US3430015A - Vacuum-type circuit interrupter having brazed joints protected from weld-inhibiting constitutent in contact structure - Google Patents

Description

Feb. 25, 1969 0'. w. CROUCH ET AL I 3,430,015

VACUUM-TYPE CIRCUIT INTERRUPTER HAVING .BRAZED JOINTS PROTECTED FROM WELD-INHIBITING CONSTITUENT IN CONTACT STRUCTURE Filed March 28. 1966' INVENTORS. DONALD W. CROUCH ROBERT CROWELL, BY Mm W ATTORNEY United States Patent 6 Claims ABSTRACT OF THE DISCLOSURE A vacuum-type circuit interrupter comprising a brazed joint in its envelope located immediately adjacent one of the contacts of the interrupter and remote from the other contact. A weld-inhibiting constituent for preventing contact welding is incorporated in the metal of the contacts but is confined to the contact remote from said brazed joint. The contact immediately adjacent the brazed joint is essentially free of the weld-inhibiting constituent.

This invention relates to a vacuum-type electric circuit interrupter that comprises relatively movable contacts and, more particularly, relates to an interrupter of this type in which objectionable contact-welding is prevented by the presence of a weld-inhibiting constituent in the metal of the contacts.

A vacuum-type circuit interrupter must have contacts that are extremely clean and devoid of oxide and other contaminating films on their surfaces. This cleanliness is required in order to impart high dielectric strength and current-interrupting capacity to the interrupter, but unfortunately the clean surface conditions are conducive to the formation of strong welds between the contacts. For preventing the formation of such welds, it is proposed in application Ser. No. 186,127 Lafierty et al., filed June 3, 1963, now Patent No. 3,246,979, and assigned to the assignee of the present invention, that a Weld-inhibiting metal be incorporated as a minor constituent in the contact metaL. This weldrinhibiting minor constituent has a low solid-state solubility in the major constituent and acts to ernbrittle any welds that are formed between the contacts so that they can be easily broken with relatively small contact-separating force. The weld-inhibiting constituent also has a lower effective freezing temperature than that of the major constituent. An example of a metal suitable for use as a weld-inhibiting minor constituent is bismuth when the major con stituent is copper or silver. Another example is lead when the major constituent is copper or silver. Other examples are set forth in the aforementioned Laiferty et al. application.

A problem that has been encountered in the manufacture of interrupters having their contacts made of such alloys is that it has been difficult to make strong vacuum-tight brazed joints in the surrounding envelope. The reason for this is not fully understood, but it is believed that traces of the minor constituent of the contact metal have found their way into these joints during their formation and have impaired the soundness of the joints. Even joints that are spaced from the contacts and out of direct contact with the contacts have been eifected.

An object of our invention is to prevent the brazed 3,430,015 Patented Feb. 25, 1969 joints of the interrupter from being impaired by the presence of a weld-inhibiting minor constituent in the contact metal, while still retaining a high resistance to contact-welding.

In carrying out the invention in one form, we include the weld-inhibiting minor constituent in only one of the two contacts. The other contact is made of a metal that 's essentially free of the weld-inhibiting constituent prior to interrupter operation. The interrupter includes brazed vacuum-tight joints, but none of these joints is located close to the contact containing the weld-inhibiting constituent. Any brazed vacuum-tight joint that is located near a contact is located near the contact that is free of the weld-inhibiting constituent. This joint, as well as the other brazed vacuum-tight joints, is remote from the contact containing the weld-inhibiting constituent.

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 is a side-elevational view partly in section showing a vacuum interrupter embodying one form of the invention.

Referring now to the drawing, the illustrated vacuumtype circuit interrupter comprises a sealed envelope 11 that is evacuated to a pressure of 10- millimeters of mercury or lower. This envelope 11 comprises a tubular ceramic housing 12 and a pair of metal end caps 13 and 14 joined to the housing 12 in vacuum-tight relationship at its respective opposite ends. Suitable ceramic-tometal joints 15 are used for joining the end caps 13 and 14 to the ceramic housing 12.

Projecting into the envelope through its end caps 13 and 14 are a pair of relatively movable electrodes and 21. The upper electrode 20 is a stationary electrode that is joined to the upper end cap 13 by means of a vacuum-tight brazed joint 23, which will soon be referred to in greater detail. The lower electrode 21 is a movable electrode that projects freely through a central opening 24 in the lower end cap 14.

For providing a seal about the lower electrode that allows it to be moved vertically without impairing the vacuum inside the envelope 11, a flexible tubular metal bellows 25 surrounding the electrode 21 is provided. This bellows 25 has its lower end joined to the lower end cap 14 by a suitable vacuum-tight seal 26 and its upper end joined to the movable electrode 21 by a suitable vacuum tight brazed joint 28.

The upper electrode 20 is constituted by a circuitmaking portion, or contact, 30 and a contact-supporting rod 32 carrying the contact 30 at its lower end. The contact 30 is suitably joined to the rod 32 by a joint 34. This joint 34 is formed by providing the contact 30 with a plug portion 35 fitting into a socket 36 in the lower end of rod 32. The walls of the socket are crimped to form dimples 37 tightly fitting into an annular groove in the plug 35.

The lower electrode 21 is of essentially the same construction and comprises a circuit-making portion, or contact, 40 and a contact-supporting rod 42 carrying the contact 40 at its upper end. The contact 40 and its supporting rod 42 are shown joined together in the same manner as contact 30 and its supporting rod 32.

When the interrupter is to be closed, the lower electrode 21 is driven in an upward direction to move its contact 40 into engagement with the contact 30 of the upper electrode. The dotted line 40a indicates the position of the lower contact 40 when it engages the upper contact. This engagement permits current to flow through the interrupter via the electrodes 20 and 21 through the contacts 30 and 40. Assuming that the contacts are engaged, when an interrupting operation is to take place, the lower electrode 21 is driven downwardly to separate lower contact 40 from upper contact 30. This establishes an are between the two contacts. Assuming an alternating current circuit, the arc persists until about the time of the first natural current zero, at which time the arc vanishes and the vacuum recovers its dielectric strength, thereby preventing the are from reestablishing.

For condensing the metal vapors generated by the arc and for preventing these vapors from condensing on the internal surfaces of insulator 12, a tubular metal shield 50 is provided. This shield 50 surrounds the contacts 30 and 40 and is spaced therefrom. It is shown supported from the lower end cap 14. A cup-shaped auxiliary shield 52 surrounds the upper end of the shield 50 to provide additional means for condensing any vapors generated by the arc and traveling toward the insulating housing 12. This cup-shaped shield 52 has a central opening surrounding the upper electrode 20 and is joined thereto by means of the brazed vacuum-tight joint 23.

An additional shield is shown at 54. This shield is an annular metal disc which protects the bellows 25 from damage by the hot arcing products. This shield is joined to the electrode 21 at the brazed joint 28.

As pointed out hereinabove, the contacts 30 and 40 have clean surfaces that are essentially free of oxide and other contaminating films. When the contacts are driven together, there is a tendency for a weld to form between the surfaces in view of their cleanliness, the high pressures involved, and the arcing which occasionally occurs on closing.

For preventing a strong weld from occurring, it has been proposed in the aforementioned Lafferty et al. application that a weld-inhibiting metal be incorporated as a minor constituent in the contact metal. This weldinhibiting minor constituent has a low solid state solubility in the major constituent and acts to drastically weaken through embrittlement any Welds that are formed between the contacts so that they can be easily and cleanly broken with a relatively small contact-separating force. Examples of metals suitable for use as weld-inhibiting minor constituents are bismuth and lead when the major constituent is either copper or silver. In other words, alloys such as copper-bismuth, copper-lead, silverbismuth, and silver-lead can be used for the contacts 30 and 40 to prevent objectionable contact-welding. The minor constituent is listed second in each of these alloys.

A problem that has been encountered in the manufacture of interrupters having their contacts made of such alloys is that it has been diflicult to make strong vacuumtight brazed joints in the surrounding envelope. The reason for this is not fully understood, but it is believed that traces of the minor constituent of the contact metal have found their way into these joints during their forma tion and have rendered them slightly porous.

In this respect, the joints 23 and 28 are formed by placing shims or rings of brazing material between or near the parts to be joined and by heating the entire interrupter to a temperature in excess of the melting point of the brazing alloy. A typical brazing alloy is a coppersilver eutectic, which has a melting point of 780 C. To form a brazed joint with such an alloy, the temperature of the interrupter is raised to slightly over 800 0., thereby melting the brazing alloy and causing it to flow into all openings in the joint. Thereafter, the temperature is reduced to solidify the molten metal and form the vacuum-tight seal.

It is believed that during the period when the temperature is high, traces of the weld-inhibiting constituent in the contact metal find their way into the molten brazed metal. Even though weld inhibitors such as bismuth and lead have very little solid-state solubility in the copper or silver of the brazed metal, there is appreciable liquidstate solubility, and it is believed that some of the bismuth or lead dissolves in the molten brazed metal. When the brazed metal solidifies upon cooling, the bismuth or lead comes out of solution and enters the grain boundaries of the brazed metal, and for some reason, not fully understood, produces pores in the brazed metal. This joint-impairment has been observed even when the joint is located at a point spaced from the contact.

We have been able to overcome this problem and still have a high resistance to contact-welding by relying upon several cooperating features. One of these cooperating features is that we incorporate the weld-inhibiting minor constituent in only the contact 30. The other contact 40 is maintained essentially free of the weld-inhibiting minor constituent, at least prior to an interrupting operation. More specifically, in one embodiment of the invention, we make the contact 30 of a copper-bismuth alloy and the contact 40 of pure copper. Preferably, the bismuth content of the copper-bismuth alloy is held below a few percent by weight. A second one of these cooperating features is that the brazed joints (23 and 28) are located remote from the particular contact (30) that contains the weld-inhibiting constituent. In order to provide a compact interrupter, it is necessary to locate one of these two brazed joints (23 and 28) close to one of the contacts, but we are able to do this and still have a sound joint because we locate this joint (28) close to the particular contact (40) that is free of weld-inhibiting metal. Joint 28 is remote from the other contact 30, which contains the weld-inhibiting constituent, and this remoteness protects the joint from the weld-inhibiting constituent in contact 30. In an interrupter that uses about 0.5 percent bismuth as a weld-inhibitor for predominantly copper contacts, we have found that excellent vacuum-tight brazed joints can be formed at 28 and 23 if the bismuth is incorporated only in the material of contact 30.

Even though the antiweld constituent is provided in only one of the two contacts, it is still capable of effectively preventing a strong weld from forming between the contacts when the contacts are engaged.

If the percentage of the weld-inhibiting constituent is high and the volume of the envelope 11 is very low, even though the weld-inhibiting constituent is confined solely to contact 30, we have found that good vacuum-tight joints are difficult to consistently obtain. For example, in an interrupter of the design illustrated having a volume of about 5 cubic inches, when copper-bismuth is used for contact 30 and pure copper for contact 40, we have found that when greater than about 5% bismuth is included in contact 30, good joints at 28 and 23 are difficult to consistently obtain with a copper-silver eutectic brazing alloy. For larger volume interrupters, however, higher percentages of bismuth can be tolerated.

The brazed joints 28 and .23 can be of any suitable conventional form. In the illustrated interrupter, they are of the form disclosed and claimed in application Ser. No. 342,757, Crouch, filed Jan. 31, 1964, now Patent No. 3,290,772, and assigned to the assignee of the present invention. Thus, the upper joint 23 comprises a pair of spaced-apart positioning rings 60 surroundings the electrode 20 and disposed at opposite sides of the plate structure 13, 52. Before the joint is formed, shims or rings of brazed metal are disposed adjacent or between the parts of the joint. Then the entire interrupter, including the joint 23, is heated to melt the braze metal and cause it to fill all openings in the joint. Upon subsequent cooling, the braze metal solidifies, forming a bond with the adjacent parts and a vacuum-tight seal between the parts. The lower joint 28 is made in the same manner during the aforementioned heating operation.

The aforementioned heating operation is also used as a bake-out for removing contaminants from the surfaces of the interrupter parts. The envelope 11 is suitably evacuated during or immediately following this heating operation.

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

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

1. A vacuum-type circuit interrupter comprising:

(a) a highly evacuated envelope,

(b) a pair of relatively movable electrodes projecting into said envelope and, respectively, comprising contacts located within the envelope for engaging each other,

(c) a first brazed joint forming a vacuum-tight seal between one of said electrodes and said envelope,

(d) a second brazed joint forming a vacuum-tight seal between the other of said electrodes and said envelope,

(e) said first brazed joint being located immediately adjacent the contact of said first electrode but remote from the contact of said second electrode,

(f) said second brazed joint being located remote from both of said contacts,

(g) the contact of said second electrode being formed of an alloy containing a small percentage of a weldinhibiting metal constituent that em-brittles any welds formed between said contacts when engaged,

(h) the contact of said first electrode being formed of a material that is essentially free of said weldinhi biting contstituent prior to operation of said interrupter,

(i) said first brazed joint including brazing metal in which said weld-inhibiting metal has appreciable liquid-state solubility but is substantially insoluble in the solid state.

4. The vacuum-type circuit interrupter of claim 1 in which:

(a) said alloy is selected from a group consisting essentially of copper-bismuth, copper-lead, silverbismuth and silver-lead, and

(b) the material of the contact of said first electrode is selected from a group consisting essentially of copper and silver.

5. The vacuum-type circuit interrupter of claim 1 in which the contact of said second electrode is formed of copper-bismuth and the contact of said first electrode is formed of pure copper.

6. The vacuum-type circuit interrupter of claim 5 in which the bismuth content of the copper-bismuth alloy is less than five percent by weight of the alloy.

References Cited UNITED STATES PATENTS 2,975,256 3/1961 Lee et al.

2,979,587 4/ 1961 Jennings.

3,082,307 3/1963 Greenwood et al. 200144 3,185,800 5/1965 Titus 200-444 3,246,979 4/1966 Latferty et a1. 200--144 XR 3,280,286 10/1966 Ranhei-m 200-144 3,321,599 5/1967 Lee 200144 ROBERT S. MACON, Primary Examiner.

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