CA1252828A - Air flow system for a microwave oven - Google Patents
Air flow system for a microwave ovenInfo
- Publication number
- CA1252828A CA1252828A CA000507830A CA507830A CA1252828A CA 1252828 A CA1252828 A CA 1252828A CA 000507830 A CA000507830 A CA 000507830A CA 507830 A CA507830 A CA 507830A CA 1252828 A CA1252828 A CA 1252828A
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- Prior art keywords
- air
- magnetron
- compartment
- transformer
- cavity
- Prior art date
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Abstract
ABSTRACT OF THE DISCLOSURE
A microwave oven having a forced air flow system wherein the blower output is coupled directly to the power transformer of the high voltage power supply. The power transformer is positioned in an enclosure that houses the magnetron and may also encase other electrical components such as the high voltage capacitor, the high voltage rectifier, an oven light, and a fuse board. The blower is positioned outside the en-closure in a compartment housing the microwave oven controls, Air is drawn into the control compartment by the blower and then forced through a chute in the enclosure directly at the power transformer. By static pressure differential, air exhausts the enclosure through a plurality of predetermined paths. One path flows through the fins of the magnetron to an exhaust port. Another path flows through perforations into the cavity to remove moisture and rotate a microwave antenna.
A microwave oven having a forced air flow system wherein the blower output is coupled directly to the power transformer of the high voltage power supply. The power transformer is positioned in an enclosure that houses the magnetron and may also encase other electrical components such as the high voltage capacitor, the high voltage rectifier, an oven light, and a fuse board. The blower is positioned outside the en-closure in a compartment housing the microwave oven controls, Air is drawn into the control compartment by the blower and then forced through a chute in the enclosure directly at the power transformer. By static pressure differential, air exhausts the enclosure through a plurality of predetermined paths. One path flows through the fins of the magnetron to an exhaust port. Another path flows through perforations into the cavity to remove moisture and rotate a microwave antenna.
Description
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AIR FLOW SYSTEM FO:E~ A MICROWAVE OVEN
Background of ~he Invention Magnetrons are normally used as the source of microwave - energy for mlcrowave ovensO As is well known, a magnetron requires a high voltage, such as 4,000 volts, and this has conventionally been provided by a high voltage power supply which consists of just a few components such as a power trans-former, a capacitor, and a diode.
It has been w211 known that suitable cooling is required to remove internally generated heat from a magnetron. General-ly, magnetrons used for domestic microwave ovens are air cooled and have cooling fins. The standard pxior art configl~ration is to couple a blower directly to the fins of the magnetron;
this maximize~ the forced convection air transferring heat from the magnetron. At least a portion of the forced convec-tion air is ~hen typically routed through the oven cavity because it 1S desirable to remove water vapor that otherwise might condense on food, cavity walls, or the door. Also, the forced air has been used to rotate a mode stirrer or primary radiator.
It has also been well known that electrical components and particularly the power transformer o the high voltage .
power supply produce heat. In fact, app~oval agencies pro-- vide standards for maximum allowable temperatures for various components. Typical present examples for the maximum allowable , ~, . .
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temperatures are 85C for the high voltage capacitor, 100C
for the high voltage diode, 95C rise for the blower motor, 9,C rise for the filamen~ tran~former, ar,d 135C rise or the power transformer. In some prior art ovens, heat genera~
ted within the power transformer and other components is removed by naturai or free convection. In other prior art ovens, the removal of heat from the power transformer and other components has been enhanced by positioning them so that a portion of the ambient room air drawn by the magnetron blower passes across them first. In another prior art ~mbodi-ment such as shown in U.S. Patent No. 4,028,519, the magnetron and power transformer are located in a housing. The blower air first passes through the fins of the magnetron and then is dispersed into the housing where a slight pressure is created. In all of the above embodiments, the transfer of heat from the power transformer and other components has ``~ been relatively inefficient. More specifically, even in those embodiments where air has either been drawn or blown - across the power transformer or other component~, the velocity or volume of that forced convection air has been relatively small and the flow has been substantially laminar~ In the prior art, the combination o~ ineffective cooling and the maximum allowable temperature standards of the approval agen-cies has put significant limitations on the design selection of electrical components. `For example, in the low velocity
AIR FLOW SYSTEM FO:E~ A MICROWAVE OVEN
Background of ~he Invention Magnetrons are normally used as the source of microwave - energy for mlcrowave ovensO As is well known, a magnetron requires a high voltage, such as 4,000 volts, and this has conventionally been provided by a high voltage power supply which consists of just a few components such as a power trans-former, a capacitor, and a diode.
It has been w211 known that suitable cooling is required to remove internally generated heat from a magnetron. General-ly, magnetrons used for domestic microwave ovens are air cooled and have cooling fins. The standard pxior art configl~ration is to couple a blower directly to the fins of the magnetron;
this maximize~ the forced convection air transferring heat from the magnetron. At least a portion of the forced convec-tion air is ~hen typically routed through the oven cavity because it 1S desirable to remove water vapor that otherwise might condense on food, cavity walls, or the door. Also, the forced air has been used to rotate a mode stirrer or primary radiator.
It has also been well known that electrical components and particularly the power transformer o the high voltage .
power supply produce heat. In fact, app~oval agencies pro-- vide standards for maximum allowable temperatures for various components. Typical present examples for the maximum allowable , ~, . .
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temperatures are 85C for the high voltage capacitor, 100C
for the high voltage diode, 95C rise for the blower motor, 9,C rise for the filamen~ tran~former, ar,d 135C rise or the power transformer. In some prior art ovens, heat genera~
ted within the power transformer and other components is removed by naturai or free convection. In other prior art ovens, the removal of heat from the power transformer and other components has been enhanced by positioning them so that a portion of the ambient room air drawn by the magnetron blower passes across them first. In another prior art ~mbodi-ment such as shown in U.S. Patent No. 4,028,519, the magnetron and power transformer are located in a housing. The blower air first passes through the fins of the magnetron and then is dispersed into the housing where a slight pressure is created. In all of the above embodiments, the transfer of heat from the power transformer and other components has ``~ been relatively inefficient. More specifically, even in those embodiments where air has either been drawn or blown - across the power transformer or other component~, the velocity or volume of that forced convection air has been relatively small and the flow has been substantially laminar~ In the prior art, the combination o~ ineffective cooling and the maximum allowable temperature standards of the approval agen-cies has put significant limitations on the design selection of electrical components. `For example, in the low velocity
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laminar air flow environment of the prior art, relatively large power transformers had to be used to meet the approval agency requirement of 135C rise. Not only do larger trans-formers increase the cost of materials by requiring more copper and/or steel, they also increase the weight and size of the microwave oven. This results in a significant draw~
back in shipping and also makes the microwave oven much less portable within the home.
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It is an object of the invention to provide enhanced cooling of the power transformer.
Another oh~ect of the invention is to maximize heat transfer from the power transformer so that a smaller and lighter transformer can be used.
It is still another object of the invention to utilize the full orce of the blower to cool the transformer with turbulent relatively high velocity air while exhausting the forced convection air through the fins of the magnetron.
Still another object of the invention is to provide a cooling air flow rate to the power transformer which is - equal to or greater than the rate at the magnetron.
These and other objects are provided by the invention which defines a microwave oven comprising a microwave cavity, a magnetron for energizing the cavity with microwave energy, a power supply for activating the magnetron, the power supply comprlsing a transformer, means for providing a directed flow of forced convection cooling air against the trans-former, and means for routing at least a portion of the alr from the transformer to the magnetron. It may be pre-ferable that the providing means comprise a blower having an output substantially directed at the transformer. Also, it msy be prefersble that the routing means comprise a plenum enclosing the transformer, the air being directed - .
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from the blower into the plenum against the transformer.
Also, it may be preferable that at least a portion of the air in the plenum be exhausted through the fins of the magneton. By directed flow of forced convection cooling air, it is meant that most of the forced air from a blower ei~her be drawn across or directed at the transformer, The invention may also be practiced by a microwave oven comprising a microwave cavity, a magnetron for energizing the cavity with microwave energy, a power supply coupled to the magnetron, the power supply comprising a transformer, a blower for providing a flow of air for cooling the trans-former and the magnetron, the rate of said air flow at said transformer being equal to or greater than the rate of said air flow at said magnetron. The rate of air flow could be defined in terms of weight per time or volume per time. It may be preferable that the oven further comprise an enclosure encasing the transformer and the magnetron, the blower being positioned outside the enclosure and providing the flow of air through an aperture in the enclosure directly at the , .. . .
transformer, the enclosure having an exhaust port adjacent to the magnetron wherein at least a portion of the air being exhousted through the exhaust port first flows through the magnetron.
The invention also defines a microwave oven comprising ~5 a microwave cavity, a compartment outside the cavity, a _5_ . ' , -. , . . ~ ... . .... .. . .. . ..
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magnetron for energizing the cavity with microwave energy, the magnetron having cooling fins and being positioned in the compartment, a magnetron power supply comprising a trans-former positioned in the compartment, the compartment having a first aperture adjacent to the transformer, a blower for directing a flow of forced convection cooling air through the aperture directly against the transformer, and the compartment having a passageway for exhausting air from the compartment, the passageway passing through the cooling fins of the magnetron. The compartment or enclosure as it may be referred to may preferably be substantially sealed. Also, a portion of the air forced into the compartment or enclosure - may be exhausted through the cavity to remove water moisture.
The invention may further be practiced by a microwave oven comprising a microwave cavity, a chamber outside the cavity divided into first and second compartments by a ~_ partition having an aperture, a blower positioned in the first compartment, the blower having its o~tput coupled to the aperture for providing a flow of forced convection air from the first compartment into the second compartment, a magnetron for energizing the cavity with microwave energy, the magnetron being positioned in the second compartment, a high voltage power supply for activating the magnetron, the power supply including a power transformer positioned in the second compartment adjacent to the aperture wherein . ~ .
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the flow of forced convection air from the blower is substantially directed first against the transformer upon entering the second compartment and the second compartment having an exhaust port for exhausting at ~east a portion of the air from the second compartment, the exhaust air being routed through the magnetron.
The invention may further be practiced by the method of cooling a power transformer and magnetron of a microwave oven comprising the steps of encasing a transformer in an enclosure, providing a flow of forced convection air directed at the transformer, and exhausting at least a portion of the air from the enclosure past the magnetron.
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Brief Description of the Drawings The foregoing objects and advantages will be more fully understood by reading the Description of the Preferred Embodi-ment with reference to the drawings wherein.
FIG. 1 is a front perspective view of a microwave oven;
FIG. 2 is a side rear partially broken away perspective view of the microwave oven with the top and side casing removed;
FIG. 3 is a front sectional view of the oven;
FIG. 4 is an exploded view of the microwave feed assembly, cradle, and grease shield;
FIG. 5 is a sectioned elevation view taken along the cradle;
FIG. 6 is a side elevation view taken along line 6-6 of iS FIG. 3;
FIG. 7 is a top view of the microwave feed assembly;
FIG. 8 is a bottom view of the microwave feed assembly;
FIG. 9 is a top view of the grease shield depicting the alignment of the microwave feed assembly; and FIG. 10 is a front view of th- grease shield.
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Description of the Preferred Embodim~nt Referring to FIG. 1, a front perspective view of a microwave oven 10 is shown, The cavity 12 of microwave oven 10 is generally defined by side walls 14 "back wall 16, floor 5- 18 (FIG. 3), ceiling 20 and door 22 which preferably includes a window 24 for observing the cooking. Door 22.also includes a suitable choke for preventing the escape of microwave energy from microwave cavity 12. A microwave transparent support tray 26 is elevated above floor 18. A control panel 27 including control knobs 28 is provided for activating and controlling the source of microwave energy such as magnetron 30 (FIG. 2) in conventional manner. Top and side casing 32 ` surrounds'the outside of microwave oven 10.
Refer,ring to FIG. 2, there is shown a partially broken-away right rear perspective view of microwave oven 10 with top and side casing 32 removed. FIG. 3 shows a front ~ ~ s~ctional view of microwave oven 10. Waveguide 34, which is parallel to back wall 16, consists of a horizontal section ''34a runnlng across the top of ceiling 20 and a vertical section 34b along the right side wall 14. The output probe 36 of magnetron 30 inserts into vertical section 34b to which the magnetron 30 is mounted. When magnetron 30 is activated in response to control knobs 28, microwave energy at a fre~uency such as, for example, 2450 MHz travels up waveguide vertical section 34b, across waveguide horizontal _g_ ., .. ... ~ , .
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, ~;2~f~1 section 34a and excites antenna probe 38 for coupling into cavity 12. Waveguide section 34a extends considerably past antenna probe 38 to increase the rigidity of ceiling 20.
The precise length of waveguide section 34a i5 selected to optimize the impedance match and power transfer to antenna probe 38.
Referring to FIG. 4, an exploded view oE microwave feed assembly 40, mounting cradle 42 and grease shield 44 are shown. Referring to FIGS. 5 and 6, sectional elevation views taken along cradle 44 and along line 6-6 of FIG. ~ are shown. Cradle 42, which is molded from a microwave trans-parent material such as plastic, has two legs 46a and 46b _ with a horizontal cross bar or arm 48. Each leg 46a and 46b has a pin 50 or fastening post which inserts up through a small hole 52 in ceiling 20 for engagement by a suitable fastener such as a nut 54 or clip. Leg 46a has wings 56 which extend to platforms 58 which seat against ceiling 20, to secure the attitude of cradle 42. Horizontal arm 48 is here shown with diagonal struts 60 and side slat,$ 62 to make it rigid. Ar,~ 48 of cradle 42 is mounted oblique to waveguide ~, 34 and extends underneath circular aperture 64 which communi-' cates from waveguide horizontal section 34a to cavity 12.
A vertlcal support post 66 with braces 68 e%~ends ~pwardly from hor,izontal arm 48 and inserts through aperture 64. The 25 ~ oblique mounting of cradle 42 helps to prevent bending or .. :: . I
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"oil canning" of ceiling 20 al~ ring the ali~nment of sup-porting post 66 and aperture 64. More specifically, the angle between cradle 42 and the axis of waveguide 34 is relatively small to more securely locate supporting post 66 in the center of aperture 54; in one embodiment where cradle 42 was mounted perpendicular to waveguide 34, ceiling 20 buckled slightly and the downward bow altered the alignment between supporting post 66 and aperture 64. For illustration, cradle 42 may have a length of approximately 10 inches and legs 46a and 46b may space horizontal arm 48 about 1 1/8 inches from ceiling 20. Also, supporting post 66 may extend up into waveguide 34 approximat~ly 1/~ inch and have an - upper flat diameter of 0.175 in~hes taperiny outwardly to a diameter of 0.196 inches approximately 3/4 inches below.
Now, with additional reference to FIGS. 7 and 8, there are shown top and bottom views, respectively, of microwave feed assembly 40 which includes antenna probe 38, conductive strip 70 and microwave transparent turbine 72. As shown best in FIG. 5, antenna probe 38 is a cylindrica~ stub having a bottom axial bore 74 which is placed over support post 66.
The roof 76 of bore 74 which supports microwave feed assembly 40 on support post 66 has a dow~wardly extending ball 78 or spherically shaped protrusion f~r minimizing the contact area and rotational friction between the bearing surfaces. As an example, bore 74 may have a len~th of approximately 0.8 inches ~ . .
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with a top diameter of 0,ls inches tapering outwardly to approximately 0.21 inches. The outer diame~er of antenna probe 38 may be 1/2 inch. The bottom of antenna probe 38 has a flange 82 which inserts through hole 84 in conductive strip 70 and then is secured thereto by an orbital peen.
Still referring to FIGS. 5, 7, and 8, conductive strip 70 includes a flag or patch antenna 86, a strip conductor 88 and a strip support 90 or strip connector. Conductive strip 70 is punched or cut from a flat sheet o~ aluminum alloy having a thickness o' approximately .032 inches. ~lag antenna 86 defines a rectzngle having rounded corners. In the direction of strip con~uctor 88, flag antenna 86 has a dimension of 1.281 inches ~ith a transverse dimension of 1.968 inches. From hole 8'., strip conductor 88 has a hori-zontal length of approximately 2 inches to a downwardly bent section 92:which spaces flag antenna 86 approximately 0.5 ~- . inches below the plane of .he rest of strip conductor 88.
Strip conductor 8B is spaced approximately 0.3 inches from ceiling 20 so flag antenna 86 is spaced ap.pr~ximately.
0.8 inches. Strip conductor 88 has two outwardly extending tabs 94 on opposing sides approximately .625 inches from hole 84. Tabs 94, which extend outwardly approximately .375 inches for a distance of 0~5 inches enhance the impedance match between antenna probe 38 and flag antenna 86. Strip support 90 has a segment 96 approximately 1.3 inches long to ;: -12-~ ~ 2~
a curved anchor segment 98. Cor.ductive strip 70 has three holes 100 to connect it to turb ne 72. More specifically, turbine 72 has a horizontal dis:~ 102 with three bosses 104 which are aligned with holes 100. In connecting plastic turbine 72 to conductive strip 70, the bosses 104 are respec-tively inserted up through holes 100 and then meltod or ultrasonically welded for permanent attachment. On the bottom side of disk 102, turbine 72 has a pl~rality of ver tical radial vanes 106 or paddl~s which cause microwave feed assembly 40 to rotate when impinged by a stream of air as will be described in detail later herein. Vanes 106 may have weights 108 to balance mic~owave feed assembly 40 about its vertical axis of rotation through hole 84 and antenna probe 38. The radius of each v~.~e 106 from the axis of rotation is approximately 3.75 i~ches. Disk 102 has a central aperture 110 through which orbi.al peened flange 82 extends.
~_ Also, disk 102 has a notch 112 where vertical section 92 of strip conductor 88 extends down-~ardly.
In operation, the microwave energy in waveguide 34 excites microwave currents on antenna probe 38 thereby co- -axially coupling the energy down antenna probe 38 through circular aperture 64 to the ~unction with conductive stxip 70. Most o~ the microwave curr~nt conducts in the direction of strip conductor 88 rather tha~ ~trip support 90 because the described geometry of strip support 90 has a high input .
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of anchor segment g8 are joined in parallel to provide a low impedance which transforms to a high impedance at the input to strip support 90 which is approximately a quarter wave-length away. The two arms of anchor segment 98 should not total a half wavelength because that could cause high field strength and possible arcing if cavity 12 were operated without a load. There is very little radiation of microwave energy along strip conductor 88 because, as shown best in FIG. S, it is closely spaced to ceiling 20 which functions as a ground plane or plate. More specifically, strip con-ductor 88 is parallel and spaced approximately 0.3 inches or less than one-eighth wavelength from from ceiling 20 so that it is equivalent to an air dielectric microstrip line.
According to well-known principles, the closely spaced ground plate or reflector maximizes transmission and minimizes -radiation. Strip support 90 is also closely spaced to ceiling 20 so that the relativel~ small currents traveling in that -direction would have a high radiation resistance.
The microwave c~rrents travel down the 0.~-inch vertical length of strip section 92 to flag~antenna 86. Because the flag antenna 86 is spaced approximately 0.8 inches or sub-stantially more than one-eighth wavelength below ceiling 20 which functions as a ground plane, flag antenna 86 is an effective radiator of microwave energy. Radiation from flag - .
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antenna 86 is also enhanced because i~ i5 resonant thereby increasing the current flow. The long dimension of the rectangle of flag antenna 86 w;~ich is 1.968 inches long is resonant at a frequency Fl below the operating frequency;
accordingly, the current in that direction lags the excita-tion voltage. The short dimension of the rectangle of flag antenna 86 which is 1.281 inches long is resonant at a fre-~uency F2 above the operating frequency; accordingly, the current in that direction leads the excitation voltage.
With these rectangle dimensions in the described cavity environment, flag antenna 86 provides a pattern with sub-stantially circular polarization over a relatively large angle. More specifically, the orthogonal components radiated from flag antenna 86 are su~stcntially equal in magnitude 1~ and their phases have a difference of approximately ~0~.
The dimensions of flag antenna 86 were empirically determined ~- by using rectangles of different dimensions as radiating antennas and measuring the relative signal strengths and -- phases received with a linearly polarized antenna alternately held parallel to the two ortho~onal directions of the rec-tangles. The results of the tests were plotted as functions of the lengths of the sides. ~ntensity contours of equal magnitude and 90 phase differences were estimated from the data to obtain the optimum dim~nsions.
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Ideally, the maximum orthogonal current amplitudes and the radiated components at resonant freque~cies Fl and'F2 are equal such that Fl and F2 could be dis~laced such that their respective half power points occur a.'the operating frequency so that the current of one leads the excitation by 45 and the current of the other lags the excitation voltage by 45. In this way, the currents in the orthogonal direc-tions would have a phase differential of 90. However, even if the maximum'radiated components at resonant frequencies Fl and F2 are unequal, the dimensions of the rectangle of flag antenna 86 can be selected such that resonant frequen-cies Fl and ~2 are moved up or down from t~e ideal Fl and F2 ~ frequencies to compensate. More specifica'ly, for a strong Fl excitation, resonant frequencies Fl and F2 are moved lS down in frequency to provide e~ual radiated components with a phase difference of gO at the operating frequency; for a ~_ strong F2 excitation, resonant frequencies ~l and F2 are moved up in frequency to provide equal radiated components with a phase d1fferen~e of 90 at the operating ~requency.
Many other embodiments of flag antennas and feed struc-tures could be used to radiate substantially circular polari-zation into cavity 12. For example, flag antenna 86 could be oblique to strip section 92 rather than at a right angle.
Also, strip section 92 could be perpendicu'ar to the short side rather than to the long side or'it ~i5ht be connected , .
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` ' ' anywhere along either side. Further, the width of strip section 92, here 0.5 inches, could be different and it could have a different length. Alsol a plurality of flag antennas 86 could be used~ It should be recognize~, however, that the optimum dimensions of flag 2ntenna 86 are a function of the operating environment. Accordingly, each one of the described modifications would alter resonant freguencies Fl and F2 and the currents in the two orthogonal directions parallel to the sides of the rectangle. As a result, the dimensions of the flag antenna ~6 should be optimized for circular polarization with each configuration.
Because flag antenna 86 is supported by strip conductor 88 rather than a solid dielectr-c as is commonly used in microstrip patch antennas for rzdars, ~lag antenna 86 can be moved freely with respect to ceiling 20 which functions as its ground plane. More specifically, as will be described in detail later herein, flag antenna 86 is rotated about the axis of antenna probe 38 thereby moving the radiated pattern in an offset circular path. Also, the air dielectric between the flag antenna 86 and celling 20 stores less energy than a solid dielectric. Accordingly, the operating bandwidth for both radiation and circular polarization i5 enhanced. ~hile the mi~crowave oven operates in o~ly a narrow band of allowed frequencies, the resonant frequency of the flag antenna 86 is det~ermined, among other things, by its physical size and ..
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spacing from ceiling 20. Accordingly, the wider bandwidth increases the tolerance on dimensional variations and posi tioning of flag antenna 86.
Fla~ antenna 86 has a lower profile than a helical S antenna having a cylindrical segment for providing circular polarization by gradually increasing the spacing from the ground`plane. The low profile increases the usable volume of cavity 12 and also permits flag antenna 86 to be rotated above cross arm 48 of a support cradle 42 which has rela-tively short legs 46a and 46b.
Again referring to FIG. 2, divider panel 120 separates the chamber 114 behind control panel 27 into a front control _ compartment 116 and a back ~ower supply compartment 118 or plenum. More specifically, control compartment 116 is l; bounded by control panel 27, partition or divider panel 120, top and side casing 32, cavity side wall 14, and bottom 122. Blower 124 which includes scroll 125 and motor 126 is positioned in control compartment 116 and, as will be des-cribed in detail, provides a forced flow of air for cooling components, for rotating microwave ~eed assembly 40, and for removing moisture from cavi~y 12. Control compartment 116 also encases conventional controls 128 such as relays, timers, circuit boards, and interlocks for microwave oven 10. The intake air for blower 124 r.ay be drawn or sucked into control 2i compartment 116 through a variety of passageways such as, :
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~ 2~3 for example, openings 132 in bottom 122, Only a relatively small amount of heat is generated in the control compartment 116 and that heat is effectively removed by intake ~ir 134 being drawn across blower motor 126 and controls 128 on its way to scroll 125.
Power supply compartment 118 is an enclosure generally defined by divider panel 120, to? and side casing 32, cavity side wall 14, back wall 16 and bottom 136. The seams 138 of power supply compartment 118 are substantially sealed by gasket 140 so that a static pressure differential can be built up between the power supp~y compartment 118 or plenum and the outside~ In addition to power supply components, power supply compartment 118 ho;-ses magnetron 30 and may also enclose other components such as a fuse board and an oven light (not shown). More specifically, power supply compartment 118 is here shown housing power transformer 142 ``~ and condenser 144 which, along with a rectifier 145, are part of a conventional voltage doubler circuit for providing - high.voltage such as 4,000 volts to magnetron 30; Also, conventional fllament transformer 146 is shown mounted to back wall 16 in power supply compartment 118. The forced air from blower 124 is coupled directly to a chute or aperture 148 through divider panel 120. Power transformer 142 is mounted adjacent to aperture 148 and transverse to the direction of flow of forced convection air from aperture 148. Accordingly, .
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turbulent and relatively high velocity cooling air from blower 124 impinges power transformer 142 first upon entering'power supply compartment 118. As a result, the rate of heat flow by convection from power transormer 142 is greater than in prior art microwave ovens, For example, in one prior art embodiment where relatively low velocity substantially laminar air was drawn across a particular power transformer on its way to the blower, the power transformer had a temperature rise in the ranqe from 120C to 131C. That same power transformer 142, however, was found to have a temperature rise of only about 64C when mounted in power supply compartment 118 as - ¦
described herein. As a result of more effective cooling/
- the size of the power transformer 142 can be reduced without exceeding the maximum allowable temperature standard~ of approval agencies. The reduced size not only lowers the cost of materials, but it results in less weight which equates to significant advantages in shipping and portability.
Another advantage of the above-described embodiment is that , the blower motor 126 is positioned in control compartment 116 isolated from the higher heat producing components in the power supply compartment 118. Filament transformer 146 along with condenser 144 and rectifier 145, which are mounted on discharge air duct lS0,, are cooled by the forced convection air after it flows across power transormer 142.
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, The convection air forced throuyh aper~ure 148 by blow~r 124 creates a static pressure di.-ferential between the interior of power supply c~partrent 118 or plenum and the outside. Accordingly, air flows outwardly from power supply compartment 118 by any a~allablé ~ath. Because the seams 138 of power suppiy compartment 118 are subs~antially sealed against the passage o air, the air primarily exhausts from power supply compartment 118 through three predetermined and well defined paths or ~assageways. The first path is through cooling fins 152 of m~gnetron 30 which communicate through discharge air duct 150 to exhaust ports 154 on back wall 16. The air flow through th.is first path has to be large enough to provide adequate cooling of magnetron 30.
The second path enters perLorations 156 in waveguide vertical section 34b and flows down the wzveguide where i~ entérs cavity 12 through aperture 64; the relatively small air flow ~_ ln this second path provides some cooling of the magnetron output probe 36. The third path enters cavity 12 through a -- set of perforations 158 in side W211 14 near cei~ ing ~0. As will be described in detaiL ~ the functions of the air flow along the third path are to rotate microwave feed assem~ly 40 and to remove moisture from cavity 12. Because all o the air entering power supply compartment 118 is directed through aperture 148, the rate of zir flo-~ at the power transformer 142 is equal to or greater than ~e rate of air flow at :. .
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magnetron 30. For example, the cubic feet per minute of air flowing through aperture 148 is approximately equal to sum of the cubic feet per minute air flow rates through the three described exit paths. Because the air flowing through magne-tron 30 is only one of those exit paths, its rate would never be larger than the input rate through aperture 148.
As described earlier herein, FIG. 4 shows an exploded view including grease shield 44 which may be a plastic molded part. FIG. 6 shows a side sectioned elevation view of cavity 12 and grease shield 44. FIGS~ 9 and lO respectively show top and front views of grease shield 44. FIG. 9 also depicts the alignment of microwave feed assembly 40. Grease shield 44 directs the flow and distrlbution of air in cavity 12 so as to rotate microwave feed assembly 40 and exhaust moisture from cavity 12. Because a significant portion of the forced air from blower l24 is exhausted through magnetron 30 to `~ exhaust port 154l there is only a limited amount of air forced through perforations 158 into cavity 12. Accordingly, grease shield~44 must make efficient and effective uso of ~he air in performing its intended ~unctions. Grease shield 44 has a substantially horizontal panel 160 which divides cavity 12 into an upper region or chamber 162 housing micro-wave feed structure 40 and a lower cooking region or chamber 164. The air from perforations 158 flows by static press~re differential into nozzle or chute 166 formed by horizontal ~ .
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panel 160, ceiling 20 and interconnecting vertical parti~ions 168 and 170 or dividers. The spacing between partitions 168 and 170 may be closer at the inside s~ch as 2.125 inches as compared to 2.5 inches so that the air is concentrated into a relatively high velocity stream of air directed tangen~ially àt the vanes 106 of turbine 72. Panel 160 has an oblique channel 172 aligned for receiving the arm 48 of cradle 42.
If cradle 42 were not recessed in channel 172, the stream of air driving turbine 72 would become turbulent upon impinging side slat 62. Laminar flow, on the other hand, helps to increase the speed of rotation of microwave feed assembly 40. In fact, because of substantially laminar flow and the low rotational friction provided by ball 78, microwave feed assem~ly may rotate at approximate}y 120 rpm. This fast rotation is especially advantageous because the scan rate of leakage testing equipment used during manufacturing is determined by the rate of antenna rotation. For example, at 120 rpm, the scan rate along the door can be 2 inches per - second. At 60 rpm, however, the scan must be l inch per second.
Channel 172 is enough wider than arm 48 of cradle 42 so that grease shield 44 can easily be installed or removed without contacting or disturbing cradle 42. More specifically, as descri~ed best with reference to FIG. 6, grease shield 44 is initially installed and then later removed and replaced `::
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for cleaning by inserting tabs 174 into suitable receptacles 176, here slots in ceiling 20, and then pushing bac~ward in thumb holds 178a and 178b to slightly bend grease shield 44 so that front tabs 180 can be inserted or snapped over flange S 182 of cavity 12. If the tolerance of channel 172 and cradle 42 were too small, installation and removal of grease shield 44 would be encumbered.
Partition 168 extends approximately out to channel 172 while partition 170 stops short of the rotational circum-ference defined by vanes 106. Wings 56 of leg 46a approxi-mately conform with the gap 184 between partition 168 and partition 186. Accordingly, most of the air stream driving _ turbine 72 continues inside partition 186 rather than being ` directed through gap 184.
lS Partition 186 and a portion of partition 168 adjacent channel 172 are arcs concentric to the rotational circum-ference defined by vanes 106. ~ach arc has a radius of . . .
4.125 inches. From a point approximately 15 to the fron~
of a lateral axis through the center of rotation, the height of partition 186 has a downward 'inear slope from approxi-mately 1.123 inches where it contacts ceiling 20 to approxi-mately .087 inches at its juncture with channel 172 on the right. Accordingly, the air driving stream flows out over partition 186 and it is disperseA with relative uniformity toward the front 188 of grease s;nield 44. More specifically, ~. ' ' , . .
~ ~ 2~3 where the air stream is strongest, the space between ceiling 20 and partition 186 is smallest to limit the air flowing toward the left front. As the air stream gets weaker in the counter clockwise direction due to dissipation, the space between ceiling 20 and partition 186 gets larger to equalize the air flowing toward th~ right front.
The uniformly dispersed flow of air from partition 186 travels forwar~ to trou~h 190 defined by vertical side 192 and sloped front side 194. Vertical side 192 has a plurality of lateral slots 196 and sloped side 194 has a plurality of front-to-back slots 198. The air entering trough 190 by static pressure differential flows from upper chamber 162 to lower cooking chamber 164 throu~h slots 196 and 198. The path initiated from slots 198 flows down along the door and is important to prevent condensation of moisture on window 24 that would reduce visibility of cavity 12. Although - - partition 186 is sloped downwardly to provide substantially uniform distribution of air f~ow down along door 22, it was -found that the air flow ad~acent to the left side was slightly less than the rest resulting in some` condensatïon there.
Accordingly, an aperture 200 was located in the left thumb hold 178a and the distribution of air flow down door 22 was made more uniform. As shown best in FIG. 6, the path of air flow initiated from slots 196 is directed back along the underside 202 o grease sheild 44 to ef~iciently and directly .
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l, exhaust moisture rising from the food. More specifically, if the rising moisture is exhausted directly rather than being dispersed throughout the cavity, the limited air flow is utilized more effectively and efficiently. The air carrying moisture from cavity 12 passes back along the underside 202 and then exhausts through a set of perforations 204 in the ceiling 20 behind the back partition 206 of grease shield 44. The bottom of back partition 206 extends downwardly below the horizontal plane of panel 160 so as to shield per- ¦
forations 204 from direct splatter of grease. In addition to at least portions of partitions 168, 170, 186 and 206, spacers 208 contact ceiling 20 so as to limit the warping of _ grease shield 44. Side slats 210 and ribs 212 function as ` braces to increase the rigidity of grease shield 44. The front side 214 of channel 172 is sloped so as to reduce the turbulence of air flowing across it; the laminar flow provides a more effective use of the limited air flow.
As shown best in FIG. 2, the air exhausting cavity 12 up through perforations 204 flows laterally between the top of ceiling 20 and the top casing 32 to the left side. Then, the air flows downwardly to oven exhaust vents 216. In con-formance with safety requirements, the indirect flow of air .
from cavity 12 prevents flames from coming out of microwave oven 10 in the unlikely event that a fire was burning in the cavity 12.
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This concludes the description of the preferred embodi-ment. A reading of it by those skilled in the art will bring to mind many modifications and alterations without departing from the spirit and scope of the invention.
Accordingly, it is intended that the invention only be limited by the foilowing claims.
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laminar air flow environment of the prior art, relatively large power transformers had to be used to meet the approval agency requirement of 135C rise. Not only do larger trans-formers increase the cost of materials by requiring more copper and/or steel, they also increase the weight and size of the microwave oven. This results in a significant draw~
back in shipping and also makes the microwave oven much less portable within the home.
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It is an object of the invention to provide enhanced cooling of the power transformer.
Another oh~ect of the invention is to maximize heat transfer from the power transformer so that a smaller and lighter transformer can be used.
It is still another object of the invention to utilize the full orce of the blower to cool the transformer with turbulent relatively high velocity air while exhausting the forced convection air through the fins of the magnetron.
Still another object of the invention is to provide a cooling air flow rate to the power transformer which is - equal to or greater than the rate at the magnetron.
These and other objects are provided by the invention which defines a microwave oven comprising a microwave cavity, a magnetron for energizing the cavity with microwave energy, a power supply for activating the magnetron, the power supply comprlsing a transformer, means for providing a directed flow of forced convection cooling air against the trans-former, and means for routing at least a portion of the alr from the transformer to the magnetron. It may be pre-ferable that the providing means comprise a blower having an output substantially directed at the transformer. Also, it msy be prefersble that the routing means comprise a plenum enclosing the transformer, the air being directed - .
~ . ., , ~,. .. .
from the blower into the plenum against the transformer.
Also, it may be preferable that at least a portion of the air in the plenum be exhausted through the fins of the magneton. By directed flow of forced convection cooling air, it is meant that most of the forced air from a blower ei~her be drawn across or directed at the transformer, The invention may also be practiced by a microwave oven comprising a microwave cavity, a magnetron for energizing the cavity with microwave energy, a power supply coupled to the magnetron, the power supply comprising a transformer, a blower for providing a flow of air for cooling the trans-former and the magnetron, the rate of said air flow at said transformer being equal to or greater than the rate of said air flow at said magnetron. The rate of air flow could be defined in terms of weight per time or volume per time. It may be preferable that the oven further comprise an enclosure encasing the transformer and the magnetron, the blower being positioned outside the enclosure and providing the flow of air through an aperture in the enclosure directly at the , .. . .
transformer, the enclosure having an exhaust port adjacent to the magnetron wherein at least a portion of the air being exhousted through the exhaust port first flows through the magnetron.
The invention also defines a microwave oven comprising ~5 a microwave cavity, a compartment outside the cavity, a _5_ . ' , -. , . . ~ ... . .... .. . .. . ..
,.
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magnetron for energizing the cavity with microwave energy, the magnetron having cooling fins and being positioned in the compartment, a magnetron power supply comprising a trans-former positioned in the compartment, the compartment having a first aperture adjacent to the transformer, a blower for directing a flow of forced convection cooling air through the aperture directly against the transformer, and the compartment having a passageway for exhausting air from the compartment, the passageway passing through the cooling fins of the magnetron. The compartment or enclosure as it may be referred to may preferably be substantially sealed. Also, a portion of the air forced into the compartment or enclosure - may be exhausted through the cavity to remove water moisture.
The invention may further be practiced by a microwave oven comprising a microwave cavity, a chamber outside the cavity divided into first and second compartments by a ~_ partition having an aperture, a blower positioned in the first compartment, the blower having its o~tput coupled to the aperture for providing a flow of forced convection air from the first compartment into the second compartment, a magnetron for energizing the cavity with microwave energy, the magnetron being positioned in the second compartment, a high voltage power supply for activating the magnetron, the power supply including a power transformer positioned in the second compartment adjacent to the aperture wherein . ~ .
, . .
the flow of forced convection air from the blower is substantially directed first against the transformer upon entering the second compartment and the second compartment having an exhaust port for exhausting at ~east a portion of the air from the second compartment, the exhaust air being routed through the magnetron.
The invention may further be practiced by the method of cooling a power transformer and magnetron of a microwave oven comprising the steps of encasing a transformer in an enclosure, providing a flow of forced convection air directed at the transformer, and exhausting at least a portion of the air from the enclosure past the magnetron.
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Brief Description of the Drawings The foregoing objects and advantages will be more fully understood by reading the Description of the Preferred Embodi-ment with reference to the drawings wherein.
FIG. 1 is a front perspective view of a microwave oven;
FIG. 2 is a side rear partially broken away perspective view of the microwave oven with the top and side casing removed;
FIG. 3 is a front sectional view of the oven;
FIG. 4 is an exploded view of the microwave feed assembly, cradle, and grease shield;
FIG. 5 is a sectioned elevation view taken along the cradle;
FIG. 6 is a side elevation view taken along line 6-6 of iS FIG. 3;
FIG. 7 is a top view of the microwave feed assembly;
FIG. 8 is a bottom view of the microwave feed assembly;
FIG. 9 is a top view of the grease shield depicting the alignment of the microwave feed assembly; and FIG. 10 is a front view of th- grease shield.
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Description of the Preferred Embodim~nt Referring to FIG. 1, a front perspective view of a microwave oven 10 is shown, The cavity 12 of microwave oven 10 is generally defined by side walls 14 "back wall 16, floor 5- 18 (FIG. 3), ceiling 20 and door 22 which preferably includes a window 24 for observing the cooking. Door 22.also includes a suitable choke for preventing the escape of microwave energy from microwave cavity 12. A microwave transparent support tray 26 is elevated above floor 18. A control panel 27 including control knobs 28 is provided for activating and controlling the source of microwave energy such as magnetron 30 (FIG. 2) in conventional manner. Top and side casing 32 ` surrounds'the outside of microwave oven 10.
Refer,ring to FIG. 2, there is shown a partially broken-away right rear perspective view of microwave oven 10 with top and side casing 32 removed. FIG. 3 shows a front ~ ~ s~ctional view of microwave oven 10. Waveguide 34, which is parallel to back wall 16, consists of a horizontal section ''34a runnlng across the top of ceiling 20 and a vertical section 34b along the right side wall 14. The output probe 36 of magnetron 30 inserts into vertical section 34b to which the magnetron 30 is mounted. When magnetron 30 is activated in response to control knobs 28, microwave energy at a fre~uency such as, for example, 2450 MHz travels up waveguide vertical section 34b, across waveguide horizontal _g_ ., .. ... ~ , .
' ~:
, ~;2~f~1 section 34a and excites antenna probe 38 for coupling into cavity 12. Waveguide section 34a extends considerably past antenna probe 38 to increase the rigidity of ceiling 20.
The precise length of waveguide section 34a i5 selected to optimize the impedance match and power transfer to antenna probe 38.
Referring to FIG. 4, an exploded view oE microwave feed assembly 40, mounting cradle 42 and grease shield 44 are shown. Referring to FIGS. 5 and 6, sectional elevation views taken along cradle 44 and along line 6-6 of FIG. ~ are shown. Cradle 42, which is molded from a microwave trans-parent material such as plastic, has two legs 46a and 46b _ with a horizontal cross bar or arm 48. Each leg 46a and 46b has a pin 50 or fastening post which inserts up through a small hole 52 in ceiling 20 for engagement by a suitable fastener such as a nut 54 or clip. Leg 46a has wings 56 which extend to platforms 58 which seat against ceiling 20, to secure the attitude of cradle 42. Horizontal arm 48 is here shown with diagonal struts 60 and side slat,$ 62 to make it rigid. Ar,~ 48 of cradle 42 is mounted oblique to waveguide ~, 34 and extends underneath circular aperture 64 which communi-' cates from waveguide horizontal section 34a to cavity 12.
A vertlcal support post 66 with braces 68 e%~ends ~pwardly from hor,izontal arm 48 and inserts through aperture 64. The 25 ~ oblique mounting of cradle 42 helps to prevent bending or .. :: . I
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"oil canning" of ceiling 20 al~ ring the ali~nment of sup-porting post 66 and aperture 64. More specifically, the angle between cradle 42 and the axis of waveguide 34 is relatively small to more securely locate supporting post 66 in the center of aperture 54; in one embodiment where cradle 42 was mounted perpendicular to waveguide 34, ceiling 20 buckled slightly and the downward bow altered the alignment between supporting post 66 and aperture 64. For illustration, cradle 42 may have a length of approximately 10 inches and legs 46a and 46b may space horizontal arm 48 about 1 1/8 inches from ceiling 20. Also, supporting post 66 may extend up into waveguide 34 approximat~ly 1/~ inch and have an - upper flat diameter of 0.175 in~hes taperiny outwardly to a diameter of 0.196 inches approximately 3/4 inches below.
Now, with additional reference to FIGS. 7 and 8, there are shown top and bottom views, respectively, of microwave feed assembly 40 which includes antenna probe 38, conductive strip 70 and microwave transparent turbine 72. As shown best in FIG. 5, antenna probe 38 is a cylindrica~ stub having a bottom axial bore 74 which is placed over support post 66.
The roof 76 of bore 74 which supports microwave feed assembly 40 on support post 66 has a dow~wardly extending ball 78 or spherically shaped protrusion f~r minimizing the contact area and rotational friction between the bearing surfaces. As an example, bore 74 may have a len~th of approximately 0.8 inches ~ . .
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with a top diameter of 0,ls inches tapering outwardly to approximately 0.21 inches. The outer diame~er of antenna probe 38 may be 1/2 inch. The bottom of antenna probe 38 has a flange 82 which inserts through hole 84 in conductive strip 70 and then is secured thereto by an orbital peen.
Still referring to FIGS. 5, 7, and 8, conductive strip 70 includes a flag or patch antenna 86, a strip conductor 88 and a strip support 90 or strip connector. Conductive strip 70 is punched or cut from a flat sheet o~ aluminum alloy having a thickness o' approximately .032 inches. ~lag antenna 86 defines a rectzngle having rounded corners. In the direction of strip con~uctor 88, flag antenna 86 has a dimension of 1.281 inches ~ith a transverse dimension of 1.968 inches. From hole 8'., strip conductor 88 has a hori-zontal length of approximately 2 inches to a downwardly bent section 92:which spaces flag antenna 86 approximately 0.5 ~- . inches below the plane of .he rest of strip conductor 88.
Strip conductor 8B is spaced approximately 0.3 inches from ceiling 20 so flag antenna 86 is spaced ap.pr~ximately.
0.8 inches. Strip conductor 88 has two outwardly extending tabs 94 on opposing sides approximately .625 inches from hole 84. Tabs 94, which extend outwardly approximately .375 inches for a distance of 0~5 inches enhance the impedance match between antenna probe 38 and flag antenna 86. Strip support 90 has a segment 96 approximately 1.3 inches long to ;: -12-~ ~ 2~
a curved anchor segment 98. Cor.ductive strip 70 has three holes 100 to connect it to turb ne 72. More specifically, turbine 72 has a horizontal dis:~ 102 with three bosses 104 which are aligned with holes 100. In connecting plastic turbine 72 to conductive strip 70, the bosses 104 are respec-tively inserted up through holes 100 and then meltod or ultrasonically welded for permanent attachment. On the bottom side of disk 102, turbine 72 has a pl~rality of ver tical radial vanes 106 or paddl~s which cause microwave feed assembly 40 to rotate when impinged by a stream of air as will be described in detail later herein. Vanes 106 may have weights 108 to balance mic~owave feed assembly 40 about its vertical axis of rotation through hole 84 and antenna probe 38. The radius of each v~.~e 106 from the axis of rotation is approximately 3.75 i~ches. Disk 102 has a central aperture 110 through which orbi.al peened flange 82 extends.
~_ Also, disk 102 has a notch 112 where vertical section 92 of strip conductor 88 extends down-~ardly.
In operation, the microwave energy in waveguide 34 excites microwave currents on antenna probe 38 thereby co- -axially coupling the energy down antenna probe 38 through circular aperture 64 to the ~unction with conductive stxip 70. Most o~ the microwave curr~nt conducts in the direction of strip conductor 88 rather tha~ ~trip support 90 because the described geometry of strip support 90 has a high input .
. -' , ~ ~ 2 impedance. More specifically, the oppositely directed arm~
of anchor segment g8 are joined in parallel to provide a low impedance which transforms to a high impedance at the input to strip support 90 which is approximately a quarter wave-length away. The two arms of anchor segment 98 should not total a half wavelength because that could cause high field strength and possible arcing if cavity 12 were operated without a load. There is very little radiation of microwave energy along strip conductor 88 because, as shown best in FIG. S, it is closely spaced to ceiling 20 which functions as a ground plane or plate. More specifically, strip con-ductor 88 is parallel and spaced approximately 0.3 inches or less than one-eighth wavelength from from ceiling 20 so that it is equivalent to an air dielectric microstrip line.
According to well-known principles, the closely spaced ground plate or reflector maximizes transmission and minimizes -radiation. Strip support 90 is also closely spaced to ceiling 20 so that the relativel~ small currents traveling in that -direction would have a high radiation resistance.
The microwave c~rrents travel down the 0.~-inch vertical length of strip section 92 to flag~antenna 86. Because the flag antenna 86 is spaced approximately 0.8 inches or sub-stantially more than one-eighth wavelength below ceiling 20 which functions as a ground plane, flag antenna 86 is an effective radiator of microwave energy. Radiation from flag - .
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antenna 86 is also enhanced because i~ i5 resonant thereby increasing the current flow. The long dimension of the rectangle of flag antenna 86 w;~ich is 1.968 inches long is resonant at a frequency Fl below the operating frequency;
accordingly, the current in that direction lags the excita-tion voltage. The short dimension of the rectangle of flag antenna 86 which is 1.281 inches long is resonant at a fre-~uency F2 above the operating frequency; accordingly, the current in that direction leads the excitation voltage.
With these rectangle dimensions in the described cavity environment, flag antenna 86 provides a pattern with sub-stantially circular polarization over a relatively large angle. More specifically, the orthogonal components radiated from flag antenna 86 are su~stcntially equal in magnitude 1~ and their phases have a difference of approximately ~0~.
The dimensions of flag antenna 86 were empirically determined ~- by using rectangles of different dimensions as radiating antennas and measuring the relative signal strengths and -- phases received with a linearly polarized antenna alternately held parallel to the two ortho~onal directions of the rec-tangles. The results of the tests were plotted as functions of the lengths of the sides. ~ntensity contours of equal magnitude and 90 phase differences were estimated from the data to obtain the optimum dim~nsions.
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Ideally, the maximum orthogonal current amplitudes and the radiated components at resonant freque~cies Fl and'F2 are equal such that Fl and F2 could be dis~laced such that their respective half power points occur a.'the operating frequency so that the current of one leads the excitation by 45 and the current of the other lags the excitation voltage by 45. In this way, the currents in the orthogonal direc-tions would have a phase differential of 90. However, even if the maximum'radiated components at resonant frequencies Fl and F2 are unequal, the dimensions of the rectangle of flag antenna 86 can be selected such that resonant frequen-cies Fl and ~2 are moved up or down from t~e ideal Fl and F2 ~ frequencies to compensate. More specifica'ly, for a strong Fl excitation, resonant frequencies Fl and F2 are moved lS down in frequency to provide e~ual radiated components with a phase difference of gO at the operating frequency; for a ~_ strong F2 excitation, resonant frequencies ~l and F2 are moved up in frequency to provide equal radiated components with a phase d1fferen~e of 90 at the operating ~requency.
Many other embodiments of flag antennas and feed struc-tures could be used to radiate substantially circular polari-zation into cavity 12. For example, flag antenna 86 could be oblique to strip section 92 rather than at a right angle.
Also, strip section 92 could be perpendicu'ar to the short side rather than to the long side or'it ~i5ht be connected , .
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` ' ' anywhere along either side. Further, the width of strip section 92, here 0.5 inches, could be different and it could have a different length. Alsol a plurality of flag antennas 86 could be used~ It should be recognize~, however, that the optimum dimensions of flag 2ntenna 86 are a function of the operating environment. Accordingly, each one of the described modifications would alter resonant freguencies Fl and F2 and the currents in the two orthogonal directions parallel to the sides of the rectangle. As a result, the dimensions of the flag antenna ~6 should be optimized for circular polarization with each configuration.
Because flag antenna 86 is supported by strip conductor 88 rather than a solid dielectr-c as is commonly used in microstrip patch antennas for rzdars, ~lag antenna 86 can be moved freely with respect to ceiling 20 which functions as its ground plane. More specifically, as will be described in detail later herein, flag antenna 86 is rotated about the axis of antenna probe 38 thereby moving the radiated pattern in an offset circular path. Also, the air dielectric between the flag antenna 86 and celling 20 stores less energy than a solid dielectric. Accordingly, the operating bandwidth for both radiation and circular polarization i5 enhanced. ~hile the mi~crowave oven operates in o~ly a narrow band of allowed frequencies, the resonant frequency of the flag antenna 86 is det~ermined, among other things, by its physical size and ..
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spacing from ceiling 20. Accordingly, the wider bandwidth increases the tolerance on dimensional variations and posi tioning of flag antenna 86.
Fla~ antenna 86 has a lower profile than a helical S antenna having a cylindrical segment for providing circular polarization by gradually increasing the spacing from the ground`plane. The low profile increases the usable volume of cavity 12 and also permits flag antenna 86 to be rotated above cross arm 48 of a support cradle 42 which has rela-tively short legs 46a and 46b.
Again referring to FIG. 2, divider panel 120 separates the chamber 114 behind control panel 27 into a front control _ compartment 116 and a back ~ower supply compartment 118 or plenum. More specifically, control compartment 116 is l; bounded by control panel 27, partition or divider panel 120, top and side casing 32, cavity side wall 14, and bottom 122. Blower 124 which includes scroll 125 and motor 126 is positioned in control compartment 116 and, as will be des-cribed in detail, provides a forced flow of air for cooling components, for rotating microwave ~eed assembly 40, and for removing moisture from cavi~y 12. Control compartment 116 also encases conventional controls 128 such as relays, timers, circuit boards, and interlocks for microwave oven 10. The intake air for blower 124 r.ay be drawn or sucked into control 2i compartment 116 through a variety of passageways such as, :
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~ 2~3 for example, openings 132 in bottom 122, Only a relatively small amount of heat is generated in the control compartment 116 and that heat is effectively removed by intake ~ir 134 being drawn across blower motor 126 and controls 128 on its way to scroll 125.
Power supply compartment 118 is an enclosure generally defined by divider panel 120, to? and side casing 32, cavity side wall 14, back wall 16 and bottom 136. The seams 138 of power supply compartment 118 are substantially sealed by gasket 140 so that a static pressure differential can be built up between the power supp~y compartment 118 or plenum and the outside~ In addition to power supply components, power supply compartment 118 ho;-ses magnetron 30 and may also enclose other components such as a fuse board and an oven light (not shown). More specifically, power supply compartment 118 is here shown housing power transformer 142 ``~ and condenser 144 which, along with a rectifier 145, are part of a conventional voltage doubler circuit for providing - high.voltage such as 4,000 volts to magnetron 30; Also, conventional fllament transformer 146 is shown mounted to back wall 16 in power supply compartment 118. The forced air from blower 124 is coupled directly to a chute or aperture 148 through divider panel 120. Power transformer 142 is mounted adjacent to aperture 148 and transverse to the direction of flow of forced convection air from aperture 148. Accordingly, .
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turbulent and relatively high velocity cooling air from blower 124 impinges power transformer 142 first upon entering'power supply compartment 118. As a result, the rate of heat flow by convection from power transormer 142 is greater than in prior art microwave ovens, For example, in one prior art embodiment where relatively low velocity substantially laminar air was drawn across a particular power transformer on its way to the blower, the power transformer had a temperature rise in the ranqe from 120C to 131C. That same power transformer 142, however, was found to have a temperature rise of only about 64C when mounted in power supply compartment 118 as - ¦
described herein. As a result of more effective cooling/
- the size of the power transformer 142 can be reduced without exceeding the maximum allowable temperature standard~ of approval agencies. The reduced size not only lowers the cost of materials, but it results in less weight which equates to significant advantages in shipping and portability.
Another advantage of the above-described embodiment is that , the blower motor 126 is positioned in control compartment 116 isolated from the higher heat producing components in the power supply compartment 118. Filament transformer 146 along with condenser 144 and rectifier 145, which are mounted on discharge air duct lS0,, are cooled by the forced convection air after it flows across power transormer 142.
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, The convection air forced throuyh aper~ure 148 by blow~r 124 creates a static pressure di.-ferential between the interior of power supply c~partrent 118 or plenum and the outside. Accordingly, air flows outwardly from power supply compartment 118 by any a~allablé ~ath. Because the seams 138 of power suppiy compartment 118 are subs~antially sealed against the passage o air, the air primarily exhausts from power supply compartment 118 through three predetermined and well defined paths or ~assageways. The first path is through cooling fins 152 of m~gnetron 30 which communicate through discharge air duct 150 to exhaust ports 154 on back wall 16. The air flow through th.is first path has to be large enough to provide adequate cooling of magnetron 30.
The second path enters perLorations 156 in waveguide vertical section 34b and flows down the wzveguide where i~ entérs cavity 12 through aperture 64; the relatively small air flow ~_ ln this second path provides some cooling of the magnetron output probe 36. The third path enters cavity 12 through a -- set of perforations 158 in side W211 14 near cei~ ing ~0. As will be described in detaiL ~ the functions of the air flow along the third path are to rotate microwave feed assem~ly 40 and to remove moisture from cavity 12. Because all o the air entering power supply compartment 118 is directed through aperture 148, the rate of zir flo-~ at the power transformer 142 is equal to or greater than ~e rate of air flow at :. .
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magnetron 30. For example, the cubic feet per minute of air flowing through aperture 148 is approximately equal to sum of the cubic feet per minute air flow rates through the three described exit paths. Because the air flowing through magne-tron 30 is only one of those exit paths, its rate would never be larger than the input rate through aperture 148.
As described earlier herein, FIG. 4 shows an exploded view including grease shield 44 which may be a plastic molded part. FIG. 6 shows a side sectioned elevation view of cavity 12 and grease shield 44. FIGS~ 9 and lO respectively show top and front views of grease shield 44. FIG. 9 also depicts the alignment of microwave feed assembly 40. Grease shield 44 directs the flow and distrlbution of air in cavity 12 so as to rotate microwave feed assembly 40 and exhaust moisture from cavity 12. Because a significant portion of the forced air from blower l24 is exhausted through magnetron 30 to `~ exhaust port 154l there is only a limited amount of air forced through perforations 158 into cavity 12. Accordingly, grease shield~44 must make efficient and effective uso of ~he air in performing its intended ~unctions. Grease shield 44 has a substantially horizontal panel 160 which divides cavity 12 into an upper region or chamber 162 housing micro-wave feed structure 40 and a lower cooking region or chamber 164. The air from perforations 158 flows by static press~re differential into nozzle or chute 166 formed by horizontal ~ .
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panel 160, ceiling 20 and interconnecting vertical parti~ions 168 and 170 or dividers. The spacing between partitions 168 and 170 may be closer at the inside s~ch as 2.125 inches as compared to 2.5 inches so that the air is concentrated into a relatively high velocity stream of air directed tangen~ially àt the vanes 106 of turbine 72. Panel 160 has an oblique channel 172 aligned for receiving the arm 48 of cradle 42.
If cradle 42 were not recessed in channel 172, the stream of air driving turbine 72 would become turbulent upon impinging side slat 62. Laminar flow, on the other hand, helps to increase the speed of rotation of microwave feed assembly 40. In fact, because of substantially laminar flow and the low rotational friction provided by ball 78, microwave feed assem~ly may rotate at approximate}y 120 rpm. This fast rotation is especially advantageous because the scan rate of leakage testing equipment used during manufacturing is determined by the rate of antenna rotation. For example, at 120 rpm, the scan rate along the door can be 2 inches per - second. At 60 rpm, however, the scan must be l inch per second.
Channel 172 is enough wider than arm 48 of cradle 42 so that grease shield 44 can easily be installed or removed without contacting or disturbing cradle 42. More specifically, as descri~ed best with reference to FIG. 6, grease shield 44 is initially installed and then later removed and replaced `::
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for cleaning by inserting tabs 174 into suitable receptacles 176, here slots in ceiling 20, and then pushing bac~ward in thumb holds 178a and 178b to slightly bend grease shield 44 so that front tabs 180 can be inserted or snapped over flange S 182 of cavity 12. If the tolerance of channel 172 and cradle 42 were too small, installation and removal of grease shield 44 would be encumbered.
Partition 168 extends approximately out to channel 172 while partition 170 stops short of the rotational circum-ference defined by vanes 106. Wings 56 of leg 46a approxi-mately conform with the gap 184 between partition 168 and partition 186. Accordingly, most of the air stream driving _ turbine 72 continues inside partition 186 rather than being ` directed through gap 184.
lS Partition 186 and a portion of partition 168 adjacent channel 172 are arcs concentric to the rotational circum-ference defined by vanes 106. ~ach arc has a radius of . . .
4.125 inches. From a point approximately 15 to the fron~
of a lateral axis through the center of rotation, the height of partition 186 has a downward 'inear slope from approxi-mately 1.123 inches where it contacts ceiling 20 to approxi-mately .087 inches at its juncture with channel 172 on the right. Accordingly, the air driving stream flows out over partition 186 and it is disperseA with relative uniformity toward the front 188 of grease s;nield 44. More specifically, ~. ' ' , . .
~ ~ 2~3 where the air stream is strongest, the space between ceiling 20 and partition 186 is smallest to limit the air flowing toward the left front. As the air stream gets weaker in the counter clockwise direction due to dissipation, the space between ceiling 20 and partition 186 gets larger to equalize the air flowing toward th~ right front.
The uniformly dispersed flow of air from partition 186 travels forwar~ to trou~h 190 defined by vertical side 192 and sloped front side 194. Vertical side 192 has a plurality of lateral slots 196 and sloped side 194 has a plurality of front-to-back slots 198. The air entering trough 190 by static pressure differential flows from upper chamber 162 to lower cooking chamber 164 throu~h slots 196 and 198. The path initiated from slots 198 flows down along the door and is important to prevent condensation of moisture on window 24 that would reduce visibility of cavity 12. Although - - partition 186 is sloped downwardly to provide substantially uniform distribution of air f~ow down along door 22, it was -found that the air flow ad~acent to the left side was slightly less than the rest resulting in some` condensatïon there.
Accordingly, an aperture 200 was located in the left thumb hold 178a and the distribution of air flow down door 22 was made more uniform. As shown best in FIG. 6, the path of air flow initiated from slots 196 is directed back along the underside 202 o grease sheild 44 to ef~iciently and directly .
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.. ~, ,' ~,.
l, exhaust moisture rising from the food. More specifically, if the rising moisture is exhausted directly rather than being dispersed throughout the cavity, the limited air flow is utilized more effectively and efficiently. The air carrying moisture from cavity 12 passes back along the underside 202 and then exhausts through a set of perforations 204 in the ceiling 20 behind the back partition 206 of grease shield 44. The bottom of back partition 206 extends downwardly below the horizontal plane of panel 160 so as to shield per- ¦
forations 204 from direct splatter of grease. In addition to at least portions of partitions 168, 170, 186 and 206, spacers 208 contact ceiling 20 so as to limit the warping of _ grease shield 44. Side slats 210 and ribs 212 function as ` braces to increase the rigidity of grease shield 44. The front side 214 of channel 172 is sloped so as to reduce the turbulence of air flowing across it; the laminar flow provides a more effective use of the limited air flow.
As shown best in FIG. 2, the air exhausting cavity 12 up through perforations 204 flows laterally between the top of ceiling 20 and the top casing 32 to the left side. Then, the air flows downwardly to oven exhaust vents 216. In con-formance with safety requirements, the indirect flow of air .
from cavity 12 prevents flames from coming out of microwave oven 10 in the unlikely event that a fire was burning in the cavity 12.
`. `:
.
- :
-:
:. :
~5~8~
This concludes the description of the preferred embodi-ment. A reading of it by those skilled in the art will bring to mind many modifications and alterations without departing from the spirit and scope of the invention.
Accordingly, it is intended that the invention only be limited by the foilowing claims.
I
' . .. : .
`,` ~:
- : ~
Claims (10)
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. A microwave oven comprising;
a microwave cavity;
a magnetron for energizing said cavity with microwave energy;
a power supply for activating said magnetron, said power supply comprising a transformer;
a blower having an output substantially directed at said transformer for providing a directed flow of forced convection cooling air against said transformer; and means for routing at least a portion of said air from said transformer to said magnetron.
a microwave cavity;
a magnetron for energizing said cavity with microwave energy;
a power supply for activating said magnetron, said power supply comprising a transformer;
a blower having an output substantially directed at said transformer for providing a directed flow of forced convection cooling air against said transformer; and means for routing at least a portion of said air from said transformer to said magnetron.
2. The oven recited in Claim 1 wherein said routing means comprises a plenum enclosing said transformer, said air being directed from said blower into said plenum against said transformer.
3. The oven recited in Claim 2 wherein at least a portion of said air in said plenum is exhausted through said magnetron.
4. A microwave oven, comprising;
a microwave cavity;
a compartment outside said cavity;
a magnetron for energizing said cavity with microwave energy, said magnetron having cooling fins and being positioned in said compartment;
a magnetron power supply comprising a transformer positioned in said compartment;
said compartment having a first aperture adjacent to said transformer;
a blower for directing a flow of forced convection cooling air through said aperture directly against said trans-former; and said compartment having a passageway for exhausting air from said compartment, said passageway passing through said cooling fins of said magnetron.
a microwave cavity;
a compartment outside said cavity;
a magnetron for energizing said cavity with microwave energy, said magnetron having cooling fins and being positioned in said compartment;
a magnetron power supply comprising a transformer positioned in said compartment;
said compartment having a first aperture adjacent to said transformer;
a blower for directing a flow of forced convection cooling air through said aperture directly against said trans-former; and said compartment having a passageway for exhausting air from said compartment, said passageway passing through said cooling fins of said magnetron.
5. A microwave oven, comprising;
a microwave cavity;
a substantially sealed enclosure having a cooling air input aperture and an air exhaust port;
a magnetron for energizing said cavity with microwave energy;
a power supply coupled to said magnetron, said power supply comprising a transformer positioned within said enclosure adjacent to said air input aperture;
a blower positioned outside said enclosure for provid-ing a flow of forced convection air through said input aperture into said enclosure, said flow being substantially directed at said transformer; and said magnetron having cooling fins positioned so that at least a portion of said air being forced into said enclosure by said blower is routed through said fins to said air exhaust port.
a microwave cavity;
a substantially sealed enclosure having a cooling air input aperture and an air exhaust port;
a magnetron for energizing said cavity with microwave energy;
a power supply coupled to said magnetron, said power supply comprising a transformer positioned within said enclosure adjacent to said air input aperture;
a blower positioned outside said enclosure for provid-ing a flow of forced convection air through said input aperture into said enclosure, said flow being substantially directed at said transformer; and said magnetron having cooling fins positioned so that at least a portion of said air being forced into said enclosure by said blower is routed through said fins to said air exhaust port.
6. The oven recited in Claim 5 further comprising a passageway from said enclosure into said cavity for routing a portion of said flow of forced convection air from said enclosure into said cavity for removing water vapor.
7. A microwave oven, comprising;
a microwave cavity;
a chamber outside said cavity divided into first and second compartments by a partition having an aperture;
a blower positioned in said first compartment, said blower having its output coupled to said aperture for providing a flow of forced convection air from said first compartment into said second compartment;
a magnetron for energizing said cavity with microwave energy, said magnetron being positioned in said second compartment;
a high voltage power supply for activating said magne-tron, said power supply comprising a power transformer positioned in said second compartment adjacent to said aperture wherein said flow of forced convection air from said blower is substantially directed first against said transformer upon entering said second compartment; and said second compartment having an exhaust port for exhausting at least a portion of said air from said second compartment, said exhaust air being routed through said magnetron.
a microwave cavity;
a chamber outside said cavity divided into first and second compartments by a partition having an aperture;
a blower positioned in said first compartment, said blower having its output coupled to said aperture for providing a flow of forced convection air from said first compartment into said second compartment;
a magnetron for energizing said cavity with microwave energy, said magnetron being positioned in said second compartment;
a high voltage power supply for activating said magne-tron, said power supply comprising a power transformer positioned in said second compartment adjacent to said aperture wherein said flow of forced convection air from said blower is substantially directed first against said transformer upon entering said second compartment; and said second compartment having an exhaust port for exhausting at least a portion of said air from said second compartment, said exhaust air being routed through said magnetron.
8. The oven recited in Claim 7 wherein said magnetron has fins, said oven further comprising a duct connected from said magnetron to said exhaust port wherein said exhaust air flows by static pressure differential through said fins.
9. The oven recited in Claim 8 further comprising a second exhaust port from said second compartment, said second exhaust port communicating with said cavity wherein a portion of the air from said second compartment flows through said cavity for removing water vapor.
10. The method of cooling a power transformer and magnetron of a microwave oven, comprising the steps of:
encasing said transformer in an enclosure;
directing a flow of forced convection air from a blower at said transformer; and exhausting at least a portion of said air from said enclosure past said magnetron.
encasing said transformer in an enclosure;
directing a flow of forced convection air from a blower at said transformer; and exhausting at least a portion of said air from said enclosure past said magnetron.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US73143885A | 1985-05-07 | 1985-05-07 | |
| US731,438 | 1985-05-07 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1252828A true CA1252828A (en) | 1989-04-18 |
Family
ID=24939506
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000507830A Expired CA1252828A (en) | 1985-05-07 | 1986-04-29 | Air flow system for a microwave oven |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1252828A (en) |
-
1986
- 1986-04-29 CA CA000507830A patent/CA1252828A/en not_active Expired
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