally heat a liquid contained in a tall container.

A 13th aspect of the present invention provides a high-frequency heater having a microwave generator, a waveguide for transmitting microwaves generated by the microwave generator, and an opening for emitting the microwaves from the waveguide into a heating chamber, to heat an object placed in the heating chamber. The waveguide is rectangular in cross section and has an annular part. An H-plane of the annular part is orthogonal to the center axis thereof. The H-plane of the annular part is circumferentially divided, to form an annular fixed wave guide fixed to the heating chamber and an annular movable waveguide. The microwave generator is connected to the fixed waveguide, and the opening is connected to the wall of the movable waveguide, which forms an E-plane, directly or indirectly through a waveguide.

The 13th aspect employs the annular part having a rectangular cross section and whose H-plane is orthogonal to the center axis thereof. This aspect connects the opening to the wall corresponding to the E-plane of the movable waveguide. As a result, a top plate of the annular part serves as the bottom of the heating chamber, so that the waveguide is easily assembled to the bottom of the heating chamber. The opening is easily positioned around the rotation center of the movable waveguide.

A 14th aspect of the present invention provides a high-frequency heater having a microwave generator, a waveguide for transmitting microwaves generated by the microwave generator, and an opening for emitting the microwaves from the waveguide into a heating chamber, to heat an object placed in the heating chamber. The waveguide is rectangular in cross section and has an annular part. An E-plane of the annular part is orthogonal to the center axis of the annular part. An H-plane of the annular part is circumferentially divided into an annular fixed wave guide fixed to the heating chamber and an annular movable waveguide. The microwave generator is connected to the fixed waveguide, and the opening is connected to the movable waveguide.

The 14th aspect employs the annular part having a rectangular cross section and whose E-plane is orthogonal to the center axis thereof. As a result, the outer diameter of the annular part is reduced, and a rotary plate of the waveguide serves as a sidewall of the heating chamber so that the annular part is easily assembled to the sidewall of the heating chamber.

A 15th aspect of the present invention provides a high-frequency heater having a microwave generator, a waveguide for transmitting microwaves generated by the microwave generator, and an opening for emitting the microwaves from the waveguide into a heating chamber, to heat an object placed in the heating chamber. The waveguide is rectangular in cross section and has an annular part. An E-plane of the annular part is orthogonal to the center axis thereof. An H-plane of the annular part is circumferentially divided, to form an annular fixed waveguide fixed'to the heating chamber and an annular movable waveguide. The microwave generator is connected to the fixed waveguide, and the opening is connected to the wall corresponding to the H-plane of the movable waveguide directly or indirectly through a waveguide.

The 15th aspect employs the annular part having a rectangular cross section and whose E-plane is orthogonal to the center axis thereof. This aspect connects the opening to the wall corresponding to the H-plane of the movable waveguide directly or indirectly through a waveguide. As a result, a rotary plate of the annular part serves as a sidewall of the heating chamber so that the waveguide is easily assembled to the side of the heating chamber. The opening may be positioned between the movable waveguide and the rotation center thereof. The position of the opening on the sidewall of the heating chamber is changed by turning the movable waveguide, to change a heating mode depending on the kind of an object to heat, thereby efficiently and uniformly heating the object.

A 16th aspect of the present invention employs any one of the arrangements of the 11th to 15th aspects, and makes a circumferential electric length of the annular waveguide part about n x A g / 2 where n is an optional integer and A g is a wavelength in the waveguide.

The 16th aspect makes a circumferential electric length about n x l g / 2 so that microwaves that transmit in a clockwise direction and microwaves that transmit in a counterclockwise direction have the same phase at n locations. When the opening is positioned at any one of the n locations, the strength of microwaves emitted from the opening into the heating chamber increases. The movable waveguide is turned to shift the opening to the n locations as time passes, to efficiently heat an object according to the kind of the object.

A 17th aspect of the present invention employs any one of the arrangements of the 11th to 15th aspects, and makes a circumferential electric length of the annular waveguide part (n + (1/m)) x I g where n and m are each optional integer and l g is a wavelength in the waveguide. The 17th aspect employs m openings.

The. 17th aspect makes a circumferential electric length of the annular waveguide (n + (1/m)) x I g. If m 2 and if the two openings are symmetrically arranged with respect to a rotary shaft so that their phases are shifted from each other by A g / 4, the two openings complementarily operate. Namely, when the microwave output of one of the openings is strong, that of the other is weak. As a result, the openings always emit microwaves into the heating chamber irrespective of the rotational angle of the movable waveguide, to uniformly heat an object. In addition, an operation point with respect to the microwave generator is unchanged.

An 18th aspect of the present invention employs any one of the arrangements of the 11th to 17th aspects. The waveguide having a rectangular cross section is a ridge waveguide according to this aspect.

The 18th aspect employs a ridge waveguide, which involves a lower cut-off frequency than a wave guide having a rectangular cross section and the same external size.

Namely, a wavelength l g in the ridge waveguide is shorter than that in the waveguide having a rectangular cross section with respect to microwaves of the same frequency.

Accordingly, the ridge waveguide is compact and is easily assembled.

A 19th aspect of the present invention employs any one of the arrangements of the 11th to 17th aspects, and bends an edge of the annular waveguide part along a separation line at the center of an H-plane, to form a bent section having a predetermined width.

The 19th aspect forms the bent section along the separation line at the center of the H-plane of the annular waveguide part, to achieve the same effect as the ridge waveguide.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a perspective view showing the inside of a high-frequency heater according to a first embodiment of the present invention; Figs. 2(A) to 2(D) show the conditions of an object heated by the heater of the first embodiment and the output waveforms of openings of the same heater; Figs. 3(A) to 3(C) show the conditions of an object heated by a comparison heater and the output waveforms of openings of the comparison heater; Fig. 4 is an enlarged view showing the output waveforms of Figs. 2(C) and 2(D); Fig. 5 is a flowchart of heating and controlling steps according to the first embodiment; Figs. 6(A) to 6(D) show the conditions of an object heated by a heater according to a second embodiment of the present invention and the output waveforms of openings of the same heater;; Figs. 7(A) to 7(C) show the conditions of an object heated by a comparison heater and the output waveforms of openings of the comparison heater; Figs. 8(A) to 8(C) show output waveforms of the openings of the second embodiment; Figs. 9(A) and 9(B) show output patterns of openings according to a third embodiment of the present invention; Figs. 10 (A) and 10 (B) show output waveforms involving ON and OFF intervals; Fig. 11 is a perspective view showing the inside of a high-frequency heater according to a fourth embodiment of the present invention; Fig. 12 is a flowchart of heating and controlling steps according to the fourth embodiment; Figs. 13(A) to 13(C) show output patterns of openings according to a fifth embodiment of the present invention; ; Figs. 14(A) to 14(C) show output patterns of openings according to a sixth embodiment of the present invention; Figs. 15(A) and 15(B) show output patterns of openings according to the sixth embodiment; Figs. 16(A) and 16(B) show thawing states according to a seventh embodiment of the present invention; Figs. 17(A) to 17(C) are exploded views showing a change-over switch according to an eighth embodiment of the present invention; Fig. 18 is a partly broken perspective view showing an annular waveguide having a rectangular cross section of a high-frequency heater according to the present invention; Fig. 19 is a vertical section showing the waveguide of Fig. 18; Fig. 20 is a sectional view showing the waveguide of Fig. 18 fitted to a heating chamber;; Fig. 21 is a partly broken perspective view showing another annular waveguide having a rectangular cross section according to the present invention; Fig. 22 is a vertical section showing the waveguide of Fig. 21; Fig. 23 is a sectional view showing the waveguide of Fig. 21 fitted to a heating chamber; Fig. 24 is a partly broken perspective view showing still another annular waveguide having a rectangular cross section according to the present invention; Fig. 25 is a partly broken perspective view showing still another annular waveguide having a rectangular cross section according to the present invention; Fig. 26 is a partly broken perspective view showing still another annular waveguide having a rectangular cross section according to the present invention; and Fig. 27 is a partly broken perspective view showing still another annular waveguide having a rectangular cross section according to the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS The embodiments of the present invention will be explained with reference to the drawings. Figure 1 shows a microwave oven as an example of a high-frequency heater.

The microwave oven has a heating chamber 1, a sidewall 2, and a magnetron 4 arranged on the outer surface of the sidewall 2. The magnetron 4 generates microwaves. The microwaves are propagated to an upper waveguide 6 and a lower waveguide 7 through a change-over switch 12. The magnetron 4 and switch 12 are controlled by a controller (not shown) such as a microcomputer. An end of the waveguide 6 has an upper opening 9 for emitting the microwaves. The opening 9 is positioned at upper part of the sidewall 2 and is open to the heating chamber 1. The lower waveguide 7 is connected to a bottom waveguide 8 arranged under the heating chamber 1. An end of the waveguide 8 has a lower opening 10, which is positioned substantially at the center of a bottom 3 of the heating chamber 1 and is open to the heating chamber 1. A turntable 11 is made from material that transmits microwaves.

The change-over switch 12 directs microwaves alternately to the waveguides 6 and 7, to heat an object placed in the heating chamber 1. When the change-over switch 12 is changed to the lower waveguide 7, microwaves from the magnetron 4 pass through a waveguide 5, the change-over switch 12, and the waveguides 7 and 8 and are emitted into the heating chamber 1 from the opening 10.

When the change-over switch 12 is changed to the upper waveguide 6, microwaves from the magnetron 4 pass through the waveguide 5, switch 12, and waveguide 6 and are emitted into the heating chamber 1 from the opening 9.

Figures 2(A) to 2(D) show 15 frozen shaomais and results of thawing and heating the shaomais with the microwave oven of Fig. 1 by emitting microwaves alternately from the upper and lower openings 9 and 10. Figures 3(A) to 3(C) show a comparison that thaws and warms 15 frozen shaomais by emitting microwaves only from the upper opening 9 toward them. In Figs. 2(B) and 3(B), an outermost temperature is an average temperature of eight peripheral shaomais, and an inner temperature is an average temperature of six shaomais arranged between the peripheral shaomais and a center shaomai. The alternate heating by the upper and lower openings 9 and 10 reduces a heating time from 4 minutes and 20 seconds of Fig. 3(B) to three minutes and 40 seconds of Fig. 2(B).A difference between the highest and lowest temperatures of the shaomais completely warmed decreases from 29 degrees centigrade of Fig. 3(B) to 21.5 degrees centigrade of Fig. 2(B). In this way, the alternate operation of the upper and lower openings 9 and 10 by the change-over switch 12 shortens a cooking time and reduces temperature difference.

Figures 4(A) and 4(B) are enlarged views of Figs. 2(C) and 2(D). Figure 5(A) is a flowchart showing steps of heating food (15 shaomais) on the turntable 11 in the heating chamber 1, and Fig. 5(B) shows a subprogram for a timer.

The 15 shaomais are set in the heating chamber 1. A total heating period T is set. Step 20 divides the total heating period T into T1, T2, and T3 such that T1 > T2 = T3. The periods T1, T2, and T3 are continuous in this order. During the heating period T1, the upper opening 9 heats the food for an interval tl, and the lower opening 10 heats the food for an interval tl'. Steps 21 and 22 determine these intervals tl and tl'. During the heating period T2, the upper opening 9 heats the food for an interval t2, and the lower opening 10 heats the food for an interval t2'. Steps 23 and 24 determine these intervals t2 and t2'.During the heating period T3, the upper opening 9 heats the food for an interval t3, and the lower opening 10 heats the food for-an interval t3'. Steps 25 and 26 determine these intervals t3 and t3'. Steps 27 through 39 alternate the openings 9 and 10 according to the determined intervals, to completely heat the 15 shaomais. The heating patterns of Figs. 4(A) and 4(B) shorten a cooking time and reduces temperature difference among the food.

In this way, a microwave heater according to the first embodiment has at least two openings and a change-over switch. The openings are optionally positioned on, for example, the ceiling, bottom, sidewalls, and back of a heating chamber of the heater. The change-over switch directs microwaves alternately toward the openings. The first embodiment divides a total heating period of food by n into heating periods T1 to Tn. During each of the periods T1 to Tn, the change-over switch alternates the openings. The heating periods are set to T1 > = T2 > = T3 > = ... > = Tn so that the openings are more frequently alternated as a heating operation progresses.

Figures 6 to 8 show the second embodiment of the present invention. This embodiment employs the same microwave oven of Fig. 1 and gradually reduces output energy per unit time from the magnetron 4 to the openings 9 and 10 as heating time passes. Figure 6(A) shows a block of tuna of 500 grams heated in the heating chamber 1 by alternating the openings 9 and 10 through the change-over switch 12. Figure 6(B) shows data of the tuna block before and after the heating operation. Figure 6(C) shows heating patterns carried out by the upper opening 9, and Fig. 6(D) shows heating patterns carried out by the lower opening 10.

Figures 7(A) to 7(C) show a comparison, in which Fig. 7(A) shows a block of tuna of 500 grams heated only by the upper opening 9, Fig. 7(B) shows data of the tuna block before and after the heating operation, and Fig. 7(C) shows heating patterns carried out by the upper opening 9.

Heating the food by alternating the upper and lower openings 9 and 10 shortens a heating time from six minutes and 48 seconds of Fig. 7(B) to four minutes and 19 seconds of Fig. 6(B), and reduces a temperature difference from 14 degrees centigrade to eight degrees centigrade. In this way, this embodiment decreases output energy per unit time as heating time passes while alternating the upper and lower openings 9 and 10 through the change-over switch 12, to shorten a cooking time and reduces temperature difference.

The second embodiment will be more precisely explained with reference to Figs. 8(A) to 8(C). Food is placed on the turntable 11 in the heating chamber 1. A weight sensor detects the weight of the food, and the kind of the food is entered through a control panel. According to these data, total output energy EO is calculated. According to the total output energy EO and a maximum output value Pmax, a total heating period T is calculated on the basis that output energy is gradually reduced as shown in Fig. 8(A).

T = 2 x EO / Pmax The total heating period T is divided by n, and the output energy Ek of the "k"th heating period is calculated as follows: Ek = 2 x EO (n - k + 1/2) / n2 The upper opening 9 emits microwaves of Ek*1/2, and the lower opening 10 emits microwaves of Ek*1/2. Figures 8(B) and 8(C) show heating patterns of the upper and lower openings 9 and 10, respectively. Heating food according to these heating patterns shortens a cooking time and reduces temperature difference. If the last heating period is shorter than n seconds after dividing the total heating period T by n, the last heating period is discarded.

The second embodiment reduces output energy according to time. This does not limit the present invention. For example, a total heating period is divided by n. In the divided heating periods, totals of output energy of the upper and lower openings are El, E2, ..., En. When El > = E2 > = ... > = En, it is possible to shorten a cooking time and reduce temperature difference.

Figures 9 and 10 show the third embodiment of the present invention. This embodiment employs the microwave oven of Fig. 1. If output energy required for each opening is lower than a variable range of the output of the magnetron 4, the third embodiment lowers an average output of the magnetron to the required output energy by combining ON and OFF intervals. Figure 9(A) shows required heating patterns for the upper opening 9, and Fig. 9(B) shows required heating patterns for the lower opening 10. When the magnetron 4 is incapable of lowering its output below 400 W, heating patterns of Figs. 10 (A) and 10(B) are actually used for the openings 9 and 10, to heat food.For example, if one of the openings is required to provide output P, which is less than 400 W, for t seconds, the following calculations are made: ton = t * P / Pmin toff = t - ton where Pmin is the minimum output, which is 400 W in this example, ton is ON time for the output Pmin, and toff is OFF time. The ON time and OFF time are combined to form the period t.

The third embodiment will be explained more precisely with reference to Figs. 10 (A) and 10(B) in which Fig. 10 (A) shows the output of the upper opening 9 and Fig. 10(B) shows the output of the lower opening 10. In Fig. 9(A), the upper opening 9 is required to emit energy of 300 W for 20 seconds from an 80-second point up to a 100-second point after the start of heating. In this case, the opening 9 is activated at 400 W for 15 seconds from the 80-second point up to a 95-second point after the start of heating, and is deactivated for the following five seconds, as shown in Fig. 10(A). - As a result, an average output of the opening 9 for 20 seconds from the 80-second point up to the 100second point after the start of heating becomes 300 W.

Similarly, the lower opening 10 is required to emit energy of 300 W for 20 seconds from the 100-second point up to a 120-second point after the start of heating, as shown in Fig. 9(B). In this case, the opening 10 is activated at 400 W for 15 seconds from the 100-second point up to a 115 second point after the start of heating, and is deactivated for the following five seconds, as shown in Fig. 10(B). As a result, an average output of the opening 10 for 20 seconds from the 100-second point up to the 120-second point after the start of heating becomes 300 W.

Figures 11 and 12 show the fourth embodiment of the present invention. In Fig. 11, a microwave oven according to the fourth embodiment has a heating chamber 1 and a temperature detector 15 arranged on top of the heating chamber 1. The temperature detector 15 detects the surface temperature of food placed on a turntable 11 disposed in the heating chamber 1. Other arrangements of the fourth embodiment are the same as those of the first embodiment of Fig. 1. Figure 12 is a flowchart showing the steps of heating 15 frozen shaomais by the upper and lower openings 9 and 10 according to the heating patterns of Fig. 4. The 15 frozen shaomais are placed on the turntable 11 in the heating chamber 1. A total heating period T is calculated.

Firstly, the upper and lower openings 9 and 10 are alternated once to heat the food. The temperature detector 15 determines whether or not the temperature Tf of the food has reached 2 to 3 degrees centigrade. These processes are carried out in steps 40 to 50. A heating period from the start to this point is Ta, and a heating period from this point to the end is Tb. During the heating period Tb, the upper and lower openings 9 and 10 are alternated twice.

Namely, steps 51 to 58 are repeated twice. A reference mark "A" in step 59 indicates a subprogram involving the steps 51 to 58. These heating processes shorten a cooking time and reduce temperature difference. In this way, the temperature of food detected by the temperature detector 15 is used to set the heating intervals of the upper and lower openings 9 and 10, the frequencies per unit time of the openings, and the magnitude of the output of the openings.

The fourth embodiment heats food with optimum heating patterns according to the temperature of the food.

Figure 13 shows the fifth embodiment of the present invention. This embodiment employs the microwave oven of Fig. 1. This embodiment supplies the output energy of the magnetron 4 alternately to the openings 9 and 10 while changing the ratio of output energy per unit time between the openings 9 and 10 as heating time passes. Food is placed on the turntable 11 in the heating chamber 1. The weight sensor detects the weight of the food, and the kind of the food is entered through the control panel.

According to these data, total output energy EO is calculated. According to the total output energy EO and a constant output P, a total heating period T is calculated as shown in Fig. 13(A).

T = EO / P The total heating period T is divided by n, and the output energy Ek of the "k"th heating period is calculated as follows: Ek = P x n The upper opening 9 emits microwaves of a x Ek, and the lower opening 10 emits microwaves of b x Ek. Here, a and b are variables and a + b = 1, a > 0, and b > O. Figure 13(B) shows heating patterns of the upper opening 9, and Fig. 13(C) shows heating patterns of the lower opening 10.

It is preferable to make the variable b smaller when thawing, for example, frozen ground beef. These heating patterns shorten a cooking time and reduces temperature difference. If the last heating period is shorter than n seconds after dividing the total heating period T by n, the last heating period is discarded.

Although the output level is constant in the above example, it may be decreased in proportion to time. The ratio of output energy between the openings may be changed according to the kind of food to cook. The temperature detector 15 of the fourth embodiment may be employed to change the output per unit time of the openings according to the conditions of food.

Figures 14(A) to 14(C) and 15(A) and 15(B) show the sixth embodiment of the present invention. This embodiment employs the microwave oven of Fig. 1. This embodiment increases the frequency of alternating operations per unit time of the openings while reducing the output thereof.

This embodiment also changes the ratio of output energy between the openings while alternating the openings. Food is placed on the turntable 11 in the heating chamber 1.

The weight sensor detects the weight of the food, and the kind of the food is entered through the control panel.

According to these data, total output energy EO is calculated. According to the total output energy EO and an output waveform of Fig. 14(A), a total heating period T is calculated. The total heating period T is divided by 3 into heating periods T1, T2, and T3 with T1 > T2 = T3.

During the heating period T1, output energy is El. During this heating period T1, the upper opening 9 emits energy of a x El for a period of tl, and the lower opening 10 emits energy of b x El for a period of tl'. Here, a + b = 1, a > 0, and b > O. In the heating period T1, a = 1/4, and b = 3/4. When tl + tl' = T1, no OFF interval is set in the heating period T1, as shown in Fig. 14(B) for the upper opening 9 and Fig. 14(C) for the lower opening 10. When tl + tl' < T1, OFF intervals are set in the heating period T1 as shown in Fig. 15(A) for the upper opening 9 and Fig.

15(B) for the lower opening 10. These heating patterns are selected according to the kind of food to cook, to shorten a cooking time and reduce temperature difference.

Figures 16(A) and 16(B) show the seventh embodiment of the present invention. The embodiment employs the microwave oven of Fig. 1. Firstly, the lower opening 10 emits microwaves to heat frozen food, and then, the upper opening 9 emits microwaves to heat the food, to uniformly thaw the food. In Fig. 16(A), a frozen tuna block is firstly heated by the upper opening 9. Lower part (nonhatched part) of the tuna block is still frozen. Even if the lower opening 10 is activated to heat the tuna block of this condition, microwaves from the lower opening 10 usually transmit the frozen part, to insufficiently thaw the food. On the other hand, a frozen tuna block is uniformly thawed if the lower opening 10 is firstly activated as shown in Fig. 16(B).

Figure 17 shows the eighth embodiment of the present invention. A change-over switch 12 of a microwave oven includes fixed members 13 and a rotary member 14 arranged at the center of the fixed members 13. The rotary member 14 is turned to change waveguides from one to another to transmit microwaves generated by a magnetron. The changeover switch 12 involves a change-over period (a transient period) in which microwaves generated by the magnetron are not guided to none of the waveguides and are returned to the magnetron, to adversely affect the magnetron. To prevent this adverse effect, the eighth embodiment turns OFF the magnetron during the transient period and turns ON the same after the transient period.

The details of the upper and lower openings 9 and 10 according to the present invention will be explained.

Figures 18 to 20 show the details of the lower opening 10, in which Fig. 18 is a partly broken perspective view, Fig. 19 is a sectional view, and Fig. 20 is a sectional view showing essential parts.

The lower opening 10 has an annular wave guide 101 having a rectangular cross section. The waveguide 101 consists of an annular fixed waveguide lOla and an annular movable waveguide 101b. A metal top plate 102 forms the bottom of the heating chamber 1 as well as the top (Hplane) of the fixed waveguide 101a. A metal outer wall 103 is connected to the top plate 102, to form an outer face (E-plane) of the fixed waveguide l01a. A metal bottom plate 104 is connected to the outer wall 103, to form the bottom (H-plane) of the fixed waveguide lOla. The bottom plate 104 has a central circular recess in which a metal rotary bottom 105 is rotatably arranged. A metal inner wall 106 is connected to the rotary bottom 105. A metal rotary top 107 is connected to the inner wall 106. The plates 105, 106, and 107 form the movable waveguide lOlb.

The H-plane of the annular waveguide 101 is orthogonal to the center axis thereof, and the waveguide 101 is divided, along a circumferential line 108 positioned substantially at the center of the H-plane, into the fixed waveguide lOla fixed to the heating chamber 1 and the movable wave guide 101b. The E-plane of the fixed waveguide 101a is connected to the magnetron 4 through the wave guides 109 and 7 and switch 12. The top of the waveguide 109 is formed by the top plate 102, and the bottom thereof is formed by the bottom plate 104. A waveguide 110 is connected to the Eplane of the movable waveguide 101b, i.e., the inner wall 106. The other end of the waveguide 110 forms an opening opening 111. The rotary bottom 105, inner wall 106, rotary top 107, waveguide 110, and emitter opening 111 form a rotor 112.The rotor 112 has a support 114 removably engaged with a shaft 113 that is connected to a motor 115 incorporating a weight sensor. The shaft 113 passes through a hole formed on the bottom plate 104. This hole is surrounded by a cylindrical waveguide choke 116 to prevent microwaves from leaking outside from the hole. A turntable 119 is made from material that transmits microwaves. A container 120 accommodates an object 121 to be heated.

The operation of the microwave oven will be explained.

The circumferential separation line 108 is formed substantially at the center of the H-plane of the annular waveguide 101, so that only a little amount of microwaves escape through the line 108 into the heating chamber 1.

When the magnetron 4 is connected to the lower opening 10 through the change-over switch 12, most of microwaves generated by the magnetron 4 are emitted from the opening opening 111 into the heating chamber 1. If the object 121 is placed adjacent to the emitter opening 111, the object 121 is locally heated. If the object 121 in the container 120 is a liquid, the bottom of the liquid is heated to cause a heat convection to stir itself. As a result, the liquid is uniformly heated. Namely, microwaves emitted from the emitter opening 111 are mostly absorbed by the object 121, to efficiently heat the object. If the liquid boils over, the spilled portion will stay in the center recess of the bottom plate 104 without affecting microwave circuits. Although the emitter opening 111 is at the center, it is easy to install a weight sensor.

Figures 21 to 23 show an example of the upper opening 9 according to the present invention. In this example, an E-plane of an annular waveguide having a rectangular cross section is orthogonal to the center axis of the waveguide, and an emitter opening is connected to an H-plane of an annular movable waveguide. Figure 21 is a partly broken perspective view, Fig. 22 is a sectional view, and Fig. 23 shows the position of the opening with respect to the heating chamber 1. In Fig. 23, the lower opening 10 is omitted. A waveguide 126 has a port 126a that receives microwaves. The waveguide 126 is connected to a metal outer wall plate 123 having an annular groove that forms a fixed waveguide 122a of an annular waveguide 122. A circular recess 123a extends from the fixed waveguide 122a, to receive a metal rotor 124. The rotor 124 has a rotary plate 124a that serves as a wall of the heating chamber 1.

The periphery of the rotary plate 124a forms a movable waveguide 122b. In this way, the E-plane of the annular waveguide 122 is orthogonal to a shaft 129. The waveguide 122 is divided into the fixed waveguide 122a and movable waveguide 122b along a separation line 125 located at the center of the H-plane of the waveguide 122. The waveguide 126 is connected to the H-plane of the fixed waveguide 122a. The center of the rotary plate 124a is fitted to a drive mechanism 130 through the rotary shaft 129. The rotary shaft 129 is connected to a rotation angle detector 131 such as an absolute rotary encoder, to detect the rotation angle of the rotor 124, i.e., the rotational position of an emitter opening 128. The emitter opening 128 is connected to the wall of the movable waveguide 122b through a waveguide 127.The emitter opening 128 may be directly open to the E-plane, i.e., the rotary plate 124a of the movable waveguide 122b.

The operation of this example will be explained. The separation line 125 of the waveguide 122 leaks substantially no microwaves into the heating chamber 1 because the separation line 125 is located at about the center of the H-plane of the waveguide 122. Accordingly, substantially all microwaves generated by the magnetron are emitted from the emitter opening 128 into the heating chamber 1. Turning the rotor 124 changes the position of the emitter opening 128 on the wall of the heating chamber 1, to change an excitation mode in the heating chamber 1 depending on the kind of food to cook. For example, the emitter opening 128 may be shifted to the lowest position when heating a liquid 121, which may be milk or Japanese sake, contained in a tall container 120. This results in uniformly heating the liquid.The emitter opening 128 may be continuously turned when thawing and heating frozen shaomais, sa that the emitter opening 128 may serve as a stirrer fan to efficiently and uniformly cook the food.

Figure 24 shows another example of the lower opening 10 and is based on the annular waveguide 101 of Fig. 18.

This example sets a circumferential electric length of the annular waveguide 101 having a rectangular cross section as 3.5A g (n = 7 in n x A g/2) where a wavelength in the waveguide is A g. To make the circumferential electric length 3.51 g, the frequency of microwaves generated by the magnetron is set to 2.45 GHz, and the width of the waveguide is set to 78 mm. Then, the wavelength in the waveguide is about 212 mm, and 3.51 g is 742 mm. A radius for the circumferential length is about 118 mm. Then, the inner radius rl of the annular waveguide 101 is 79 mm and the outer radius r2 thereof is 157 mm. If the cross section of the waveguide is nonlinear, it will be difficult to calculate a wavelength in the waveguide.The calculations mentioned above, however, have sufficient accuracy for the waveguide of this example.

When a circumferential electric length of the annular waveguide 101 is n x 1 g / 2, microwaves advancing in a clockwise direction and microwaves advancing in a counterclockwise direction have the same phase at n locations. When the waveguide 110 connected to the opening opening 111 is positioned at such in-phase locations, the emitter opening 111 may emit strong microwaves. If n=7, there are seven in-phase positions that are suitable for emitting microwaves. When a circumferential electric length is n x A g / 2, there are n locations suitable for emitting microwaves. The n locations suitable for the emitter opening 111 provide different excitation modes with respect to the heating chamber 1, to provide different heating characteristics, i.e., heating patterns.Required heating characteristics may be achieved by selecting one of the heating patterns, or by combining them.

Figure 25 shows still another example of the lower opening 10. This example is based on the annular waveguide 101 of Fig. 18. When a wavelength in the annular waveguide 101 is 1 g, a circumferential electric length of the waveguide 101 is 3.51 g. Namely, n = 3 and m = 2 in (n + (1/m)) x 1 g. This example involves a first opening opening 132 and a second emitter opening 133, which are symmetrical with respect to a rotary shaft (not shown).

Waveguides that connect the emitter openings 132 and 133 to the annular waveguide 101 are electrically shifted from each other by l g/4 phases. Namely, when one of the waveguides is on a loop of a standing wave, the other is on a node of the standing wave.

When a circumferential electric length of the annular waveguide 101 is (n + (1/2)) x l g, microwaves advancing in a clockwise direction and microwaves advancing in a counterclockwise direction in the waveguide 101 have the same phase at (2 x n + 1) locations. If the waveguides connected to the emitter openings 132 and 133 are positioned at such in-phase positions, microwaves emitted from the emitter openings are strong. When the first and second emitter openings 132 and 133 are positioned as shown in Fig. 25, microwaves from one of the emitter openings 132 and 133 are strong, and those from the other are weak.

Namely, the emitter openings 132 and 133 complementarily operate, to provide microwaves at any position. When the emitter openings are turned with the magnetron being activated, operation points with respect to the magnetron are unchanged. The arrangement uniformly heats even a flat object by a combination of the locally heating effect achieved by the emitter openings 132 and 133 positioned at the center of the bottom of the heating chamber and by the complementary effect achieved by the two emitter openings.

Figure 26 shows still another example of the lower opening 10. This example is based on the annular waveguide 101 having a rectangular cross section of Fig. 25. The waveguide 101 of Fig. 26 is a ridge waveguide. The ridge waveguide has a recess 134 at the center of an H-plane, to reduce the height of the waveguide thereat. The upper and lower H-planes of the ridge waveguide may be symmetrically or asymmetrically recessed. When the upper H-plane is recessed as shown in Fig. 26, foreign matters such as spilled liquid may enter the separation line 108 on the top and easily flow outside through the separation line 108 on the bottom plate. Compared with a waveguide having a rectangular cross section and the same outer size, the ridge waveguide has a lower cut-off frequency and a shorter guide wavelength I g with respect to microwaves of the same frequency.Accordingly, the ridge waveguide is more compact than the waveguide having a rectangular cross section.

Figure 27 shows another example of the upper opening 9. Similar to the example of Fig. 21, this example employs an annular waveguide having a rectangular cross section with its E-plane being orthogonal to the axis thereof. The annular waveguide 122 of this example has a bent edge 135, which has a predetermined width and extends along a separation line at the center of an H-plane. A rotary plate 124a forms a wall of the heating chamber. An emitter opening 128 is directly formed on the E-plane, i.e., the rotary plate 124a of a movable waveguide 122b. The position of the emitter opening 128 is rotatable around a rotary shaft, which is connected to a drive mechanism and a rotation angle detection mechanism. This example provides the same effect as the example of Fig. 21. In addition, the waveguide of Fig. 27 may serve as a ridge waveguide if a distance d is shorter than a distance h. This results in shortening a wavelength in the wave guide and an outer radius r2.

In summary, in a microwave oven having a heating chamber, a microwave generator for generating microwaves, a plurality of openings arranged in the heating chamber, for emitting the microwaves toward an object, and a change-over controller for directing the microwaves toward the openings, the first aspect of the present invention supplies the microwaves from the microwave generator alternately to the openings, to uniformly heat the object, shorten a cooking time, and reduce temperature difference on the object.

The second aspect of the present invention directs the microwaves from the microwave generator alternately toward the openings at switching frequencies and strength that are optimized according to the output of a temperature detector, to shorten a cooking time and reduce temperature difference on the object.

The third aspect of the present invention directs the microwaves from the microwave generator alternately toward the openings while stopping the microwave generator during transient intervals, to prevent the change-over controller and microwaves from adversely affecting the microwave generator during the transient intervals.

The fourth aspect of the present invention directs the microwaves from the microwave generator alternately toward the openings according to switch-over frequencies per unit time that are increased as heating time passes, to thereby shorten a cooking time and uniformly thaw and heat, for example, frozen shaomais.

The fifth aspect of the present invention directs the microwaves from the microwave generator alternately toward the openings, and at the same time, reduces the strength of the microwaves per unit time as heating time passes, to thereby shorten a cooking time and uniformly thaw and heat, for example, a block of frozen tuna.

The sixth aspect of the present invention properly combines ON and OFF intervals to form a unit operation period of each opening, to adjust an average of the output of each opening to a required level when the required level is below the lower limit of the variable output range of the microwave generator.

The seventh aspect of the present invention directs the microwaves from the microwave generator alternately toward the openings while changing the ratio of output energy per unit time between the openings as heating time passes, to thereby shorten a cooking time and uniformly thaw, for example, frozen ground beef.

The eighth aspect of the present invention lets the lower opening emit microwaves at first and then the upper opening emit microwaves, to uniformly thaw the object.

The ninth aspect of the present invention directs the microwaves from the microwave generator alternately toward the openings while increasing the frequency of alternations per unit time, decreasing the strength of the microwaves, and changing the ratio of output energy between the openings as heating time passes, to thereby cook a solid, semisolid, or liquid object according to optimum heating patterns, shorten a cooking time, and reduce temperature difference on the object.

The 10th aspect of the present invention employs a waveguide having an annular part. The annular part is circumferentially divided into an annular fixed waveguide fixed to the heating chamber, and an annular movable waveguide. The microwave generator is connected to the fixed waveguide, and an opening is connected to the movable waveguide. This aspect properly turns the movable waveguide to freely position the opening according to the kind of an object to heat. The annular part may be arranged at the bottom of the heating chamber with the movable waveguide being inside the fixed waveguide. The opening is positioned at the center of the movable waveguide, to efficiently uniformly locally heat a liquid contained in a tall container. Even when the annular part is arranged at the bottom of the heating chamber, it is possible to arrange a weight sensor on a shaft of the annular part.

The 11th aspect of the present invention employs a waveguide, which is rectangular in cross section and has an annular part. An H-plane of the annular part is circumferentially divided along a separation line, to form an annular fixed waveguide fixed to the heating chamber and an annular movable waveguide. The microwave generator is connected to the fixed waveguide, and an opening is connected to the movable waveguide. The separation line leaks substantially no microwaves, and therefore, almost all microwaves generated by the microwave generator are emitted from the opening. Accordingly, the 11th aspect properly locally heat an object according to the kind of the object.

The 12th aspect of the present invention employs a waveguide, which is rectangular in cross section and has an annular part. An H-plane of the annular part is orthogonal to the center axis thereof. The H-plane of the annular part is circumferentially divided, to form an annular fixed waveguide fixed to the heating chamber and an annular movable waveguide. The microwave generator is connected to the fixed waveguide, and an opening is connected to the movable waveguide. The annular part is flat, and a top plate thereof serves as the bottom of the heating chamber.

Namely, the annular part is easily assembled to the bottom of the heating chamber, to easily locally heat a liquid contained in a tall container.

The 13th aspect of the present invention employs a waveguide, which is rectangular in cross section and has an annular part. An H-plane of the annular part is orthogonal to the center axis thereof. The H-plane of the annular part is circumferentially divided, to form an annular fixed waveguide fixed to the heating chamber and an annular movable waveguide. The microwave generator is connected to the fixed waveguide, and an opening is connected to the wall of the movable waveguide, which forms an E-plane, directly or indirectly through a waveguide. A top plate of the annular part serves as the bottom of the heating chamber, so that the waveguide is easily assembled to the bottom of the heating chamber. The opening is easily positioned around the rotation center of the movable waveguide.

The 14th aspect of the present invention employs a waveguide, which is rectangular in cross section and has an annular part. An E-plane of the annular part is orthogonal to the center axis of the annular part. An H-plane of the annular part is circumferentially divided into an annular fixed waveguide fixed to the heating chamber and an annular movable waveguide. The microwave generator is connected to the fixed waveguide, and an opening is connected to the movable waveguide. The outer diameter of the annular part is reduced, and a rotary plate of the waveguide serves as a sidewall of the heating chamber so that the annular part is easily assembled to the sidewall of the heating chamber.

The 15th aspect of the present invention employs a waveguide, which is rectangular in cross section and has an annular part. An E-plane of the annular part is orthogonal to the center axis thereof. An H-plane of the annular part is circumferentially divided, to form an annular fixed wave guide fixed to the heating chamber and an annular movable waveguide. The microwave generator is connected to the fixed waveguide, and an opening is connected to the wall corresponding to the H-plane of the movable waveguide directly or indirectly through a waveguide. A rotary plate of the annular part serves as a sidewall of the heating chamber so that the waveguide is easily assembled to the side of the heating chamber. The opening may be positioned between the movable waveguide and the rotation center thereof. The position of the opening on the sidewall of the heating chamber is changed by turning the movable waveguide, to change a heating mode depending on the kind of an object to heat, thereby efficiently and uniformly heating the object.

The 16th aspect of the present invention makes a circumferenial electric length of the annular waveguide part about n x A g / 2 where n is an optional integer and X g is a wavelength in the waveguide. Microwaves that transmit in a clockwise direction and microwaves that transmit in a counterclockwise direction have the same phase at n locations. When the opening is positioned at any one of the n locations, the strength of microwaves emitted from the opening into the heating chamber increases. The movable waveguide is turned to shift the opening to the n locations as time passes, to efficiently heat an object according to the kind of the object.

The 17th aspect of the present invention makes a circumferential electric length of the annular waveguide part (n + (1/m)) x l g where n and m are each optional integer and A g is a wavelength in the waveguide. The 17th aspect employs m openings which complementarily operate.

Namely, when the microwave output of one of the openings is strong, that of another is weak. As a result, the openings always emit microwaves into the heating chamber irrespective of the rotation angle of the movable waveguide, to uniformly heat an object. Operation points with respect to the microwave generator are unchanged.

The 18th aspect of the present invention employs a ridge waveguide. The ridge waveguide has a lower cut-off frequency than a waveguide having a rectangular cross section and the same external size. Namely, a wavelength A g in the ridge waveguide is shorter than that in the waveguide having a rectangular cross section with respect to microwaves of the same frequency. Accordingly, the ridge waveguide is compact and is easily assembled.

The 19th aspect of the present invention bends an edge of an annular waveguide along a separation line at the center of an H-plane, to form a bent section having a predetermined width, to achieve the same effect as the ridge waveguide Various modifications will become possible for those skilled in t;he art after receiving the teachings of the present disclosure without departing from the scope thereof.

Claims

WHAT IS CLAIMED IS:

1. A high-frequency heater for emitting microwaves to dielectrically heat an object placed in a heating chamber, comprising: a magnetron for generating the microwaves; a plurality of openings for emitting the microwaves into the heating chamber; waveguides for transmitting the microwaves from said magnetron to said openings; change-over means for directing the microwaves from said magnetron alternately toward said openings during the heating of the object; and control means for controlling at least said changeover means such that the microwaves from said magnetron are alternately directed toward said openings according to a predetermined mode.

2. The high-frequency heater according to claim 1, wherein said control means controls said change-over means such that the microwaves are alternately directed toward said openings according to the kind of the object.

3. The high-frequency heater according to claim 1, further comprising temperature detection means for detecting the temperature of the object, said control means controls said change-over means to direct the microwaves from said magnetron alternately toward said openings at switching frequencies and strength determined according to the output of the temperature detection means.

4. The high-frequency heater according to claim 1, wherein said control means directs the microwaves from said magnetron alternately toward said openings and stops said magnetron during a change-over interval of the alternating operation.

5. The high-frequency heater according to claim 1, wherein said control means directs the microwaves from said magnetron alternately toward said openings according to switch-over frequencies per unit time that are increased as heating time passes.

6. The high-frequency heater according to claim 1, wherein said control means directs the microwaves from said magnetron alternately toward said openings while reducing the strength of the microwaves per unit time as heating time passes.

7. The high-frequency heater according to claim 1, wherein, when a required output level of each of said openings is below the lower limit of the variable output range of said magnetron, said control means lowers the average output of each of said openings to the required output level by combining ON and OFF intervals of said openings in a predetermined period.

8. The high-frequency heater according to claim 1, wherein said control means directs the microwaves from said magnetron alternately toward said openings while changing the ratio of output energy per unit time between said openings as heating time passes.

9. The high-frequency heater according to claim 1, wherein said openings include upper and lower openings, and said control means lets the lower opening emit the microwaves at first and then the upper opening emit the microwaves.

10. The high-frequency heater according to claim 1, wherein said control means directs the microwaves from said magnetron alternately toward said openings while increasing the frequency of alternations per unit time, decreasing the strength of the microwaves, and changing the ratio of output energy between said openings as heating time passes.

11. The high-frequency heater according to claim 1, wherein said waveguides have an annular part, which is circumferentially divided into an annular fixed waveguide fixed to the heating chamber and an annular movable waveguide, said magnetron is connected to the fixed waveguide, and said opening is connected to the movable waveguide.

12. The high-frequency heater according to claim 1, wherein said waveguides have an annular part and a rectangular cross section, an H-plane of the annular part is circumferentially divided, to form an annular fixed waveguide fixed to the heating chamber and an annular movable waveguide, said magnetron is connected to the fixed waveguide, and said opening is connected to the movable waveguide.

13. The high-frequency heater according to claim 12, wherein a circumferential electric length of the annular waveguide is about n x A g / 2 where n is an optional integer and A g is a wavelength in the waveguide.

14. The high-frequency heater according to claim 12, wherein a circumferential electric length of the annular waveguide (n + (1/m)) x A g where n and m are each optional integer and A g is a wavelength in the waveguide, and there are m openings.

15. The high-frequency heater according to claim 12, wherein a ridge waveguide is employed as the waveguide having a rectangular cross section.

16. The high-frequency heater according to claim 12, wherein bends an edge of the annular waveguide along the separation line at the center of the H-plane, to form a bent section having a predetermined width.

17. The high-frequency heater according to claim 1, wherein the waveguides have an annular part and a rectangular cross section, an H-plane of the annular part is orthogonal to the center axis thereof, the H-plane of the annular part is circumferentially divided, to form an annular fixed wave guide fixed to the heating chamber and an annular movable waveguide, said magnetron is connected to the fixed waveguide, and said opening is connected to the movable waveguide.

18. The high-frequency heater according to claim 1, wherein said waveguides have an annular part and a rectangular cross section, an H-plane of the annular part is orthogonal to the center axis thereof, the H-plane of the annular part is circumferentially divided, to form an annular fixed waveguide fixed to the heating chamber and an annular movable waveguide, said magnetron is connected to the fixed waveguide, and said opening is connected to the wall of the movable waveguide forming an E-plane directly or indirectly through a waveguide.

19. The high-frequency heater according to claim 1, wherein said waveguides have an annular part and a rectangular cross section, an E-plane of the annular part is orthogonal to the center axis thereof, an H-plane of the annular part is circumferentially divided, to form an annular fixed waveguide fixed to the heating chamber and an annular movable waveguide, said magnetron is connected to the fixed waveguide, and said opening is connected to the movable waveguide.

20. The high-frequency heater according to claim 1, wherein said waveguides have an annular part and a rectangular cross section, an E-plane of the annular part is orthogonal to the center axis thereof, an H-plane of the annular part is circumferentially divided, to form an annular fixed wave guide fixed to the heating chamber and an annular movable waveguide, said magnetron is connected to the fixed waveguide, and said opening is connected to the wall corresponding to the H-plane of the movable wave guide directly or indirectly through a waveguide.

21. A high-frequency heater having a microwave generator, a waveguide for transmitting microwaves generated by the microwave generator, and an opening for emitting the microwaves from the waveguide into a heating chamber, to heat an object placed in the heating chamber, comprising: an annular part of the waveguide, said annular part being circumferentially divided into an annular fixed waveguide fixed to the heating chamber and an annular movable waveguide, the microwave generator being connected to the fixed waveguide, and the opening being connected to the movable waveguide.

22. A high-frequency heater substantially as hereinbefore described with reference to the accompanying drawings.