EP0171170A2

EP0171170A2 - Microwave oven

Info

Publication number: EP0171170A2
Application number: EP85304692A
Authority: EP
Inventors: Bernard Frederick Fellerman; Stephen John Newton; David Wellcome
Original assignee: Thorn EMI Appliances Ltd; Kenwood Ltd
Current assignee: Kenwood Ltd
Priority date: 1984-08-02
Filing date: 1985-07-01
Publication date: 1986-02-12
Also published as: EP0171170A3; GB8419730D0; AU4468385A; CA1236173A

Abstract

A microwave oven has a standard control circuit (1) and magnetron (2). To enable the oven to operate from a 24V d.c. battery power supply (9), such as that of a lorry, car, etc., the oven includes a 50 Hz inverter (11) and a transformer (12) for generating an a.c. power supply of mains frequency and voltage to drive a fan motor (5) and a timer motor (7) in the circuit (1), and a 150 Hz inverter (13) and another transformer (20) for generating an a.c. power supply of higher frequency and different voltages to drive the magnetron (2) and its associated heater (10). The oven also includes a battery sensing circuit (17), which senses fluctuations in the battery voltage level and a control (18) adjusts current to the magnetron (2) accordingly. The control (18) also de-energises the magnetron (2) if the battery voltage level goes low.

Description

This invention relates to microwave ovens.
Conventional microwave ovens generally operate from the mains power supply, which does not usually create undue problems, if they are required to be used in domestic surroundings where the mains power supply is readily available.
However, it may be desirable to use microwave ovens, which are known to cook, or simply heat, food rapidly and efficiently, in other circumstances where the mains power supply is not so readily available, such as in lorry cabs, boats, caravans, etc.
It is therefore an object of the present invention to provide a microwave oven suitable for use in circumstances where the mains power supply is not readily available.
According to the present invention there is provided a microwave oven including a control circuit and characterised in that said circuit includes first inverter means for generating an a.c. power supply to drive first components of said oven, which are required to operate substantially at mains frequency, and second inverter means for generating an a.c. power supply to drive further components of the oven, which are capable of operating at a frequency substantially higher than mains frequency, thereby enabling said oven to operate from a d.c. power supply.
The control circuit preferably also includes first transformer means for generating mains voltage to drive the first components and second transformer means for generating an appropriate voltage to drive each of the further components.
The control circuit preferably further includes means for monitoring the voltage level of the d.c. power supply and means for adjusting the power supplied to the components in dependence on the voltage level so monitored.
It can thus be envisaged that the microwave oven, in accordance with the present invention, may be driven from a battery power supply, such as that of a lorry, car, etc.
The invention will now be further described by way of example only with reference to the accompanying drawings, wherein:
Figure 1 shows a schematic circuit diagram of one embodiment of the invention, and
Figure;2 shows one implementation of the embodiment shown.
The circuit diagram shown in Figure 1 includes a standard microwave control circuit 1 and a standard magnetron 2. The circuit 1 includes a magnetron thermal cut-out 3, an oven lamp 4, a fan motor 5 for driving a fan (not shown) to cool the magnetron 2, a cooking lamp 6, and a timer motor 7 with an associated timer switch 8.
To provide power to the circuit 1 a 24V d.c. battery power supply 9 is connected thereto, which battery 9 may be that of a lorry, car, etc.
However, the fan motor 5, cooking lamp 6 and the timer motor 7, as well as the magnetron 2 and its associated heater filament 10, are required to be run at a higher voltage and from an a.c. power supply.
The fan motor 5 and timer motor 7 are required to be run by mains voltage, i.e. 240V, and those that are currently available and suitable for the purpose in hand are frequency-conscious and need to be operated close to the usual mains frequency of 50 Hz. Thus to provide the a.c. power needed to run these components, a 50 Hz inverter 11 and an appropriate transformer 12 are connected in the circuit between the terminals of the battery 9 and the oven components 5 and 7.
The magnetron 2 and the heater filament 10 may however be run at a higher, and thus more efficient, frequency than the components 5 and 7, so that a 150 Hz inverter and EHT driver 13 is provided to drive the magnetron 2 and filament 10 from the battery 9.
A transformer 20 comprising a primary winding 21 and two secondary windings 14 and 15 change the voltage from the 24V battery supply 9 to voltages suitable to drive respectively the filament 10 and magnetron 2. The heater filament 10 requires a voltage of approximately 3.4V and the magnetron 2 requires a voltage of approximately 4000V.
To drive the magnetron 2, the secondary windings 15, which are preferably of high leakage reactance, may be arranged to provide a voltage of 2000V and a half wave voltage doubler 16 may be included to provide the required voltage of 4000V. Alternatively, the transformer 15 may be arranged to provide the full 4000V. An average current of 50A is required to drive the magnetron 2 and in the present embodiment the peak current supplied to the magnetron is approximately 100A.
If the battery 9 is being used to drive a lorry, for example, it is necessary to monitor the voltage level of the battery 9 during operation of the microwave oven.
To this end, there is provided a voltage regulation in the form of a battery sensing circuit 17, which monitors the battery voltage, and a control circuit 18, which provides pulse width modulation of the a.c. square waveform generated by the inverter 13, the modulated waveform being passed to the primary windings 21 of transformer 20.
If the battery sensing circuit 17 senses a slight fluctuation in the battery voltage, the current to the magnetron 2 is adjusted to compensate for this fluctuation.
If, however, the battery voltage goes low, the control 18 will cut out the magnetron 2 completely and therefore stop operation of the microwave oven.
A further sensing circuit 19 is provided to monitor the current to the fan motor 5, as operation of the fan during operation of the magnetron 2 is essential to prevent overheating of the magnetron. If the circuit 19 senses that the fan motor drive has failed, the control circuit 18 will again cut out the magnetron 2.
Use of the 150 Hz inverter 13 to drive the magnetron 2, which is capable of operating at relatively high frequencies, improves the efficiency of the oven and also enables the physical size of the transformer 15 to be reduced considerably, in relation to its use with a 50 Hz inverter.
The frequency at which the magnetron 2 operates may be as high as 400 Hz. However, 150 Hz is preferable, because whilst it is still considerably higher than 50 Hz to increase efficiency, it enables (as compared with 400 Hz operation) less expensive transistors, having limited switching times, to be used.
The heater filament 10 may be driven from the 50 Hz inverter 11 rather than the 150 Hz inverter 13 if preferred.
In addition to the oven components 5 and 7, the oven lamp 4 in the standard circuit 1 may also be driven from the 240V a.c. supply 11.
The cooking lamp 6 in the circuit 1 may be driven directly from the 24V d.c. battery supply 9.
Figure 2 shows one implementation of the embodiment shown in Figure 1, including the transformers 12 and 20 and indicating the circuit parts 11, 13 and 17 to 19. Inputs A and B to the primary windings of transformers 21 and 12 respectively, as also shown in Figure 1, lead to the positive terminal of the battery 9. Secondary windings 14 and 15 of the transformer 20 are connected respectively to the filament 10 and the magnetron 2, and the secondary windings of the transformer 12 are connected to the conventional microwave control circuit 1, as shown in Figure 1.
It can thus be envisaged that the present microwave oven is a modified standard oven, which may be driven from a d.c. battery power supply, which may typically be a lorry or car battery, thereby removing the previously imposed constraint of having to run the microwave oven from a mains power supply.

Claims

(1) A microwave oven including a control circuit and characterised in that said circuit includes first inverter means (11) for generating an a.c. power supply to drive first components (5, 7) of said oven, which are required to operate substantially at mains frequency, and second inverter means (13) for generating an a.c. power supply to drive further components (2, 10) of the oven, which are capable of operating at a frequency substantially higher than mains frequency, thereby enabling said oven to operate from a d.c. power supply (9).
(2) An oven as claimed in claim 1 wherein said circuit includes first transformer means (12) for generating mains voltage to drive the first components (5, 7) and second transformer means (20) for generating a voltage suitable to drive each of said further components (2, 10).
(3) An oven as claimed in claim 1 or 2 wherein said circuit includes means (17) for monitoring the voltage level of said d.c. power supply (9) and means (18) for adjusting power supplied to one or more of said components (2, 5, 7, 10), in dependence on said voltage level.
(4) An oven as claimed in claim 3 wherein said means (18) for adjusting power supplied to one or more of said components (2, 5, 7, 10) is arranged to disconnect said power supply, if said voltage level drops below a predetermined value.
(5) An oven as claimed in any preceding claim wherein said components (2, 5, 7, 10) include a magnetron (2) and a fan (5) for preventing overheating of said magnetron (2), and said control circuit further includes means (19) for monitoring operation of said fan (5) and for de-energising said magnetron (2) if a failure in operation of said fan (5) is detected.