CA1157104A - Support for rotatable air driven mode stirrer - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

A mode stirrer arrangement for a microwave oven is disclosed wherein the stirrer is driven by means of an air flow which does not impinge on the mode stirring blades themselves. The mode stirrer blades are carried on one end of a rotatable shaft and positioned in the path of micro-wave energy entering the oven cooking cavity. The other end of the shaft carries a drive fan which is separated from the mode stirrer blades by an air imprevious wall and a stream of air is directed onto the drive fan to thereby rotate the shaft and mode stirrer blades. The stream of air onto the drive fan is provided via an air channel which communicates with the magnetron cooling blower.
An upstanding cylindrical bearing made of a dielectric material supports the shaft for rotation, the top surface of the bearing being tapered to provide a seat of reduced contact area to reduce frictional losses during rotation.

Description

7104 9D-RG-1419~

BACKGROUND OF THE INVENTI~N
1. Field of the Invention -This application is directed to microwave ovens and to mode stirrers for use in microwave ovens and, more particularly, to a novel mounting arrangement for supporting a mode stirrer device for rotation in the path of microwave energy entering the cooking cavity of a microwave oven.

2. Description of the Prior Art One well known problem associated with conventional microwave ovens concerns the uneven distribution of micro-wave energy within the cooking cavity. The result of such unevenness has been the creation of "hot spots" and "cold spots" at different finite areas of the oven. For many types of foods, cooking results are unsatisfactory under such conditions because some portions of the food may be completely cooked while others are barely warmed.
One explanation for the non-uniform cooking pattern is that electromagnetic standing wave patterns, known as "modes," are set up within the cooking cavity. When such a standing wave pattern is set up, the intensities of the electric and magnetic fields vary greatly with position.
The precise configuration of the standing wave or mode pattern during a cooking cycle is dependent on a multitude of factors among which are the characteristics of the microwave energy source of the dimensions and makeup of the cavity and the loading effect of different types and quantities of food whichare placed in the cooking cavity.
In an effort to alleviate the problem of non-uniform energy distribution, a great many approaches have been tried with varying degrees of success. The most common approach involves the use of a socalled "mode stirrer"
or mode chang~r which typically resembles a fan having ~ 1 -- r,~-.

~ 7 1()4 9D~RG-14198 metal blades. The mode stirrer rotates and may be placed either within the cooking cavity itself (usually protected by a cover constructed of a material transparent to micro-wave ovens) or, to conserve space within the cooking cavity, may be mounted within a recess formed in one of the cooking cavity walls, normally the top.
The most common method of turning the blades of such mode stirrers is to attach the blades to a shaft which extends into the microwave oven cooking cavity thrcugh the top wall of the enclosure. An electric motor is coupled to the end of the shaft opposite the blades to impart a rotary motion thereto (U.S. Patent No. 3,783,219 to Takeda dated January 1, 1974 and U.S. Pat.No. 2,813,185 to Smith dated November 12, 1957). This arrangement has as its main disadvantage the need for a separate motor to drive the mode stirrer blades. Additionally, care must be taken to prevent the microwave energy from escaping through the shaft opening to damage or destroy the motor.
Yet another prior art arrangement is shown in U.S.
Patents Nos. 4,019,010 to Tanaka et al dated April 19, 1977,

3,471,671 to Ouschner dated October 7, 1969 and 3,991,295 to Akiyoshi dated November 9, 1976, in which the mode stirrer blades themselves are driven by an air stream used to cool the oven magnetron. The advantage of this arrangement is that it does away with the need for a separate motor by utilizing an already present air stream used to cool the magnetron. However, by passing the air stream over the mode stirring blades themselves severe restrictions are placed on the shape and angular orientation of these blades. More specifically, since the blades driven by the air stream also perform the additional function of changing modes within the oven cavity, a compromise blade design which satisfactorily 9D-RG-1419~
5 J ~34 performs both func~ions, albeit each in a less than optimum manner, must be used. Secondly, this design ordinarily requires the use of an air flow path through the oven cavity itself, and this presents an additional variable to deal with in properly designing the oven by requiring a minimum amount of air circulation to obtain satisfactory mode mixing.
Japanese patent 542,531 issued in 1979 provides for a mode stirrer arrangement wherein the stirrer is driven by means of an air flow which does not impinge on the mode stirrer blades themselves. The mode stirrer blades are carried on one end of a rotatable shaft and positioned in the path of microwave energy entering the oven cooking cavity.
The other end of the shaft carries a drive fan which is separated from the mode stirrer blades by an air impervious wall and a stream of air is directed onto the drive fan to thereby rotate the shaft and mode stirrer blades. The stream of air onto the driven fan is provided via an air channel which communicates with the magnetron cooling blower.
This application is directed to a mounting and support arrangement for use in connection with a mode stirrer of the type disclosed in the aforementioned Japanese patent which provides for adequate rotation of the stirrer with a relatively low volume of driving air while concurrently reducing leakage of microwave energy out of the oven cavity to an acceptable level.
OB3ECTS AND S~MMARY OF THE INVENTION
It is therefore an ob~ect of the invention to provide an improved mode stirrer or mode changing arrangement for use in microwave ovens.
A further object is the provision of a mode stirrer drive arrangement which produces relatively low noise level during ~7~4 9~-RG-14198 o~eration while conc~rently reducing the lea~age of microwave enerqy out of the oven cavity.

A further object is the provision of a mounting arrangement for a mode stirrer which reduces the frictional losses attendant to rotation and thereby permits rotation thereof at an acceptable rate when driven by a relatively small volume air flow.
These and other objects are accomplished by a mode stirrer arrangement for a microwave oven wherein the stirrer is driven by means of an air flow which does not impinge on the mode stirring blades them-selves. The mode stirrer blades are carried on one end of a rotatable shaft and positioned in the path of microwave energy entering the oven cooking cavity. The other end of the shaft carries a drive fan which is separated from the mode stirrer blades by an air impervious wall and a stream of air is directed onto the drive fan to thereby rotate the shaft and mode stirrer blades. The stream of air onto the drive fan is provided via an air channel which communicates with the magnetron cooling blower.
An upstanding cylindrical bearing made of a dielectric plastic material supports the shaft for rotation, the top surface of the bearing being tapered to provide a seat of reduced contact area to reduce frictional losses during rotation. The bearing is supported coaxially from a metal bushing carried by the top wall of a mode stirrer cavity, the bushing, in combination with the bearing and shaft, being selected to limit escape of microwave energy from the cooking cavity.

BRIEF DESCRIPTION OF THE DRAWINGS
.
The invention, both as to its organization and the principles of operation, together with further objects and advantages thereof, will best be understood by reference to the following specification taken in conjunctian with the ~ccompanying drawings in which:
FIG. 1 is a front illustration of an oven embodying the prin-ciples of the invention with portions of the oven broken away to show ~ ~L~7 3L()~ 9D-RG-14198 --the overal1 construction of the mode stirrer and the ftow of air from the magnetron cooling blower in the electronic component compartment into contact with the mode stirrer drive fan; and FIG. 2 is an elevational view of the oven of FIG. 1 with the top wall broken away to show the mode stirrer and the air conduits for providing an air stream to drive the mode s~irrer; and FIG. 3 is a sectional view taken on the line 3-3 of FIG. 2 and showing the details of the mode stirrer assembly and associated air con-duits.

DESCR~PTION OF THE PREFERRED EM80DIMENT
~ eferring now to FIGS. 1-3, there is generally illustrated a microwave oven generally designated by the numeral 20, construct~d in accordance and embodying the features of the present invention.
It will be understood that numerous co~ponents in addition to those illustrated are required in a complete microwave oven, but for clarity of illustration and brevity of description, only those compon-ents believed essential to the understanding of the invention will be described in detail.
The countertop oven 20 is adapted for placement on the top of a table or counter and is housed in a cabinet which includes an upstand-ing front panel 21~ a rear wall 22, a top wall 23, a bottom wall 24, and a pair of opposed sidPwalls 26 and 27. Mounted within the oven cavity and alongside the front panel 21 is a heating enclosure or cooking caYity generally desi~nat~d by the numeral 30, including a top wall 31, a bottom wall 32, a rear wall 33 and a pair of opposed sidewalls 34 and 36.
The front of the heating enclosure 30 is closed by a door 35 which, in the closed position thereof, forms the front wall of the heating enclosure 33, which enclosure is generally in the fonm of a 3Q rectangular parallelepiped. The door 35 includes a handle 37 and is ~ 73L()4 9D-R~-14198 hinged on the side 29 thereof remote from the handle to allow for easy access to the oven cavity 30. The panel 21 is provided with control knobs 38 for operating the oven. Thus, the interior of the oven cabinet is constructed to include a cooking cavity 30 defined by the walls 31, 32, 33, 34 and 36 within the oven cavity and an electrical controi cabinet 25 between the heating enclosure 30 and the oven cabinet walls 22, 23, 24 and 27. The walls 31, 32, 33, 34 and 36, as well as the interior wall of the door 35, are made of a conductive material so as to confine the microwave energy within the cavity 30. ~he oven door 35 is also provided around the inner periphery thereof with a conventional ~icrowave energy seal (not shown) to prevent the escape of microwave energy from the heating enclosure 30 in use.
As indicated hereinbefore, the electrical control compartment 25 includes the various power supply and control circuits for operating the microwave oven including a magnetron 40 which is adapted to produce microwave energy having a frequency of approximately 2450 MHz at the output probe 39 thereof when coupled to a suitable source of power. It should be understood that a suitable high voltage DC power supply (not shown) must be provided to operate the magnetron 40, such power supplies being well known in the art. Microwave energy from magnetron 40 is coupled by the probe 39 into one end of a waveguide 41. A conductive, short-circuiting plate 38 closing off the end of the waveguide 41 is spaced approximately one-sixth wavelength from the probe 39. As is conventional, the waveguide 41 is so dimensioned as to propagate 2450 MHz microwave energy in the TE1~ mode.
The excitation system of the mi~rowave oven further includes a mode stirrer cavity 43 formed between a cover member 4g and the top wall 31 of the cavity 30. The cover member 4~ includes upstanding sidewalls 44 and a top wall 45, the top wall 45 having a rectangular opening 46 therein for connection to the end o~ the waveguide 41 remote from the probe 39. The mode stirrer cavity 43 has its major dimension oriented ~L~L~;7~ ~14 90-RG-14198 horizontally and has a vertical dimension of less than one-half wave-length, and a horizontal extent of a plurality of half wavelengths. The cavity 43 is substantially square in cross section and is dimensioned to support a plurality of half-waves of standing wave energy in each of the two orthagonal directions. While the illustrated mode stirrer cavity 43 ls generally square in cross section, other shapes, for example, circular, may be employed. The waveguide 41 and mode stirrer cavity 43 are qener-ally conventional in design, and more specific infonmation as to their construction may be obtained from u.s. Pat. No.4,144,436.
The top wall 31 of the cooking cavity 30 is provided with an opening or feed port 54 through which microwaYe energy is propagated from the mode stirrer cavity 43 into the cooking cavity 30. The feed port 54, while shown as being physically opened, may be closed with any material known in the art to be pervious to microwave energy.
A mode stirrer assembly 60 is supported above the feed port 54. The assembly 60 includes a mode stirrer fan 61 including metal fan blades 62 which is mounted for rotation within the mode stirrer cavity 43 ~uxtaposed to the entrance port 54. The blades 62 are conventional in dèsign and are shaped to cooperate with the excitation syste~ in effectively changin~ mode patterns in the oven cavity 30. The mode stirrer blades 62 are mounted on one end of a dielectric mounting shaft 63 by any suitable means, such as a bolt 64 carried on a threaded end of the shaft 63. r~he top wall 45 of the cover member 4~ includes an indented or depressed portion generally cylindrical in cross section and having a sidewall 6~ and base wall 68. The diameter of the coni-cal depression decreases in the direction of the top wall 31.
The end of the shaft 63 oppositP the mode stirrer blades 62 has attached thereto a metal drive fan or impeller 70 which pro~ects slightly above the top wall of the mode stirrer cavity 43 and is attached thereto by means of a screw 66. The hub portion of the drive fan 70 rests on a washer 72, which in turn rests on an upstanding cylindrical support ~ 71~)4 9D-RG-14198 column or bearing 71. Both the washer 72 and the support bearing 71 are made of a non-metal, plastic material having high heat resistance charac-teristics. Preferably, these elements are made from Teflon, a synthetic flouride resin, which also provides low frictional losses during rotation of the mode stirrer. The bearing 71 has an inner diameter on1y slightiy larger than the diameter of the shaft 63 to firmly support the shaft vertically during rotation. The support bearing 71 is tapered inwardly and upwardly adjacent the washer 72 to fonm a ridge or seat of reduced contact area with the underside of washer 72 to further decrease friction-al losses during rotation. The support bearing 71 has fonmed at the endremote from the drive fan a widened axial bore which fits tightly over an upstanding cylindrical portion of a metal bushing 69 to coaxially align the bearing and bushing. The bushing 69 also includes a flat portîon extending outwardly from the shaft, which flat portion lies in a plane parallel to and in contact with the wall 68 of the mods stirrer housing. The bushing 69 is appropriately attached by any suitable means to the top w~ll 31 of the cavity 30. The bushing 69 functions to limit the passage of microwave energy from the cavity 30. For this purpose the height and diameter of the cylindrical portion of the bushing 69 are selected to be substantially non-transmissive of microwave energy at the ~requency used in the oven. The vertical height of the drive fan 70 is fixed essentially by the vertical height of the support column 71 and may be adjusted accordingly.
A blower 80, which may be of the conventional squirrel cage type and driven by a suitab1e motor 81, is located in the electrical component compartment to provide a cooling air flow oYer the magnetron 40. For this purpose, the magnetron 40 is positioned in a generally L-shaped main air chamber 75 bounded by an L-shaped extension 76 of the top wall 31 on the top and by a similarly shaped wall section 77 on the bottom. rhe chamber 75 is further defined on its sides (referring to FIGS. 1 and 2) by walls 7~, 78, 8~, 86, 87 and 36. The wall 78 is pro-~LJL~ 3L~) ~ 9D-RG-!4198 vided with an opening 73 through which a main air stream from blower 80 enters the main air chamber 75, as illustrated generally by the arrows in the drawing. A portion of the air stream directed by the blower 80 toward the magnetron 40 fonms a secondary air stream 82 which enters the oven cavity 30 through perfQrations or openings 84 in the sidewall 36 of the cooking cavity 30. A suitable exit opening (not shown) is also provided in another cavity wall to serve as an exit for the air stream entering through opening 84 to provide a continuous circulation of air throu~h the cooking cavity 30 during the cooking process, as is convent10nal.
An inlet port 90 is provided in the wall section 76 of the main air chamber 75 and an air directing conduit or duct 84 is provided to create a channel 88 between the top walls of the mode stirrer cavity 43 and the conduit 84. The channel 88 ls in air communication with the blower 80 via thP chamber 7~ and the port 90 and is open at the end thereof remote from the blower 80 at an exit port 91 at the front of the oven (FIG. 3) which is closed by an air pervious grille 92 to thereby provide an air stream 83 along-the channel 88.
As can be seen, the drive fan or impeller 70 is located in the air stream 83 intermediate the entrance port 90 and the exit port 91 with the ma~or surfaces of the fan blades oriented orthogonally to the flow of air stream 83 along the channel 88. A pair of air directing baffles 95 and 96 are positioned in the duct 84 to direct the air stream 83 into the drive fan 7Q.
During operation, air from blower 80 operates to cool the magnetron 40 and ventilate the oven cooking cavity 30 by means of air stream 82. Concurrently, the air flow 83 along the channel 88 is directed into contact with the drive fan 70 of the mode stirrer assembly and then out of the oven cabinet via exit port 9t. This causes the fan 70 to rotate and consequently the shaft 63 and the mode stirrer blades ~,jL~r7~L~ ~ 9D-RG-1~198 62. Since the mode stirrer blades 62 are located in the mode stirrer cavity 43 and are thus separated by the top wall 45 of the cover member 49 from the air stream 83, no air from stream 83 impinges thereon.
Also, during operation, the magnetron 40 generates microwave energy which is coupled to the cavity 30 via the waveguide 41 and mode stirrer cavity 43. Since the rotating mode stirrer blades 62 are located in the cavity 43, the field pattern of the microwave energy propagated into the cooking cavity 30 is continuously altered to thereby produce a more unifonm distribution of energy and more unifonm cookin~.
lo While a specifie embodlment of the invention has been 111u-strated and described herein, it is realized that numerous modifications and changes will occur to those ski11ed in the art. For example, while the mode stirrer cavity 43 is illustrated and described as being mounted at the top of the oven cavity, and is further illustrated and described as extending horizontally, it will be apparent that the mode stirrer cavity could be mounted on any outside wall of the cooking cavity 30 without departing from the invention. The mode stirrer assembly in such a construction would be mounted in the same relative position to the mode stirrer cavity and suitable ducting would be utilized to create a stream of air to impinge upon the drive blades of the mode stirrer without impinging on the mode stirrer blades 62.
Since numerous changes may be made in the above described apparatus, and di~erent embodiments of the invention may be made with-out departing fram the spirit thereo~, it is intended that all matter contained in the ~oregoing description or shown in the accumpanying drawings shall be interpreted as illustrative and not in a limiting sense.

Claims (4)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A microwave energy stirrer for a microwave oven, said stirrer including stirrer blades mounted for rotational movement in the path of microwave energy into said oven, a dielectric mounting shaft, said blades mounted ad-jacent one end of said shaft, a drive fan carried by said shaft adjacent the end remote from said stirrer blades, said stirrer blades adapted when rotated to change the microwave energy field pattern in said oven, air channel-ing means for directing a flow of air into contact with said drive fan to rotate said fan and consequently said shaft and mode stirrer blades, and an elongated cylindrical dielectric bearing having an inner diameter only slightly larger than the diameter of said shaft for supporting said shaft for rotation in a vertical position, said drive fan resting on the top surface of said support, the length of said support selected to position said drive fan in said flow.

2. The combination recited in Claim 1 further including an energy stirrer chamber above said oven having a top wall, an upstanding metal bushing attached to said top wall, said shaft passing through said bushing, said bearing supported from said bushing, and said stirrer blades mounted in said chamber.

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3. The combination recited in Claim 2 wherein said bearing is tapered inwardly and upwardly to provide a seat of reduced surface area to support said drive fan.

4. The combination recited in Claim 3 wherein said drive fan is made of metal and further including a dielectric washer intermediate said bearing and said fan to reduce friction losses during rotation.