US3492454A - Electronic oven - Google Patents

Description

Jan. 27, 1970 P. H. SMITH 3,492,454

ELECTRONIC OVEN- Original Filed March 26, 1965 2 Sheets-Sheet 1 5 INVENTOR. m 46m BY PE lVDL E T OIV, NE UMAN .SE/BOLO 8 WILLIAMS Jan. 27, 1970 P. H. SMITH 3,492,454

ELECTRONIC OVEN Original Filed March 26, 1965 2 Sheets-Sheet 2 zl w l L. Z A60 f;v x ,2 INVENTOR. (a :2 4 MM BY PEA/DLETON, NEUMA/V SE/BOLD a W/L L 64/115 United States Patent 3,492,454 ELECTRONIC OVEN Peter H. Smith, Maidenhead, England, assignor to J. Lyons & Company, Limited, London, England, a corporation of England Original application Mar. 26, 1965, Ser. No. 444,942, now Patent No. 3,373,259, dated Mar. 12, 1968. Divided and this application Dec. 22, 1967, Ser. No. 740,400 Claims priority, application Great Britain, Feb. 9, 1965, 5,573/ 65 Int. Cl. H05!) 9/06 US. Cl. 21910.55 5 Claims ABSTRACT OF THE DISCLOSURE This application is a division of application Ser. No. 444,942, filed by Peter H. Smith on Mar. 26, 1965, now Patent No. 3,373,259.

This invention relates to electronic ovens and more particularly to such ovens which are adapted for cooking small prepackaged items such as sandwiches and the like.

It has been recognized in the prior art that the field intensity of wave energy within the cooking or heating chamber of an electronic oven is dependent not only upon the load represented by the food to be heated, but also by the physical size and shape of the heating chamber. Typically, variations in field intensity occur within the heating chamber, and are represented by locations within the heating chamber of relatively high field intensity, and other locations of relatively low field intensity. This variation in field strength within the heating chamber results in certain portions of the food being heated to a greater extent than others.

It has been recognized that one way of evening the field strength within the heating chamber is to provide two inputs or antennas where wave energy can enter the heating compartment. One difficulty with this approach is that some of the wave energy radiated by one antenna is received by the other, and transmitted back toward the wave energy source or sources. The result is a high standing wave ratio on the legs of the waveguide between the antennas and the wave energy sources.

When a single source is employed to drive both antennas, an additional problem arises from the division of power from a single source into two parts to drive the two antennas. In the prior art it has been impossible to make such a power division without reducing performance of the system in some way.

For example, in order to reduce the characteristic impedance of the waveguide legs connected to the two outputs of the power divider, it has been known to reduce the width of the narrower wall of the waveguide. However, this results in an increase in the electric field strength and may cause electrical flash over in the guide, especially at any bends in the waveguide. Flash over may also occur at the location of screws in the waveguide which are employed for tuning purposes. In electronic ovens it is important to have match impedances to maintain a low standing wave ratio, and moreover the match ice must exist over a relatively wide bandwidth. The frequency of oscillation of a magnetron, commonly used as the source of wave energy for such ovens, varies in response to changes in its load, and such load changes accompany the process of heating or cooking food. A high standing wave ratio is objectional not only because the energy reflected back to the source might damage the same, but also it is a characteristic of poor efliciency, since all the available energy is not necessarily being absorbed by the food to be heated.

It has also been proposed to employ elliptically polarized wave energy to heat the food within the heating chamber, and one apparatus for performing this function is disclosed and claimed in copending Smith application Ser. No. 254,925, filed Jan. 30, 1963. The present invention comprises in part an improvement of the apparatus there disclosed.

By use of the novel power divider of the present invention, the energy output of a single magnetron may be coupled to a waveguide assembly terminating in a pair of antennas disposed on opposite sides of the heating chamber of the oven. The characteristic impedance of each of the legs of the waveguide assembly is the same as that of the magnetron, and matched impedance operation is provided over a relatively large range of frequencies, such as is encountered during typical operation of the electronic oven in which the magnetron is employed. In addition, the Wave energy introduced into the heating chamber from the two antennas is out of phase, to produce elliptically polarized energy within the chamber.

Another advantage of the present invention pertains to the disposition of vapors generated in the heating chamber when the food within such chamber is being heated. It is desirable to prevent these vapors from entering the waveguide where they could damage the apparatus by causing corrosion, andthe build up of moisture at a high voltage point along the waveguide which may cause arcing and damage the magnetron or the waveguide walls. It is also desirable to exhaust the vapors from the oven chamber to the atmosphere in order to prevent severe condensation within the chamber during the heating process.

During the operation of electronic ovens, it is necessary to insure that a dangerous amount of wave energy does not escape from the heating chamber. Prior ovens have been provided with interlock means comprising microswitches or the like, the contacts of which are adapted to open when the door of the heating chamber is opened. In the event that the contacts of such a switch might be welded shut, however, the opening of the door would be ineffective to shut off the power to the magnetron, and, in this event, a dangerous amount of wave energy could escape from the oven. It is therefore desirable to provide means to insure that this cannot happen. The present invention accomplishes this object by providing interlock means in the form of a switch which is adapted to prevent opening of the door in the event that the switch contacts are welded shut.

For the reasons discussed above, one object of the present invention is to provide an electronic oven having means adapted to present a uniform field intensity throughout the working area of the chamber.

Another object of the present invention is to provide a microwave oven having improved means for introducing elliptically polarized microwave energy into the heating chamber.

A further object of the present invention is to provide improved means for inserting wave energy into a heating chamber at oppositely disposed locations above and below the food to be heated.

Another object of the present invention is to provide a microwave oven having a waveguide assembly interconnecting a pair of antennas to a single source of wave energy, with means to provide a broad band impedance match.

A further object of the present invention is to provide a microwave oven having improved means for evacuating vapors from the heating chamber.

Another object of the present invention is to provide a microwave oven having means for preventing opening of the door of the oven unless a pair of switch contacts are first opened.

These and other objects and advantages of the present invention will become manifest upon an examination of this specification and the accompanying drawings.

In one embodiment of the present invention, there is provided a microwave oven having a heating chamber and dielectric means within the heating chamber for supporting food to be cooked. Diametrically above and below the supporting means are disposed antenna means adapted to radiate wave energy with a relative radiation phase diflerence of 90. Each of the antennas is fed by a single source of wave energy through a transmission line having power divider means adapted to provide a broad band impedance match between the magnetron and the load.

The heating chamber, the transmission line, and the wave energy source are contained within a housing, and means is provided to exhaust vapor generated in the heating chamber from the housing. The heating chamber is provided with an access door, and switch means is provided within the housing for preventing the door from opening unless the switch contacts are open.

Reference will now be made to the accompanying drawings in which:

FIG. 1 is a vertical cross-sectional view of an oven constructed in accordance with the present invention;

FIG. 2 is a cross-sectional view of a portion of the waveguide illustrated in FIG. 1, taken along the line 2-2;

FIG. 3 is a cross-sectional view of the apparatus of FIG. 2, taken along the line 3-3;

FIG. 4 is a plan view of a portion of the lower leg of the waveguide illustrated in FIG. 1;

FIG. 5 is a cross-sectional view of a portion of the upper leg of the waveguide, taken along the line 55;

FIG. 6 is a partial front elevation view of the oven of FIG. 1 with its door open;

FIG. 7 is a vertical cross-sectional view of a portion of the apparatus illustrated in FIG. 1, taken along the line 7-7; and

FIG. 8 is a vertical cross-sectional view of the apparatus of FIG. 6, taken along line 8-8.

Referring now to FIG. 1, there is illustrated an electronic oven including a housing 10 having top and bottom walls 11 and 13, respectively, a pair of end walls 15 and 17 enclosing the separate parts of the oven, except for a rectangular aperture 19 in the end wall 15 and an aperture 21 in the top wall 11. A pair of side walls 23 and 25 (FIG. 6) interconnect the side edges of the walls 11, 13, 15 and 17, to close the housing 10.

Within the housing 10 there is disposed a metallic heating chamber 27 having top and bottom walls 16 and 18, respectively, and an end wall 20, and side walls 29 and 31 (FIG. 7). The heating chamber 27 is open on its side opposite the end wall 20, and is provided with an outwardly extending peripheral flange 22 at its open end. The flange 22 overlaps and is secured to portions of the end wall 15 of the housing 10 surrounding the rectangular aperture 19.

A door 12 is hinged to the end wall 15 by a hinge 24 which has one leaf 33 secured to the end wall 15, and its other leaf 28 secured to the lower surface of the door A pad formed of resilient plastic foam or the like is secured to the inside surface of the door 12, a thin flexible metallic sheet 32 is supported by the pad 30. The sheet 32 is adapted to be pressed into tight fitting engagement with the flange 22 when the door 12 is in its closed position.

Also within the housing 10 is provided a magnetron 34 schematically shown in FIG. 1, surrounded by a casing 34. The casing 34 is in the form of a hollow circular cylinder, open at its ends, and is connected at its upper end to a conically-shaped channel 36 which extends between the casing 34 and the aperture 21. Within the channel 36 a fan 42 is connected to the shaft 40 of a motor 38 supported within the casing 34 by means not shown. When the fan 42 is rotated by the motor 38, air is drawn upwardly through the open lower end of the casing 34 past the magnetron 34 to air-cool the same.

The magnetron 34' is connected to a hollow circular mounting flange 35, and the magnetron antenna 37 extends through the mounting flange 35 into a vertical waveguide leg 39, which is secured to the end wall 20, the end wall 20 forming one of the walls of the vertical waveguide leg 39. The vertical waveguide leg 39 is connected at its upper end to an upper waveguide leg 46 and at its lower end to a lower waveguide leg 48. The upper waveguide leg 46 is secured to the upper wall 16 of the heating chamber 27, and is adapted to radiate wave energy into the heating chamber 27 through an antenna slot 50 disposed in the upper wall 16. Similarly, the lower waveguide leg 48 is disposed adjacent the lower wall 18 of the heating chamber 27, and radiates energy into the heating chamber 27 through an antenna slot 52. As illustrated in FIGS. 1 and 7, the upper wall 16 of the heating chamber 27 forms the lower wall of the upper waveguide leg 46, and the lower wall 18 forms the upper wall of the lower waveguide leg 48.

Both the upper and lower waveguide legs 46 and 48 include a bend in a vertical plane, adjacent the edges of the rear wall 20, and extend for the length of the upper and lower walls 16 and 18. The vertical waveguide leg 39 and the upper and lower waveguide legs 46 and 48 present a symmetrical waveguide assembly, with the power input at the midpoint of the assembly. The antenna slots 50 and 52 are each located at the same distance from the antenna 37, and are disposed opposite each other substantially at the center of the heating chamber 27.

It is more clearly seen in FIGS. 2 and 3 the magnetron output is coaxial, and the power input to the vertical waveguide leg 39 comprises the antenna 37 and the mounting flange 35. The vertical waveguide leg has a rectangular cross section with oppositely disposed relatively wider walls 54 and 56, and oppositely disposed relatively narrower walls 58 and 60. The coaxial output of the magnetron is connected in alignment with a circular aperture in the wall 56. A pair of segmental members 62 and 64 composed of conductive material are disposed opposite the aperture. The members 62 and 64 each have a length L approximately equal to the dimension of the width of walls 54 and 56, a thickness T equal ot the width of walls 58 and 60, and an arcuate interior contour. The spacing S between the crests of the members 62 and 64 determines the characteristic impedance of the waveguide assembly as seen by the magnetron.

It has been found that the provision of the segment members 62 and 64 provide substantially matched impedance operation over a wide band of frequencies, with a relatively low standing wave ratio.

In one embodiment of the present invention, designed to operate at a nominal frequency of 2450 megacycles, the interior dimensions of all the waveguide legs were 86 mm. by 43 mm., and the segmental members 62 and 64 were 86 mm. long, with a height H of 11.5 mm. The measured standing wave ratio was 1.13 at 2400 mc., 1.02 at 2450 mc., and 1.12 at 2500 me.

Each of the upper and lower waveguide legs 46 and 48 is provided with a tuning screw 66 by which the waveguide legs may be tuned to the proper operating condition.

Within the heating chamber 27 is disposed a platform 68 comprising a piece of sheet metal formed with folded down side walls 69 which rest on the bottom wall 18 of the heating chamber 27. A relatively large aperture 70 is provided in the central portion of sheet 68, and a dielectric turntable 72 is mounted in the aperture 70, overlapping with portions of the sheet 68 adjacent the aperture 70, and having its lower side connected to a gear 74. The gear 74 is provided with a boss which projects through the aperture 70 and connects with the turntable 72, thereby spacing the turntable 72 and the gear 74 apart by a distance which is greater than the thickness of the sheet 68.

Apparatus is provided for driving the gear 74, including a drive gear 76 mounted on a shaft 78 (FIG. 8). One end of the shaft 78 is journaled in an aperture in sheet 68, and the other end of the shaft 78 is connected to a motor 80. The motor 80 is preferably mounted by a bracket (not shown) on the front wall 15 of the housing 10. The drive gear 76 is in mesh with the gear 74 so that energization of the motor 80 is adapted to rotate the platform 72 within the heating chamber 15.

Each of the antenna slots 50 and 52 of the upper and lower legs 46 and 48 are disposed at a 45 angle relative to the longitudinal dimension of their respective waveguides as illustrated in FIGS. 4 and 5, and are disposed at 90 with respect to each other. This causes the wave energy inserted into the heating chamber 27 from the two antennas to have a relative radiation phase difference of 90 As a consequence, the electromagnetic field within the heating chamber 27 is elliptically polarized, containing E vectors at right angles to each other. This has been found to render the field distribution within the heating chamber 27 substantially more uniform than if the wave energy were polarized in a single dimension. In addition, the antenna slots 50 and 52 are disposed in opposite walls of the heating chamber 27, which also helps to make the field distribution within the heating chamber more uniform.

With the waveguide dimensions and operating frequency as described above, slots about 61 mm. long and 3 mm. wide have been found to operate very satisfactorily.

The sheet 68, in addition to supporting the platform 72 within the heating chamber, also functions to space it upwardly from the bottom wall 18 of the heating chamber 27 so that the food disposed thereon is generally in the central portion of the heating chamber 27 Thus the food disposed on the platform 74 is centrally located within the heating chamber 27, where the intensity of the radiation emanating from each of the two antennas 50 and 52 is substantially equal, thereby heating the food substantially uniformly.

The platform 72 is formed from a low-loss dielectric material such that there is little attenuation of the radiation emanating from the slot antenna 52 passing upwardly through the platform 72.

Each of the slot antennas 50 and 52 are covered with a piece of low-loss dielectric tape 82, so that the radiation emanating from the antennas may pass therethrough substantially unattenuated. The tape presents a vapor barrier between the heating chamber 27 and the waveguide, so that no vapors or moisture from the heating chamber 27 may pass into the waveguide.

In the side walls 29 and 31 of the heating chamber 27, a plurality of apertures 84 are provided, opening into the interior of the casing 10, so that air may pass from the interior of the housing into the heating chamber 27. The end wall 20 of the heating chamber is provided with a plurality of apertures 86 so that the air within the heating chamber 27 may pass out of the heating chamber 27, carrying with it any vapors which are generated by the heating process. From the aperture 86, the air is drawn through the open end of the casing 34 by the fan 42. This air is entrained in the stream of air cooling the magnetron 34', and the result is that any vapors generated within the heating chamber 27 are exhausted through the aperture 21 in the top wall 11 of the casing 10. The stream of air passing from the apertures 9 in the end wall 15- of the casing 10 to the open end of the casing 34 is sutficient to maintain the circulation just described through the heating chamber 27.

Referring now to FIGS. 6 and 8, positive interlock means is illustrated, by which operation of the magnetron 34 is inhibited, unless the door 12 is in its closed position. A slot 92 is disposed in the front wall of the housing 10, aligned with one of the side edges of the door 12, and an arm 94 is secured to the side of the door 12 and extends through the slot 92 into the interior of the housing 10 but outside of the heating chamber 27. An insulating member 96 is connected to the end of the arm 94 within the casing 10, and the member 96 supports a conductive switch member 98. When the door is in open position, as illustrated in FIG. 8, the switch member 98 is disposed in spaced relation with a pair of contact elements 100 which are supported by an insualting member 102 secured to the end 15 of the casing 10. When the door 12 is in closed position, however, the arm 94 and the switch member 98 rotate about the hinge 24 into the position illustrated in dashed lines in FIG. 8, where the conductive switch member 98 is brought into engagement with the contacts 100 and establishes an electrical connection therebetween. Wires 104 are connected to the contacts 100 and are incorporated in the control circuit for the magnetron so that the operation of the magnetron is inhibited unless the switch member 98 is in its lower position interconnecting the contacts 100. Such a control system is well known in the art and therefore need not be specifically described.

In the operation of the interlock formed by the arm 94 and its associated parts, the door 12 is inhibited from opening if, by some malfunction of the electrical system, the switch member 98 becomes welded to its contacts 102. It is therefore impossible to open the door 12 into the position illustrated in FIG. 8 unless the circuit including the contacts 100 has first been broken.

From the foregoing the present invention has been described with such particularity that others skilled in the art may make and use the same, and by employing common knowledge adapt the same for use under varying conditions of service without departing from the essential features of novelty thereof which are intended to be defined and secured by the appending claims.

What is claimed is:

1. An electronic oven comprising a heating chamber having upper and lower walls and side walls interconnecting said upper and lower walls, means for introducing microwave energy into said chamber through said upper and lower walls, and a rotatable platform to support material to be heated, said platform being formed of dielectric material, support means secured to a wall of said chamber at a location laterally offset from said platform for rotatably supporting said platform for rotation in the central portion of the space between said upper and lower walls, and drive means operatively coupled to the periphery of said platform for rotating said platform.

2. An electronic oven comprising a heating chamber having upper and lower walls, means for introducing microwave energy into said chamber through said upper and lower walls, a rotatable platform adapted to support material to be heated, said platform being formed of dielectric material and supported for rotation intermediate said upper and lower walls, and a metallic sheet spaced intermediate said upper and lower walls and having a circular aperture, said rotatable platform being supported by said sheet in a position covering said aperture.

3. Apparatus according to claim 2, including gear means secured to the lower surface of said platform and extending through said aperture below said sheet, said gear means having an aperture exposing the lower surface of said platform to the space below said gear to permit microwave energy introduced through said lower wall to pass through said platform to said chamber.

4. Apparatus according to claim 3, including a drive gear meshing with said gear means, a motor, and means coupling said drive gear to said motor, whereby energiZatiOn of said motor causes said platform to rotate;

5. Apparatus according to claim 4, wherein said motor is positioned below said lower Wall, and including a shaft interconnecting said drive gear and said motor, said shaft extending through an aperture in said lower wall.

References Cited UNITED STATES PATENTS Kamide 219-10.55 Johnson et al. 21910.55 Sawada 21910.55 Bohm et al. 21910.55 Ojelid 219-10.55 Smith 219--10.55 Staats 219-10.55 X

Great Britain.

15 JOSEPH V. TRUHE, Primary Examiner L. H. BENDER, Assistant Examiner

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