MICROWAVE OVEN

Abstract: A microwave oven for predominately heating of food, in which power control is obtained by pulsing of a microwave source comprised therein during sequence of operation periods. The start moment of the microwave source is sensed during each operation period, and the switch on time of the microwave source during each operation period is calculated from the start moment. Start moment related variations in supplied average power level are eliminated.

Description: The invention is directed to a microwave oven, being predominately intended for heating of food and comprising an oven cavity, a microwave source for generating microwaves in said cavity, a power source for supplying power to said microwave source, and a control device for controlling the microwave power in said cavity by means of a time controlled on/off switching of said microwave source during a sequence of operation periods. Power controlled microwave ovens are previously known in many different implementations. Accordingly, GB-A2-108 734 describes a device in which the power supplied to the microwave source, being normally a magnetron, is measured and dependent thereon the number of mains voltage cycles that are supplied to the power source is varied in order to obtain the desirable power. USP 4.420.668 is likewise based on power measurement and adaption of the operation time of the magnetron dependent thereon. EP-A2-0.043.958 described a further device which is based on power measurement by measuring current and voltage supplied to the power source. SE 7510018-0 describes a device comprising a power source of the so called switch mode type, in which the current to the magnetron is measured and in which the measured current value is used for controlling a frequency transformation in the power source and thereby to influence the power which is supplied. The above mentioned power control devices have in common, that the same are based on measurement of power and current. This means that the devices will be relatively demanding with respect to components with a consequently increased complexity and manufacturing cost. Another method for power control in microwave ovens is based on time control of the magnetron and means normally that the magnetron always operates with the maximum power and that lower power levels are obtained by so called pulsing of the magnetron, meaning that the operation time of the magnetron is subdivided into operation periods and that the magnetron is switched on during parts of the operation period in such a manner that the mean power value during an operation period corresponds to the desirable power level. If, as one example, an operation period of the length 15 seconds is used a mean power level corresponding to 60% of the maximum power may be obtained by switching on the magnetron for 9 seconds and switching off the same for 6 seconds during each operation period. This time control is useful also in combination with e.g. power control according to said patent specifications, which may be examplified by time control in combination with a power source of switch mode type of which the output power is controllable. Generally it is so, that the components comprised in a microwave supply system, e.g. transformer, magnetron, high voltage capacitor, show component tolerances which will influence the power which is supplied. In case of time control of power levels which are lower than the maximum level as described above, must be added to this the variations of the time used by the magnetron to initiate the generation of microwave ovens during an operation period after the generation of a start signal by a control device. The variations of the start moment are influenced by said component tolerances but are in first hand strongly dependent of the current mains voltage which is supplied to the microwave oven. As a consequence a deviation will arise between the nominally calculated switch-on interval of the magnetron and that which is really obtained during an operation period and consequently a corresponding deviation in the average power during the operation period. The invention is based on the understanding that in a microwave oven using time control for obtaining power levels lower than the maximum level typically more than 50% of the power deviation may be referred to the above described variations in the switch-on interval of the magnetron, and that an elimination of said variations will lead to an improved power control. One object of the invention is to obtain in a said microwave oven that the switch-on interval of the microwave source during an operation period is essentially independent of variations of the start moment of the magnetron. The object of the invention is obtained by a microwave oven of the type mentioned in the introduction, which according to the invention is characterized by means for generating an output signal which indicates the start moment of the microwave source, said means sensing a magnitude which changes when the generation of microwave power is started, and in that said control device is provided to determine the lenght of the switch-on interval of the microwave source dependent on the occurrence of said generated output signal. The invention provides for an advantageously simple and cheap solution because a measurement value of the sensed magnitude is not established but instead only the moment when a change occurs. Furhermore, a technical solution of this type is well fitted for use in a time controlled power control system as described above, because the switch-on interval of the microwave source during the operation period in a simple manner may be independent of variations of the start moment of the microwave source by being calculated from said moment of change. Preferred embodiments of the device according to the invention have the features disclosed by the following patent claims. The invention is described closer in the following with a reference to the drawings, in which: fig 1 shows a power supply system for a microwave oven according to the invention; fig 2 shows a time diagram of the generated microwave power during a number of operation periods; fig 3 shows a time diagram demonstrating the change of supplied current at the start of the magnetron; and fig a, b shows a time diagram of current and voltage supplied to the primary winding of the used transformer before respectively after start of the magnetron. Fig 1 shows a power supply system of a microwave oven according to the invention. The supply system comprises a microwave source 1, e.g. a magnetron, a transformer 2 having the primary winding 2a and the secondary winding 2b, a current switch 3 included in the primary circuit of said transformer, a current sensor 4 comprised in the primary circuit, a control device 5 for controlling the microwave power which is generated and having a control output 5a connected to a control input 3a of the current switch 3 and a control input 5b connected to an output 4a of the current sensor 4, a diode 6, and a capacitor 7. Disregarding the current sensor 4 in its cooperation with the control device 5, the disclosed power supply system is in accordance with ordinary skill and of a type which is used in microwave ovens available on the market, e.g. Philips type AWM730. For the more detailed construction is referred to this prior art. The magnetron 1 is supplied with a rectified high voltage via the transformer 2, the capacitor 7 and the diode 6. Switch on/off of the magnetron is obtained by means of the current switch 3, which may comprise a triac or a relay, under control by the control device 5. The operation time of a heating process is set by means of the control device 5 as also the desirable power level. When the power level is below the maximum power the operation time is subdivided into successive operation periods, and the control device 5 takes care of the on/off switching of the magnetron 1 by closing and interrupting the current switch 3 during the operation periods. By means of the control device 5 the switch-on interval of the magnetron during each operation period is so adapted that the average power during the operation period corresponds to the power level that has been set. This is illustrated in fig 2, showing the generated microwave power P during three operation periods of the length T. The magnetron starts at different moments t s during the operation periods, and is switched off according to prior art at one and the same moment t₀ during each period, when a nominal switch-on interval calculated from the beginning of the operation period is reached. Because the start moments vary between the operation periods the power supply during the operation periods will also vary, which is represented by the differently sized, lined areas. According to the invention the switch-on is instead calculated from the start moment t s of the magnetron, which brings with it a corresponding displacement of the switch-off moment t₀. Thereby the power contributions during the operation periods will be equally sized due to the contributions which have been marked with a dashed line, as well as the average power during the periods. The current sensor 4 comprises a so called current transformer having its primary winding connected in series with the primary winding of the transformer 2 and its secondary winding connected to the input 5b of the control device 5. Alternatively, the current in the primary circuit of the transformer may be sensed by means of a small impedance, e.g. a resistor, and then the current is measured by sensing the voltage across said impedance. A so called opto coupler may be used to obtain galvanic separation. Since only the change of current is sensed this may be easily obtained by means of a threshold circuit comprised in the sensor 4 or in the control device 5, and the threshold value of which is adjusted to a level which is adequate in comparison with the value of the current before and after start of the magnetron. An operation period is initiated by that the control device 5 generates a control signal which closes the current switch 3. This occurs at the moment t₁ in the current-time diagramme in fig 3. Through the current sensor 4 is then passed a comparatively small current up to the moment t s , when the magnetron starts and the current makes a jump and thereafter stabilizes at a comparatively high level. The interval t₁-t s varies strongly among other things dependent on the actual mains voltage. As illustrated in fig 2 the influence of said variations on the average power during an operation period is eliminated by calculating the switch-on interval of the magnetron during the operation period from the magnetron start moment t s , which is sensed during each operation period, instead of being calculated from the moment t₁ when a control signal is sent from the control device 5 to the current switch 3. From fig 3 is also clear that a sensing of the magnetron start is comparatively uncritical because the difference is great in the current level before and after magnetron start. Either the peak value or the RMS-value of the current may be sensed. Fig 4a and 4b illustrate an alternative way of sensing the change of current. In fig 4a the upper graph shows the current progress before magnetron start, and the lower graph shows the mains voltage, while the upper graph in fig 4b shows the current after magnetron start and the lower graph the mains voltage. From fig 4a is clear that the current before magnetron start reaches its maximum value at the zero transition of the mains voltage, while fig 4b shows that the maximum of the absolute value of the current after the magnetron start and the mains voltage appear simultaneously. The magentron start may consequently be detected by a phase comparison between the phase of the current through the primary winding of the transformer 2 and the phase of the mains voltage. The start moment of the magnetron may alternatively be obtained by sensing any other magnitude which changes at the start of the magnetron. This may be obtained by sensing the magnetic field in the high voltage transformer 2, e.g. by means of an extra winding in connection with the transformer or the iron core thereof. Alternatively, it is possible to use a magnetic field sensing element, like a Hall element. According to fig 1 the current is sensed in the primary winding of the transformer 2. Alternatively it is possible to sense the current correspondingly in the secondary winding of the transformer 2. A further possibility to establish the magnetron start is to detect by means of a microwave field sensor the appearance of microwaves in the oven cavity or in the wave guide connection between the magnetron and the oven cavity. In all alternative embodiments a magnetron start indicating signal is generated, which is supplied to the control device in correspondence with the embodiment according to fig 1 described above.

Claims: A microwave oven, being predominantly intended for heating of food and comprising an oven cavity, a microwave source for generating microwaves in said cavity, a power source for supplying power to said microwave source, and a control device (5) for controlling the microwave power in said cavity by means of a time controlled on/off switching of the microwave source during a sequence of operation periods, each including a switch-on interval characterized by means (4) for generating an output signal which indicates the start moment of said microwave source, said means (4) sensing a magnitude which changes when the generation of microwave power is started, and in that said control device (5) is arranged to determine the length of the switch-on interval of said microwave source dependent on the occurrence of said output signal. A microwave oven as claimed in claim 1, characterized in that said means (4) comprises a current sensing coil connected into the primary circuit of a mains transformer comprised in said power source. A microwave oven as claimed in claim 1, characterized in that said means (4) comprises an impedance which is connected in series with the primary circuit of a transformer comprised in said power source, and means for sensing a change of voltage across said impedance. A microwave oven as claimed in claim 1, characterized in that said means (4) comprises a device for sensing the change of magnetic field in a mains transformer comprised in said power source. A microwave oven as claimed in claim 1, characterized in that said means (4) comprises a microwave field detector provided in connection with said microwave source or said oven cavity.

Inventors: IDEBRO, MATS, JOELSSON, BO GUNNAR
Applicants: WHIRLPOOL EUROPE B.V.
Application year: 1989
Main classification: H05B6
IPC codes: H05B6, F24C7