Patent Grant
8835822
The present invention is directed to a field director assembly which prevents arcing when used in an unloaded microwave oven.
Subject matter disclosed herein is disclosed in the following copending applications filed contemporaneously herewith and assigned to the assignee of the present invention:
Arc-Resistant Microwave Susceptor Assembly, U.S. application Ser. No. 11/641,276, filed Dec. 18, 2006;
Microwave Susceptor Assembly Having Overheating Protection, U.S. application Ser. No. 11/641,929, filed Dec. 18, 2006; and
Field Director Assembly Having Overheating Protection, U.S. application Ser. No. 11/641,370, filed Dec. 18, 2006.
Microwave ovens use electromagnetic energy at frequencies that vibrate molecules within a food product to produce heat. The heat so generated warms or cooks the food. However, the food is not raised to a sufficiently high temperature to brown its surface to a crisp texture (and still keep the food edible).
To achieve these visual and tactile aesthetics a susceptor formed of a substrate having a lossy susceptor material thereon may be placed adjacent to the surface of the food. When exposed to microwave energy the material of the susceptor is heated to a temperature sufficient to cause the food's surface to brown and crisp.
The walls of a microwave oven impose boundary conditions that cause the distribution of electromagnetic field energy within the volume of the oven to vary. These variations in intensity and directionality of the electromagnetic field, particularly the electric field constituent of that field, create relatively hot and cold regions in the oven. These hot and cold regions cause the food to warm or to cook unevenly. If a microwave susceptor material is present the browning and crisping effect is similarly uneven.
To counter this uneven heating effect a turntable may be used to rotate a food product along a circular path within the oven. Each portion of the food is exposed to a more uniform level of electromagnetic energy. However, the averaging effect occurs along circumferential paths and not along radial paths. Thus, the use of the turntable still creates bands of uneven heating within the food.
This effect may be more fully understood from the diagrammatic illustrations of
As may be appreciated from
Owing to the number of hot regions encountered and cold regions avoided, points J and L experience considerably more energy exposure than Point K. If the region of the food product in the vicinity of the path of point J is deemed fully cooked, then the region of the food product in the vicinity of the path of point L is likely to be overcooked or excessively browned (if a susceptor is present). On the other hand, the region of the food product in the vicinity of the path of point K is likely to be undercooked.
Since non-uniform cooking due to the presence of hot and cold regions is undesirable it has been found advantageous to employ a susceptor assembly formed by the combination of a field director structure with a susceptor. The field director structure includes one or more vanes, each having a conductive portion on a paperboard support. The field director structure mitigates the effects of regions of relatively high and low electric field intensity within a microwave oven by redirecting and relocating these regions so that food warms, cooks and browns more uniformly. Use of the field director structure alone (i.e., without a susceptor) has also been found advantageous.
When a susceptor assembly is placed in an “unloaded” microwave oven (i.e., an oven without a food product or other article being present) and the oven is energized deleterious problems of overheating of the susceptor, and/or overheating of the field director structure, and/or arcing have been observed.
By “overheating of the susceptor” (or similar terms) it is meant heating of the lossy susceptor material to the extent that the susceptor substrate burns.
“Overheating of the field director structure” (or similar terms) means heating of the paperboard support of the vanes to the extent that it burns. Such overheating may be caused by either the heat generated by a lossy susceptor material or by arcing.
“Arcing” (or similar terms) is an electrical discharge occurring when a high intensity electric field exceeds the breakdown threshold of air. Arcing typically occurs in the vicinity of the electrically conductive portions of the vanes, particularly along the edges, and especially at any sharp corners. Arcing may cause the paperboard support of the vanes to discolor, to char, or, in the extreme, to ignite and to burn.
Most common expedients to prevent arcing are impractical in microwave oven applications. These expedients are also not suitable for disposable packaging for convenience foods.
In view of the foregoing it is believed advantageous to provide a field director structure and a susceptor assembly incorporating the same that prevents the occurrence of arcing, the occurrence of overheating of the field director, and the occurrence of overheating of the susceptor.
The present invention is directed to a field director assembly that prevents arcing when placed in an “unloaded” microwave oven, i.e., an oven without a food product or other article being present. The microwave oven is operative to generate a standing electromagnetic wave having a predetermined wavelength.
The field director assembly includes a generally planar support member having one or more vanes mechanically connected thereto. Each vane has an electrically conductive surface that is generally rectangular in shape with a predetermined length and width dimension.
In accordance with the present invention the electrically conductive portion of each vane is disposed at least a predetermined close distance from the planar support member. In the preferred instance the predetermined close distance is defined by a border of a lower conductivity material disposed between the conductive portion of the vane and the support member. The predetermined close distance lies in the range from 0.025 times the wavelength to 0.1 times the wavelength. Preferably, the border surrounds the conductive portion.
In addition to the disposition of the electrically conductive portion of each vane at the predetermined close distance from the support member, in accordance with one embodiment of the invention the corners of the electrically conductive portion are rounded at a radius up to and including one half of the width dimension of the conductive portion. In accordance with an alternate embodiment of the invention, instead of being rounded, the electrically conductive portion of the vane may be covered with an electrically non-conducting material selected from the group consisting of a polyimide tape, a polyacrylic spray coating and a polytetrafluoroethylene spray coating. In accordance with yet another alternate embodiment of the invention, instead of being rounded or covered, the electrically conductive portion of the vane may be may be formed from a metallic foil less than 0.1 millimeter in thickness with the foil folded over to at least a double thickness along its perimeter.
The invention will be more fully understood from the following detailed description, taken in connection with the accompanying drawings, which form a part of this application and in which:
Throughout the following detailed description similar reference characters refers to similar elements in all figures of the drawings.
With reference to
The susceptor assembly 10 comprises a conventional, generally planar susceptor 12 having a field director structure generally indicated at reference numeral 14 connected thereto. As will be developed herein the field director structure 14 is useful for redirecting and relocating the regions of high and low electric field intensity of the standing wave pattern within the volume of the oven. When used in conjunction with a turntable the positions of the redirected and relocated regions change continuously, further improving the uniformity of warming, cooking or browning of a food product placed on a susceptor assembly 10 that includes the field director structure 16.
In the embodiment shown in
The planar susceptor 12 shown in the figures is generally circular in outline although it may exhibit any predetermined desired form consistent with the food product to be warmed, cooked or browned within the oven M. As shown in the circled detail portion of
The substrate 12S may be made from any of a variety of materials conventionally used for this purpose, such as cardboard, paperboard, fiber glass or a polymeric material such as polyethylene terephlate, heat stabilized polyethylene terephlate, polyethylene ester ketone, polyethylene naphthalate, cellophane, polyimides, polyetherimides, polyesterimides, polyarylates, polyamides, polyolefins, polyaramids or polycyclohexylenedimethylene terephthalate. The substrate 12S may be omitted if the electrically lossy layer 12C is self-supporting.
The field director structure 14 includes one or more vanes 16. In the embodiment illustrated in
For purposes of illustration the vanes shown in
The front and back of each vane define a surface area 16S. In
At least a portion of the surface of the front and/or the back of each of the vane(s) 16 is electrically conductive. Any region of drawing
Each vane has an edge 16F extending between a first end 16D and a second end 16E. The edge 16F of a vane may exhibit any of a variety of contours. For example, the edge 16F of a vane may be straight, as illustrated by the vanes 16-1 to 16-3. Alternatively, the edge 16F of a vane may be bent or folded along one or more bend or fold line(s) 16L as suggested by the vane 16-4. Moreover, the contour of the edge 16F of a vane may be curved, as suggested by the vanes 16-5 (
A vane may have its first end 16D and its second end 16E disposed at any predetermined respective points of origin and termination on the planar susceptor 12. The distance along the edge 16F of a vane between its first end 16D and its second end 16E defines the edge length of the vane. The vanes in the field director structure 14 may have any desired edge length, subject to the proviso regarding the length of the conductive portion 16C mentioned below.
The vanes 16 may be integrally constructed from an electrically conductive foil or other material. In such a case the entire surface 16S of the vane is electrically conductive (e.g., as shown in
Alternatively, a vane may be constructed as a layered structure formed from a dielectric substrate with an electrically conductive material laminated or coated over some or all of the front and/or back of its surface area. One form of construction could utilize a paperboard substrate to which an adhesive-backed electrically conductive foil tape is applied.
If provided over less than the full surface area of a vane the electrically conductive portion 16C may itself exhibit any convenient shape, e.g., trapezoidal (as shown for vanes 16-2 and 16-3) or rectangular (as shown for vanes 16-4 and 16-5 and vane 16-1′ in
Whatever the shape of the conductive portion it may be desirable to radius or “round-off” corners to avoid arcing, as will be developed in connection with
Selection of the shape and the length of the electrically conductive portion of the vane and the spacing of the conductor portion from the susceptor plane and other vanes permits the field attenuating effect of the vane to be more precisely tailored.
Wherever its points of origin and termination a vane may also be arranged to pass through the geometric center 10C.
The vanes 16 extend in a generally radial direction with respect to the geometric center 10C of the susceptor assembly 10. The vanes 16 may be angularly spaced about the center 10C at equal or unequal angles of separation. For example, the angle 18 between the vanes 16-1 and 16-2 may be smaller than the angle 20 between the vanes 16-2 and 16-3.
It should be appreciated that the term “generally radial” (or similar terms) does not require that each vane must lie exactly on a radius emanating from the center 10C. For example, vanes may be either offset or inclined with respect to the radius.
Each vane 16 is physically (i.e., mechanically) connected to the planar susceptor 12 at one or more connection points. A connection between a vane 16 and the planar susceptor 12 may be a fixed connection or a flexible articulating connection.
A fixed connection is shown in
A flexible articulating connection is shown in
Whatever the form of construction, configuration of the vane's surface area, shape of the conductive portion, edge contour of the vane, edge length of the vane, length of the conductive portion on the vane, path of the vane with respect to the center of the susceptor, and the orientation of the vane with respect to plane of the susceptor, the electrically conductive portion 16C of the vane 16 must be disposed no farther than a predetermined close distance from the electrically lossy layer 12C of the planar susceptor 12. In general the predetermined close distance should be no greater than a distance approximating 0.25 times the wavelength of the electromagnetic energy generated in the oven. It should be understood that so long as a food product or other article is present the predetermined close distance can be zero, meaning that the conductive portion 16C of the vane abuts electrically against the lossy layer 12C of the planar susceptor.
In a typical implementation, shown in
Alternatively, as seen from
The planar susceptor 12 and a surface area 16S of a vane 16 intersect along a line of intersection 12L extending in a generally transverse direction with respect to the planar susceptor 12. When intersected with the planar susceptor 12, a straight-edged vane 16 will produce a straight line of intersection 12L. A vane 16 having a bent edge or curved edge, when intersected with the planar susceptor 12, will produce a bent or curved line of intersection 12L, respectively. The magnitude of the bend angle or the shape of curvature of the line of intersection, as the case may be, will depend upon the angle of inclination of the vane to the planar susceptor. Whether the line of intersection is a straight line, a bent line or a curved line, the extension of the conductive surface of the vane will lie along the line of intersection.
Having described the various structural details of a susceptor assembly 10 in accordance with the present invention, its effect on a standing electromagnetic wave may now be discussed.
An electromagnetic wave is composed of mutually orthogonal oscillating magnetic and electric fields. At any given instant a standing electromagnetic wave includes an electric field constituent Ē. At any instant the electric field constituent Ē is oriented in a given direction in the Cartesian space and may have any given value.
The electric field Ē is itself resolvable into three component vectors, viz., Ēx, Ēy, Ēz. Each component vector is oriented along its respective corresponding coordinate axis. Depending upon the value of the electric field Ē each component vector has a predetermined value of “x”, “y” or “z” units, as the case may be.
One corollary of Faraday's Law of Electromagnetism is the boundary condition that the tangential electric field at the interface surface between two media must be continuous across that surface. A particular example of such a media interface is that between a perfect conductor and air. By definition, a perfect conductor must have a zero electric field within it. Therefore, in particular, the tangential component of the electric field just inside the conductor surface must be zero. Hence, from the above asserted boundary continuity condition, the tangential electric field in the air just outside the conductor must also be zero. So we have the general rule that the tangential component of the electric field at the surface of a perfect conductor is always zero. If the conductor is good, but not perfect, then the tangential component of the electric field at the surface may be nonzero, but it remains very small. Thus, any electric field existing just outside the surface of a good conductor must be substantially normal to that surface.
The application of this physical law mandates that within that surface area of the vane 16 having the conductive portion 16C only the component vector of the electric field that is oriented perpendicular to that surface, viz., the vector Ēy, is permitted to exist.
The component vectors of the electric field lying in any plane tangent to the surface of the vane, (viz., the vector Ēy, and the vector Ēz) are not permitted. In
If the conductive portion 16C of the vane 16 were in electrical contact with the lossy layer 12C the value of the component vector Ex lying along the line of intersection 12L and the value of the component vector Ēz would be zero, for the reasons just discussed. However, the conductive portion 16C is not in electrical contact with the lossy layer 12C, but is instead spaced therefrom by the distance D. The conductive portion of the surface of the vane nevertheless exerts an attenuating effect having its most pronounced action in the extension of the conductive portion of the surface of the vane.
Thus, the component vectors Ēx and Ēz of the electric field of the wave have only attenuated intensities “xa” and “za”. The intensity values “xa” and “za” are each some intensity value less than “x” and “z”, respectively. Attenuation of the electric field component of the electromagnetic wave in the plane tangent to the surface of the vane results in enhancement of the component of the electric field oriented perpendicular to the conductive portion of the surface of the vane. Thus, the component vector Ēy has an enhanced intensity value “ye” greater than the intensity value than “y”.
The degree of attenuation of the vector component Ēx is dependent upon the magnitude of the distance D and the orientation of the conductive portion 16C relative to the lossy layer 12C. The attenuation effect is most pronounced when the distance D is less than one-quarter (0.25) wavelength, for a typical microwave oven a distance of about three centimeters (3 cm). At an angle of inclination less than ninety degrees the permitted field (i.e., the field normal to the conductive surface of the vane) will itself have components acting in the susceptor plane.
This effect is utilized by the susceptor assembly 10 of the present invention to redirect and relocate the regions of relatively high electric field intensity within a microwave oven.
Consider the situation at Position 1, near where the vane first encounters the hot region H2. For the reasons explained earlier only an electric field vector having an attenuated intensity is permitted to exist in the segment of the hot region H2 overlaid by the vane 16. However, even though only an attenuated field is permitted to exist the energy content of the electric field cannot merely disappear. Instead, the attenuating action in the region extending from the conductive portion of the vane manifests itself by causing the electric field energy to relocate from its original location A on the planar susceptor 12 to a displaced location A′. This energy relocation is illustrated by the displacement arrow D.
As the rotational sweep carries the vane 16 to Position 2 a similar result obtains. The attenuating action of the vane again permits only an attenuated field to exist in the region extending from the conductive portion of the vane. The energy in the electric field energy originally located at location B on the planar susceptor 12 displaces to location B′, as suggested by the displacement arrow D′.
Similar energy relocations and redirections occur as the vane 16 sweeps through all of the regions H1 through H5 (
The use of the present invention in a microwave oven having a mode stirrer apparatus will result in the same effect.
It is clear from
As a result, warming, cooking and browning of a food product placed on the susceptor assembly 10 will be improved over the situation extant in the prior art.
The support member may be connected to all or some of the vanes.
If desired, the vanes 164-1 and 164-4 may themselves be connected by a length of a non-conductive member 164N. The member 164N is shown in
In a second aspect, the invention is directed to various implementations of a collapsible self-supporting field director structure embodying the teachings of the present invention.
In
In the field director structure 146 shown in
In the field director structure 148 shown in
The field director structure 149 shown in
Although the vanes in each of the embodiments illustrated in
It should also be appreciated that a field director structure of the present invention need not be made collapsible, but instead may be made self-supporting through the use of a suitable non-conductive support member.
It should also further be appreciated that any embodiment of a field director structure falling within the scope of the present invention may be used with a separate planar susceptor (earlier described). It should also be appreciated that for some food products it may be desirable to place a second planar susceptor above the food product or to wrap the food product with a flexible susceptor.
The operation of the field director structure and a susceptor assembly in accordance with the present invention may be understood more clearly from the following examples.
Introduction
For all of the following examples commercially available microwavable pizzas (DiGiorno® Microwave Four Cheese Pizza, 280 grams) were used in the cooking experiments.
A planar susceptor comprised of a thin layer of vapor-deposited aluminum sandwiched between a polyester film and paperboard was provided with the pizza in the package. This planar susceptor was used with various implementations of the field director structure of the present invention, as will be discussed. The edge of the paperboard provided was shaped to form an inverted U-shape cooking tray to space the planar susceptor approximately 2.5 cm above a turntable in the microwave oven. A crisping ring (intended for browning the edges of the pizza) provided with the pizza in the package was not used.
In all examples the planar susceptor was placed directly upon a turntable of a microwave oven. In all examples frozen pizzas were placed directly on the planar susceptor and cooked at full power for 5 minutes, except for Example 5, which was cooked in a lower power over for 7.5 minutes.
For comparison purposes one group of three pizzas was cooked using only the planar susceptor without a field director structure, and another group of three pizzas was cooked using the planar susceptor with a field director structure of the present invention.
The vanes of each field director were constructed using aluminum foil of 0.002 inch (0.05 millimeter) thickness, paperboard, and tape.
For Examples 1 through 7 the field director structure was placed in the space under the planar susceptor. For Example 8 the field director structure was positioned above the pizza.
Browning and Browning Profile Measurements
The percent browned and the browning profile of the pizza bottom crust were measured following a procedure described in Papadakis, S. E., et al. “A Versatile and Inexpensive Technique for Measuring Color of Foods,” Food Technology, 54 (12) pp. 48-51 (2000). A lighting system was set up and a digital camera (Nikon, model D1) was used to acquire images of the bottom crust after cooking. A commercially available image and graphics software program was used to convert color parameters to the L-a-b color model, the preferred color model for food research. Following the suggestion from the referenced procedure the percent browned area was defined as percent of pixels with a lightness L value of less than 153 (on a lightness scale of 0 to 255, 255 being the lightest). Following the methodology described in the referenced procedure the browning profile (i.e., the percent browned area as a function of radial position) was calculated.
The image of the bottom crust was divided into multiple concentric annular rings and the mean L value was calculated for each annular ring.
The following examples are believed to illustrate the improvements in browning and browning uniformity that resulted from the use of different field director structures of the present invention.
A DiGiorno® Microwave Four Cheese Pizza was cooked in an 1100-watt General Electric (GE) brand microwave oven, Model Number JES1036WF001, in the manner described in the introduction. When a field director was employed, the field director structure in accordance with
After cooking an image of the bottom crust was acquired with the digital camera, as described. From the image data the percent browned area was calculated using the procedures described. The average percent browned area for the pizzas cooked without a field director was determined to be 40.3%. The average percent browned area for the pizzas cooked with a field director was determined to be 60.5%.
The experiment described in Example 1 was repeated in four microwave ovens of different manufacturers. The oven manufacturer, model number, full power wattage, and cooking time for each example are summarized in Table 1. The table reports the percent browned area achieved with and without a field director. It should be noted that the percent browned area was improved in all cases.
A DiGiorno® Microwave Four Cheese Pizza, 280 gram, was cooked in an 1100-watt Sharp brand oven, Model R-630DW. When a field director structure was employed, the field director structure in accordance with
After cooking an image of the bottom crust was acquired with the digital camera and the percent browned area was calculated, all as described.
The average percent browned area for the pizzas cooked without a field director was 55.2%. The average percent browned area for the pizzas cooked with the field director was determined to be 73.8%. The browning profile, was plotted and is shown in
The experiment described in Example 6 was repeated using a 1300-watt Panasonic brand oven, Model NN5760WA. The average percent browned area for the pizza cooked without a field director was 50.3%. The average percent browned area for the pizzas cooked with a field director structure was determined to be 51.7%. The substantially uniform browning profile that follows from the use of the present invention may be observed from the plot shown in
The experiment described in Example 1 was repeated in a 700-watt Goldstar brand microwave oven, Model MAL783W. When a field director structure was employed, the field director structure in accordance with
After cooking (for 7.5 minutes at full power of the oven used) an image of the bottom crust was acquired with the digital camera and the percent browned area was calculated, all as described.
The percent browned area for the pizza cooked without a field director was 31.5%. The percent browned area for the pizza cooked with a field director was 65.1%.
When a microwave susceptor assembly such as described above is placed in an “unloaded” microwave oven (i.e., an oven without a food product or other article being present) several deleterious problems have been observed. The problems are particularly acute in high wattage ovens (i.e., ovens having power ratings typically greater than nine hundred watts). In some instances the microwave susceptor assembly may overheat even when an article is present.
As the lossy layer 12C of the planar susceptor 12 overheats, melting or charring of the substrate 12S may occur. The susceptor may overheat to the extent that the susceptor substrate burns. The conductive portions of the vanes of the field director structure may arc, particularly along the edges and especially at the corners. The arcing causes the non-conductive (typically paperboard) support of the vanes to discolor, to char or to overheat to the extent that it ignites into flames. Overheating of the field director structure may also be caused by overheating of the susceptor material.
Accordingly, it is believed advantageous to provide a field director structure and a susceptor assembly incorporating the same that is “abuse-tolerant”, that is, a structure that prevents the occurrence of arcing, and/or the occurrence of overheating of the field director, and/or the occurrence of overheating of the susceptor.
The susceptor assembly 1010 includes a generally planar susceptor 12 having a substrate 12B with an electrically lossy layer 12C, as described earlier in connection with
The field director structure 1410 has at least one but preferably a plurality of vanes 1610 each mechanically connected to the planar susceptor 12. Each vane 1610-1 through 1610-8 shown in
It should be understood that the field director structure 1410 may alternatively be used in combination with a planar non-conductive support member 32 to define a field director assembly generally indicated by the reference character 31.
Each vane 1610 has a surface 1610S which is identified for clarity of illustration only for the vane 1610-6. At least a portion 1610C of the surface 1610S of each vane is electrically conductive. As will be described the electrically conductive portion 1610C of each vane 1610 is positioned with respect to the planar susceptor 12 and configured in various ways to prevent overheating and arcing problems.
The conductive portion 1610C of each vane 1610 has a first end 1510D and a second end 1510E. Again for clarity the ends are indicated only on vane 1610-6. The distance between the first and second ends 1510D and 1510E defines a predetermined length dimension for the conductive portion 1610C. The conductive portion 1610C of each vane also exhibits a predetermined width dimension. As previously described (e.g., in conjunction with
The vane 1610-1 has a conductive portion 1610C-1 that occupies the entire rectangular surface. The conductive portion 1610C-1 abuts the planar susceptor 12. The vane 1610-1 is typical of a vane structure that would overheat when used in an unloaded oven. A susceptor 12, when used with a field director structure having a vane 1610-1, may also overheat resulting in melting or charring of the susceptor substrate 12S. The conductive portion of the vane 1610-1 may arc along its edges or at its corners.
The conductive portion 1610C-2 of the vane 1610-2 is also rectangular in shape. This conductive portion 1610C-2 occupies only a portion of the vane surface, leaving part of the substrate 1610N exposed to define a border 19L along the bottom edge. The conductive portion 1610C-2 abuts the planar susceptor 12. The structure of the vane 1610-2 has been shown to limit but not to eliminate overheating of the vane and susceptor when used in an unloaded oven (Examples 36, 39). When used with a field director structure having a vane 1610-2 the susceptor 12 may also overheat, resulting in melting or charring of the substrate 12S.
As will be developed the vanes 1610-3 through 1610-5, 1610-7 and 1610-8 exemplify various positions and/or configurations of the conductive portions 1610C in accordance with the present invention that the problems of overheating of the susceptor, and/or overheating of the field director, and/or arcing are prevented.
Vane 1610-3 is an example of a vane in which the substrate 1610N abuts the planar susceptor 12. In this instance the conductive portion 1610C-3 is positioned on the vane such that a top border 19T of non-conductive substrate material is exposed along the edge of the vane adjacent to the susceptor 12. The border 19T serves to space the conductive portion 1610C-3 of the vane 1610-3 a predetermined close distance 21D away from the susceptor 12. The dimension 21D, measured in a direction orthogonal to the plane of the susceptor 12, lies in a range from 0.025 to 0.1 times the wavelength of the standing electromagnetic wave produced in the microwave oven in which the susceptor assembly 1010 is being used. That is, the dimension 21D should be at least 0.025 times the wavelength. Further, the dimension 21D should be no greater than 0.1 times that wavelength (that is, the dimension 21D≦0.1 times that wavelength). It should noted that the maximum distance 17D referred to earlier and the maximum distance shown by reference character D in
The conductive portion 1610C-4 of the vane 1610-4 is sized such that part of its substrate 1610N is exposed to define radially inner and outer borders 19D and 19E, respectively. In addition an upper border 19T and a lower border 19L of substrate material 16N are exposed.
Vane 1610-5 is an example of a vane in which the conductive portion 1610C-5 is generally rectangular (similar to the conductive portion 1610C-4) but with rounded corners. The corners may be rounded at a radius dimension 15R up to and including one-half of the width dimension of the conductive portion 1610C-5 (i.e., 15R≦0.5 width). When the corners are rounded the length of the conductive portion is defined by the radial extent of the conductive portion. The vane 1610-5 also has borders 19T, 19L, 19D, 19E (similar to those shown about the vane 1610C-4). The dimension of the lower border 19L is indicated by the reference character 21L.
Vane 1610-6 also exhibits a conductive portion 1610C-6 with rounded corners. However, the conductive portion 1610C-6 extends the full width of the vane and abuts the planar susceptor 12. It is not spaced a predetermined close distance away from the planar susceptor 12.
The vane 1610-7 is an example of a vane having an electrically conductive portion 1610C-7 made of a metallic foil that is folded as indicated at 1610C-7F to define at least a double thickness along its perimeter. Borders 19T, 19L, 19D, 19E (similar to those shown about the vane 1610C-4) are present along the perimeter of the conductive portion 1610C-7.
The vane 1610-8 has a conductive portion 1610C-8 that occupies its entire rectangular surface. For this vane the requisite spacing 21D of the conductive portion 1610C-8 from the susceptor 12 is achieved by using a mounting arrangement in which the vane is physically set apart from the susceptor.
Of course, it should also be appreciated that the requisite spacing 21D may also be achieved by the sum of the set apart distance from the susceptor and the border width of an appropriately sized bordered vane (i.e., vane 1610-3, 1610-4, 1610-5, or 1610-7).
As indicated in
It has been found that disposing the first end 1510D of the conductive portion 1610C of each of the vanes at the predetermined separation distance 21S from the geometric center 12C of the planar susceptor 12 mitigates the occurrence of overheating of the susceptor in the vicinity of the susceptor center (Examples 18, 19, 20-22). Disposing the electrically conductive portion of the vane the predetermined close distance 21D from the electrically lossy layer of the planar susceptor (however that spacing is achieved) has also been found to mitigate the occurrence of overheating of the susceptor (Examples 35, 37). Further mitigation of the occurrence of susceptor overheating may be achieved by the provision of the lower border 19L (Examples 36, 39).
In accordance with the present invention the combination of the disposition of the conductive portions of the vanes at the predetermined separation distance 21S together with the disposition of the conductive portions of the vanes at the predetermined close distance 21D from the planar susceptor prevents the occurrence of overheating of the susceptor when used in an unloaded microwave oven.
Also in accordance with the present invention disposing the electrically conductive portion of the vane at the predetermined close distance 21D from the electrically lossy layer of the planar susceptor and rounding the corners of the conductive portion with the radius 15R prevents the occurrence of arcing when used in an unloaded microwave oven.
Further in accord with the invention the occurrence of arcing in an unloaded microwave oven is prevented by disposing the electrically conductive portion of the vane at the predetermined close distance 21D from the electrically lossy layer of the planar susceptor and covering the conductive portion of any of the vanes 1610-3 through 1610-5, 1610-7, 1610-8 with an electrically non-conductive material such as a polyacrylic or a polytetrafluoroethylene spray coating or a polyimide tape.
Still further in accordance with the invention disposing the electrically conductive portion of the vane at the predetermined close distance 21D from the electrically lossy layer of the planar susceptor and increasing the thickness of the perimeter of a thin foil conductive portion (in the manner shown on the vane 1610-7) prevents the occurrence of arcing when used in an unloaded oven.
The following examples describe experiments that were conducted to determine parameters that mitigate or eliminate the overheating and/or arcing problems. A General Electric, model JES1456BJ01, 1100 watt microwave oven was used in Examples 9 through 23. The tests were conducted with the oven unloaded, i.e., no food product or other article was present in the oven. These Examples are summarized in Table 2 herein.
Example 9 was a control example with no borders and no rounding of corners of the conductive portion of a single vane.
Examples 10-13 and 14-17 tested the effect of a non-conductive covering on the conductive portion of a single vane. In Examples 10-13 the conductive portion was ¾″ (0.75″; 19 mm) wide with rounded corners; in Examples 14-17 the conductive portion was 1″ (25.4 mm) wide with rounded corners.
Examples 18-20 tested the effect of varying the center gap between radially opposite conductive portions on arcing and overheating.
Examples 21-22 tested alternate materials for the conductive portions. Example 23 tested the effect of fire retardant treatment of the paperboard on arcing and burning.
In this example a single vane was configured and positioned with respect to the susceptor in accordance with vane 1610-1 of
In these examples the single vane was configured and positioned with respect to the susceptor in accordance with vane 1610-5 of
Examples 10 through 12 provided a protective covering of an electrically non-conductive material over the aluminum conductive portion in an effort to prevent arcing. An uncovered version, Example 13, was also tested as a control.
Each vane had a conductive portion 3½″ (3.5″; 88.9 mm) long and ¾″ (0.75″; 19.2 mm) wide cut from the same adhesive backed 0.002″ (0.05 mm) thick aluminum foil used in Example 9, applied to a 4″×1″ (101.6 by 25.4 mm) rectangle of the same cellulose paperboard as in Example 9. The conductive portion was ¾″ (0.75″; 19.2 mm) wide in order to insure the non-conductive covering covered all of the edges of the aluminum conductive portion. A top border of ⅛″ (0.125″; 3.2 mm) of paperboard was exposed above the conductive portion. A ⅛″ (0.125″; 3.2 mm) border dimension was about 0.025 times the wavelength. The conductive portion had all corners rounded at a radius of ⅜″ (0.375″; 9.6 mm).
A lower border of ⅛″ (0.125″; 3.2 mm) of paperboard was also exposed below the conductive portion and ¼″ (0.25″; 6.4 mm) border of paperboard was exposed on each end.
Different non-conductive materials were used as the coverings, as follows:
None of the vanes showed any arcing when exposed unloaded in a microwave oven for two minutes.
In these examples a single vane was configured and positioned with respect to the susceptor in accordance with vane 1610-6 of
Examples 14 through 16 evaluated the same non-conductive protective coverings disposed over the aluminum conductive portion as in Examples 10 through 12, respectively, but with the aluminum conductive portion being the same 1″ (25.4 mm) width as the paperboard. Again, an uncovered version, Example 17, was tested as a control. In each of these examples the conductive portion was 3½″ (3.5″; 88.9 mm) long by 1″ (25.4 mm) wide adhesive backed 0.002″ (0.05 mm) thick aluminum foil applied to a 4″ by 1″ (101.6 mm by 25.4) rectangle of the cellulose paperboard as was used in Examples 10-13. The conductive portion had all corners rounded at a radius of ½″ (0.5″; 12.7 mm) and had a ¼″ (0.25″; 6.4 mm) border of exposed paperboard on both of the ends.
Different non-conductive materials were used as the coverings, as follows:
In Examples 15 and 16 the surface of the conductive portion was covered by the polyacrylic or polytetrafluoroethylene spray coating, respectively. The top and bottom edges of the aluminum conductive portion were covered only by incidental over-spray of the polyacrylic or polytetrafluoroethylene coatings.
In Examples 14, 16 and 17 the bottom edge of the conductive portion arced in the center. This arcing occurred very shortly after being exposed unloaded in the microwave oven. In Example 15 no arcing occurred.
More particularly, the results of the experiments were as follows:
In this example each of the six vanes of the field director of
As shown in
Each of three vane blanks was then bent in the middle to form a V-shape and positioned under a susceptor with the apex of each V at the center of the susceptor, thus defining a separation distance 21S (
There was no discernible arcing when this susceptor assembly was exposed unloaded in the microwave oven, but the assembly did burst into flames when the paperboard substrate in the center overheated in forty-seven seconds.
In this example each of the six vanes of the field director of
The vanes in this Example were constructed in the same manner as in Example 18 from vane blanks as illustrated in
As in Example 18 three of these V-folded vane blanks were glued to the underside of a susceptor defining a separation distance 21S (
Again, there were no discernible arcs when this susceptor assembly was exposed in the microwave oven unloaded, but the assembly did burst into flames when the paperboard vanes in the center overheated in one minute, eighteen seconds.
In this example each of the six vanes of the field director of
The vanes in this Example were also constructed in the same manner as in Examples 18 and 19 from vane blanks as illustrated in
As in Examples 18 and 19 three of these V-folded vane blanks were glued to the underside of a susceptor defining a separation distance 21S (
There was no arcing and no burning when this susceptor assembly was exposed in the microwave oven for five minutes.
The test of Example 20 was repeated using conductive portions as shown in
There was no arcing and no burning when this susceptor assembly was exposed unloaded in the microwave oven for five minutes.
The test of Example 20 was repeated using conductive portions as shown in
There was no arcing and no burning when this susceptor assembly was exposed unloaded in the microwave oven for five minutes. The aluminum foil of this tape performed acceptably but the adhesive loosened.
The application of a fire retardant composition to avoid spontaneous burning of the vanes was tested as Example 23. The fire retardant used was an aqueous based resin known as Paper Seal™ from Flame Seals Products of Houston, Tex. The susceptor assembly was constructed as in Example 18 with a ¾″ (0.75″; 19.2 mm) gap in the center between each pair of conductive portions as shown in
The paperboard blanks were dipped into a bath of the fire retardant liquid and allowed to dry for a day before adhering the conductive portions and assembling the susceptor assembly.
There were no arcs when an unloaded susceptor assembly was exposed in the microwave oven for five minutes. Unlike Example 18 the assembly did not burst into flames, suggesting that a fire retardant treatment of the paperboard was sufficient to prevent burning.
The tests of Examples 9 through 23 are summarized in Table 2.
Observations from Examples 9 to 23 were:
In the following Examples 24-64 a susceptor assembly similar to that shown in
The Examples 24-50 and Examples 61-64 were conducted to assess the effect of various vane designs in eliminating overheating susceptor during pizza cooking in various microwave ovens. The remaining examples (viz., Examples 51-60) were conducted to assess the effect of various vane designs on browning of the pizza cooked in various microwave ovens.
As shown in
The susceptor assemblies tested had substrates formed from various materials. Four different susceptor substrate materials were tested in combination with two different thicknesses of metallization that formed the lossy conductive layer.
The conductive portion of each vane was made using an adhesive backed 0.002″ (0.05 mm) thick aluminum foil applied to a cellulose paperboard vane from International Paper as described previously in connections with Examples 9-20. Each conductive portion was 3½″ (3.5″; 88.9 mm) in length but of different widths. Tables 3, 4A, 4B and 5 each contain a column of alphabetic designators indicating the “Vane type” tested. Each designator indicates a vane type as depicted in
Tables 3, 4A, 4B and 5 also contain a column of alpha-numeric designators indicating the “Oven” used for the test. Each designator corresponds to a particular microwave oven manufacturer and model, as follows:
Tables 3, 4A, 4B and 5 contain a column indicating the “Susceptor” (i.e., substrate 12S and layer 12C) used.
The Susceptor in some of the examples contained in Tables 3, 4A and 4B below is identified as “Control”. The “Control” susceptor was that provided with the DiGiorno® Microwave Four Cheese Pizza (280 grams) mentioned earlier. The “Control” susceptor included a paperboard substrate.
The “Susceptor” in some of the examples contained in Tables 3 and 5 below is identified by a reference designation comprising hyphenated first and second numeric values. The first numeric value represents the polymeric substrate material of the susceptor, while the second numeric value denotes the thickness of the susceptor lossy layer metallization (vacuum deposited aluminum) based upon its measured optical density.
The first numeric value denotes the polymeric substrate material, as follows:
The second numeric value represents the optical density thickness measurement of the metallized coating of vacuum deposited aluminum, as follows:
Thus, for Example 29 in Table 3, a susceptor designated “12-3” indicates the susceptor had a substrate of 300 gauge polyethylene terephalate heat stabilized film (Melinex® ST-507 film) (as denoted by the first numeric “12”) and that the aluminum vacuum deposited metallization had an optical density of 0.3 (as denoted by the second numeric “3”).
A susceptor assembly with Type A vanes (as described above) was used to cook DiGiorno® Microwave Four Cheese Pizza (280 grams) in either the S-1000″ or the F-950 oven. As may be seen in Table 3 four types of susceptor substrate materials were used. The cooking time was varied from 5 to 6 minutes. All vaned susceptor assemblies consistently overheated in the center. The severity of the overheating increased with cooking time for each susceptor substrate material used. Examples of the overheating included burned and melted spots on the surface of the susceptor that in some cases resulted in transport of the melted susceptor material to the bottom of the pizza, as may be seen in
In Examples 35 to 40 addition of a ¼″ (0.25″; 6.4 mm) border of paperboard on either top or bottom of the conductive portion of the vane was tested to assess its potential to eliminate the overheating in the center of the susceptor. As summarized in Table 3 below, in this series of tests DiGiorno® Microwave Four Cheese Pizza was cooked an S-1000 microwave oven for 6 minutes using susceptors having 12-3 substrates. Field director assemblies exhibit different vane types A, B, C, D, E and F were tested. Example 35 utilized a type B vane; Example 36 utilized a type C vane; Example 37 utilized a type D vane; Example 38 utilized a type E vane; Example 39 utilized a type F vane; and Example 40 utilized a type A vane.
The results are summarized in Table 3.
Table 3 illustrates that for vaned susceptors having a separation distance defined between the inner of the conductive portion and the geometric center of the susceptor the addition of a top border between the susceptor and the top edge of the conductive portion of the vane structure (vane Types B and E) consistently prevented overheating of the susceptor. Vaned susceptors without any border (vane Types A and D) consistently led to overheating in the center of the susceptor. Vaned susceptors having a lower border (but no top border) of non-conductive material along the conductive portion of the vane (vane Types C and F) somewhat reduced the severity of the susceptor overheating, but did not eliminate this problem completely. These results of Examples 35-40 are illustrated in
A series of cooking tests were performed with five microwave ovens identified above. The tests used susceptors with vane types A and B to assess the effect of the addition of a top ¼″ (0.25″; 6.4 mm) wide paperboard border along the conductive portion of the vane. Examples 41-50 (summarized in Table 4A) and Examples 51-60 (summarized in Table 4B) respectively used the same test conditions. Examples 41-50 assessed overheating.
Examples 51-60 assessed the overall microwave cooking performance, specifically the ability of this configuration of the susceptor assembly to brown uniformly the bottom of a pizza. Percent browning (“% browning”) of a pizza was measured in the same manner as described in connection with Examples 1 through 8. The measured % browning was averaged over three pizza samples.
The results shown in Tables 4A and 4B indicated that for vaned susceptors having a separation distance defined between the inner of the conductive portion and the geometric center of the susceptor the addition of a top ¼″ (0.25″; 6.4 mm) paperboard border along the conductive portion of the vane (Type B) consistently prevented overheating in the center of the susceptor. However, as seen in Table 4B the overall cooking performance of a susceptor with vane type B decreased (as evidenced by lower average percent browning).
Examples 61-64 evaluated the effect of the width of the top paperboard border between the susceptor and the top edge of the conductive portion of the vane on susceptor overheating. This series of tests was also performed with DiGiorno® Microwave Four Cheese Pizza cooked for 6 minutes in an S-1000 microwave oven. The susceptor assemblies had 12-3 substrate materials and vane types A, B, G and H.
These results of Examples 61-64 are illustrated in
These test indicated that for vaned susceptors having a separation distance defined between the inner of the conductive portion and the geometric center of the susceptor a top paperboard border of at least ⅛″ (0.125″; 3.2 mm) (i.e., vane types B and G) between susceptor and the top edge of the conductive portion of the vane structure was required to prevent overheating of the susceptor.
Overall, the conclusions drawn from Examples 24 through 64 for vaned susceptors having a separation distance defined between the inner of the conductive portion and the geometric center of the susceptor were:
When a field director structure having one or more conductive portions is present in an energized microwave oven (either with or without the presence of a susceptor) the conductive portion(s) cause a disturbance of the standing wave electric field in the oven. The conductive portion(s) concentrate the electric field along their edges, producing local electric field intensities that are much higher than the base electric field within the oven, i.e., the field intensity before the introduction of the conductive portion(s). So long as the oven is loaded these higher field intensities are usually insufficient to cause breakdown of air.
However, when the oven is unloaded (i.e., no food or other article is present) the base electric field increases to a level above that extant when the food or other article is present. In the unloaded case the local intensity of the field along the edge of a conductive portion may be sufficiently high to exceed the breakdown threshold of the air causing an electric discharge in the form of an arc to occur.
It is believed that when a field director structure is used without a susceptor present a conductive portion should be spaced by a border of a lower conductivity material (e.g., a dielectric) at least a predetermined close distance from the planar support member. Preferably the border surrounds the conductive portion. The presence of the border reduces the local electric field intensity at the edges. The magnitude of this reduction is approximated by the following formula:
EI′=EI/(∈r′2+∈r″2)1/2
When the field director is used with a susceptor the lossy layer of the susceptor also plays a part in preventing arcing. The lossy layer absorbs part of the microwave energy in the oven and converts it to heat. This absorption reduces the electric field intensity in the oven. The heat flows into a food product or other article present.
However when the oven is unloaded there is no food product or other article present in the oven to dissipate the heat generated by the lossy layer. This results in rapid overheating that damages the lossy layer and causes its electrical conductivity to drop significantly. This reduces the ability of the lossy layer to absorb the microwave energy.
Without this absorption by the lossy layer the electric field intensity in the oven increases and the high field intensity condition along the edge of a conductive portion may then exceed the breakdown threshold of the air, causing an electric discharge in the form of an arc to occur.
It is believed that when the conductive portion(s) of the field director structure is spaced from the lossy layer by a border of a dielectric material, the border reduces the local electric field intensity at the edges.
Prevention of Overheating
When a field director structure having two conductive portions is present in an energized microwave oven a concentrated field is created in the space between these conductive portions. When a material having a moderate dielectric loss factor, such as a paperboard planar support member or a susceptor, is placed in or near the region between the conductive portions the concentrated field causes this material to rapidly heat. The concentration of the field is a function of the spacing apart of the conductive portions. If the conductive portions are close enough together this concentrated field may cause the material to overheat sufficiently to burst into flames, as is the case for paperboard. Increasing the spacing between the conductive portions reduces this field concentration and thus prevents overheating.
Those skilled in the art, having the benefit of the teachings of the present invention may impart modifications thereto. Such modifications are to be construed as lying within the scope of the present invention, as defined by the appended claims.
This application claims the benefit of U.S. Provisional Applications; 60/841,088 which was filed 29 Aug. 2006, and 60/751,544, which was filed 19 Dec. 2005 and are incorporated as a part hereof for all purposes.
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