Vacuum switch tube

Abstract: Disclosed herein is a vacuum switch tube which comprises a highly conductive member provided on a back surface of a main electrode. The highly conductive member prevents an arc formed upon breaking a large current from locally concentrating or stagnating, and promotes rapid and smooth movement of the arc from the main electrode to an auxiliary electrode. Marked enhancement and stabilization of the performance of the vacuum switch tube in breaking a large current, as well as a smaller construction, are thereby realized.

Description: Field of the Invention This invention relates to a vacuum switch tube used for switching a large electric current. Description of the Prior Art EP-A-0 025 371 defining the closest prior art from which the present invention proceeds discloses a vacuum switch having a highly conductive reinforcement member being higher in electric conductivity than a contact portion and a main surface portion. Such an arrangement leads to an improvement of a current-carrying characteristic, but not to an improvement of the breaking characteristic. Furthermore, from EP-A-0 121 180 a vacuum interrupter is known wherein a contact-making portion is made of material of 20 to 60 % IACS electrical conductivity whereas a magnetically arc-rotating portion comprising fingers surrounding the contact-making portion is made of material of 10 to 20 % IACS electrical conductivity. So, the electrical conductivity of the contact-making portion is greater than that of the surrounding arc-rotating portion. This known arrangement is effective at an extremely high pressure level under high voltages like 84 kV wherein a voltage proof characteristic of the material itself is largely effective and, thus, critical as to the electrical characteristics. Therefore, material having a low voltage proof level like Cu are desired to a necessarily use with a small amount. Then, the amount of voltage proof material in the outer portion of the electrode is reduced with regard to that in the contact portion. Fig. 1 is a sectional view of a further conventional vacuum switch tube, as disclosed in JP-A-45-29935 (1970) for instance, and a sectional view taken along line A-A′ of Fig. 1 is shown in Fig. 2. In the figures, a vacuum vessel 1 is formed to maintain therein a high vacuum of a pressure of 10⁻⁴ Torr or below. A stationary electrode rod 5 is disposed with one end portion fixed to a stationary-side end plate 2 of the vessel 1. A movable electrode rod 6 is fixed to one end of a bellows 4, and is passed through a movable-side end plate 3, to which the other end of the bellows 4 is fixed, so that the movable electrode rod 6 is movable vertically relative to the end plate 3. The stationary electrode rod 5 and the movable electrode rod 6 are provided respectively with a stationary electrode 7 and a movable electrode 8, each of which comprises a main electrode 7a, 8a provided at a central portion thereof and an auxiliary electrode 7b, 8b provided at the periphery of the central portion and connected to the respective electrode rods 5, 6. The main electrodes 7a and 8a are each provided with a recessed portion 7c, 8c at a central portion thereof and an annular portion 7d, 8d at the peripheral portion thereof. The auxiliary electrodes 7b and 8b are provided with spiral grooves 7e and 8e, respectively. For adsorbing a metal vapor emitted from each of the electrodes, a shield plate 9 is provided. The conventional vacuum switch tube mentioned above operates as follows. To make the vacuum switch tube, the movable electrode rod 6 is moved upward to bring the main electrodes 7a, 8a into contact with each other at the annular portions 7d, 8d. Upon the contact a current flows through the path constituted of the stationary electrode rod 5-auxiliary electrode 7b-main electrode 7a-main electrode 8a-auxiliary electrode 8b-movable electrode rod 6. To break the vacuum switch tube, on the other hand, the movable electrode rod 6 is moved downward to separate the main electrode 8a from the main electrode 7a, thereby breaking the current. In this case, when the magnitude of the current flowing is of the order of the load current, separation of the annular portion 8d from the annular portion 7d completes the break of the flowing current in that region. When the flowing current is a heavy current, as in the case of a shortcircuit or the like, the separation of the annular portion 8d from the annular portion 7d is accompanied by an arc therebetween. The arc moves outward with respect to the center axis of the electrodes due to the effect of a magnetic field developed by an external wiring or the like. Upon reaching the auxiliary electrodes 7b, 8b, the arc is given a rotating force by the spiral grooves 7e, 8e, and is brought into a rotational motion around the center axis while moving further outward. This process prevents the arc from stagnating locally to damage the electrodes or generate a metal vapor. The main electrodes 7a, 8a, which make contact with each other to serve as a current-passing portion when the vacuum switch tube is made, constitute an arc-extinguishing portion at the time of breaking a current of the order of the load current. Therefore, a material requiring a small tripping force against welding thereof and having a small chopping current value is selected for the main electrodes 7a, 8a. For the auxiliary electrodes 7b, 8b, on the other hand, a material is selected which is capable of breaking a large current and has superior withstand voltage performance. Furthermore, the main electrodes 7a, 8a, auxiliary electrodes 7b, 8b, stationary electrode rods 5 and movable electrode rod 6 are jointed to each other generally by brazing using a Cu-Ag brazing filler metal in a hydrogen atmosphere or in a vacuum. The vacuum switch tube according to the prior art, constructed as mentioned above, has the problem as follows. Where the material constituting the main electrodes 7a, 8a is quite different from the material constituting the auxiliary electrodes 7b, 8b, as for example in the case where the main electrodes 7a, 8a are formed of a material containing a large amount of a low melting point metal whereas the auxiliary electrodes 7b, 8b are formed of a high withstand voltage material, a metal vapor is liable to be emitted from the low melting point metal at the time of breaking a heavy current, making it difficult for the arc to move to the auxiliary electrodes 7b, 8b. As a result, the arc stagnates at the main electrodes 7a, 8a, thereby heavily damaging the electrodes. It is therefore impossible to obtain a stable break performance for large currents. SUMMARY OF THE INVENTION It is an object of this invention, aimed at overcoming the above-mentioned difficulties, to provide a vacuum switch tube which has an electrode structure ensuring stable break of large currents and is small in size and economical. To attain the above object, a vacuum switch tube according to this invention comprises a highly conductive member on a non-opposed surface of each one of a stationary electrode and a movable electrode opposed to each other, namely, on either or both of back surfaces of a main electrode and an auxiliary electrode, at least on the back surface of the main electrode, of each one of the stationary and movable electrodes. In the vacuum switch tube, furthermore, the electric conductivities σ a , σ b and σ h of the main electrode, auxiliary electrode and highly conductive member are so selected that σ a < σ b < σ h . This, together with an improvement in electrode shape, prevents positively the local concentration or stagnation of an arc formed upon the break of an electric current, whereby stable performance in breaking a large current is realized. Other objects and effects of this invention will become apparent from the following detailed description of some embodiments of this invention, referring to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a sectional side view of a vacuum switch tube according to the prior art; Fig. 2 is a sectional plan view taken along line A-A′ of Fig. 1; Fig. 3 is a sectional side view of a vacuum switch tube according to one embodiment of this invention; Fig. 4 is a sectional side view of a stationary electrode shown in Fig. 3; Fig. 5 is a sectional side view of a stationary electrode in a vacuum switch tube according to another embodiment of the invention; and Fig. 6 is a characteristic diagram showing the performance of the vacuum switch tube shown in Fig. 5. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of this invention will now be described. In Figs. 3 and 4, the components or portions the same as or equivalent to the corresponding ones in Figs. 1 and 2 are denoted by the same reference signs as in Figs. 1 and 2, and the explanation of those components or portions will be omitted. In Figs. 3 and 4, mutually opposed surfaces of main electrodes 7a, 8a are each provided with a flat portion 7f, 8f of a diameter Da at a central portion thereof and with a taper portion 7g, 8g at a peripheral portion thereof. On back surfaces of the main electrodes 7a, 8a are provided highly conductive members of a diameter Dh, which are formed of copper or the like and jointed a stationary electrode rod 5 and a movable electrode rod 6, respectively. Db denotes the diameter of auxiliary electrodes 7b, 8b, whereas L₁ and L₂ denotes the paths of arc currents flowing through the main electrodes 7a, 8a and the auxiliary electrodes 7b, 8b, respectively. The electric conductivities σ a , σ b and σ h of the main electrodes 7a, 8a, auxiliary electrodes 7b, 8b and highly conductive members 7h, 8b are so selected that σ a < σ b < σ h . Here, Cu-20Cr-Bi is used as a material for the main electrodes 7a, 8a, whereas Cu-(10-60)Cr or Cu-20Cr is used as a material for the auxiliary electrodes 7b, 8b, and 99.9Cu is used as a material for the highly conductive members 7h, 8h. The relative values of the conductivities of the materials are roughly in the ratio σ a : σ b : σ h ≈ 0.3 : 0.7 : 1. When a material containing at least 10% of a low melting point metal, such as Bi and Te, is used for the main electrodes 7a, 8a, a mixed powder of the electrode material may be compression-molded onto a copper base, followed by integral forming, as described in Japanese Patent Application Laid-Open (KOKAI) No. 59-3822 (1984). It is thereby possible to obtain a stock in which the electrode material and the highly conductive member 7h, 8h of copper are jointed to each other in a body through a thermal reaction, and the stock is capable of being used after mechanically processed. When the above-mentioned electrode material is used for the auxiliary electrodes 7b, 8b, it is possible to obtain a smaller vacuum switch tube capable of breaking a large current, with a higher withstand voltage and a lower chopping current value, as compared to the case where the auxiliary electrodes 7b, 8b themselves are formed of copper to constitute the highly conductive members. The operation of the above-mentioned vacuum switch tube will now be explained. To make the vacuum switch, the movable electrode rod is moved upward to bring the main electrodes 7a, 8a into contact with each other at the flat portions 7f, 8f thereof. The current path L₁ upon the contact is the stationary electrode rod 5-highly conductive member 7h-main electrode 7a-main electrode 8a-highly conductive member 8h-movable electrode rod 6. To break a current of the order of the load current, the movable electrode rod 6 is moved downward to separate the flat portions 7f, 8f from each other, whereby the breaking is completed in that region. When a material containing a large amount of the low melting point metal mentioned above is used for the main electrodes 7a, 8a, it is possible to obtain a low chopping current value of not more than 1 A. At the time of breaking a large current, as for example in the case of a shortcircuit, separation of the flat portions 7f, 8f from each other causes formation of an arc at that portion, and the arc is moved outward by an electromagnetic force generated by an external wiring or the like. The movement takes place smoothly between the main electrodes 7a, 8a formed of the same material, and the arc is further moved rapidly from the flat portions 7f, 8f to the taper portions 7g, 8g. Because the mutually opposed surfaces of the main electrodes 7a, 8a in this embodiment comprises the flat portions 7f, 8f and the taper portions 7g, 8g extending from the flat portions, there is no possibility of the concentration or stagnation of the arc, which would occur in the conventional vacuum switch tube shown in Figs. 1 and 2 at stepped portions formed by the recessed portions 7c, 8c or the annular portions 7d, 8d of the main electrodes. Thus, in this embodiment, the movement of the arc takes place rapidly and smoothly. The arc moved to the taper parts 7g, 8g is capable of being further moved smoothly, without stagnating at the auxiliary electrodes 7b, 8b formed of a material different from the material of the main electrodes 7a, 8a. The reason is as follows. Because the conductivities σ a , σ b and σ h of the main electrodes 7a, 8a, auxiliary electrodes 7b, 8b and highly conductive members 7h, 8h are so selected as to satisfy the relationship σ a < σ b < σ h , the current flows through the path of the stationary electrode rod 5-highly conductive member 7h-auxiliary electrode 7b-auxiliary electrode 8b-highly conductive member 8h-movable electrode rod 6 as indicated by the current path L₂, the movement of the arc from the taper portions 7g, 8g to the auxiliary electrodes 7b, 8b is effected smoothly through the highly conductive members 7h, 8h. Though in the above embodiment the highly conductive members 7h, 8h are provided on only the back surfaces of the main electrodes 7a, 8a, the highly conductive members may be provided over the back surfaces of both the main electrodes 7a, 8a and the auxiliary electrodes 7b, 8b, as shown in Fig. 5, in which case the performance in breaking a large current is further enhanced. Namely, in the embodiment shown in Fig. 4, Da < Dh < Db, whereas in the embodiment shown in Fig. 5, Da < Dh = Db. The higher the diameter ratio Dh/Da, the easier the movement of the arc from the current path L₁ to the current path L₂. The maximum value of the diameter ratio Dh/Da is limited to Db/Da, for the following reason. The material of the auxiliary electrodes 7b, 8b comprising the Cu-Cr alloy as mentioned above is superior, in shortcircuit break performance and withstand voltage performance, to the material of the highly conductive members 7h, 8h comprising Cu, so that outer peripheral portions of the opposed surfaces of the auxiliary electrodes 7b, 8b should be formed of the material of the auxiliary electrodes 7b, 8b. Fig. 6 is a characteristic diagram showing the shortcircuit cutoff performance ratio and withstand voltage performance ratio, for the case where the materials of the above-mentioned compositions are used for the electrodes. In the diagram, the shortcircuit break performance ratio is indicated by taking the value at Dh/Da = 1 as 100%, and the withstand voltage performance ratio is indicated by taking the value at Dh/Da = Db/Da as 100%. It is clearly seen from the diagram that the effect of this invention is displayed when the ratio Dh/Da is in the range 1 ≦ Dh/Da ≦ Db/Da. The compositions of the materials for the three portions, i.e., main electrode 7a, 8a, auxiliary electrode 7b, 8b and highly conductive member 7b, 8b, are not limited to the above-mentioned. The materials for the three portions may be, for instance, CuCrCr₂O₃, CuCr and Cu, respectively, or AgWC, CuCr and Cu, respectively, or CuC, CuCr and Cu, respectively. With such combinations of materials, the same effect as in the above embodiment is obtainable. Moreover, the ratio of the conductivities σ a , σ b and σ h of the above-mentioned three portions is not limited to the above-mentioned numerical value, insofar as the conductivities σ a , σ b and σ h satisfy the relationship σ a < σ b < σ h .

Claims: A vacuum switch tube comprising: a stationary electrode (7) provided in a vacuum vessel (1), said stationary electrode (7) comprising a main electrode (7a) and an auxiliary electrode (7b) disposed at the periphery of the main electrode (7a), and a movable electrode (8) which is provided in the vacuum vessel (1), comprises a main electrode (8a) and an auxiliary electrode (8b) disposed at the periphery of the main electrode (8a), is opposed to said stationary electrode (7), is capable of being moved toward and away from said stationary electrode (7), and is separated from and brought into contact with said stationary electrode (7) to thereby switch a current off and on, wherein, in each one of said stationary electrode (7) and said movable electrode (8), a highly conductive member (7h, 8h) is additionally provided on a back surface of the each one electrode opposite to a surface of the each one electrode facing the other electrode, at least on the back surface of the main electrode (7a, 8a), and the electric conductivities σ a , σ b , and σ h of the main electrode (7a, 8a), the auxiliary electrode (7b, 8b) and the highly conductive member (7h, 8h) are in the relationship of the following inequality: σ a < σ h , σ b < σ h ; characterized in that the electric conductivities σ a and σ b of the main electrode (7a, 8a), and the auxiliary electrode (7b, 8b) are in the relationship of the following inequality: σ a < σ b . The vacuum switch tube as set forth in claim 1, wherein the highly conductive member (7h, 8h) in each one of the stationary electrode (7) and the movable electrode (8) is provided over the back surfaces of the main (7a, 8a) and auxiliary electrodes (7b, 8b) of the each one electrode, and an outer peripheral portion of the surface of the each one electrode (7, 8) opposed to the other electrode is formed of the same material as the auxiliary electrode (7b, 8b) of the each one electrode (7, 8). The vacuum switch tube as set forth in claim 1 or 2, wherein, of each one of the main electrodes (7a, 8a), the surface opposed to the other main electrode (8a) has a central portion formed as a flat portion (7f, 8f), and the periphery of the flat portion (7f, 8f) is formed as a taper portion (7g, 8g) slanted away from the other main electrode. The vacuum switch tube as set forth in claim 1 or 2, wherein the main electrode (7a, 8a) and highly conductive member (7h, 8h) of each one of the stationary (7) and movable electrodes (8) are jointed to each other in a body by compression molding of a mixed powder of a main electrode material containing at least 10% of a low melting point metal, such as Bi and Te, onto a copper base used as a material for the high conductive member (7h, 8h). The vacuum switch tube as set forth in claim 1 or 2, wherein the auxiliary electrode (7b, 8b) and highly conductive member (7h, 8h) of each one of the stationary (7) and movable (8) electrodes are jointed to each other in a body by compression molding of an auxiliary electrode material containing at least 10% of a low melting point metal, such as Bi and Te, onto a copper base used as a material for the highly conductive member. The vacuum switch tube as set forth in claim 1 or 2, wherein materials for the main electrode (7a, 8a), auxiliary electrode (7b, 8b) and highly conductive member (7h, 8h) are respectively Cu-20Cr-Bi, Cu-(10-60)Cr and 99.9Cu, or CuCrCr₂O₃, CuCr and Cu, or AgWC, CuCr and Cu, or CuC, CuCr and Cu.

Inventors: INAGAKI KOICHI C O MITSUBISHI, INAGAKI, KOICHI C/O MITSUBISHI DENKI K.K.
Applicants: MITSUBISHI ELECTRIC CORP, MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1989
Main classification: H01H33
IPC codes: H01H33