Microwave oven with invertor control power source

Abstract: A microwave oven with an invertor control power source produces a microwave by the use of the magnetron (MAG) in relation to the high frequency power (Pi) generated by the invertor control power source (1). The heat sensitive ferreit beads (FB) provided on the lead wire (11) for transmitting the high frequency power (Pi) to the magnetron (MAG) changes the inductance of the wire (11) from the high level (3µH) to the low level (0µ) at around a threshold temperature (T₀) lower than the working equilibrium temperature (T eq ) of the magnetron (MAG). The cathode filament current (I₂f and I₂ℓ) remains within the allowable range, and the moding of the magnetron is prevented. The magnetron (MAG) can operate in a long life time.

Description: 1. Field of the Invention The present invention relates to a microwave oven with an inverter control power source and, more particularly, to the microwave oven being capable of adjusting the power to the magnetron at the level selected by the user. 2. Description of the Prior Art In general, the microwave oven with an inverter control power source produces the high voltage power of high frequency (some tens kilo hertz) and the cathode filament current for driving the magnetron. Simultaneously, the inverter control power source detects the high voltage power supplied to the magnetron and controls the high voltage power and the cathode filament current so that the high voltage power stays within the allowable range. Both the high voltage power and the cathode filament current are supplied to the magnetron through the high voltage coil of high voltage transformer and the driving coil for the cathode filament. The high voltage coil and the driving coil for the cathode filament are directly connected to the magnetron by the connecting wire, or a ferrite bead is provided around the connecting wire so as to give an inductance for limiting the power supplied to the magnetron. However, the magnetron generates heat by the power loss of the microwave as well as by receiving the reflected microwave due to the impedance unmatching with respect to the load inside the oven. The cooling fan is employed to cause the heated magnetron cool down so as to keep the magnetron working stably above a suitable temperature, hereinafter referred to as a "working equilibrium temperature". According to Fig. 7, I₁f and I₁ℓ in dotted lines show the cathode filament currents when the microwave oven with the conventional ferrite bead is operated under the maximum rated power and under the minimum rated power with slight load, respectively, and I₀f and I₀ℓ in dashed lines show the cathode filament current when the microwave oven with no ferrite bead is operated under the maximum rated power and under the minimum rated power with a slight load, respectively. As apparent from the curve, a large current flows through the cathode filament of the magnetron just after the magnetron starts the operation. The current in the cathode filament is reduced until the oscillation becomes stable, i.e., during the temperature increase of the magnetron from the room temperature up to the working equilibrium temperature, at which the cathode filament current becomes stable. It is obvious, from Fig. 7, that the cathode filament current I₀f exceeds the upper limit UL during the initial stage of operation under the maximum rated power in the case where the cathode filament driving coil is directly connected to the magnetron through a connecting wire. When the cathode filament current I₀f exceeds the upper limit UL, a lattice defect of the metallic material will be observed in the surface of the cathode filament which causes the abnormal oscillation or a so-called "moding" of the magnetron, resulting in the very short service life time of the magnetron. On the other hands, the cathode filament current I₁ℓ falls below the lower limit LL during the operation under the minimum power with a slight load after reaching the working equilibrium temperature, in the case where the ferrite bead is mounted on the connecting wire to provide a predetermined inductance. When the cathode filament current I₁ℓ falls below the lower limit LL, there will be a self heat generation by the microwave reflection due to a slight cooking load and insufficient thermal electrons which cause the moding of the magnetron, resulting in very short life time of the magnetron. Thus, the microwave oven of the art has a problem such that the life time of magnetron is shortened due to the excessive current just after the operation start or to the insufficient current after reaching the working equilibrium temperatures. SUMMARY OF THE INVENTION The object of the present invention is therefore to provide a microwave oven with an inverter control power source which solves these problems. The present invention has been developed with a view to substantially solving the above described disadvantages and has for its essential object to provide an improved microwave oven with the inverter control power source. In order to achieve the aforementioned objective, a microwave oven with an inverter control power source comprises a power transmitting means for supplying a high frequency power, a microwave generating means for producing a microwave in relation to the high frequency power, the microwave generating means operating stably at a temperature above a working equilibrium temperature, and an impedance changing means mounted on the power transmitting means near the microwave generating means for changing an impedance of the power transmitting means between a first level impedance and a second level impedance being lower than the first level impedance. The first level impedance is established when the impedance changing means is heated below a predetermined threshold temperature. The second level impedance is established when the impedance changing means is heated above the threshold temperature. The threshold temperature is lower than the working equilibrium temperature. BRIEF DESCRIPTION OF THE DRAWINGS These and other objects and features of the present invention will become clear from the following description taken in conjunction with the preferred embodiments thereof with reference to the accompanying drawings throughout which like parts are designated by like reference numerals, and in which: Figs. 1 and 2 are illustrations showing an inner construction of the microwave oven according to the present invention; Fig. 3 is an schematic perspective view showing a front appearance of the microwave oven of Fig. 1; Fig. 4 is a perspective view showing a heat sensitive ferrite bead mounted on a high voltage wire of the microwave oven according to the present invention; Fig. 5 is a graph showing the inductance change with respect to temperature change of the high voltage wire of Fig. 4 caused by the heat sensitive ferrite bead; Fig. 6 is an electric circuit diagram showing the details of the microwave oven, according to the present invention; and Fig. 7 is a graph showing relationship between the cathode filament current and time of the microwave oven according to the present invention, in comparison with that of the prior art. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to Fig. 3, a microwave oven with a inverter control power source according to the present invention is shown. The microwave oven includes a rectangular box-like main body 30 having an opening at one side to define a cavity 6, a keyboard 20 for entering commands by a user, a door 9 for shielding microwave, and a turn table 7 for receiving a heating object. Referring to Figs. 1 and 2, an inside structure of the microwave oven body 30 is schematically illustrated. In the back space of the keyboard 20, a power source 1 with an inverter control 1, a magnetron MAG, a fan 3 (Fig. 2) for cooling the magnetron MAG, an air duct 4 (Fig. 1) for guiding the cooling air flow generated by the fan 3 to the outside, a waveguide 5 for introducing a microwave from the magnetron MAG into the cavity 6 are provided. A motor 8 for driving the turn table 7 is also provided at the bottom center of the cavity 6. Referring to Fig. 6, an electric circuit of the microwave oven according to the present invention is shown. The power source 1 includes a microcomputer CPU2 operated by the keyboard 20, an interface transistor Tr₃, a driving transformer DT, a power transistor PT r , and a circuit PTD for driving the power transistor PT r . A high voltage transformer HVT includes a primary coil PC o and a driving coil DC o wound on the primary side, and a coil Fi for driving a cathode filament 10, a high voltage coil HC o , and a sensing coil SCo wound on the secondary side. The coil Fi is connected to the cathode filament 10 of the magnetron MAG by high voltage wires 11 and 12 via magnetron terminals 13. A high voltage capacitor HCap and a high voltage diode HD for rectifying a current from the coil HC₀ are connected with the first end of high voltage coil HC o in a series. The first end of high voltage coil HC o is further connected with a high voltage wire 11 of the high voltage transformer HVT via the high voltage capacitor Hcap. The second end of high voltage coil HC o is connected with the magnetron MAG through the ground. The high voltage wire 11 is inserted into a heat sensitive ferrite bead FB, as shown in Fig. 4. The ferrite bead FB is fixed to a yoke 2 by a fixture 14, as shown in Figs. 1 and 2. Referring to Fig. 5, relationship between temperature and inductance of the high voltage wire 11 connected to the magnetron MAG is shown. A solid line L₂ represents the inductance in the high voltage wire 11 mounted with the head sensitive ferrite bead FB according to the present invention. Line L₂ shows a plat of about 3 µH at temperatures between the room temperature, e.g., 25 °C and 60 °C, rapidly decreases to nearly zero µH at about 70 °C, and remains at a constant level of inductance thereafter. It is noted that the temperature at which the inductance becomes stable at the low level is the working equilibrium temperature T eq of the magnetron, and the temperature T₀ which is at about 70 °C is the threshold temperature of the ferrite bead FB. According to the preferred embodiment, the threshold temperature T₀ should be between about 60 °C and 80 °C.A dotted line L₁ represents the inductance of the high voltage wire 11 provided by a conventional ferrite bead having no threshold temperature between the room temperature RT and the equilibrium temperature L eq . Line L₁ shows the same plat of 3µH as that of the solid line L₂ at the temperatures up to about 60 °C and is maintained at about level of 3 µH even in the temperature range between 60 °C and 100 °C. A dashed line L₀ represents the inductance of the high voltage wire 11 without any ferrite bead. In this preferred embodiment, for example, T₀ is set at 70 °C and the inverter control power source 1 works at a frequency of some tens of kilo hertz. Therefore, the ferrite bead FB causes the high voltage wire 11 to produce an inductance of approximately 3 µH when the temperature at the magnetron MAG is between the room temperature RT and the threshold temperature T₀. When the temperature at the magnetron MAG is higher than the threshold temperature T₀, the heat sensitive ferrite bead FB causes the high voltage wire 11 to produce almost no inductance. The inverter control power source 1, according to Fig. 6, produces the power for the microwave oven system from the commercial power supply of, e.g., 120 V at 60 Hz. The line from the commercial power supply is connected to a line noise filter LNF which is in turn connected to a bridge rectifier BD₁. The output of bridge rectifier BD₁ is connected to a choke coil L, and capacitor SCap. Thus, the commercial power is rectified to a direct current which is supplied to the power transistor PT r . Furthermore, the rectifier bridge BD₁ is coupled with a current transformer CT which induces an alternating current which is fed to a bridge rectifier BD₂. A direct current as produced by the rectifier bridge BD₂ is applied to a resistor R₁, and in turn to a capacitor C1. Thus, the produced direct current is supplied to the microcomputer CPU2 for controlling the microwave oven. The microcomputer CPU2 drives an interface transistor Tr₃ and the power transistor driving circuit PTD according to the instructions from the keyboard 20. The power transistor drive circuit PTD drives the power transistor PT r , through a driving coil DC₀ The power transistor PT r generates a high frequency alternating current in the primary coil PC₀ of the high voltage transformer HVT to produce an induction power Pi of high frequency in the secondary coil HC₀ of the high voltage transformer HVT and also in the coil Fi for driving cathode filament 10. The induction power Pi is supplied through the wire 11 mounted with the heat sensitive ferrite bead FB to the magnetron MAG for producing a microwave power Pm. The microcomputer CPU2 detects the induction power Pi by the sensing coil SC₀ provided on the secondary side of the high voltage transformer HVT. A resistor R₂ and a diode D₁ are provided along a line between the sensing coil SC₀ and the microcomputer CPU2. The microcomputer CPU2 controls each element in the inverter control power source 1 so that the induction power Pi stays within an predetermined allowable range, according to the detected induction power Pi. Immediately after the microwave oven is turned on, the temperature of the magnetron MAG is at the room temperature RT. At this stage, because the inductance of the wire is 3 µH by the ferrite bead FB, the current through the cathode filament 10 is reduced by 1 to 2 amperes, when compared with the current through wire 11 without the heat sensitive ferrite bead FB. Referring to Fig. 7, the relationship between the current through cathode filament 10 and time is shown. I₂f and I₂ℓ in solid lines show the cathode filament currents in the case where the microwave oven with the heat sensitive ferrite bead FB according to the present invention is operated under the maximum rated power load, and in the case where the same is operated under the minimum rated power with a slight load, respectively. When the magnetron starts its operation but is still at about the room temperature RT, the cathode filament current I₂f according to the present invention remains under the upper limit UL because the cathode filament current I₂f is reduced by 1 to 2 amperes, under the effect of the heat sensitive ferrite beads FB. Furthermore, when the temperature of the magnetron MAG increases above the threshold temperature T₀, the heat sensitive ferrite bead FB causes the inductance change in the wire 11 to almost zero, resulting in coincidence of lines I₂f and I₂ℓ with I₀f and I₀ℓ, respectively. Thus, even when the microwave oven is operated under the minimum power with a slight load, the cathode filament current I₂f will not fall below the lower limit LL. It is to be noted that each of respective points α and β on solid lines I₂f and I₂ℓ represents specific points in which the magnetron MAG reaches threshold temperatures (T₀) under a maximum rated power and a minimum power with a slight load, respectively. Thus, the microwave oven with the inverter power control according to the present invention can control the cathode filament current to be within a predetermined allowable range during the operation and realize the long life time of the magnetron. Although the present invention has been fully described in connection with the preferred embodiment thereof with reference to the accompanying drawings, it is to be noted that various changes and modifications are apparent to those skilled in the art. Such changes and modifications are to be understood as included within the scope of the present invention as defined by the appended claims unless they depart therefrom.

Claims: A microwave oven with an inverter control power source comprising: a power transmitting means (11) for supplying a high frequency power (Pi); a microwave generating means (MAG) for producing a microwave (Pm) in relation to said high frequency power (Pi), said microwave generating means (MAG) operating stably at a temperature above a working equilibrium temperature (T eq ); characterized by an impedance changing means (FB) mounted on said power transmitting means (11) near said microwave generating means (MAG) for changing an impedance of said power transmitting means (11) between a first level impedance (3µH) and a second level impedance (0µH) being lower than the first level impedance (3µH), said first level impedance (3µH) being established when said impedance changing means (FB) is heated below a predetermined threshold temperature (T₀), said second level impedance (0µH) being established when said impedance changing means (FB) is heated above said threshold temperature (T₀), said threshold temperature (T₀) being lower than said working equilibrium temperature (T eq ). A microwave oven with an inverter control power source as claimed in Claim 1, wherein said power transmitting means (11) comprises a high frequency power generating means (1) and a transmission line (11). A microwave oven with an inverter control power source as claimed in Claim 2, wherein said high frequency power generating means (1) is an inverter control power source (1). A microwave oven with an inverter control power source as claimed in Claim 3, wherein said inverter control power source (1) monitors and controls said high frequency power (Pi) through said microwave generating (MAG) so as to keep said high frequency power (Pi) within a predetermined allowable range. A microwave oven with an inverter control power source as claimed in Claim 1, wherein said microwave generating means (MAG) is a magnetron (MAG). A microwave oven with an inverter control power source as claimed in Claim 1, wherein said transmission line (1) is a lead wire (11). A microwave oven with an inverter control power source as claimed in Claim 1, wherein said impedance changing (FB) is a heat sensitive ferrite bead (FB).

Inventors: FUKUDA EIJI, KITAGUCHI YOSHIKAZU, MINAKAWA HIROSHI, FUKUDA, EIJI, KITAGUCHI, YOSHIKAZU, MINAKAWA, HIROSHI
Applicants: SHARP KK, SHARP KABUSHIKI KAISHA
Application year: 1991
Main classification: H05B6
IPC codes: H05B6, G05D23, H01J25