US3485978A

US3485978A - Vacuum switch

Info

Publication number: US3485978A
Application number: US590773A
Authority: US
Inventors: Duncan Holmes Grindell
Original assignee: Associated Electrical Industries Ltd
Current assignee: Associated Electrical Industries Ltd
Priority date: 1965-11-17
Filing date: 1966-10-31
Publication date: 1969-12-23
Anticipated expiration: 1986-12-23
Also published as: GB1142200A; FR1501306A

Description

Dec. 23, 1969 D. H. GRlNDr-:LL 3,485,978

VACUUM SWITCH Filed oct. s1, 196e United States Patent 3,485,978 VACUUM SWITCH Duncan Holmes Grindell, Rugby, England, assignor to Associated Electrical Industries Limited, London, Eugland, a British Company Filed Oct. 31, 1966, Ser. No. 590,773 Claims priority, application Great Britain, Nov. 17, 1965, 48,875/ 65 Int. Cl. H01h 9/30, 33/00, 3/ 00 U.S. Cl. 200-144 13 Claims ABSTRACT 0F THE DISCLOSURE The present invention relates to electrical vacuum switches adapted to interrupt alternating currents of at least some tens of amperes and below a few thousands of amperes.

A Vacuum switch comprises an evacuated envelope, two conductors extending within said envelope with their inner ends adjacent, and means for providing relative movement between said conductors so that said inner ends can abut and can be separated in order to interrupt a current flowing from one conductor into the other conductor.

It has been found desirable that the adjacent inner ends of the conductors should be formed with contact surfaces for the most part of metal of higher-arcing-voltage, for example molybdenum or tungsten, but formed with reservoirs containing a non-uid lower-arcing-voltage metal or alloy.

The physical contact between the contacts when they abut is formed between the portions of the contacting surfaces made from the higher-arcing-voltage material, and this material is sufficiently strong to maintain the required contact pressure between the contacts. When the contacts are separated an arc will be formed between the contacting surfaces and will continue to burn until the current is fully interrupted, which normally occurs at or close to the rst current zero in the applied alternating current waveform. The negative or cathode end of the arc tends to burn preferentially on the lower-arcing-voltage material, and will travel quickly to such material. This lower arcing-voltage material therefore tends to vaporise, and the arc current is carried by the ionised vapour. The higher-arcing-voltage material does not tend to wear away, and in fact becomes coated with an adherent layer or film of the lower-arCing-voltage material which provides further protection. The travel of the moving contact relative to the xed contact does not change, even after a large number of operations of the switch.

In known designs for contacts in vacuum switches each contacting surface is formed with a plurality of reservoir recesses of circular or similar shape and spaced apart over the contacting surfaces. In use either the arc ends must stay in the recesses in which they originate o-r the arc must move across the contacting surfaces, so that the ends of the arc will repeatedly move from one recess to another. Unless the surface between the recesses has a substantial deposit of the lower-arcing-voltage, the arc ends will remain in the original recesses.

An object of the present invention is the provision of Patented Dec. 23, 1969 ICC vacuum switches having improved contacts including improved lower-arcing-voltage material.

According to the present invention, a vacuum switch comprises an evacuated envelope; a rst conductor extending within the envelope and having an inner end; a second conductor extending within the envelope and having an inner end disposed adjacent the inner end of the first conductor; operating means by which relative movement between the two conductors can be produced to move their inner ends into and out of contact with one another; a contact face on the first conductor inner end; a iirst zone of the contact face, of higher-arcing-voltage material; and a second zone of the contact face, of lowerarcing-voltage material in the form of an alloy consisting essentially of from 70-95% by weight antimony with the remainder bismuth apart from incidental impurities and mere adulterants; the zone of higher-arcingvoltage material being level with or projecting slightly beyondthe zone of lower-arcing-voltage material to provide an effective contact surface.

For optimum results, the alloy should contain less than 50 parts per million of metallic impurities, and in the inished switch should be -substantially free from gases and gas forming impurities.

In a preferred alloy the antimony content is about The contact preferably is used in combination with another similar contact, and preferably the corresponding Zones on the two contacts are of equal size and extend coterminously.

The contact face of the contact may be formed with at least one annular groove containing lower-arcing-Voltage material.

The invention will now be described, by way of example, with reference to the accompanying drawing, in which:

FIGURE 1 is a sectional side elevation of a vacuum switch including a contact according to the invention and with a normal rating of 300 amperes at 3.3 kv. and with an overload rating of at least 2000 amperes at 3.3 kv.; and

FIGURE 2 is a plan view of the lower contact illustrated in FIGURE 1.

The vacuum switch comprises an envelope including a cylindrical body portion 1 of electrically insulating material, glass in this instance, and two metal end plates 2 and 3 suitably sealed to the body portion. Two

conductors

4 and 5 extend within the envelope from opposite ends -so that their inner ends, which are provided with contacts 6 and 7 respectively, are adjacent. Conductor 4 extends through end plate 2 in a gas tight manner. Conductor 5 extends freely through an aperture 8 in end plate 3 and is connected thereto by a bellows member 9 which provides a gas tight seal but permits longitudinal movement of conductor 5. A sleeve 10 carried by plate 3 and provided with a bush 11 provides a self-lubrication guide for the conductor 5. The envelope is adapted to be evacuated by any suitable method after assembly of all the components. The contacts 6 and 7 are encircled by a cylindrical shield 15 which is provided with four extern-al lugs 17 which are secured to the body portion 1 by inwardly nipped portions 19. A metallic mesh 20 on the inside of shield 15 enhances the adhesion of evaporated contact material.

Each of the contacts 6 and 7 has a body made from a higher-arcing-voltage material, for example molybdenum, and the contacting surface of each contact is formed with a central circular recess 21 land with a concentric annular groove 23, substantially filled with lower-arcingvoltage material.

The grooves in the contacting surfaces of the contacts 6 and 7 are aligned so that when the contacts .abut the k3 portions of the contacting surfaces made from higherarcing-voltage material are in contact, although a thin film of the reservoir alloy will appear in use. With this Varrangement the required contact pressure can be applied without damaging the contacting surfaces.

Each contact 6 and 7 is formed of molybdenum, has an overall diameter of one inch, the central recess 21 has a diameter of 1A inch, and the groove 23 has a width of /s inch and outer and inner diameters of 3A inch and 1/2 inch respectively. The lower-arcing-voltage material used in the recess 21 and the groove 23 is an alloy of 90 parts by weight of antimony with parts by weight of bismuth.

In order to assist in the provision of this filling, the molybdenum contacts are machined with an additional axially extending peripheral lip. The machined contacts are vacuum fired at 900 degrees centrigrade after degreasing and electrolysing. The contacts are then nickel plated, again vacuum fired brazed with a silver/copper eutectic material to the

conductors

4 and 5, which are of oxygen free copper, and the nickel is removed from the outer periphery of the contacts. The prepared alloy of 90 parts antimony and l0 parts bismuth by' weight is received in the form of broken bar, and further broken into random shaped pieces with a maximum dimension of about 1A inch. These are placed within the encircling additional axially extending peripheral lip mentioned above, and melted into the recess 21 and the groove 23 in an atmosphere of pure, dry hydrogen, the arrangement being such that the hydrogen can circulate about the pieces of the alloy and reduce any oxides whic-h may be present. This is found to produce fillings free from blowholes and leaves the contacts clean. The iilled contact is then machined to remove the peripheral lip and to expose the molybdenum in the regions surrounding recess 21 and groove Z3. A nickel plated interface remains between the regions of exposed molybdenum and both the recess region 21 and the groove region 23. The fillings of the central recess 21 and the groove 23 are then undercut or recessed by about 0.005 inch to ensure that during the period when the switch is closed the contacts will engage over their molybdenum surfaces, which are coated in use with a film of the iiller alloy. This avoids welding between the filler alloy surfaces.

With contacts of this construction when an arc is formed between the contacting surfaces each end of the arc will be located in a mass of lower-arcing-voltage material in one of the grooves. If the ends of the arc move over the contacting surfaces, or when one arc splits into two or more arcs, the ends of the arc or arcs can still be located on the mass of low-arcing-voltage material and need not pass over the portions of the contacting surfaces formed from higher-arcing-voltage material.

The switch described is able to meet the onerous working conditions inside the envelope, and under test conditions has been found to allow 2 106 interruptions of 300 amperes with 0.004 to 0.008 inches eroded from each contact per l06 interruptions. The molybdenum contact surfaces are hard wearing, have a high melting point, and require a high arcing potential. The novel alloy used as the ller material does not alloy in use with the molybdenum to produce deleterious alloys. Whereas antimony is found to have a large crystal grain size, which has the result that when the switch opens two antimony grains respectively on the two contacts can adhere and one pull the other out of its matrix, the novel alloy has a finer grain structure and is not subject to this defect. Since the novel alloy expands as it cools, and since it is put in place in a molten form, the alloy filler is tightly fixed in place in its recess or groove. The alloy has a low-arcingvoltage compared with the molybdenum, which causes the arc struck between the two molybdenum contacts to transfer very quickly to the alloy filler; since the arcing voltage is only about l0 volts at vthe currents involved, the heat dissipation in the switch is relatively low giving 4 consequently small contact damage; it has a relatively low tensile strength, so that welds which occur between parts respectively on the two contacts readily break; it has a high vapour pressure at the arc temperature but a sufficiently low vapour pressure and high meltingr point to allow bake-out at a temperature of between 350 to 400 degrees centrigrade in treating the assembled switch, so that no problem arises, during bake-out and in service, of serious vaporisation or melting of the filler material. It does not readily oxidise and it is stable. Its low thermal conductivity ensures eiiicient vaporisation and ensures that much of the heat generated in the arc will .remain in the aro and will not be dissipated in the mass of filler material and passed on to the molybdenum contacts. Its properties are such that it can supportalow current arc of 1 ampere or less so that the arc persists until the'current has fallen very nearly to zero on the alternating current waveform. Thus the arc is not extinguished prematurely so excessive voltages are not generated in inductive circuits. Since the alloy Iis in the form of a solid solution of two low arcing voltage metals, no intermetallic compounds appear in use to contaminate the working surfaces of the switch. After much use, during which some of the filler material vapourises, the surface of the iiller material remains uncontaminatedl since both the antimony and the bismuth evaporate, leaving no scum or crust.

What I claim is: 1. A vacuum switch comprising: (a) an evacuated envelope; (b) a iirst conductor extending within the envelope and having an inner end; 4

(c) a second conductor extending within the envelope and having an inner end disposed adjacent the inner end of the Iirst conductor;

(d) operating means by which relative movement between the two conductors can be produced to move their inner ends into and out of contact with one another;

(e) a contact face on the iirst conductor inner end;

(f) a first zone of the contact face, of higher-arcingvoltage material; and

(g) a second zone of the contact face, of lower-arcingvoltage material in the form of an alloy comprising consisting essentially of from 70 to 95% by weight antimony with the remainder bismuth apart from incidental impurities; the zone of hig'her-arcing-voltage material being level with or projecting slightly beyond the zone of lower-aroing-voltage material to provide an effective contact surface.

2. A vacuum switch according to claim 1, in which the lower-arcing-voltage material contains less than 50 parts per million of metallic impurities.

3. A vacuum switch accor-ding to claim 1, in which the antimony content is about 4. A vacuum switch according to claim 1, in which the higher-arcing-voltage material is molybdenum.

5. A vacuum switch according to claim 1, in which the higher-arcing-voltage material is tungsten.

6. A vacuum switch according to claim 1,.in which the contact is used in combination with another similar contact, and the corresponding zones on the two contacts are of equal size and extend coterminously.

7. A vacuum switch according to claim v1, in which the contact face is formed with at least one annular groove containing lower-arcing-voltage material. y

8. A vacuum switch according to claim 7, in which:

(a) the contact face is circular with an overall diameter of about one inch; y (b) the higher-arcing-voltage material is molybdenum; (c) the annular groove iscircular with an louter diameter of about 3A inch and an inner diameter of about 1/2 inch; and

(d) the lower-arcing-voltage material has an antimony content of about 90%.

9. A vacuum switch according to claim 8, in which the contact face is also formed lwith a central circular recess with a diameter of about 1A inch containing lower-arcingvoltage material having an antimony content of about 90%.

10. A vacuum switch according to claim 1, in which the rst and second zones of the contact face are provided by respective regions of the rst conductor inner end and these regions have a nickel-plated interface between them.

11. A vacuum switch according to claim 1, in which the evacuated envelope includes an end plate having an aperture through which one of the conductors freely extends and a bellows member connecting the end plate to, and permitting said relative movement of, that conductor, and in which a sleeve carried by the end plate has a bush providing a self-lubrication guide for that conductor.

12. A vacuum switch according to claim 1, in which the evacuated envelope includes a cylindrical body portion of glass and the contacts are encircled by a cylindrical shield having external lugs which are located in the body portion by inwardly nipped dimple portions of the glass.

13. A vacuum switch according to claim 1, in which the 4contacts are encircled by a cylindrical shield having on its inside a metallic mesh for enhancing the adhesion of evaporated contact material to the shield.

References Cited UNITED STATES PATENTS 2,900,476 8/1959 Reece. 3,014,107 12/1961 Cobine et al. 1,720,756 7/1929 Behrens 75-149 ROBERT S. MACON, Primary Examiner U.S. Cl. X.R.