High frequency cooking device having electromagnetic induction heater

Abstract: A cooking heater comprises a housing provided thereon with an operation control panel including a plurality of function keys, a cooking room (2) disposed in the housing, the cooking room having a bottom formed by a non-magnetic metal mesh (9) having size in a range from 10 mesh to 25 mesh, a turntable (7) disposed within the heating room for mounting material (5) to be cooked, a magnetron (3) for generating microwave, a wave guide (4) for guiding said microwave into the cooking room, an electromagnetic induction coil (10) disposed below the non-magnetic metal mesh and means (11) for selectively coupling electric power to the magnetron or the induction coil. The non-magnetic metal mesh (9) may be sandwitched between two layers of dielectric material to form a laminated structure to thereby absorb any thermal deformation of the mesh. The turntable (7) is preferably formed of dielectric material. The cooking heater may further comprise a warning means for setting a preheating time and producing an audible warning signal when the preheating time elapses and wherein the function keys includes a preheating key and the warning means is actuated by depression of the preheating key.

Description: The present invention relates to a cooking heater according to the preamble of claim 1 and which is known from EP-A-0 291 093. An example of a conventional electromagnetic cooking heater is shown in Fig. 5, which employs a high frequency induction heater, or microwave oven. In Fig. 5, a housing 1 includes therein a heating room 2, a magnetron 3 for generating high frequency electromagnetic wave, a wave guide 4 for guiding the microwave into the heating room 2 in which a material 5 to be cooked is disposed, a flying pan 6 which can be removed from the heating room and used for other cooking operations externally of the heater, a turntable 7 on which the flying pan can be mounted and a motor 8 for driving the turntable 7. For example, there may a case where the flying pan 6 is first heated by an external heat source such as gas oven to scorch the material 5 thereon and thereafter inserted into the heating room 2 to heat it uniformly with high frequency electromagnetic wave (typically, 2.45 GHz) generated by the magnetron 3, while rotating the turntable 7 by the motor 8. In the so-called flying pan cooking system mentioned above, it is necessary to bring the flying pan from the external oven for scorching to the electromagnetic heater which may be remote from the external oven. Another conventional cooking heater comprises a high frequency heating device and an electromagnetic induction heating device provided integrally on the high frequency heating device. In this heater, material is induction-heated in the electromagnetic induction heater to scorch it and then moved into the high frequency heating device for internal heating. This also requires to move the material from the upper electromagnetic device to the high frequency device, which is troublesome. Further, since a distance between an electromagnetic induction heating coil and a magnetic container is selected as large in view of thermal insulation, coupling efficiency of magnetic flux is low and hence heating efficency is relatively low compared with the cooking with a gas range. Although there is another conventional device in which a shiethed heater is provided within a heating room to providing scorching, the heating efficiency thereof is also low. Furthermore, it is known from EP-A-0 291 093 to provide a rotary antenna connected to the waveguide for guiding said microwaves into said cooking room at a bottom-side of said cooking room below said electromagnetic induction coil. In this case, the electromagnetic induction coil is protected by a complicated layer structure using aluminium honeycomb course in a perforated disc shape. EP-A-0 318 645 discloses a composite cooking system having microwave heating and induction heating. EP-A-0 199 264 discloses a high frequency heating apparatus with an electric heating device, wherein a turntable is disposed within the cooking room for mounting material to be cooked. SUMMARY OF THE INVENTION An object of the present invention is to provide a cooking heater capable of scorching a material with high frequency induction heating and then heating the same with electromagnetic induction heating without moving the same. Since the material to be cooked is not required to be moved from one cooking device to another, the time required for cooking can be reduced. In order to achieve the above object, a cooking heater having the features of claim 1 is provided. The metal mesh may be sandwitched between two layers of insulating material and the turntable may be formed of dielectric material. A key for generating an audible signal may be provided on a control panel of the cooking heater for signalling a completion of preheating of the container in the heating room. The metal mesh constituting the bottom of the heating room serves to restrict leakage of microwave (typically, 2.45 GHz) for microwave heating to a permissible range and to allow electromagnetic wave (typically, about 20 to 30 KHz) for electromagnetic induction heating to pass into the heating room, so that both the microwave heating and the electromagnetic induction heating can be done on a material within the same heating room. The "electromagnetic induction heating" is a heating due to eddy current loss in a surface portion of a cooking container of dielectric material induced by electromagnetic field produced by the coil. Thus, the metal mesh formed from thin metal wires hardly forms eddy current paths and hence is hardly heated, so that it is possible to induce eddy current loss in a surface portion of the material to be cooked as if high frequency magnetic field produced by the coil passes therethrough. Any deformation of the metal mesh due to thermal expansion thereof caused by thus minimized eddy current loss can be absorbed by the dielectric layers sandwitching it. The dielectric turntable may be not affected by the electromagnetic induction heating and its dielectric loss for microwave frequency is minimized. By actuating the signalling key on the control panel of the cooking heater, the cooking can be started efficiently upon the signal generated when the preheating of the container within the heating room completes, resulting in a reduction of cooking time. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 shows a construction of a cooking heater according to an embodiment of the present invention; Fig. 2 is an electric circuit diagram of the cooking heater shown in Fig. 1; Fig. 3 shows a construction of a cooking heater according to another embodiment of the present invention; Fig. 4 illustrates generation eddy current in a metal mesh formed by punching of a metal sheet; and Fig. 5 shows a construction microwave cooking heater. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described in detail with reference to Fig. 1 showing a construction of a cooking heater and Fig. 2 showing an electric circuit thereof. In Figs. 1 and 2, a cooking heater 1 includes a heating room 2 provided with a wave guide 4 for connecting a magnetron 3 thereto. A bottom portion of the heating room 2 is constituted with a non-magnetic metal mesh 9. A turntable 7 is disposed within the heating room 2, which is driven by a turntable motor 8 through a shaft thereof penerating the metal mesh 2. An induction coil 10 is disposed beneath the metal mesh 9. A material food 5 is disposed on a container 6a of magnetic material such as iron which, in turn, is disposed on the turntable 7 which may be of dielectric material. A high voltage transformer 13 for driving the magnetron 3, a switching power source 12 connectable to an external commercial a.c. 100V power source and a double-pole, double-throw relay 11 having relay contacts 11a and 11b for selectively connecting the commercial power source to the high voltage transformer 13 or the induction coil 10 to he commercial power source, through the relay 12. The high voltage transformer 13 itself is well known and so it is shown in Fig. 2 schematically. The switching power source 12 is composed of a full wave rectifier 20, a smoothing capacitor 21, a load circuit including a series circuit of an inductor 22, a capacitor 23 which consitutes together with a primary winding of the high voltage transformer 13 a tank circuit and a collector-emitter circuit of a switching transister 24. It further includes a drive signal generator 25 for driving the switching transister 24. In operation, an a.c. 100V is rectified by the rectifier 20 and supplied to either the high voltage transformer 13 or the electromagnetic induction coil 10. The selection therebetween is performed manually through the relay 11. It may be possible to provide a selection key on the control panel. When the high voltage transformer 13 is selected, energy accumlated in the tank circuit is transformed thereby to produce a high voltage by which the magnetron 3 generates high frequency electromagnetic wave having frequency typically of 2.45 GHz suitable to excite water molecules of the material to be cooked. When the electromagnetic induction coil 10 is selected, the material 5 is heated by eddy current loss in its surface portion. In order to heat the container 6a of magnetic material by the coil 10, the turntable 7 and the bottom of the heating room 2 must be made of non-magnetic material which is not affected by electromagnetic wave having frequency of 20 to 30 KHz. On the contrary, in order to confine microwave (2.45 GHz) generated by the magnetron 3 within the heating room 2, the bottom of the heating room 2 must be of metal. The inventors of this invention have found that a mesh formed by plane-weaving thin stainless steel wires (JIS SUS304) which are non-magnetic material blocks 2.45 GHz microwave while coupling 20 to 30 KHz electromagnetic wave without loss. Further, it has been found that, in order to shield such microwave within a permissible range, the size of the stainless steel mesh is at least 10 lines per 2.54 cm (mesh) and that, in order to restrict loss of 20-30 KHz wave to a few percent or lower, the size must be 25 lines per 2.54 cm (mesh) at most. Therefore, by selecting the mesh size of the bottom of the heating room 2 from a range from 10 to 25 lines per 2.54 cm (mesh), it is possible to perform the high frequency (2.45 GHz) induction heating and the electromagnetic (20-30 KHz) induction heating within the same heating room. In a case where such bottom mesh is formed from a punched metal sheet, it is heated too much within a very short time due to eddy current flowing around each punched hole as shown by arrows in Fig. 4. An example of cooking with the heater constructed as mentioned above will be described with reference to heating of meat. At first, a flying pan 6a is put on the turntable 7 and a "flying pan" key on the control panel of the cooking heater is depressed so that the relay contacts 11a and 11b are thrawn onto the side of the electromagnetic induction coil 10. After preheating the flying pan 6a for about 2 minutes, the pan 6a is oiled and then a meat is put thereon and suitably scorched by a subsequent electromagnetic induction heating by the coil 10 to fix protein of the surface portion of the meat. By restarting the cooking after the scorching step completes, the relay contacts 11a and 11b automatically connect the power source to the magnetron 3 to heat an internal portion of the meat by microwave heating. Thus, a well cooked steak is obtained without loss of meat juice within a shorter heating time compared with that required for the conventional cooker. Fig. 3 shows another embodiment of the mesh bottom of the heating room. In Fig. 3, the mesh 9 is sandwitched between layers 14 of electrically insulating material and fixed in place by means of silicone addhesive. Although the mesh 9 is relatively hardly heated by electromagnetic induction, it is somewhat heated and deformed thereby. Such deformation of the mesh is absorbed by the insulating layers 14. The turntable 7 of the present invention may be formed of dielectric material so that there is no effect of electromagnetic wave of 20 to 30 KHz thereon. For dielectric material usable in the present invention, ceramic material is preferable since it is possible to minimize dielectric loss at 2.45 GHz. A warning means such as buzzer may be provided on the cooking heater according to any of the mentioned embodiments and a warning key may be provided on the control panel thereof. The warning means may be capable of setting a preheating time of the pan 6a and, when the key is actuated, audible signal is generated by the buzzer when the preheating time lapses. Thus, it is possible to perform a cooking without any time gap between the completion of preheating and a start of the cooking, contributing to a shortening of cooking time.

Claims: A cooking heater comprising: a housing (1) provided thereon with an operation control panel including a plurality of function keys, a cooking room (2) disposed in said housing (1), a magnetron (3) for generating microwaves, an electromagnetic induction coil (10), and means (11a, 11b) for selectively coupling electric power to said magnetron (3) or said induction coil (10), wherein a wave guide (4) is provided for guiding said microwaves into said cooking room (2), and said electromagnetic induction coil (10) is disposed below a bottom of said cooking room (2), characterized in that said cooking room (2) has said bottom formed by a non-magnetic metal mesh (9) having a size in a range of 10 to 25 lines/2.54 cm (mesh), a turntable (7) is disposed within said cooking room (2) for mounting material (5) to be cooked, and said wave guide (4) is provided at a non-bottom side of said cooking room (2). The cooking heater claimed in claim 1, wherein said non-magnetic metal mesh (9) is sandwiched between two layers of dielectric material (14) to form a laminated structure. The cooking heater claimed in claim 1 or 2, wherein said non-magnetic metal mesh (9) is formed by plane-weaving thin stainless steel wires. The cooking heater claimed in claim 1 or 2, wherein said turntable (7) is formed of dielectric material. The cooking heater claimed in claim 1 or 2, further comprising a warning means for setting a preheating time and producing an audible warning signal when the preheating time elapses and wherein said function keys includes a preheating key and said warning means is actuated by depression of said preheating key.

Inventors: OGASAWARA TOSHIO C O MITSUBISH, OGASAWARA, TOSHIO, C/O MITSUBISHI ELECTRIC
Applicants: MITSUBISHI ELECTRIC CORP, MITSUBISHI ELECTRIC HOME APPL, MITSUBISHI DENKI KABUSHIKI KAISHA, MITSUBISHI ELECTRIC HOME APPLIANCE CO., LTD
Application year: 1991
Main classification: H05B6
IPC codes: H05B11, H05B6, F24C7, F24C1