Electric heating – Microwave heating – With diverse-type heating
Patent
1997-03-03
1998-04-21
Leung, Philip H.
Electric heating
Microwave heating
With diverse-type heating
219685, 219715, 219721, 219757, H05B 668
Patent
active
057420327
DESCRIPTION:
BRIEF SUMMARY
This invention relates to microwave ovens.
According to one aspect of the invention there is provided a microwave oven having a cavity to receive food to be cooked, a magnetron for delivering microwave power to the cavity, a transformer for supplying the magnetron with the necessary high voltage, the transformer having a primary winding, characterised in that a resistance is connected in series with the primary winding, the resistive heating produced by primary current flowing through the resistance being utilised to heat the air in the cavity.
Said resistance is preferably provided by an impedance having a reactive component (mainly inductive) in addition to the resistive component. The resistive heating may be introduced to the cavity as a forced flow of air which is blown over the impedance by a fan. The impedance may be positioned adjacent an aperture in a rear wall of the cavity, the fan drawing air from the cavity, through the aperture, over the impedance and then recirculating the heated air into the cavity through apertures in the rear wall of the cavity.
The fan may serve to cool the magnetron, in which case the air is heated by being blown over the magnetron and then further heated by being blown over the impedance, before being forced into the oven cavity.
The oven may have a temperature sensing device (eg a thermistor) for sensing the temperature of the air and the temperature sensing device may be located in the region where the air leaves the oven cavity. The variation of detected temperature with time will be characteristic of the thermal load of a food item being cooked in the cavity, leading to the possibility of achieving automatic control of the cooking process by the use of a microprocessor which senses the variation of the hot air temperature with time.
The oven may have, in addition to the impedance, a main resistive heating element over which air is drawn, and in this case the resistive heating produced by the air forced over the impedance can supplement the heating produced by the air forced over the main heating element.
The magnitude of the resistance may be adjustable so as to vary the magnitude of the primary current and, in turn, the power produced by the magnetron. By utilising this feature, the microwave power delivered to the cavity can be readily controlled.
According to another aspect of the invention there is provided a microwave oven having a cavity to receive food items to be cooked, a magnetron for delivering microwave power to the cavity, a transformer for delivering electrical power to the magnetron, an electrical resistance heating element and a fan for forcing hot air through the cavity, wherein the transformer has a primary winding and an electrical resistance is connected in series with the winding so that primary current in the electrical resistance produces a resistive heating effect which is added to the heat introduced into the cavity by the hot air flow.
According to a further aspect of the invention there is provided a microwave oven having a cavity to receive food to be cooked, a magnetron for delivering microwave power to the cavity, a transformer for supplying the magnetron with the necessary high voltage, the transformer having a primary winding, characterised in that a resistance is connected in series with the primary winding, the magnitude of the resistance being variable to vary the microwave power produced by the magnetron.
The invention will now be further described, by way of example, with reference to the accompanying drawings, in which:
FIG. 1 is a circuit diagram of a first embodiment of the microwave oven according to the invention,
FIG. 2 is a rear view of the oven of FIG. 1,
FIG. 3 shows a rear wall of the oven of FIG. 2, viewed from inside the oven cavity,
FIG. 4 is a circuit diagram of a second embodiment of microwave oven according to the invention,
FIG. 5 is a schematic perspective view showing the structure of the oven of FIG. 4,
FIG. 6 is a graph for explaining the operation of the oven of FIGS. 4 and 5,
FIG. 7 is a graph for explaini
REFERENCES:
patent: 4198553 (1980-04-01), Dills
patent: 4337384 (1982-06-01), Tanaka et al.
patent: 4369347 (1983-01-01), Shin
patent: 4647746 (1987-03-01), Eke
patent: 4798927 (1989-01-01), Kaminaka
Leung Philip H.
Microwave Ovens Limited
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