US3497652A

US3497652A - Vacuum-type circuit interrupter with contact material containing a minor percentage of aluminum

Info

Publication number: US3497652A
Application number: US773980A
Authority: US
Inventors: Fordyce H Horn
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1968-11-01
Filing date: 1968-11-01
Publication date: 1970-02-24
Anticipated expiration: 1987-02-24
Also published as: DE1954589A1; FR2022398A1; DE1954589B2; DE1954589C3; GB1270719A; JPS4747946B1

Description

Feb. 24, 1970 R ET AL 3,497,652

VACUUM-TYPE CIRCUIT INTERRUPTER WITH CONTACT MATERIAL CONTAINING:A MINOR v PERCENTAGE OF-"ALUMINUM Filed Nov. 1, 1968 United States Patent US. Cl. 200-144 8 Claims ABSTRACT OF THE DISCLOSURE A vacuum-type circuit interrupter comprising a pair of contacts relatively movable into and out of engagement, the contacts having circuit-making and breaking regions formed of an alloy consisting essentially of copper-aluminum and bismuth, the aluminum being present in a quantity of between 9% and 15% by weight of the copperaluminum and the bismuth being present in a quantity of a few percent or less by weight of the total alloy.

BACKGROUND OF THE INVENTION This invention relates to a vacuum-type circuit interrupter and, more particularly, to contact structure for such an interrupter.

In US. Patent 3,246,979, Laiferty et al., assigned to the assignee of the present invention, it is proposed that the contacts of a vacuum interrupter be formed of an alloy consisting essentially of a major constituent which which is a good-conductivity, nonrefractory metal and a minor constituent which is a metal having a lower freezing temperature than the major constituent and little or no solid-state solubility in the major constituent, the minor constituent being present in a quantity of a few percent or less by weight of the alloy. Examples of such alloys are copper-bismuth, copper-lead, copper-tellurium, silverbismuth, silver-lead, and silver-tellurium, each alloy containing a few percent or less by weight of the second mentioned, or minor, constituent.

Vacuum interrupters having contacts of these alloys can interrupt high inductive currents (e.g., in excess of 8,000 amperes symmetrical R.M.S.) at rated voltage, can carry and close against such currents without producing objectionable contact-welds, and can successfully withstand impulse crest voltages of at least 95 kv. and continuous 60 cycle voltages of at least 36 kv. R.M.S. when their contacts are fully separated. These voltages are those which indoor oilless circuit breakers rated at 7.2 kv. and 13.8 kv. must be capable of withstanding if they are to meet the requirements of the National Electrical Manufactures Association (NEMA) Standards for Power Circuit Breakers, Publication 364-1954, March 1954, revised November 1955, part 2, page 5.

For higher voltage ratings and for certain switching operations, such as capacitance switching, even more diflicult dielectric strength requirements are imposed upon the interrupter. One severe measure of an interrupters ability to meet these more difiicult dielectric strength requirements is its ability to withstand a high transient voltage immediately following a contact-separating operation which fractures a weld between the contacts, especially the type of weld formed by closing on several thousand amperes or more current.

Although entirely satisfactory for many circuit applications, vacuum interrupters having their contact-making and breaking regions consisting of copper-bismuth, copper-lead or the other materials disclosed in the aforesaid Lal'ferty et al. patent have not been able to meet this latter test as well as might be desired. It appears that fracture of the weld between the contacts leaves surface irregularities that detract from the ability of the intercontact gap to withstand the high transient voltage.

SUMMARY An object of the present invention is to provide a vacuum interrupter that has improved ability to withstand high voltages immediately following a contact-separating operation that fractures a weld between the contacts.

Another object is to consistently achieve a high dielectric strength between the contacts immediately following contact-separation which follows closing on high inrush currents.

Another object is to provide a vacuum interrupter capable of performing as set forth in the immediately preceding paragraphs and also capable (1) of interrupting high inductive currents of 8,000 or more amperes R.M.S. symmetrical, (2) of being free from objectionable contact-welding, and (3) of meeting the high dielectric strength requirements imposed by conventional standards at rated circuit voltages of 14.4 kv. and higher. (The above NEMA Standards require that an oilless circuit breaker rated at 14.4 kv. be capable of withstanding kv, peak impulse voltage and 50 kv. R.M.S. continuous voltage.)

Another object is to provide vacuum interrupter contacts capable of performing as set forth in the above objects and also capable of being more easily and less expensively made than the beryllium-containing contacts disclosed in our pending application S.N. 647,646. The latter application was filed on June 21, 1967, and is assigned to the assignee of the present invention.

In carrying out our invention in one form we provide the vacuum interrupter with contacts having their circuitmaking and breaking regions made of an alloy of copperaluminum and bismuth. The aluminum is present in a quantity of between 9% and 15 by weight of the copper-aluminum, and the bismuth is present in a quantity of a few percent or less by weight of the total alloy. In a preferred form of the invention, the aluminum is present in a quantity of between 11 and 13 percent by weight of the copper-aluminum.

BRIEF DESCRIPTION OF DRAWINGS For a better understanding of our invention, reference may be had to the following description taken in conjunction with the accompanying drawings, wherein:

FIG. 1 is a sectional view of a vacuum-type circuit interrupter embodying one form of our invention.

FIG. 2 is an enlarged perspective view of one of the contacts of the interrupter of FIG. 1.

DESCRIPTION OF PREFERRED EMBODIMENT Referring now to the interrupter to FIG. 1, there is shown a highly evacuated envelope 10 comprising a casing 11 of a suitable insulating material, such as glass, and a pair of metallic end caps 12 and 13, closing off the ends of the casing. Suitable seals 14 are provided be tween the end caps and the casing to render the envelope 10 vacuum-tight. The normal pressure within the envelope 10 under static conditions is lower than 10* mm. of

" mercury so that a reasonable assurance is had that the mean free path for electrons will be longer than the potential breakdown paths in the envelope.

The internal insulating surfaces of casing 11 are protected from the condensation of arc-generated metal vapors thereon by means of a tubular metallic shield 15 suitably supported on the casing 11 and preferably isolated from both end caps 12 and 13. This shield acts in a well-known manner to intercept arc-generated metallic vapors before they can reach the casing 11.

Located within the envelope is a pair of separable contacts 17 and 18, shown in their engaged or closedcircuit position. The upper contact 17 is a stationary contact suitably attached to a conductive rod 17a, which at its upper end is united to the upper end cap 12. The lower contact 18 is a movable contact joined to a conductive operating rod 18a which is suitably mounted for vertical movement. Downward motion of the contact 18 separates the contacts and opens the interrupter, whereas return movement of contact 18 reengages the contacts and thus closes the interrupter. A typical gap length when the con tacts are fully-open is about /2 inch. The operating rod 181: projects through an opening in the lower end cap 13, and a flexible metallic bellows 20 provides a seal about the rod 18a to allow for vertical movement of the rod without impairing the vacuum inside the envelope 10. As shown in FIG. 1, the bellows 20 is secured in sealed relationship at its respective opposite ends to the operating rod 18a and the lower end cap 13.

All of the internal parts of the interrupter are substantially free of surface contaminants. These clean surfaces are obtained by suitably processing the interrupter, as by baking it out during its evacuation. A typical bakeout temperature is 400 C. In addition, the contacts 17 and 18 are effectively freed of gases absorbed internally of the contact body so as to preclude evolution of these gases during high current arcing. The manner in which these internal gases are removed will be referred to in more detail hereinafter.

Although our invention is not limited to any particular contact configuration, we prefer to use the contact configuration disclosed and claimed in US. Patent 2,949,520, Schneider, assigned to the assignee of the present invention. Accordingly, each contact is of a disk shape and has one of its major surfaces facing the other contact. The central region of each contact is formed with a recess 29 in this major surface and an annular circuitmaking and circuit-breaking area 30 surrounds this recess. These annular circuit-making and breaking areas 30 abut against each other when the contacts are in their closed position of FIG. 1, and are of such a diameter that the current flowing through the closed contacts follows a loop-shaped path L, as is indicated by the dotted lines of FIG. 1. Current flowing through this loop-shaped path has a magnetic effect which acts in a known manner to lengthen the loop. As a result, when the contacts are separated to form an are between the areas 30, the magnetic effect of the current flowing through the path L will impel the arc radially outward.

As the arc terminals move toward the outer periphery of the disks 17 and 18, the arc is subjected to a circumferentially-acting magnetic force that tends to cause the arc to move circumferentially about the central axes of the disks. This circumferentially-acting magnetic force is preferably produced by a series of slots 32 provided in the disks and extending from the outer periphery of the disks radially inward by generally spiral paths, as is shown in FIG. 2. These slots 32 correspond to similarly designated slots in the aforementioned Schneider patent and thus, force the current flowing to or from an arc terminal located at substantially any angular point on the outer peripheral region of the disk to follow a path that has a net component extending generally tangentially with respect to the periphery in the vicinity of the arc. This tangential configuration of the current path results in the development of a net tangential force component, which tends to drive the arc in a circumferential direction about the contacts. In certain cases, the arc may divide into a series of parallel arcs, and these parallel arcs move rapidly about the contact surface in a manner similar to that described hereinabove.

One of the problems that the present invention is concerned with is providing a single-break vacuum interrupter of the general type described up to this point that is capable of meeting the conventional NEMA specification for an oilless circuit breaker having a voltage ratlng of at least 14.4 kv. and an interrupting rating of at least 8000 amperes R.M.S. symmetrical and is also capable of consistently withstanding high transient voltages applied immediately following a contact-separating operation that fractures a weld between the contacts. In referring to high transient voltages, we are referring to those typically encountered in switching and interrupting circuits rated at 14.4 kv. and higher; and in referring to a weld between the contacts, we are particularly concerned with those welds such as might be formed when an arc carrying inrush currents of several thousand amperes is developed during contact-closing.

We have found that these requirements can be met by forming the circuit-making and circuit-breaking portions 30 of the vacuum interrupter contacts of an alloy consisting of copper-aluminum and bismuth, the aluminum being present in a quantity of between 9 and 15 percent by weight of the copper-aluminum and the bismuth being present in a quantity of a few percent or less by weight of the total alloy. A specific alloy which has shown exceptional ability to meet these requirements is a copperaluminum bismuth alloy consisting essentially of copper, aluminum in a quantity of 12 percent by weight of the copper-aluminum, and bismuth in a quantity of 1% by weight of the total alloy. This material is referred to hereinafter as Cu-Al-Bi (12% Al).

The material of the prior art found most satisfactory for the contacts of the vacuum interrupter has been an alloy of copper-bismuth consisting of copper and a few percent or less of bismuth by weight, e.g., 0.5 percent. A vacuum interrupter with contacts of this material can meet the conventional NEMA specification for oilless circuit breakers rated at 14.4 kv. and 8000 amperes R.M.S. interrupting current, but it has not been able to withstand as consistently as might be desired a high voltage transient applied following a contact-separating operation which fractures a weld between the contacts.

To provide a measure of this dielectric strength after weld fracture, we have conducted an extensive series of the following tests on vacuum interrupters with different contact materials, but otherwise of comparable design. First, the contacts of a given vacuum interrupter were driven together against currents sufficiently high to produce a weld between the contacts; and then the contacts were opened under no-current conditions to fracture the weld, and the open gap was stressed with a voltage of a wave form simulating a switching surge. This voltage had an available crest value high enough to break down the gap each time it was applied, and the instaneous voltage at the instant of breakdown was recorded. The available crest value was about 230 kv., and the rate of rise was such that this crest would be reached in microseconds in the absence of a breakdown. The current against which the contacts were closed Was the same in each test, being 3000 amperes peak current. These test results were plotted on probability paper, with the voltage stress required for breakdown being plotted against the. probability of breakdown. For given probabilities of breakdown, it was found that with contacts of Cu-Al-Bi (12% Al) the inter-contact gap could withstand a voltage stress approximately 300% of that withstandable by a gap between contacts of Cu- Bi (.5 Bi). For example, a voltage stress of kv./in. produced a 50% probability of breakdown with the Cu-Bi contacts, but about 315 kv./in. (or about 300% of the Cu-Bi voltage stress) was needed to produce the same probability of breakdown with the same size- Cu-Al-Bi (12% Al) contacts. It will therefore be apparent that the Cu-Al-Bi (12% Al) contacts exhibit greatly improved performance over CuBi (.5% Bi) from the standpoint of dielectric strength after weld fracture.

Metallographic studies have been made in an effort to determine the structural differences that are responsible for the improved performance of the copper-aluminumbismuth alloys containing 9 to 15% aluminum by weight of the copper-aluminum. When the amount of aluminum is reduced below about 9%, the grains of the alloy are quite large and a considerable portion of the aluminum is in solid solution with the copper. The bismuth forms a thin film around each of these grains and has a marked embrittling effect on the alloy. For higher quantities of aluminum, considerable amounts of aluminum are present with copper as the eutectoid composition (88 Cu-12 Al), which appears as a fine dispersion throughout the grains and the grain boundaries. The presence of this fine dispersion produces a better distribution of the bismuth inside the grains and greatly reduces bismuth segregation at the grain boundaries. The bismuth, while still being available as a weld-inhibiting agent, has much less of an embrittling effect on the parent alloy, and the result is improved mechanical strength and ductility. These improvements in mechanical strength and ductility are believed to contribute to improved dielectric strength because they reduce the possibility that discrete particles of metal will be pulled out of the opposite contact when the contacts are separated following such minor contact-welding as does occur. Moreover, even though the bismuth is not concentrated at the grain boundary in the parent alloy, it is still present, distributed throughout the grain structure and is available to segregate in the weld zone caused by arcing. This segregated bismuth weakens the weld by forming a weak interface along which the contacts can easily separate when subsequently opened. This further reduces the possibility that a discrete particle of metal will be pulled out of the opposite contact along a grain boundary. By reducing this possibility, we are able to maintain contact surfaces of greater smoothness with fewer protuberances of a size that would encourage a dielectric breakdown.

When the quanity of aluminum is increased beyond about 15 percent, the alloy contains a high percentage of the gamma phase and the bismuth is not dispersed in the desired manner, both resulting in excessive brittleness. Such brittleness can lead to cracking on closing impact, resulting in loose pieces that can adversely affect dielectric strength. We therefore limit the aluminum content to about 15 percent by Weight of the copper-aluminum.

Contacts of copper-aluminum-bismuth alloy have also shown an exceptional resistance to cold-welding, i.e., welding together under the influence of high pressure forcing the contacts together with no arcing between the contacts. For example, a series of tests have been made in which contacts of different materials have been forced together with 3000 pounds of force, and then separated to fracture any weld present between them. The force required to separate them is measured. With contacts of the plain copper-bismuth Cu-Bi (.5% Bi) referred to hereinabove, welds requiring approximately 80 lbs. of force for their fracture were developed. With contactsformed of Cu-Al-Bi (12% Al), no substantial welds were formed. With contacts formed of Cu-Al-Bi A1), again no substantial welds were formed. This freedom from substantial cold-welding is a significant advantage not only because it reduces the force necessary to separate the contacts but also because it reduces the likelihood that protuberances will be formed at the fractured weld which could impair the dielectric strength.

As pointed out hereinabove, another condition that can lead to contact-welding is that accompanying closing the circuit interrupter against heavy currents. When the contacts are driven into closed position, they often bounce apart a short distance immediately after initial impact and then rebound toward each other, aided by the closing force applied to the movable contact. An arc is drawn when the contacts first bounce apart, and this arc melts adjacent surface portions of the contacts so that when they reengage, a molten film is present at the interface. When arcing ceases following reengagement, the energy input into the contact interface drops sharply, and the film at the interface thus quickly cools to a solid state. The result is the formation of a weld between the two contacts. The higher the arcing current, the larger the surface area that will be covered by the molten film and hence the larger and stronger the weld ordinarily will be. The welds formed under these conditions will be referred to as hot welds.

For determining the relative strengths of welds that are formed under these conditions, clean contacts of various materials were driven together under high current arcing conditions, and the force required for their subsequent separation was measured. To prevent the formation of oxide or other films on the contacts, these tests were run in an inert atmosphere of argon, which provides ambient conditions with respect to oxidation closely simulating those present under high vacuum conditions. With contacts of plain copper, an opening force of 5000 pounds was typically required to fracture the weld and separate the contacts; with contacts of Cu-Bi .5 Bi), an opening force of 200 pounds was typically required; with Cu-Al-Bi (12% Al) an opening force of only 0 to 10 pounds was typically required.

This greatly improved resistance to the formation of both cold welds and hot welds is an unexpected property of the contacts of copper-aluminum-bismuth having an aluminum content in the vicinity of the preferred 12% by weight of the copper-aluminum.

Although we have described the invention specifically with respect to contacts of a copper-base alloy containing bismuth as a weld-inhibiting agent, the invention in its broader aspects is also considered to be applicable to copper-base alloy contacts which contain the other weldinhibiting agents of the aforesaid Lalferty et al. patent. For example, copper-base alloy contacts with lead or tellurium can be dielectrically improved by adding aluminum in an amount of 9 to 15 percent by weight of the copper-' aluminum alloy. In each of these materials, the weldinhibiting agent is substantially insoluble in the solid state in both copper and aluminum and has a lower freezing temperature than copper-aluminum. Similarly, silverbase alloy contacts with bismuth or lead weld-inhibitors can be improved dielectrically by including aluminum in approximately 5 to 10% by weight of the silver-aluminum alloy. Similarly, a nickel-base alloy with a bismuth weld-inhibitor can be improved dielectrically by including aluminum in approximately 8 to 13% by weight of the nickel-aluminum alloy. In referring to these alloys hereinafter, the first-mentioned metal constituent of an alloy is referred to as the primary metal of the alloy.

In referring to weld-inhibiting agents that are substantially insoluble in the other constituents in the solid state, we are referring to weld-inhibiting metals that have a solid-state solubility in the other constituents of less than about two percent by weight of the alloy considered at the eutectic temperature of said alloy or the freezing temperature of the minor constituent if there is no eutectic.

In preparing these contact materials, each separate constituent first should ,be suitably processed to free it of sorbed gases and other contaminants, as, for example, by the zone-refining process described in US. Patent No. 3,234,351, Hebb, assigned to the assignee of the present invention. The constituents are then melted and appropriately mixed together while they are in the liquid state, after which the temperature is lowered to cause the constituents to solidify and form the solid alloy.

In our aforesaid application S.N. 647,646, we disclose and claim a vacuum interrupter having contacts of copperberyllium-bismuth containing 1% bismuth by weight of the total alloy and 7% beryllium by weight of the copperberyllium. While a vacuum interrupter with such contacts can withstand even higher voltages after weld fracture than a corresponding interrupter having the contacts of the present application, the contacts of the present application have the advantage of being less expensive and much easier to make. In this connection, aluminum is less expensive than beryllium, requires no special precautions against toxicity, as are required with beryllium, and is much easier to purify and process in view of its lower melting point and reduce reactivity with crucible materials.

While we have shown and described particular embodiments 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.

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

1. A vacuum-type electric circuit interrupter (a) an envelope evacuated to a pressure of 10- mm.

of mercury or less,

(b) a pair of contacts within said envelope relatively movable into and out of engagement,

(c) said contacts being substantially free of absorbed gases and surface contaminants,

(d) at least one of said contacts having circuit-making and breaking regions formed of an alloy consisting essentially of copper-aluminum and bismuth,

(e) the aluminum being present in a quantity of between 9 and 15 percent by weight of the copperaluminum, and

'(f) the bismuth being present in a quantity of less than about percent by weight of the total alloy and in a suificient quantity to inhibit contact-welding, and being distributed throughout said alloy.

2. The vacuum-type circuit interrupter of claim 1 in which the aluminum is present in a quantity of between 11 and 13 percent by weight of the copper-aluminum.

3. The vacuum-type circuit interrupter of claim 1 in which the aluminum is present in a quantity of substantially 12 percent by weight of the copper-aluminum.

4. A vacuum type electric circuit interrupter compris- (a) an envelope evacuated to a pressure of mm.

of mercury or less,

(d) at least one of said contacts having circuit-making and breaking regions formed of an alloy consisting essentially of copper-aluminum and a weld-inhibiting metal having substantially no solid-state solubility in copper or aluminum and having an effective freezing temperature below that of copper-aluminum,

(f) the weld-inhibiting metal being present'in a quantity of less than about five percent by weight of the total alloy and in a quantity suflicient to inhibit contact-welding, and being distributed throughout said alloy.

. 5. A vacuum-type electric circuit interrupter comprismg:

(a) an envelope evacuated to a pressure of 10- mm.

of mercury or loss,

comprisiting metal.

(d) at least one of said contacts having circuit-making and breaking regions formed of an alloy consisting essentially of copper-aluminum-bismuth, or silveraluminum-bismuth, or nickel-aluminum-bismuth,

(e) the aluminum being present in a quantity of: between 9 and 15 percent by weight of the copperaluminum in the case of copper-aluminum-bismuth, between 5 and 10 percent by weight of the silveraluminum in the case of silver-aluminum-bismuth, and between 8 and 13 percent by weight of the nickel-aluminum in the case of nickel-aluminumbismuth,

(f) the bismuth being present in a quantity of less than about five percent by weight of the total alloy, in a quantity sufficient to inhibit contact-welding, and being distributed throughout said alloy.

6. A vacuum-type electric circuit interrupter compris- (a) an envelope evacuated to a pressure of 10* mm.

of mercury or less,

(d) at least one of said contacts having circuit-making and breaking regions formed of an alloy consisting essentially of (1) copper-aluminum containing 9 to 15 percent by weight of aluminum, Or (2) silveraluminum containing 5 to 10 percent by weight of aluminum, or (3) nickel-aluminum containing 8 to 13 percent by weight of aluminum, and a weld-inhibiting metal,

(e) said weld-inhibiting metal having substantially no solid-state solubility in the primary metal of the alloy or in aluminum and having an effective freezing temperature below that of the primary metal-aluminum alloy, I

(f) said weld-inhibiting metal being present in a quantity less than about five percent by weight of the total alloy and in a quantity sufiicient to inhibit contactwelding, and being distributed throughout said total alloy.

7. The interrupter of claim 6 in which said alloy consists essentially of silver-aluminum and said weld-inhibiting metal.

8. The interrupter of claim 6 in which said alloy consists essentially of nickel-aluminum and said weld-inhib- References Cited UNITED STATES PATENTS 3,014,108 12/1961 Cobine et al. 3,140,373 7/1964 Horn.

3,246,979 4/ 1966 Lafferty et al.

ROBERT S. MACON, Primary Examiner US. Cl. X.R. 200166