Microwave Oven

Abstract

A portable microwave oven in which the magnetron used to generate microwave energy is operated at half its rated load with power supplied thereto through a special power supply circuit which is designed to limit input current to 7.5 amps permitting a lighter transformer to be used and permitting the magnetron to dissipate reflected power under no load condition. The oven also includes an improved tuned waveguide and an improved stirrer for equally distributing energy in the oven cavity.

Description

BACKGROUND OF THE INVENTION

This invention relates to microwave ovens in general and more particularly to an improved portable microwave oven which may be operated on house current without requiring any wiring modifications for the installation.

Because of their speed and ease of operation, microwave ovens have become very popular in the last few years. Unlike conventional ovens wherein the food to be cooked is heated at high temperatures for long periods of time, the microwave oven uses microwave energy to excite the molecules within the food to obtain faster cooking. Generally, in such ovens a magnetron or the like is used to provide microwave energy through a wave guide into the oven cabinet. Typically this energy will be at a frequency of about 2,450 megacycles. The microwave energy which is generated and directed into the oven cavity will strike the food therein which is sometimes referred to as the load. As the microwave energy enters the food, it sets up electrical fields and the water molecules in the food try to align themselves with these fields. However, the waves are cyclic and tend to reverse their polarity of direction every half cycle which means that the water molecules in the load shift their positions 4,900,000,000 times per second. The friction restricting this high speed movement between the molecules produces the heat.

In prior art microwave ovens, a voltage and/or current was generally required which made custom wiring of the oven necessary. That is the oven could not be moved around from location to location such as from the kitchen to a family room or patio. In addition to the special wiring considerations such microwave ovens are heavy and not easy to move.

A further problem in prior art microwave ovens is the requirement of having special means to dissipate energy reflected back from the oven cavity and the need for special shielding around the oven door. Thus, it can be seen that there is need for a truly portable microwave oven which can be used in different areas of the house or moved from location to location. In addition, such a microwave oven must also be safe and must provide good heating characteristics.

SUMMARY OF THE INVENTION

The present invention provides such a microwave oven which is safe, efficient and can be operated anywhere in the household. Since it is to be a portable oven, the oven cavity is made somewhat smaller than those of conventional microwave ovens. The magnetron of a rating for a much larger oven is used and operated at approximately half power with power supplied from a special half-wave doubler circuit which draws only 7.5 amperes of current. Radiation from the magnetron is provided through a specially designed wave guide of drawn aluminum with folded flanges on all sides to prevent radiation leakage and containing a tuning stub for impedance matching of the cavity and wave guide to the magnetron output. Additional features include a door which has a metal to metal contact seal made through the use of a floating plate thus avoid special radiation sealing and an improved stirrer assembly comprising two discs tied together with polypropylene straps which act as a plurality of rotating antennae to distribute the microwave field evenly within the oven cavity to create an even distribution of energy therein. Because the magnetron is operated at such a low power level and through the use of a voltage doubler circuit, the size of the transformer and thus its weight can be greatly reduced. In addition, operation of the magnetron at half power, permits it to absorb reflected energy when operating at no load without overloading. Thus special means to dissipate energy at no load are not needed. This materially decreases the dissipation requirements placed on the oven design and thus makes possible a lighter oven. The unique door construction provides a design which is easy to implement as does the manner of constructing the stirrer assembly.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded view of the microwave oven of the present invention.

FIG. 2 is an exploded view of the door of the present invention.

FIG. 3a is a plan view of the stirrer of FIG. 1.

FIG. 3b is a cross-sectional view of the stirrer of FIG. 3a.

FIG. 4a is a plan view of the wave guide of the present invention.

FIG. 4b is a side view in section of the wave guide of FIG. 4a.

FIG. 4c is an end view in section of the wave guide of FIG. 3a.

FIG. 5 is an electric schematic of the oven of FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 is an exploded view of the oven of the present invention. It includes a base member 11 to which the remainder of the oven parts are mounted. As shown, there is mounted on the base a transformer 13 which will supply the power to the magnetron. The power to the transformer and the other electrical portions of the oven is provided by a line cord 15 which will be secured through a hole 17 in the back of member 11. Also shown mounted on the member 11 is a bracket 19 used to support the oven cavity as will be described below. A portion 21 of the bottom part of the member 11 will contain ventilating holes for cooling the magnetron. In order that the base does not rest on a solid surface, four feet 23, of which only one is shown, are attached to the bottom of member 11 with suitable screws 25. On each side of the bottom of member 11 is mounted a lever arm 27 loaded by a spring 29 which will attach to the oven door when in place.

For hanging the door, hinges 31 are secured at each side of the bottom of member 11. A front portion 33 is attached to member 11 and contains an opening 35 through which the oven cavity 37 may be inserted to rest on the member 33 and the bracket 19. Mounted to the front plate 33 is a terminal strip 39 used in making the oven electrical connections and a timer 41 used in controlling the time which power is applied to the magnetron. Mounted next to the timer is a diode 43. The use of these and other electrical components will be explained in connection for the schematic diagram below. The front portion 33 contains a hole 45 through which the latch of the door will extend. Mounted behind this hole is a catch 47 which has installed in it a microswitch 49 which will prevent the magnetron from being turned on unless the latch is closed. The oven cavity 31 contains ventilation holes 51 on one side over which a rubber frame 53 is mounted. This will direct ventilation from the holes 51 to openings 55 in the cover 57 which will eventually be placed over the final assembly. Also mounted to the side of the oven cavity 37 by a bracket 59 is a second microswitch 61 which is arranged to contact the lever arm 27 and be closed only when the oven door is properly closed. After insertion through the opening 35 in the front 33, the cavity 37 has mounted on top of it the wave guide 63 to which is attached the magnetron assembly 65. As shown, magnetron assembly 65 includes a fan 67 used for cooling. Air, after cooling the magnetron, then passes through openings 66 and 68 in an air duct 70 which is placed over the top of magnetron 65. Air from opening 66 is exhausted through a louver 72 and then deflected by a deflector 74 attached to the back of the base. The remaining air is directed into the oven cavity from opening 68 through holes identical to holes 51 described above. It cools the cavity and then exhausts through holes 51 and louver opening 55 in the cover. The fan will be located just above holes 21 so that adequate ventilation is provided. Also mounted within the cavity is the stirrer assembly 69 to be described in detail below. Stirrer assembly 69 is fastened to the shaft 70 of a motor 73 through a hole 71. Motor 73 is used to rotate the stirrer assembly 69. A suitable bearing 75 is placed in the hole 71 prior to insertion of shaft 70 therethrough.

The cover 57 contains openings 77 into which handles 79 are inserted to permit ease of moving the completed oven. A plate 81 is mounted in the top portion of the cavity 37 to prevent contact physically with the rotating stirrer 69. Plate 81 rests on notches 82 in the side of cavity 37 and is held by inserts 84 pushed through holes 86 and corresponding holes in the cavity. Also mounted to the top of cavity 37 is a capacitor 72 which is held in place by a bracket 76. A bottom plate 83 having feet 88, upon which the food to be cooked may be placed into the bottom of the cavity 37. A decorative top piece 85 will be secured across the top of the front plate 33. Along the side where the timer is mounted, an addtional decorative piece 87 containing the timer legend and time divisions will be mounted. Inserted into the portion 87 is a pilot light 89 to indicate when the oven is operating. The shaft 91 of timer 41 will extend through the hole in the cover 87 and have a knob 93 placed thereon. The oven door 95, which may be solid or have a see-through portion 97 as shown, is attached to the hinge brackets 31 and the lever arms 27.

The base member 11 and front 35 will preferably be made of stainless steel. The cavity 37 will preferably be made of drawn aluminum and the cover 57 of vinyl clad aluminum. Plate 83 may be made of fiberglass reinforced polyester to prevent any radiation loss therein. The decorative portions 85 and 87 may be constructed of molded Lexan.

FIG. 2 illustrates an exploded view of the door construction. The door is basically made up of three members. These comprise a door frame member 101 made of molded Lexan, an aluminum back plate 103 and a stainless steel sealing plate 105. A gasket 106 is placed around the back plate 105 which is then solidly affixed to the frame 101 with self-tapping screws 107. The sealing plate is secured to the backing plate with machine screws 109 which screw into inserts 111 placed in suitable holes 113 in the back plate. The sealing plate is free to move in and out against the gasket with the insert 111 moving in the holes 113 in the frame 101 and the holes 114 in the back plate 103. This permits a good metal seal between the surface of the sealing plate 105 and the surface of the front plate 33 of FIG. 1 and avoids the need for special provisions to prevent radiation leakage around the door. As shown, the plate 105 has a see-through panel 115 attached to it as does the frame assembly 101. If desired, these portions may equally well be made solid. The back plate 103 contains a warning label 117 to warn against closing the oven door unless the sealing plate is installed. Above plates 103 and 105, a latch retaining plate 116 is installed which hold the door latch 118.

A plan view of the stirrer 69 of FIG. 1 is shown on FIG. 3a, and a cross sectional view on FIG. 3b. It comprises a stamped aluminum center disc 121 to which is attached a stirrer ring 123 containing 8 projecting portions 125. The disc 121 and ring 123 are attached by four polypropylene straps 127 riveted to each of the discs 121 and ring 123 with rivets 129. This in effect forms a capacitor with eight rotating antennae.

FIGS. 4a, b and c illustrate the construction of the wave guide 63 of FIG. 1. It comprises a base plate 131 containing a first hole 133 which communicates with the magnetron and a second hold 135 which communicates with the oven cavity. With the wave guide installed, the stirrer 69 will be arranged so that a portion of it rotates beneath the opening 135 to evenly distribute the radiation. Mounted within the wave guide to the base plate 131 is a wave guide fastener/tuner 137. This permits tuning the system to avoid unnecessary losses resulting in more efficient operation. After installation of the tuner 137 the top portion 139 of the wave guide is placed over the base plate 131 and the edges of the base plate brought around and sealed to make a good radiation seal. As shown, the seal 141 extends completely around the wave guide thereby avoiding any radiation leakage. Suitable holes 143 are provided in the wave guide to permit attaching it to the top of the cavity 37 of FIG. 1 in the manner shown thereon. Both the base 135 and cover 139 are of drawn aluminum resulting in extreme ease of manufacture.

The electrical schematic of the oven of FIG. 1 is shown on FIG. 5. Power is provided by the line cord 115 with one side 155 of the line being provided directly to one side of the primary of a transformer 151 and the other side of the line to one side of a switch S1 which is installed in the timer 41 which may be any conventional timer of the type which closes a switch when set and opens the switch at the end of the preset time. From the other side of switch S1 a connection is made to a switch S2 which is the door latch switch 49 shown on FIG. 1. From the other side of switch S2 a switch S3 is connected. This corresponds to the switch 61 of FIG. 1. The other side of switch S3 is then provided through a thermal switch ST which will be mounted to the magnetron to sense overheating conditions to the other side of the primary of transformer 151. The timer motor 153 is connected across the one side of the line 155 and the output of switch S2. There is also shown across line 155 and the output of switch S1 a lamp 157 which may be used as a cavity lamp. This is the lamp 157 also shown on FIG. 1 next to the cavity. Across the line 155 and the output of switch S1 is a motor 159. This motor is the motor which drives the fan 67 of the magnetron assembly 65 of FIG. 1. Across the output of switch S2 and line 155 is a second motor 161 which corresponds to the motor 73 driving the stirrer 69 indicator light 163 which indicates oven operation. The transformer 151 has a first secondary 165 and a second secondary 167. The first secondary winding 165 is connected to the filament of the magnetron 169 supplying filament power thereto through a suitable filtering network 171. The secondary 167 is connected through a capacitor 173 and diode 173 arranged to act as a voltage doubler the output of which is provided to the magnetron 169 by a line 177. This will provide a high voltage e.g., 4,000 volts output to activate the magnetron. The transformer and capacitor are selected so as to limit the current drawn by the transformer to 7.5 amperes. Magnetron 169 will have a rating of twice this input power and thus will be able to dissipate reflected energy.

In operation, the food to be heated is placed within the oven cavity 37 and the door 95 closed and latched. If the door is properly closed and latched, both switches S2 and S3 will be closed. With nothing further the oven will not operate since switch S1 will be open. The timer may then be set to the desired cooking time closing switch S1. (Preferably timer 153 is set prior to closing the door thereby closing switch S1. In that case operation begins as soon as the door is closed and latched to close switches S2 and S3.) With all three switches closed power is provided to the primary of the transformer 155. Power is also provided to the cavity light 157, the fan motor on the magnetron 159 and the stirrer motor 161. The neon light 163 will light indicating that the oven is operating. Note, that if the light 157 is installed, it is preferable to set the timer prior to inserting the food with the door open, so that the oven cavity will be lighted while the food is being inserted.

The voltage across the primary of transformer 151 will cause the required outputs at the secondaries 165 and 167 resulting in both a filament voltage and high voltage being supplied to the magnetron 169. The magnetron will then begin operation and will output high frequency microwaves which will be directed through the wave guide 63, which has been properly tuned with the tuning stub 137 to obtain matched impedence. The stirrer 69 will be rotating and will act as a plurality of capacitive antennae to distribute the energy evenly throughout the oven cavity 37. The microwave energy will set up vibrations in the food as described above causing it to be heated. This process will continue for as long as the door is closed and the timer is running. If the timer reaches the end of its time, it will open switch S1 resulting in the magnetron and all the motors being stopped and all indicators extinguished. If before the timer finishes its cycle, the door is open, switches S2 and S3 will open stopping the magnetron from operating and extinguishing lamp 63 and stopping stirrer motor 161. However, motor 159 will continue to operate the fan 67 until the timer reaches the end of its cycle. As noted above, the magnetron 169 is selected to operate at about half its rated load. The remainder of the circuit is designed so as to draw no more than 7.5 amperes so that the oven may be truly portable. Since the magnetron is operating at half power, even under no load conditions i.e., with no food or load in the oven and all energy reflected back to the magnetron 169, the magnetron will be able to dissipate this power. By so limiting the magnetron operation and the current it uses a smaller transformer and in addition less shielding is required thereby materially reducing weight.

Thus, an improved lightweight portable oven has been shown. Although a specific embodiment has been illustrated and described, it will be obvious to those skilled in the art that various modifications may be made without departing from the spirit of the invention which is intended to be limited solely by the appended claims.

Claims

1. A portable microwave oven comprising:

a. a supporting frame;

b. an oven cavity attached to said frame and having an opening to insert a load;

c. means to generate microwave energy, said means including a magnetron which has a power rating approximately twice that required and means to cause said magnetron to operate at approximately half its rated power;

d. a wave guide coupling said microwave energy to said cavity;

e. a stirrer arranged to intercept and distribute the microwave energy within said cavity;

f. means to control operation of said generating means; and

g. means to seal off the opening of said cavity comprising:

1. a frame hinged to said supporting frame;

2. a backing plate securely attached to said hinged frame; and

3. a sealing plate flexibly attached to said backing plate, so that it may

2. The invention according to claim 1, wherein said means to cause said magnetron to operate comprises a circuit to supply said magnetron with a

3. The invention according to claim 2, wherein said circuit to supply said magnetron with a voltage and current causing it to operate at approximately one half its rated power is designed to draw no more than

4. The invention according to claim 3, wherein said circuit comprises:

a. a transformer having a primary, a first low voltage secondary coupled to the filament of said magnetron and a second high voltage secondary;

b. a capacitor having one side coupled to one side of said second secondary;

c. a diode having one side coupled to the other side of said capacitor and its other side to ground; and

d. means coupling the junction of said capacitor and diode as the high

5. The invention according to claim 4, wherein said control means comprise:

a. a timer including a first switch; and

b. at least a second switch adapted to be closed only when said cavity is covered; said first and second switches being wired in series with said

6. The invention according to claim 1 wherein said door includes

7. A portable microwave oven comprising:

a. a supporting frame;

b. an oven cavity attached to said frame and having an opening to insert a load;

c. a magnetron

d. circuit means to supply power to said magnetron;

e. a wave guide coupling said microwave energy to said cavity;

f. a stirrer to intercept and distribute the microwave energy in said cavity; said stirrer comprising:

a. a metallic center disk having a plurality of radially projecting arms;

b. an annular metallic ring, larger than said disk, having a plurality of projections radial thereto, at least two of which are aligned with two corresponding projections on said disk; and

c. at least two non-conductive straps coupling said disk and said ring.

g. means to rotate said stirrer;

h. means to control the supply of power to said circuit means; and

8. The invention according to claim 7 wherein said door comprises:

a. a frame hinged to said supporting frame;

b. a backing plate securely attached to said hinged frame; and

c. a sealing plate flexibly attached to said backing plate, so that it may

9. The invention according to claim 8 wherein said door includes

10. The invention according to claim 7 wherein said wave guide comprises:

a) a base plate having a first hole to communicate with said generating means, and a second hole to communicate with said cavity;

b) a mounting and tuning bracket attached to said base plate; and

c) a cover portion having bottom flanges placed atop said base plate and sealed thereto by portions of said base plate being folded over said

11. The invention according to claim 7, wherein said ring and disk are made

12. The invention according to claim 11, wherein said disk contains four projections, said ring contains eight projections, each projecting radially in two directions; and said ring and disk are attached to each other by four straps, each strap being riveted to said ring and said disk.

13. A portable microwave oven comprising:

a. a supporting frame;

b. an oven cavity attached to said frame and having an opening to insert a load;

c. means to generate microwave energy, said means including a magnetron which has a power rating approximately twice that required and means to cause said magnetron to operate at approximately half its rated power;

d. a wave guide coupling said microwave energy to said cavity;

e. a stirrer arranged to intercept and distribute the microwave energy within said cavity and comprising:

1. a metallic center disk having a plurality of radially projecting arms;

2. an annular metallic ring, larger than said disk, having a plurality of projections radial thereto, at least two of which are aligned with two corresponding projections on said disk; and

3. at least two non-conductive straps coupling said disk and said ring;

f. means to control operation of said generating means; and

14. A portable microwave oven comprising:

a. a supporting frame;

b. an oven cavity attached to said frame and having an opening to insert a load;

c. means to generate microwave energy, said means including a magnetron which has a power rating approximately twice that required and means to cause said magnetron to operate at approximately half its rated power;

d. a wave guide coupling said microwave energy to said cavity and comprising:

1. a base plate having a first hole to communicate with said generating means, and a second hole to communicate with said cavity;

2. a mounting and tuning bracket attached to said base plate; and

3. a cover portion having bottom flanges placed atop said base plate and sealed thereto by portions of said base plate being folded over said flanges;

e. a stirrer arranged to intercept and distribute the microwave energy within said cavity;

f. means to control operation of said generating means; and

g. means to seal off the opening of said cavity.