US3596025A

US3596025A - Vacuum-type circuit interrupter with contacts containing a refractory metal

Info

Publication number: US3596025A
Application number: US763116A
Authority: US
Inventors: Joseph L Talento
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1968-09-27
Filing date: 1968-09-27
Publication date: 1971-07-27
Anticipated expiration: 1988-07-27
Also published as: DE1948451A1; GB1213759A; FR2019077A1

Abstract

Discloses a vacuum-type circuit interrupter having its contacts formed of a porous refractory metal matrix and an alloy of copper and bismuth filling the pores of the matrix, the percentage of bismuth being less than 2 percent by weight of the copper-bismuth alloy.

Description

United States Patent Inventor Joseph L. Tllento Media, Pl.

Sept. 27, 1968 July 27, 1971 General Electric Company Appl: No. Filed Patented Assignee VACUUM-TYPE cuzcurr mrEuuP'rnn wrru couuc'rs counmmc A REFRACTORY new.

5 Claims, 1 Drawing Flg.

US. Cl. 200/144 8, 200/166 C Int. Cl. 11011! 33/66 Field 01 Search 200/ 144.2;

[56] References Cited UNITED STATES PATENTS 2,975,255 3/1961 Lafferty ZOO/144.2 3,246,979 4/1966 Lafferty et a1. ZOO/144.2 3,305,324 2/1967 Krock et a1. 200/144 X 3,430,015 2/1969 Crouch et a1. 200/ 144.2

Primary Examiner- Robert S. Macon Attorneys-J. Wesley Haubner, William Freedman, Melvin M.

Goldenberg, Frank L. Neuhauser and Oscar B. Waddell ABSTRACT: Discloses a vacuum-type circuit interrupter having its contacts formed of a porous refractory metal matrix and an alloy of copper and bismuth filling the pores of the matrix, the percentage of bismuth being less than 2 percent by weight of the copper-bismuth alloy.

PATENTEUJULQTIST? 3,596,025

//v VIVTORJ JOSEPH L. TALE/v10,

ATTORNEY VACUUM-TYPE CIRCUIT INTERRUPTER WITH CONTACTS CONTAINING A REFRACTORY METAL This invention relates to contact structure for a vacuumtype circuit interrupter and, more particularly, relates to contact structure of this type which is mechanically strong and has a high resistance to arc erosion and to contact welding.

A type of contact material that has a high resistance to arc erosion is one which comprises a refractory metal matrix infiltrated with a high conductivity metal. The refractory metal of the matrix is typically tungsten or molybdenum or a comnound thereof and he high conductivity metal is typically copper or silver. If the contacts of a vacuum-type circuit interrupter are made of such a material, they have a tendency to weld together following a closing operation, particularly a closing operation that takes place under high current arcing conditions. One reason for this is that the surfaces of the contacts of a vacuum interrupter are kept exceptionally clean,

.and this is especially conducive to welding under the high pressures and high local temperatures present when closing during arcing.

Efforts have been made to reduce this welding tendency by increasing the percentage of the refractory metal present to very high values, e.g., 90 percent by weight, but this has been found to produce only minor improvements in weld resistance.

An object of my invention is to greatly improve the weld resistance of refractory metal matrix contacts in :a vacuum interrupter and to do this without significantly impairing the mechanical strength of the contacts and the brazability of the contacts to appropriate supports.

ln carrying out my invention in one form, I make the contacts of the vacuum interrupter of a refractory metal matrix and infiltrate the matrix with an alloy of copper and bismuth. The percentage of bismuth in the alloy is limited to less than 2 percent by weight of the alloy and is preferably less than 1 percent.

For a better understanding of the invention reference may be had to the following description taken in conjunction with the accompanying drawing, wherein the single FIGURE illustrates one embodiment of the present invention.

Referring now to the drawing, there is shown a vacuum-type circuit interrupter comprising a sealed envelope 11 evacuated to a pressure of l torr or lower. The envelope 1 1 comprises .a tubular casing 12 of insulating material and a pair of

metal end caps

13 and 14 suitably sealed to the opposite ends of casing 12.

Within the evacuated envelope 11 there are a pair of separable disc-

shaped contacts

17 and 18. Contact 17 is a stationary contact brazed to the lower end of a stationary conductive contact rod 170; and contact 18 is a movable contact brazed to the upper end of vertically movable conductive rod 180. The movable contact rod 18a projects freely through an opening in the lower end cap 14, and a flexible metal bellows 20 provides a suitable seal thereabout that allows vertical movement of the contact rod 18a without impairing the vacuum inside envelope 11.

All of the internal parts of the interrupter are substantially free of surface contaminants. hese clean surfaces are obtained by suitable conventional vacuum processing, which involves baking out the interrupter during its evacuation.

Closing of the interrupter is effected by driving the contact rod 18a in an upward direction to drive movable contact 18 into engagement with stationary contact 17. Opening is effected by returning the movable contact from its engaged position downwardly to its solid line position shown. When the contacts are separated during an opening operation, an arc is drawn therebetween. Assuming an alternating current, this are persists until a natural current zero, at which time it vanishes and is prevented from reigniting by the high dielectric strength of the vacuum in the envelope 12. A suitable metal shield 21 "of tubular form surrounds the contact to condense the metal vapors generated by the arc, thus assisting in the interrupting process.

The illustrated interrupter is intended for use in applications where it will be opened and closed frequently. To limit the arc erosion produced by such frequent operations, I include as a major component of the contact material a refractory metal such as tungsten, which because of its high melting and boiling points has less tendency to vaporize than nonrefractory metals. The tungsten is in the form of a sintered matrix of a porous character. The pores of the matrix are filled with the high-conductivity copper-base alloy soon to be described.

When the contacts are driven into engagement during closing, they may bounce apart slightly and draw an arc therebetween before being driven back into engagement. This arc tends to produce welding together of the contacts. The contacts of a vacuum interrupter have a greater tendency toward such welding than those of other type interrupters because their surfaces are exceptionally clean and free of weld-inhibiting films.

In certain contact materials, such as tungsten-copper, the refractory metal matrix will help resist such welding. To obtain as much weld resistance as possible of the refractory metal, it has been proposed to use very high percentages of refractory metal, e.g., percent by weight. But it has been found that the improvements in weld resistance resulting from such high percentages of tungsten are minor, and much greater improvements are sometimes necessary. For example, increasing the tungsten content from 75 to 90 percent in similar sets of contacts was found to reduce the force necessary to break the strongest welds from about 2,500 pounds to about 900 pounds; but this is not nearly enough of a reduction inasmuch as lam seeking maximum weld strength of less than pounds.

To obtain the desired increased weld resistance, 1 infiltrate the tungsten matrix with an alloy of copper and bismuth containingless than 2 percent bismuth by weight of the alloy. Preferably, I use less than 1 percent bismuth by weight, e.g., one-half percent. Typically, no welds at all formed with contacts of this tungsten-copper-bismuth material, and in the worst cases, only about 20 pounds was needed to break the weld. This is in marked contrast to the many hundreds of pounds needed with the tungsten-copper contacts, even those with very high tungsten content.

It has been found that my contacts of tungsten-copperbismuth containing less than 2 percent bismuth have a high mechanical strength and do not crack or fracture when subjected to the high impacts accompanying closing. Moreover, I have been able to braze these contacts to a copper support rod (17a or 18a) without significant impairment of the braze by the bismuth contained in the contact. Using a conventional brazing alloy of copper, silver, and indium, I have been able to obtain brazed joints between the contact and the contact rod that have a much greater tensile strength than any weld that might be formed between the contacts.

I am aware of certain disclosures in the prior art (US. Pat. Nos. 2,975,255-Lafferty and 3,305,324-Krock et al.) of tungsten-copper-bismuth contacts containing 5 to 20 percent bismuth by weight of the copper-bismuth alloy. But the main purpose of using these high percentages of bismuth is to reduce the tendency of the contacts to produce current chopping. The small percentages of bismuth that I use will have little or no effect on current chopping; but this is of no concern in the present circuit application, where tungstencopper, even without bismuth, is satisfactory from a current chopping viewpoint. Moreover, I have found that when such high percentages of bismuth are present in the copper, the resulting contact is mechanically quite weak and brittle and is susceptible to being cracked by the usual sharp impacts accompanying closing. For example, the typical impact strength of contacts of 75 percent tungsten 25 percent (Cu-Bi) containing one-half percent Bi by weight of the Cu-Bi alloy was found to be almost twice as high as the typical impact strength of corresponding W-Cu-Bi contacts containing 5 percent bismuth by weight of the Cu-Bi alloy. Also, if high percentages of bismuth are present, it has been found quite difficult to make a good brazed joint between each contact and its supporting rod. The brazed joints have been found quite weak, due probably to the embrittling effect of the bismuth from the adjacent contact. As pointed out above, I have overcome these problems by using less than 2 percent bismuth by weight of the copper-bismuth alloy.

My contacts are made by sintering refractory metal particles in hydrogen and then infiltrating the sintered mass with a substantially gas-free alloy of copper-bismuth in a hydrogen atmosphere at a temperature above the melting point of the copper (about 2,200 E). The resulting composite is then cooled and thereafter degassed by heating to a temperature slightly above the melting point of copper, about 2,000 F., in an argon atmosphere.

While I have shown and described a particular embodiment of my invention, it will be obvious to those skilled in the art that various changes and modifications may be made without departing from my invention in its broader aspects; and l, therefore, intend in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

l. A vacuum-type circuit interrupter comprising a highly evacuated envelope and a pair of contacts within said envelope which are relatively movable into and out of engagement with each other, each of said contacts comprising a porous matrix of a refractory metal and an alloy consisting essentially of copper and bismuth filling the pores of said matrix, the percentage of bismuth being less than 2 percent by weight of the copper-bismuth alloy, said refractory metal constituting the major percentage by weight of the contact material; a pair of contact-support members, one for each contact, within said envelope; and a brazed joint between each of said contacts and its associated support member, said brazed joint being in contact with the porous matrix of said contact; the brazed joint having a tensile strength substantially greater than any welds forming between the contacts of said interrupter during operation thereof, thereby rendering said brazed joint capable of acting as the sole means for attaching said contact to its support member.

2. The vacuum-type circuit interrupter of claim 1 in which the percentage of bismuth is less than 1 percent by weight of the copper-bismuth alloy.

3. The vacuum-type circuit interrupter of claim 1 in which said refractory metal is tungsten, molybdenum, or a compound thereof.

4. The vacuum-type circuit interrupter of claim 1 in which said refractory metal is tungsten.

5. The vacuum-type circuit interrupter of claim 2 in which said refractory metal is tungsten.

Claims

1. A vacuum-type circuit interrupter comprising a highly evacuated envelope and a pair of contacts within said envelope which are relatively movable into and out of engagement with each other, each of said contacts comprising a porous matrix of a refractory metal and an alloy consisting essentially of copper and bismuth filling the pores of said matrix, the percentage of bismuth being less than 2 percent by weight of the copper-bismuth alloy, said refractory metal constituting the major percentage by weight of the contact material; a pair of contact-support members, one for each contact, within said envelope; and a brazed joint between each of said contacts and its associated support member, said brazed joint being in contact with the porous matrix of said contact; the brazed joint having a tensile strength substantially greater than any welds forming between the contacts of said interrupter during operation thereof, thereby rendering said brazed joint capable of acting as the sole means for attaching said contact to its support member.