JPH0945251A - Magnetron - Google Patents

Abstract

(57) Abstract: A magnetron having stable oscillation and high load stability when driven by an inverter power supply. In a magnetron for a microwave oven having an oscillation frequency of 2450 MHz, a ratio of an outer diameter of a cathode to an inner end distance between vanes positioned with the cathode sandwiched is 0.445 to.
In the range of 0.460, and assuming that the diameter of the filament constituting the cathode including the carbonized layer is D and the thickness of the carbonized layer is t, 0.04 <t / D <0.09 is set. There is.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a magnetron suitable for use with an inverter power supply, for example, for a microwave oven.

[0002]

2. Description of the Related Art A direct heating type having a structure in which a filament is spirally wound is used for a cathode of a magnetron for a microwave oven or the like. Thorium-tungsten (Th-W) is usually used as the material of the filament. Thorium-tungsten has the characteristics that it has a high electron emission density and that it can operate instantly.

By the way, as a power source of a magnetron for a microwave oven, a magnetic resonance type power source composed of a magnetic leakage type transformer and a high voltage capacitor has been used so far. In the case of the magnetic resonance type power source, the control for the magnetron output was only 400 to 600 W, which was a substantially constant power, or the control for intermittently controlling the output. For this reason, it has not been possible to sufficiently deal with a complicated cooking menu that requires a wide range of output power.

Therefore, recently, a high frequency inverter type power supply (hereinafter referred to as an inverter power supply) has been used. The inverter power supply has the following features when viewed from the magnetron side.

1) The filament voltage is affected by the high voltage on the secondary side of the power supply. Therefore, when the anode voltage of the magnetron changes, the filament voltage changes in conjunction with it.

2) The output can be controlled over a wide range by controlling the anode current flowing through the anode of the magnetron.

[0007]

When the inverter power supply having the above characteristics is used as a power supply for a magnetron, the following problems occur.

1) Ferrite magnets are used in magnetrons for microwave ovens and the like. The thermal demagnetization of a ferrite magnet is large, and when the anode temperature rises, the anode voltage drops. Also, the filament voltage decreases due to the decrease in the anode voltage,
The filament current (If) decreases. The decrease in filament current (If) may reach 2 to 3 A as the operating time passes, as shown in FIG. 6, and the oscillation becomes unstable. The left vertical axis in FIG. 6 is the filament current If (A), the right vertical axis is the estimated filament temperature (° C.), and the horizontal axis is the time (minutes). The solid line a is the filament current If, and the dotted line b is the filament temperature.

2) Magnetron has a reverse electron impact.
The electron reverse impact increases as the anode current increases, which increases the filament temperature. Therefore, when the output is reduced, the anode current is reduced and the reverse impact is reduced. As a result, the filament temperature decreases as shown in FIG. Figure 7
The vertical axis represents the filament temperature Tf (° C.), the horizontal axis represents the filament current If (A), and the output is 300 W, 600
The characteristics in the case of W and 700 W are the curves a, b and c, respectively.
It shows with. Therefore, when the anode voltage drops,
When the output is reduced and the anode current is reduced, the filament temperature becomes too low and oscillation cannot be sustained. A pattern of Po in FIG. 6 shows that the filament current decreases when the output is reduced.

In order to avoid the above-mentioned problem, that is, the reduction of the filament current, for example, the filament temperature (filament current) at the initial stage of operation is set high.
However, when the filament temperature is increased, the cathode made of thorium-tungsten is deformed. Such deformation can be solved to some extent by selecting a material of thorium / tungsten, adding an additive, and selecting heat treatment conditions appropriately. However, when the temperature of the filament becomes high, there is a problem that the decarburization during use accelerates the disappearance speed of the carburized layer and shortens the life.

From the above circumstances, when an inverter power source is used as a power source for driving the magnetron, the upper limit of the filament current has an influence on the life of the carburized layer, and the lower limit thereof has an influence on unstable oscillation due to insufficient thermoelectron emission.
Each needs to be considered.

By the way, if the carburized layer of the cathode filament is thickened, the life can be extended. However, in the case of the magnetic resonance type power supply, when the intermittent operation (repeated operation for 5 seconds / non-operation for 5 seconds, for example), the filament voltage is repeatedly turned on and off at the same time, and the temperature change of the filament becomes large. Further, as the repetition becomes faster, the shape of the carbonized layer on the surface is destroyed, the surface area is increased, and the temperature of the filament is lowered. As a result, the electron emission capability deteriorates rapidly. For this reason, the carburized layer could not be made too thick in the case of the magnetic resonance type power supply.

On the other hand, the inverter power supply can adjust the output by controlling the anode current while keeping the filament current flowing. Therefore, the carburized layer can be formed thick.

In the case of the magnetron described in Japanese Patent Publication No. 60-53418 and Japanese Patent Publication No. 61-12615, the filament structure is such that the thickness of the carbonized layer is less than 55% of the wire diameter, and the wire diameter is Is as thick as 0.6 to 0.8 mm, which is not suitable for recent compact magnetrons. In the latter case, the thickness of the carburized layer is in a wide range of 10 to 60 μm. It is considered that this thickness is suitable for inverter power supplies.

Further, as a parameter of the life of the cathode related to the amount of thermionic emission, for example, when the filament voltage (filament current) is gradually decreased while the normal oscillation is continued, the oscillation becomes unstable. A filament voltage value (hereinafter referred to as Ef ″) is considered. Ef ″ is
It rises with the passage of time. Therefore, the filament voltage E that maintains the electron emission increases with time.

By the way, in modes other than repeating the intermittent operation in a short time, if the carburized layer is thick, the initial E
Although f ″ is relatively high, the change in Ef ″ over time is small. For example, in the case where the carburized layer has a normal thickness, in the mode of operating for 3 minutes / non-operating for 1 minute,
After about 2000 hours, the change of Ef ″ is 1.0V to 2.0V, while the thickened carburized layer is
It is as small as 1.8V to 1.85V.

As described above, if the carburized layer is thickened, the life is extended, but there is a problem that the initial Ef "becomes high. In the inverter power supply, since the filament voltage has a characteristic of decreasing with the operation, the lower Ef is more suitable. It is thought that

In consideration of the above-mentioned circumstances, the present invention avoids the instability of oscillation due to the decrease of the filament current during the driving operation and sets the initial filament voltage high when the inverter power supply is used. The purpose of the present invention is to provide a magnetron whose life can be maintained as before.

[0019]

The present invention comprises an anode cylinder,
A cathode, which is located along the axis of the anode cylinder and has a filament wound spirally, ten vanes radially arranged inside the anode cylinder, and an electric field is formed in the internal space of the anode cylinder. And a ratio of an inner diameter of the anode cylinder to an inner end distance between vanes facing each other with the cathode interposed therebetween is set to about 3.9 times, and an oscillation frequency is 245.
In a 0 MHz band magnetron, the ratio of the outer diameter of the cathode to the inner end distance between the vanes is 0.445 to 0.46.
When the diameter of the filaments constituting the cathode including the carbonized layer is D and the thickness of the carbonized layer is t, the value is set to 0.04 <t / D <0.09. .

According to the above construction, the initial filament voltage Ef "at which the oscillation becomes unstable becomes low, and the change in the filament voltage Ef" due to the lapse of use time is small, so that the oscillation becomes unstable due to the decrease of the filament current. Can be avoided and a highly reliable magnetron can be realized.

Note that the above-described structure has a great effect when the inverter power supply is used as the power supply of the magnetron.
Such an inverter power source is often used in a magnetron for a microwave oven, and thus the present invention has a configuration suitable for a magnetron for a microwave oven.

[0022]

BEST MODE FOR CARRYING OUT THE INVENTION An embodiment of the present invention will be described with reference to FIG.

Reference numeral 11 is an anode cylinder made of copper, and a cathode 12 is arranged along the axis of the anode cylinder 11. The cathode 12 is
Direct heating type with a spirally wound filament, the upper end of which is the end shield 13 and the lower end of which is the end shield 14.
It is connected to the. Inside the anode cylinder 11, the cathode 12
A plurality of vanes 15 are formed in a radial pattern toward each other to form a cavity resonator. In addition, a pair of strap rings 16 having different diameters along the upper and lower sides of each vane 15 are provided.
7 are provided. The strap rings 16 and 17 are
Every other vane 15 is connected.

At both open ends of the anode cylinder 11,
Substantially funnel-shaped iron pole pieces 18 and 19 are fixed, one pole piece 18 is covered with a cylindrical metal container 20, and the other pole piece 19 is covered with a similar cylindrical metal container 21. . The metal containers 20 and 21 are made of iron material, and their surfaces are nickel-plated. In addition,
The metal containers 20 and 21 are hermetically welded to the anode cylinder 11,
It constitutes a vacuum container. Also, the pole pieces 18, 1
Cylindrical ferrite magnets 22 and 23 are arranged on the outer side of 9. The ferrite magnets 22 and 23 are the anode cylinder 1
An electric field is formed in the inner space of 1 to form an electron action space S around the cathode 12.

An antenna lead 24 is led out from one of the vanes 15, penetrates the pole piece 18, and extends into the metal container 20. The ratio of the inner diameter of the anode cylinder 11 to the inner end distance (2ra) between the vanes 15 located on the opposite sides of the cathode 12 is set to about 3.9 times, and the oscillation frequency is for a microwave oven in the 2450 MHz band. The magnetron is constructed.

In the magnetron having the above-mentioned structure, the structure of the filament forming the cathode 12 will be described with reference to FIG. Reference numeral 12a is a core wire of the filament, which is formed of, for example, thorium-tungsten, and the carbon wire 12b is coated on the core wire 12a. When the diameter including the carbonized layer 12b is D and the thickness of the carbonized layer 12b is t, 0.04 <t
The diameter D and the thickness t are set so that /D<0.09. In this case, preferably 0.05 <t / D <
It is 0.08. The cathode 12 is formed by spirally winding a filament having such a cross-sectional structure.

In the case of the magnetron of a household microwave oven,
D is 0.4 to 0.6 mm, preferably 0.44 to 0.
The range is 52 mm. Therefore, for example, D is 0.5
In the case of mm, t is 20 to 45 μm, preferably 25 to
It becomes 40 μm.

When manufacturing the cathode filament, the carburization rate (cold resistance of the filament after carburization / cold resistance of the filament before carburization) is set to 120 to 140%. The relationship between the carburization rate of the cathode filament and the carbonized layer / wire diameter is shown in FIG. The horizontal axis of FIG. 3 is the carburization rate (%), and the vertical axis is t / D (%).

By the way, in the magnetron having the structure shown in FIG. 1, the outer diameter of the cathode 12 (outer diameter of the spiral portion forming the cathode) is set to 2 rc, and the vanes between the vanes facing each other with the cathode 12 sandwiched therebetween. When the inner end distance is 2ra, the value of the ratio (2rc / 2ra) of both is 0.445 to
If it is selected in the range of 0.460, as shown in FIG.
The value of has decreased. The horizontal axis of FIG. 4 indicates the ratio (2rc / 2r
In the value of a), the vertical axis is the initial Ef ″. As can be seen from this figure, when the value of the ratio becomes larger than 44.5%, Ef ″
Falls to a value close to zero. When Ef ″ = 0, it means that the oscillation does not become unstable even if the value of Ef is gradually decreased after the oscillation. However, when the value of the ratio exceeds 46%, the oscillation efficiency of the magnetron is adversely affected. Therefore, it is considered that the ratio value is effective in the range of 44.5 to 46% In this case, the load stability is also improved as shown in Fig. 5. The horizontal axis of Fig. 5 is the ratio (2rc / 2ra). And the vertical axis represents the mode boundary M.A. (A), for example, the ratio (2
The value of Ef ″ is improved to 1.2V from 1.2V, and the mode boundary is improved to 1.6A from 1.6A, when the value of rc / 2ra) is 43%. .

When driven by an inverter power source, 3
A high frequency of 0 to 50 kHz is used. Therefore, the peak value of the anode current is about 30 to 40% higher than that of a normal power source. Therefore, a magnetron having a high mode boundary is required, and the present invention is also effective in this sense.

Further, in the above-mentioned embodiment, the magnetron for a microwave oven is described as an example, but the present invention is not limited to the magnetron for a microwave oven, and the present invention can be applied to a general magnetron.

[0032]

According to the present invention, it is possible to realize a magnetron which is stable in oscillation and has high load stability when driven by an inverter power supply.

[Brief description of drawings]

FIG. 1 is a schematic cross-sectional view showing an example of an embodiment of the present invention.

FIG. 2 is a cross-sectional view of a filament used in the present invention.

FIG. 3 is a characteristic diagram illustrating the present invention.

FIG. 4 is a characteristic diagram illustrating the present invention.

FIG. 5 is a characteristic diagram illustrating the present invention.

FIG. 6 is a characteristic diagram illustrating a conventional example.

FIG. 7 is a characteristic diagram illustrating a conventional example.

[Explanation of symbols]

 11 ... Anode cylinder 12 ... Cathode 12a ... Core wire 12b ... Carbonized layer 13, 14 ... End shield 15 ... Vane 16, 17 ... Strap ring 18, 19 ... Pole piece 20, 21 ... Metal container 22, 23 ... Ferrite

Claims (1)

[Claims] 1. An anode cylinder, a cathode in which a filament is spirally wound and located along the axis of the anode cylinder, ten vanes radially arranged inside the anode cylinder, and And a ferrite magnet that forms an electric field in the inner space of the anode cylinder, and the ratio of the inner diameter of the anode cylinder to the inner end distance between the vanes facing each other with the cathode in between is set to about 3.9 times, and oscillation is performed. In a magnetron having a frequency of 2450 MHz, a ratio of an outer diameter of the cathode to an inner end distance between the vanes is in a range of 0.445 to 0.460, and a diameter including a carbonized layer of a filament constituting the cathode. D,
0.04 <t / D <, where t is the thickness of the carbonized layer
A magnetron characterized by being set to 0.09.