US 3619536 A
Abstract available in
Claims available in
Description (OCR text may contain errors)
United States Patent Helmut Boehm Thousand Oaks, Calif. 37,279
May 14, 1970 Nov. 9, 1971 Bowmar/Tic Inc. Newbury Park, Calif.
Inventor Appl. No. Filed Patented Assignee MICROWAVE OVEN WITH SEPARATELY DRIVEN ANTENNA ELEMENTS 10 Claims, 4 Drawing Figs.
US. Cl 219/1055 Int. Cl 1105b 9/06 Field ofSearch 219/1055;
References Cited UNITED STATES PATENTS 4/1962 Baird 219/l0.55X
3,494,723 2/1970 Gray 219/1055 X 2,570,798 10/1951 Gullick 331/186 X 3,104,305 9/1963 Crapuchettes 219/1055 Primary Examiner-J. V. Truhe Assistant Examiner--Hugh D. Jaeger Attorney-Perry E. Turner P0 Wt? SUPPL V I mam/W e 250/2752 NETWOEK may: mw 26 MICROWAVE OVEN WITH SEPARATELY DRIVEN ANTENNA ELEMENTS BACKGROUND OF THE INVENTION .1. Field of the Invention This invention relates to microwave ovens employing two or more microwave energy sources.
2. Description of the Prior Art It is known to secure magnetrons to an oven wall so the ends of their output probes extend to or through windows in the wall, and to operate simultaneously and to operate them alternately. Such an arrangement presumably is aimed at getting better distribution of energy injected into the oven. However, The points of energy injection are required either to be on the same wall of the cavity or on cavity walls at right angles to each other.
Accordingly, energy distribution from the magnetrons is sufficiently different that different parts of foodstuff in the cavity are subjected to markedly different microwave energy zones, with the result that desired cooking is not achieved. Because of such deficiencies, arrangements heretofore known are not satisfactory where is is desired to cook many food stuffs in great volume, e.g., in an oven of large enough capacity for several meals to be thawed and cooked simultaneously.
SUMMARY or THE INVENTION This invention is directed to an over wherein antenna elements are adjacent and parallel to opposed walls, each antenna being coupled to the output probe of a respective length of waveguide. A respective magnetron has its output probe coupled to each waveguide to effect propagation of energy to the associated antenna element. The magnetrons are operated so that opposed antenna elements radiate simultaneously to subject foodstuffs placed between them to uniform cooking. Also embraced are operations of such magnetrons from a polyphase source, or of operations for switching among a number of pairs of magnetrons for desired treatment of a large volume of foodstuffs.
- BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic illustration of a pair of antenna elements in the upper and lower portions of an oven cavity, showing respective magnetron and waveguide feeds to the elements, and showing means for operating the magnetrons from a three-phase power supply;
FIG. 2 is a schematic diagram of the transformer and rectifier networks connected between the power supply and the magnetrons; I
FIG. 3 is a graph of voltage wave form illustrations to aid in explaining the operation of the circuit of FIG. 2; and
FIG. 4 is a schematic illustration of a plurality of antenna elements in an oven cavity, each having its magnetron and waveguide feed, and showing a switching network for selectively coupling the magnetrons to the power supply.
DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS Referring to FIG. I, a pair of antenna elements l0, 12 are located adjacent and parallel to opposed walls of an oven cavity 14, as the top and bottom walls in this illustration. The cavity has the usual door (not shown) through which to insert food stuff into the cavity to be cooked with microwave energy from the antenna elements above and below it. As shown, the an tennas l0, 12 are connected to the outer ends of respective output probes l6, 18 extending from waveguide 20, 22. At the other ends of these waveguides are respective magnetrons 24, 26 which are adapted to inject microwave energy into the waveguides, as via probes indicated at 28, 30, for propagation along the wave guides to permit the energy to be coupled via the output probes I6, 18 to the antenna elements 10, 12.
Tlle magnetrons 24, 26 are coupled through respective transformer and rectifier networks 32, 34 to a three-phase power supply source 36. Referring to FIG. 2 along with FIG. 1,
the powerlines 40, 42, 44 from the three-phase source are shown connected to the primary windings respective transformers 46, 48, 50 in network 32 and of respective transformers 52, 54, 56 in network 34. In network 32, respective full-wave rectifiers 60, 62, 64 are connected to the secondaries of the transformers 46, 48,50, with respective corresponding terminals of the rectifiers being connected in circuit with the heater of the magnetron 24 and the secondary of the associated heater transformer 66. The remaining corresponding terminals of the rectifiers are coupled to a point of reference or ground potential, as is the anode of the magnetron 24.
Similarly, in network 34, full-wave rectifiers 70, 72, 74 are connected to the secondaries of the transformers 52, 54, 56, with corresponding respective terminals of the rectifiers being connected in circuit with the heater of the magnetron 26 and the associated heater transformer 76, and with the remaining rectifier terminals being coupled, as is the anode of the mag netron 26, to reference or ground potential.
Referring to FIG. 3 along with FIGS. 1 and 2, the abovedescribed arrangement is one in which the pulse waveform of each magnetron is essentially the combined ripple frequencies of the full-wave rectifiers in the associated networks 32, 34. In this connection, FIG. 3 illustrates magnetron conduction at 2,800 volts. Thus, for the magnetron 24, the magnetron waveform is that shown in solidlines above the 2,800-volt level, and corresponds to the peaks 77, 78, 79 of the outputs of the full-wave rectifiers 60, 62, 64 in network 32. It will be apparent that the waveform of the magnetron 26 is the same.
Reference will be made to FIG. 3 in pointing up the contrasts in the operations of the magnetrons 24, 26 in prior art practices and in the system of this invention. In previously known arrangements, the magnetron is operated from a fullwave rectifier, and is turned on when the rectifier output reaches the appropriate level, e.g., the 2,800-volt level shown, and is turned off when such output falls below that level. The zero level shown in FIG. 3 is that from which each half-cycle output of such a rectifier starts and to which it returns. Accordingly, the magnetron is turned on and off twice during each cycle of the AC input to the transformer that supplies the rectifier. As indicated, this gives rise to undesired electrical noise and spurious radiations.
However, in the system of the invention, as each half cycle output of one rectifier falls below the 2,800-volt level, the output of another rectifier reaches that level. Accordingly, the magnetron does not turn ofi. Rather, it moves towards turnoff but is immediately pulsed on again. Thus, although the mag netron is definitely pulsed, the fact that it does not turn off means that the characteristic starting and stopping noise and ensuing spurious radiations in conventional magnetron operation are eliminated.
In a quantitative example of. the foregoing, the voltage and frequency of the input to each transformer is the same, e.g., 208volts at 400 Hz. and the corresponding repetition rate from the associated full-wave rectifier is 800 Hz. This means that the repetition rate of the current fluctuations from the outputs of the rectifiers is 2,400 Hz. which is the frequency or repetition rate reflected by the pulsating magnetron waveform.
Correspondingly, the power output from each magnetron is the sum of equal parts contributed by the associated trans former and rectifier combinations. Thus, for a full power output of 1,200 watts, the contribution of each of the three associated transformer and rectifier stages is 400 watts.
While it is preferred to operate the magnetrons as above described, it should be recognized that this invention embraces their operation at less than full power. In this regard, for the networks 32, 34, two transformers can be disconnected when it is desired to operate the magnetrons at one-third power, and one transformer can be disconnected when it is desired to operate to FIG. 2, a switch 80 is shown for connecting the line 44 to one end of the primary of the transformer 48, and a similar switch 82 is shown for connecting the line 40 to one end of the primary of the transformer 50. Similar switches 86, 88 are shown for connecting lines 44, 40 to respective ends of the primaries of transformers 54, 56 in the network 34.
When either switch 80, 82 is opened, the associated transformer is disconnected from the power source. Accordingly, the only transformers energized are the transformer 46 and that with which the associated primary switch is closed. This means that the repetition rate of the current and voltage to the magnetron is two-thirds of the maximum, i.e., 1,600 Hz. in the foregoing example of a 2,400 Hz. repetition rate for the total. When both of 'the switches 80, 82, are opened, only the transformer 46 is energized, whereby only the 800 Hz. repetition rate voltage from the rectifier 60 is applied to the magnetron. The power output of the magnetron is correspondingly less in these examples, being 800 watts when one switch is opened, and 400 watts when both switches are opened.
As will be apparent, this invention embraces the use of an suitable switching means for the above-described purposes. For example, such switching may be effected in the secondary circuits of the transformers if desired. Further, electronic switching may be employed. In this connection, the switches 80, 82, 86, 88 may be replaced with electronic switches. Further, such electronic switching may incorporate means for multiplexing the magnetron operations. In this latter regard, in one example the above-described switches may be relay operated, with the switching network adapted to periodically energize the relays for operating the switches 80, 86, and at other periods for energizing the relays for operating all of the switches. In this manner, the power levels of energy radiated throughout the cavity may be selectively controlled for such treatment of foodstuff therein as it may be desired to effect means of such switching.
FIG. 4 illustrates a modification of the system of this invention, wherein a transformer, rectifier and switching network 90 is connected between a power supply 36' and each of six magnetrons 91-96, for which respective waveguides 101-106 are provided for propagating energy from the magnetrons to respective radiating antenna elements 111-116 mounted in the top and bottom walls of a large capacity oven cavity 14'. Suitable switching means is provided in the network 90 to operate the magnetrons 91-96 in pairs. Thus, the network 90 in one arrangement effects operation of the magnetrons 91-96 in pairs, and successively, e. g. sequential multiplexing to operate magnetrons 91, 94 for a predetermined period, then magnetrons 92, 95 for a similar period, and then magnetrons 93, 96 for a similar period, such cyclic operations being repeated so that the magnetron pairs are each operated a predetermined number of periods in a given timespan. lf desired, the network 90 may include power level switching means as previously described, or means for operating the magnetrons in pairs from respective phases of the power supply.
1. In combination:
an oven cavity;
a pair of magnetrons;
means for radiating energy developed by each magnetron into said cavity,
said means including a pair of antenna elements in said cavity,
said antenna elements being adjacent and parallel to opposed walls of said cavity,
a pair of waveguides, external to said cavity, each having an output probe extending through opposed walls at one end connected to a respective antenna element,
each magnetron having an output probe adapted to inject microwave energy into a respective waveguide;
and means for operating said magnetrons simultaneously to cause said antenna elements to radiate microwave energy simultaneously toward material placed between them. 2. The combination of claim 1, wherein said operating means includes:
a three-phase power supply; a pair of transformer and rectifier networks for each phase of said power supply; and means for connecting one of each pair of networks to a respective magnetron so that each phase is applied simultaneously to both magnetrons.
3. The combination of claim 2, further including switching means for each magnetron to selectively connect thereto any combination of the networks associated therewith.
4. The combination of claim 3, wherein said switching means includes means for controlling the sequence in which the different combinations of networks are connected to the magnetron associated therewith, and the duration that each such combination is connected to the associated magnetron.
5. The combination of claim 1, wherein said operating means includes a polyphase power supply;
means for developing a full-wave rectified voltage for each phase of said power supply;
and means to apply all of said voltages to each magnetron.
6. In combination:
an oven cavity;
a pair of antenna elements in said cavity,
said antenna elements being adjacent and parallel to opposite walls of said cavity;
a pair of magnetrons for developing microwave energy;
respective waveguide means extending between each magnetron coupled to the waveguideby an output probe external to said cavity each having an output probe extending through opposed walls and at one end connected to a respective antenna element to cause energy developed by the magnetrons to be propagated to and by said antenna elements radiated throughout said cavity;
and means to pulse said magnetrons simultaneously without permitting them to be turned off.
7. In combination:
an oven cavity;
a plurality of pairs of antenna elements in said cavity,
the elements of each pair being adjacent and parallel to opposed walls of said cavity;
a plurality of pairs of magnetrons; respective waveguide means external to said cavity having an output probe between said cavity and magnetron coupling each magnetron by having an output probe extending through opposed walls and at one end connected to a respective antenna element to cause energy developed by each magnetron to be radiated by the associated antenna element into said cavity;
and means for operating said magnetrons in pairs in a predetermined sequence.
8. The combination of claim 7, wherein said operating means includes means for switching the magnetrons in a predetermined sequence for predetermined periods,
including means for pulsing the magnetrons to keep them from turning off during their respective predetermined periods of operation.
9 The combination of claim 8, including a polyphase power pp y;
and a respective transformer and full-wave rectifier network for each phase of the power supply being adapted for connection to each magnetron during the predetermined period for operation thereof.
10. The combination of claim 9, wherein networks for all phases of the power supply are connected to only one pair of the magnetrons during any predetermined period.
* 1 i i i