High-frequency heating apparatus with wave guide switching means and selective power switching means for magnetron

A high-frequency heating apparatus capable of heating uniformly a liquid load, having an economical wave guide switching unit and having a compact-sized and economical power circuit to drive a plurality of magnetrons. The high-frequency heating apparatus includes an excitation opening, provided at the bottom of a heating chamber for radiating microwaves to heat the liquid load in a container, a magnetron for generating the microwave to heat the object and a wave guide connected to a magnetron. The apparatus further includes a wave guide switching unity for switching the propagation of the microwave to one of plural wave guides. The switching unit includes a pair of circular conductive plates and a conductor disposed in the vicinity of a circumference of the circular conductive plate so as to shut off the microwave, wherein there is constructed a choke structure on the wave guide containing the circular conductive plates. The apparatus further includes a magnetron drive circuit including a filament heating power supply connected to each filament of the magnetrons, a high voltage generating element and a switching element for selectively supplying a high voltage from the high voltage generating element to an anode of the magnetrons.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a high-frequency heating apparatus for 
irradiating a microwave so as to perform dielectric heating, and it 
particularly relates to a microwave oven. 
2. Description of the Prior Art 
In a high-frequency heating apparatus, there is a method for heating to 
heat an object uniformly where the position of the object to be heated 
relative to a standing wave in a heating chamber is constantly change. 
Thus, a portion of the object to be heated thereby changes, so as to 
realize a uniform heating. For example, a stirrer fan is conventionally 
used for rotating a metal fan mounted on a ceiling, a side wall or a base 
so as to stir the microwave, a turntable method for heating the object 
while the object is being rotated, and a rotation antenna method for 
rotating an antenna which is a radiator of an electric wave. 
FIG. 1 shows a high-frequency heating apparatus employing the turntable 
method. In the high-frequency heating apparatus shown in FIG. 1, there is 
provided at a side wall 3 a magnetron for generating the microwave, and 
the microwave thus generated from the magnetron is radiated from an 
excitation opening 9 provided at an upper portion of the side wall 3 into 
a heating chamber 1 through a wave guide 7. Then, the irradiated microwave 
reaches an object 13 placed on a rotating turntable 11, so as to heat the 
object. 
In such a conventional high-frequency heating apparatus, the object 13 to 
be heated is rotated by the turntable 11. The microwave radiated from the 
above evenly irradiates the object 13, to realize a uniform heating. For 
solid and semi-solid objects, almost uniform heating can be obtained. 
However, in the case of a container filled with a liquid , there occurs a 
temperature difference between an upper portion and a lower portion 
therefor due to heat convection, so that there is heat unevenness in the 
liquid. For example, referring to FIG. 2, the temperature difference is 
significant in heating a sake in a sake bottle or a cup of milk or the 
like. This undesirable temperature difference occurs not only in the 
turntable method but also in the stirrer fan method as well as the 
rotating antenna method where in these methods the excitation opening for 
the microwave is provided in the upper portion of the heating chamber 1. 
FIG. 3 shows a temperature rise at each point A through D when the sake 
bottle is filled with water and dielectric-heated. In the same figure, a 
rate of temperature rise in the upper portion of liquid is faster than 
that in the lower portion of liquid, thus causing a temperature difference 
therebetween to become greater as time lapses. For example, suppose that 
40.degree. C. is a proper temperature for heating sake. Then, by the time 
the lowest portion D of the sake bottle becomes a temperature of 
40.degree. C., there is a temperature difference of over 15.degree. C. 
between D and the upper position A. At this stage, sake in a certain 
portion of the bottle presents a desirable temperature while other portion 
thereof does not. 
Accordingly, there is a problem in the conventional high-frequency heating 
apparatus where a significant temperature difference is caused by a liquid 
load in heating the liquid. 
In the high-frequency heating apparatus it has, in general, a single 
excitation opening. A wave guide switching device is not required for such 
the apparatus with a single excitation opening. Furthermore, even for a 
microwave oven having a plurality of excitation openings there is no wave 
guide switching device. 
However, considering the various loads in a microwave oven, it would be 
more efficient if the excitation openings are switched according to each 
load, so as to achieve an optimum heating of the load. 
Accordingly, the conventional microwave oven is not equipped with the wave 
guide switching device. Moreover, even if a usual wave guide switching 
device is implemented for the microwave oven, a cost increase, heavier 
weight thereof and a bulky size, result. These are not suitable for the 
microwave oven. 
When various loads are heated by the microwave oven, it is desirable to 
have a plurality of magnetrons in order to achieve the optimum heating of 
the load. For example, FIG. 5 shows a microwave oven with which two 
magnetrons are equipped. 
In a microwave oven having a plurality of magnetrons, there are 
conventionally provided separate power supplies for each magnetron in 
order to drive the magnetrons. Thus, each power supply is switched on and 
off separately in order to switch the excitation opening for each 
magnetron. 
FIG. 4 shows a schematic diagram of a power supply portion which drives two 
magnetrons. The primary sides of the two magnetrons 51, 53 are connected 
to transformers 57, 59, which are, in turn, connected to an a.c. power 
supply 55. More specifically, an anode of the magnetron 51 is connected to 
a high-voltage secondary winding 57a through a voltage doubler rectifying 
circuit, and a filament of the magnetron 51 is directly connected to a 
filament-use secondary winding 57b of the transformer 57, thus a high 
voltage and a filament voltage being supplied thereto respectively. In the 
similar manner, an anode of the magnetron 53 is connected to a 
high-voltage secondary winding 59a through a voltage doubler rectifying 
circuit, and a filament of the magnetron 53 is directly connected to a 
filament-use secondary winding 59b of the transformer, thus a high voltage 
and a filament voltage being supplied respectively. Primary sides of the 
transformers 57, 59 for the magnetrons 51, 53 are respectively connected 
to the a.c. power supply 55 through switches 61, 63 so that the magnetrons 
51, 53 are switched on and off by the switches 61, 63, respectively. 
However, in the above conventional microwave ovens, there are provided 
respective power circuits including transformers and rectifying circuits 
and-so on for each of the plural magnetrons, thus causing a problem of 
being not economical and of occupying a large space for mounting thereof. 
SUMMARY OF THE INVENTION 
The present invention has been made to overcome the foregoing drawbacks. 
Therefore, the objects of the present invention are to provide a 
high-frequency heating apparatus capable of uniformly heating a liquid 
load; to provide an economical wave guide switching means for a 
high-frequency heating apparatus; and to provide a microwave oven where a 
plurality of magnetrons thereof are driven by a compact-sized and 
economical power circuit. 
The first aspect of the present invention is to provide a high-frequency 
heating apparatus where a microwave is radiated into a heating chamber so 
as to heat an object therein, the apparatus comprising: an excitation 
opening, provided at the bottom of the heating chamber, for radiating the 
microwave to heat liquid load in a container; a turntable for rotating the 
object; a magnetron for generating the microwave to heat the object; and a 
wave guide connected to the magnetron and which is opened to the heating 
chamber through the excitation opening. 
The second aspect of the present invention is to provide a high-frequency 
heating apparatus having two wave guides so that the microwaves are 
propagated into two different excitation openings, comprising: wave guide 
switching means for switching a propagation of the microwave to either of 
the two wave guides, the switching means including a pair of circular 
conductive plates and a conductor disposed in the vicinity of a 
circumference of the circular conductive plate so as to shut off the 
microwave, wherein there is a choke structure on the wave guide containing 
the circular conductive plates therein. 
The third aspect of the present invention is to a high-frequency heating 
apparatus having a plurality of magnetrons, comprising a magnetron drive 
circuit comprising: a filament heating power supply connected to each 
filament of the magnetrons; means for generating a high voltage; and 
switching means for selectively supplying a high voltage from the high 
voltage generating means to an anode of the magnetrons. 
Thereby, though respective filaments for a plurality of magnetrons are 
heated by a respective filament-heating power source, high voltage is 
selectively supplied to anode thereof by way of the switching means. 
Other features and advantages of the present invention will become apparent 
from the following description taken in conjunction with the accompanying 
drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Features of the present invention will become apparent in the course of the 
following description of exemplary embodiments which are given for 
illustration of the invention and are not intended to be limiting thereof. 
Embodiments of the present invention will now be described with reference 
to the drawings. 
FIG. 5 shows an internal view of a microwave oven as a high-frequency 
heating apparatus, where a turntable (not shown) is removed. The microwave 
oven is equipped with two magnetrons 19, 21 for generating a microwaves at 
external portions of a side wall 17 of a heating chamber 15. The upper 
magnetron 19 is connected to a wave guide 23 along the side wall 17. An 
excitation opening 25 is provided at an upper end of the wave guide 23, 
situated in an upper portion of the side wall 17 and opened to the heating 
chamber 15. The microwaves radiated from the excitation opening 25 are 
used to heat a solid or semi-solid body placed on the turntable. 
The lower magnetron 21 is connected to a wave guide 29 along a base 27 of 
the heating chamber 15, and an excitation opening 31 formed at an end of 
the wave guide 29 is situated in the vicinity of a center of the base 27 
and opened to the heating chamber 15. The microwave radiated from the 
excitation opening 31 is used for heating a liquid in a liquid container, 
such as a sake bottle 32. 
FIG. 6 shows a enlarged cross section in the neighborhood of the excitation 
opening 31 which is exclusively used for heating the liquid. When heating 
a solid or semi-solid body, the turntable (not shown) is mounted on a 
shaft 35 of a RT motor 33. When heating the liquid contained in a 
container, the turntable is removed and a waterproof insulating plate 37 
permeated by the microwave is placed so that a sake bottle 32 containing a 
sake to be heated is placed thereupon. 
When heating the liquid, a key on a display panel designating a liquid load 
heating is depressed while the sake bottle 32 is kept at a position 
illustrated in FIG. 6. Then, the microwave is generated from the magnetron 
21 whereas no microwave is generated from the magnetron 19. Thereby, the 
microwave at the bottom of the sake bottle 32 is gradually absorbed by a 
base portion of the sake bottle 32 so that convection heat is caused in 
the sake bottle 32, to uniformly heat the liquid load therein. 
FIG. 7 a change in temperature according to this embodiment, where 
respective temperatures are measured at points A through D (refer to FIG. 
2). In the same figure, the temperature at respective points rise at a 
same rate, so that even heating is achieved. 
FIG. 8 shows a high-frequency heating apparatus where the sake bottle is 
placed on the turntable 11 so that the liquid inside the sake bottle can 
be evenly heated without removing the turntable 11. 
Referring to FIG. 16 and FIGS. 11 through 14, it is explained how a 
distance between the excitation opening 31 for exclusive use for heating 
liquid and the base 32 affects absorption of The microwave. 
Referring to FIG. 7 and FIGS. 11 through 14, the distance between a liquid 
container 32 and the excitation opening 31 are changed in a range of 0 mm 
to 30 mm. 
The temperature difference between A and D increases as The temperature 
rises. When the distances are 0 mm, 9 mm and 18 mm, the temperature 
difference between A and D is relatively small, whereas when the distances 
are 24 mm and 30 mm, the temperature difference between A and D becomes 
significantly large. 
In order to obtain sufficient heating, The distance is preferably made 
smaller than 18 mm. Furthermore, in order to keep the temperature 
difference at a minimum under other conditions, such as a different type 
of liquid container and an initial liquid temperature, the distance is 
preferably less than 1/8 wavelength (15 mm). 
Observing the above results where There can be considered a configuration 
in which the turntable is movable in a vertical direction, The present 
invention offers The following embodiment. 
With reference to FIG. 9 and FIG. 10, there are provided three protruding 
members 67 (FIG. 16) equi-distanced around the shaft 35 of the RT motor. A 
turntable support 61 has three openings 69 corresponding to the three 
protruded members 67. Referring to FIG. 10, there is a dap in the top end 
portion in The shaft 35. The dap is mounted to the turntable 11 so that 
the three protruded members 67 are coupled to respective centers of the 
openings 69 in a predetermined manner. 
FIGS. 15 through 17 show the second embodiment of the present invention. 
With reference to FIG. 16, there is provided a cylindrical shape metal 
sleeve 79 under one of the openings 69 of the turntable support 61. There 
is also provided a protrusion 67 around the RT motor shaft 35 
corresponding to the one of the opening 69 to which the cylindrical shape 
metal 79 is coupled. Further, there is provided above the excitation 
opening 31 of the wave guide 29 a cylindrical duct 81 having a same 
diameter with the cylindrical shape metal sleeve 79. A detection signal of 
a sensor 73, upon detecting the protrusion 67 is inputted to a motor 
control circuit 75. When the protrusion 67 is detected by the sensor 73, 
the motor control circuit 75 controls the RT motor 33 such that the 
cylindrical shape metal sleeve 79 mounted to the turntable support 61 is 
always disposed properly under the excitation opening 31 for exclusively 
heating the liquid load. It shall be appreciated that there can be mounted 
a plurality of the cylindrical shape metal sleeves 79 corresponding to the 
number of the openings 69. 
By the above configuration, when the sake bottle is placed on a designated 
load area indicated by a spot light irradiated from a flood lamp 77 and a 
key on the display panel for exclusively heating the liquid load is 
depressed, the microwave is propagated into the cylindrical shape metal 79 
from the liquid-heating excitation opening 31 through the wave guide 29. 
Then, since the microwave is confined inside the cylindrical shape metal 
sleeve 79, a significantly effective absorption of the microwave at the 
bottom of the sake bottle is achieved. Moreover, the turntable need not be 
removed, thus being more convenient compared to the first embodiment. 
Accordingly, the microwave is irradiated to the heating chamber from the 
excitation opening provided at the bottom of the heating chamber, so that 
the liquid load placed in the vicinity of the excitation opening can be 
evenly heated by a heat convection from the bottom thereof. Next, the 
present invention concerns a wave guide switching means where there are 
two wave guides extending from and connecting to a magnetron as 
illustrated in FIG. 24. FIG. 18A and FIG. 18B show a plan view and a cross 
section of wave guide switching means for the high-frequency heating 
apparatus. FIG. 19 shows a perspective and disassembling view of the wave 
guide switching means shown in FIG. 18. 
With reference to FIG. 18, a rectangular wave guide 1 extending upward is 
connected to the magnetron (not shown) and the microwave is propagated 
into the wave guide 1. A lower end of the wave guide 1 is coupled to other 
wave guides 5, 7 in a T-shape form by way of a wave guide switching unit 
3. The microwave propagated through the wave guide 1 propagates to either 
the left side wave guide 5 or the right side wave guide 7 by means of the 
wave guide switching unit 3. 
With reference to FIG. 18B, in the wave guide switching unit 3, a pair of 
circular conductive plates 9 are provided in parallel to an H-plane which 
contacts the long side of the cross section of the wave guide 1. The pair 
of the conductive plates 9 are coupled to a pair of shafts 11 so as to 
rotate about the shaft 11. 
Referring again to FIG. 18B, there is provided a first cylindrical shape 
conductor 13 between the pair of the circular conductive plates 9, so that 
the conductor 13 can rotate to any position along with rotation of the 
circular conducting plates 9. When the conductor 13 is placed in a 
position as illustrated in FIG. 18B and FIG. 15, the microwave is shielded 
from the wave guide 1. As a result, the microwave from the wave guide 1 is 
propagated only into the wave guide 7 by way of the wave guide switching 
unit 3. 
Moreover, there is provide a choke structure 15 in a space of the H-plane 
of the wave guide as illustrated in FIG. 18B. The H-plane constituting a 
part of the choke structure 15 has openings 15a. The openings 15a are 
points where a voltage of the standing wave is maximum, so that there is 
no leakage of the microwave from a space between the openings 15a and the 
shaft 11. The distance between the opening 15a and end of the choke 
structure is preferably approximately .lambda./4 to .lambda.g/4 
(illustrated with combined dotted arrows in FIG. 18B) 
Moreover, the distance between the opening 15 and an end of the 
circumference of the circular conductive plate 9 is preferably 
approximately .lambda./4 to .lambda.g/4, so that the end portion of the 
circumference of the circular conductive plate 9 becomes a short-circuit 
point and there can be maintained an electric connection with the H-plane 
of the wave guide. Accordingly, by taking dimensions of approximately 
.lambda./4 to .lambda.g/4, there can be achieved the optimum choke 
structure. Here, .lambda.g indicates a wavelength inside the wave guide 
whereas .lambda. indicates a wavelength in a normal space, and relation 
therebetween is such that .lambda.g.gtoreq..lambda.. 
In the wave guide switching unit thus configured, the circular conductive 
plate 9 is electrically connected to the H-plane, namely, the both ends of 
the first conductor 13 are connected to the H-plane, so that the wave 
guide 5 is shut off and the microwave is propagated into the other wave 
guide 7. 
FIG. 20A and FIG. 20B shows another wave guide switching means of another 
embodiment and a cross section thereof, respectively. Compared to the wave 
guide switching means shown in FIG. 18, there are provided a second 
conductor 31 90 degrees from the first conductor 13 and a third conductor 
33 between the first conductor 13 and the second conductor 31. 
By providing the first, second and third conductors 13, 33, 31, there is 
formed a pseudo wave guide surface, moreover, the three conductors form a 
sort of fairing to cover a rotating bulge portion on the circular 
conductive plates 9. In other words, an electric geometry due to the bulge 
portion is smoothed up. 
FIG. 24 shows a microwave oven employing the wave guide switching means 
shown in FIG. 20. In the same figure, there are two excitation openings 
and the microwave is selectively supplied by the wave guide switching 
means shown in FIG. 20. 
With reference to FIGS. 21 through 23, an example of a drive mechanism for 
the wave guide switching unit 3 will be described. 
Referring to FIG. 21, a first circular gear (GEAR I) is coupled to the 
rotation shaft 11 outside the switching unit 3, and the GEAR I is driven 
by a stepping motor by way of a second gear (GEAR II) coupled to a 
rotation shaft 12. Two holes are provided in the first gear in near 
circumference thereof so that the holes are detected by a photo 
interrupter (PI) (refer FIG. 22) and the conductor 13 is positioned at a 
predetermined position. A reason for using the stepping motor is because 
it has a stationary torque so that even if it is not electrically driven 
the gear remains stable. It shall be appreciated that a DC motor may 
replace the stepping motor and there can be provided a stopper as 
illustrated in FIG. 23. Thus two alternative positions can be obtained by 
switching a direction to drive the motor, so as to stabilize the movement 
of the gears. 
Accordingly, by implementing the above wave guide means where there is 
provided the first conductor between the a pair of conductive plates which 
are freely rotatable, one of two wave guides can be shut off by 
positioning the first conductor in front of a given wave guide, thus 
providing a simply built and economical wave guide switching unit. 
Moreover, since the both ends of the conductor are electrically joined to 
the wall of the wave guide, there is no arc discharge. Moreover, by 
providing a plurality of conductors, a voltage standing wave ratio (VSWR) 
of the wave guide can be suppressed to the minimum. 
Next, FIG. 25 shows a drive circuit for a microwave oven according to the 
present invention 
With reference to FIG. 25, filaments for magnetrons 111, 113 are connected 
to filament-use secondary windings 115a, 115b of a transformer 115, 
respectively, and a filament-use voltage is supplied from the secondary 
windings 115a, 115b so as to generate heat. Anodes of the magnetrons 111, 
113 are grounded. An end for each filament is connected to high-voltage 
secondary winding 115c by way of a voltage doubler rectifying circuit 115 
constituted by a switch 117, diodes 111, 113, and capacitor 119, thereby 
the high voltage from the secondary winding 115c and the increased voltage 
doubled by the voltage doubler rectifying circuit 115 are selectively 
supplied to anodes of the magnetron 111, 113 by means of the switch 117. 
The primary winding 115p is connected to an a.c. power supply by way of a 
smoothing capacitor 1121, a choke 1123 and a rectifying circuit 1125. A 
parallel circuit comprising a transistor 1131, a diode 1131 and a 
capacitor 1135 is connected between the primary winding 115p of the 
transformer 115 and the smoothing capacitor 1121, wherein the transistor 
1131 is controlled by a control circuit 1137. The control circuit 1137 is 
such that a current at a secondary side of the transformer 115 is detected 
through a converter 1139. The control circuit 1137, the transistor 1131, 
the diode 1133, the capacitor 1135 and the converter 1139 detect an output 
voltage at a secondary side of the transformer 115 and constitute together 
with the transformer 115 an inverter circuit in which the input voltage is 
converted and controlled to a predetermined a.c. voltage. 
In the magnetron drive circuit thus configured, the a.c. voltage from the 
a.c. power supply 1127 is rectified by a diode bridge rectifying circuit 
1125 and is then smoothed by the choke 1123 and the smoothing capacitor 
1121, thereafter converted to a predetermined a.c. voltage by the inverter 
circuit comprising the control circuit 1137 and so on and then supplied to 
the primary winding 115p of the transformer 115 so that a predetermined 
secondary voltage is supplied to each secondary winding. 
Thereafter, the filament voltages from the filament-use secondary winding 
115a, 115b of the transformer 115 are simultaneously supplied to filaments 
of the magnetrons 111, 113 so that both filaments of the both magnetrons 
111, 113 are simultaneously heated. 
When the switch is connected as illustrated in FIG. 25, the high voltage 
from the secondary winding 115c of the transformer 115 is 
voltage-doubler-rectified by the voltage doubler rectifying circuit 1115 
and then an end of the Filament of the magnetron 111 by way of the switch 
117 so that a high voltage is supplied to the anode of the magnetron 
connected between the end of the filament and the ground so as to drive 
the magnetron 111. 
When the switch is connected to a opposite side to the above case, the high 
voltage from the secondary winding 115c of the transformer 115 is supplied 
to a side of the magnetron 113 so that the magnetron 113 is driven then. 
As the switch 117, a switch of make-before-break (MBB) type may be 
preferably use for there will not be generated an abnormal high voltage at 
the time of switching. 
Accordingly, though each filament of plural magnetrons are heated by each 
filament heating power supply, high voltage is selectively supplied to the 
anode thereof by means of the switching means. Therefore, there will be no 
need to provide separate drive power to each magnetron as in the 
conventional practice. Thus, the present invention provides a microwave 
oven which is economical and space-conserved. Moreover, since the 
filaments are driven constantly, an operational speed for a magnetron at 
the time of selectively switching the high voltage is made significantly 
faster. 
Besides those already mentioned above, many modifications and variations of 
the above embodiments may be made without departing from the novel and 
advantageous features of the present invention. Accordingly, all such 
modifications and variations are intended to be included within the scope 
of the appended claims.