US3843856A - Contact for a vacuum switch of single phase alloy - Google Patents

Abstract

Materials for contacts of a vacuum type circuit interrupter comprising a single phase alloy which provides stable arcs and exhibits uniform chopping behavior.

Description

United States Patent [191 Attia CONTACT FOR A VACUUM SWITCH OF SINGLE PHASE ALLOY [75] Inventor: Edward A. Attia, San Jose, Calif.

[73] Assignee: Allis-Chalmers Corporation,

Milwaukee. Wis.

[22] Filed: June 4, 1973 [21] Appl. N0.: 366,868

[52] US. Cl. 200/144 B, 200/266 [5l] Int. Cl. H0lh 33/66 [58] Field of Search 200/144 B, 166 C, 262,

ZOO/264, 265, 266

[111 3,843,856 1 Oct. 22, 1974 [56] References Cited UNITED STATES PATENTS 2,975,256 3/1961 Lee et al. ZOO/144 B 3246.979 4/1966 Lafferty et al. ZOO/I44 B Primary ExaminerRobert S. Macon Attorney, Agent, or Firm-Robert C. Jones 5 7 ABSTRACT Materials for contacts of a vacuum type circuit interrupter comprising a single phase alloy which provides stable arcs and exhibits uniform chopping behavior.

3 Claims, 3 Drawing Figures 1 CONTACT FOR A VACUUM SWITCH OF SINGLE PHASE ALLOY BACKGROUND OF THE INVENTION The present invention relates to contacts utilized in vacuum type circuit interrupters and particularly to the material for such contacts. It has been discovered that a single phase alloy exhibits a uniform chopping behavior in contrast to two phase alloys which are apt to show inhomogeneous characteristics. It has also been discovered that the single phase alloy exhibits a longer are lifetime and, therefore, will provide stable arcs over the life expectancy of the interrupter in which it is utilized.

A general object of the invention is to provide a contact with satisfactory arc stability to prevent chopping in inductive circuits.

Still another object of the invention is to provide a contact with a uniform chopping level which is maintained during the life of the interrupter in which it is utilized.

Another object of the present invention is to provide a contact of a compound of a single phase alloy and with a unique chemical composition and crystallographic structure.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a sectional view through a vacuum type circuit interrupter embodying the present invention;

FIG. 2 shows a plurality of curves illustrating the dependence of the average arc lifetime on the arc current for various contact materials including the present invention; and

FIG. 3 shows a plurality of curves illustrating the change of the average arc life with are erosion for Copper 2 percent wt Bismuth and Copper Telluride contact material to demonstrate the inhomogeneous arc stability of currently used materials in comparison to Copper Telluride, the subject matter of this invention.

DESCRIPTION OF THE INVENTION Referring to FIG. 1 of the drawing, there is shown a vacuum type circuit interrupter in which the contact in accordance with the present invention is embodied and which is understood to be used in electrical circuits to interrupt relatively high amperage currents. The interrupter comprises an envelope made of vacuum type, heat resistant, electrical material such as a high grade ceramic, The envelope 10 is in the form of a cylindrical housinghaving a cylindrical axial chamber 12 therein which is provided with openings 14 and 16 at opposite ends. Electrically conductive means including a pair of relatively removable cooperating electrodes or contacts in the form of stationary contact rod 28 and a movable contact rod 30 extend into the chamber 12 through the openings 14 and 16, respectively. The end opening 14 through which the stationary contact 28 extends is sealed by means of an end plate 32 and a sealing member 33. As shown, the end plate 32 is operatively constructed to carry the stationary contact 28 in inwardly extending relationship therefrom. The opening 16 at the opposite end of the envelope 10 is sealed by an end plate 34 through which the movable contact 30 extends. The end plate 34 is provided with a central opening 36 which accommodates the movable contact rod 30. A bellow 38, tubular in form, is located at the end of the envelope and is welded at its innermost end to the movable contact rod 30 and at its lower end to the end of the depending tubular shield 40 that is disposed within the envelope.

In a vacuum type circuit interrupter the geometry of the contacts is of lesser importance than is the material of which the contacts are made.

ARC STABILITY CHARACTERISTICS OF VAC- UUM POWER INTERRUPTERS During vacuum power interruption, a metal vapor arc is formed between the contacts as they separate. The are becomes a circuit component that allows the current to continue to flow in the circuit. In an ideal a.c. circuit the arc should be stable and extinguish at first current zero. However, in a case where the arc suddenly ceases to exist, before current zero, a high voltage pulse or spike is generated across the inductive loads in the circuit. The magnitude of the voltage spike depends on the inductive load and in sufficiently high inductive circuits may easily damage the circuit components. This phenomenon is referred to as current chopping. The chopping current is considered the minimum current necessary to sustain a stable vacuum arc in a specific circuit.

A contact material capable of evolving in response to all instantaneous arcing currents above the maximum permissible chopping current, a sufficient supply of vapor to maintain the arc stable, and to provide a pressure at least as great as the arc constricting magnetic pressure of the are at its terminals is defined as a material with satisfactory arc stability. Materials which evolve excessive amounts of metal vapor are defined as materials with strong arc stability, whereas materials which chop the current at a relatively high level, above about 6 amps, are considered materials with unstable arcs. Examples of materials with strong arc stability are Antimony, Bismuth, Lead and Zinc. Materials with unstable arcs are Tungsten, Molybednum and Tantelum. The are stability is a function of the vapor pressure and the thermal conductivity.

To obtain an interrupter with low chopping level, yet have the capability to interrupt high currents, several investigations have suggested the use of duplex structures for the contacts. A duplex structure could be a relatively low vapor pressure metal, e.g., Copper, coated with relatively high vapor pressure metal, e.g., Bismuth, or a metal such as Tungsten infiltrated with another higher vapor pressure metal such as Copper, or an alloy composed of relatively high vapor pressure metal and another relatively low vapor pressure metal (e.g., Copper-Bismuth, Copper-Lead, etc.).

In duplex structures, as outlined later, the arc lifetime seems to depend on the number of power interruptions the interrupter has acquired to date. In newly processed interrupters the contact surface is rich in the high vapor pressure metal and a relatively high are stability is usually obtained; however, as the interrupter is subjected to arcing, the continued loss of the high vapor pressure metal leads to decreased stability.

Low current, low voltage, d.c. arcs between closely spaced contacts bum in a metal vapor atmosphere wherein the necessary vapor is supplied from a multiplicity of high current cathode spots, the average arc lifetime is an exponential functionof the arc current and is influenced, to a large degree, by the vapor pressure of the cathode material, and to a lesser degree, by

the geometry of the contacts, the geometry of the arcing chamber, and the reactance of the test circuit.

The average arc lifetime is a measure of the arc stability. At the same arcing time, a long duration are is associated with a strong arc stability material whereas a short duration are indicates arc instability.

The present invention is concerned with contacts having arc stability characteristics as manifested by average arc lifetime of less than 1 microsec at are current of 1 amp and in materials which may be used for vacuum power interrupter contacts.

To obtain average arc lifetimes which are representative of the chemical composition of aspecific material, various contact materials of high purity, low gas content were evaluated. The contacts evaluated were electron beam melted Tungsten, vacuum induction melted Copper, Copper 2 percent wt Bismuth, Copper 2 percent wt Tellurium, Copper Telluride (50 wt Tellurium), and Bismuth coated Copper. The latter material was prepared by coating a copper sample with Bismuth, then sintering in Hydrogen at 900 C. The contacts under investigation were thoroughly degassed in vacuum at 550 C. to decrease the noncondensable gas content to lpprn, as determined by induced arc vaporization and mass spectrometry analysis.

To determine the relative stability of arcs on the contact materials, an electrical circuit composed of a regulated power supply with open circuit voltage of 190 volt d.c., connected in series with the experimental interrupter containing the contacts, a backup switch, a v

0.7 mh inductance, and a noninductive variable resistance to control the current in the circuit was utilized. The experimental interrupter and the backup switch were electromechanically actuated and controlled by time delay relays. In operation the interrupter contacts were closed manually after which an automatic cycle was started by closing the backup switch to permit the current to flow in the circuit. After 0.5 seconds the contacts of the interrupter were parted, followed 0.5 seconds later by the opening of the backup switch. The current was controlled by the variable resistor and measured on an oscilloscope using a noninductive shunt.

Arc duration after parting of the interrupter contacts was determined by triggering the oscilloscope timed sweep from the voltage rise from zero to are voltage as the contacts part and measuring the time required for the current to return to zero. The reported are average lifetime is the statistical average of 50 operations at a constant current. I

The curves in FIG. '2 show the dependence of the average arc lifetime on the arc current for the various materials under investigation. Tungsten exhibits a high level of arc instability in comparison to Copper or Copper alloyed with Bismuth or Tellurium. The alloyed Copper, with Bismuth or Tellurium shows an improved arc stability in comparison to Copper, with the Copper- Bismuth showing a relatively high arc stability. However, when the contacts were first subjected to d.c. arc erosion at 250 A. and 0.5 sec. duration, and then the average arc lifetime measured, the Copper 2 percent wt Bismuth, Copper 2 percent wt Tellurium, Copper coated Bismuth contacts showed a tendency toward decreased arc stability. Tungsten, Copper and Copper Telluride showed no detectable change. The measured average arc lifetimes after 100 operations of 250 A., 0.5 sec. d.c. arcs are shown inFIG. 2.

Copper-Tellurium (less than 50 percent wt Tellurium) and Copper-Bismuth alloys are two phase compositions wherein a precipitate of second phase, Copper Telluride or Bismuth, is discretely distributed in the Copper matrix. During vacuum degassing of the interrupter, an inhomogeneous distribution of the second phase takes place and leads to a high concentration of the second phase at or close to the surface. The measured average arc lifetime at the surface layer is representative of Copper Telluride or Bismuth. A comparison of the curves of FIG. 2, representing the average arc lifetime versus arc current for both Copper Telluride and Copper 2 percentwt Tellurium at the sur-' face layer supports this view. Also, the measured values for the average arc lifetime, at the surface layer, for the Copper-Bismuth samples compares favorably to the values for Bismuth, as indicated by the dotted curve in FIG. 2.

A further evidence for the tendency to form inhomogeneous distribution of a second phase under a heat treatment has been previously established from results obtained by annealing 0.1 percent Bismuth-Copper alloy at 550 C. for 24 hours in air. When analyzed, it was found that the Bismuth concentration was 1.3 percent at the surface and0.04 percent at a distance 0.136 inch from the surface. I

When the contacts of there materials were subjected to operations of arc erosion (to lose 0.4-0.5 mm of their thickness) the measured average arc lifetime for the Copper-Bismuth contact and the Copper 2 percent Tellurium contact indicated that the layers below the surface were depleted from the high vapor pressure element (compare the average arc lifetimes before and after arc erosion, FIG. 2). The effect was stronger for the Copper-Bismuth contact, particularly the Bismuth coated Copper contact which showed after 100 arc erosions an average arc lifetime almost identical to that of pure Copper. FIG. 3 shows the change of the average arc lifetime for Copper-Bismuth contacts and Copper Telluride contacts after various degrees of arc erosion. Copper Telluride showed no detectable change showing that single phase materials exhibit homogeneous arc stability behavior similar to that of pure metals, e.g., Copper and Tungsten.

There is no universally accepted method to measure the chopping current, but a useful method is to compare the chopping current of various metals to the current corresponding to some average arc lifetime, say 0.1 msec. Accordingly, the average chopping current as deduced from the present measurements is 10 A., 5 A. and 0.4 A. for Tungsten, Copper and Copper Telluride, respectively. The values for Tungsten and Copper compare favorably with the average chopped current values previously obtained, i.e., 9.2 A. and 4 A., respectively.

Thus, arc stability characteristics of alloys containing high vapor pressure elements has been shown. Two phase materials are associated with inhomogeneous arc stability. Single phase alloys, on the other hand, exhibit a homogeneous arc stability character. 7

Copper Telluride was discovered as a materialwith homogeneous arc stability and with average chopping current much less than that of pure Copper. Its chopping current is comparable with various high vapor pressure elements, e.g. Bismuth, Lead, Zinc and Cadmium, yet it has the advantage of a higher melting point (857C). I

Contacts, especially contacts for utilization in vacuum type circuit interrupters, are especially vulnerable to erosion and when constructed of two phase composition alloys may exhibit inhomogeneous arc stability. Therefore, the discovery of an alloy that is single phase with definite crystallographic structure which behaves like single phase pure metals to exhibit a homogeneous arc stability throughout the life expectancy of the contacts is a marked advancement in vacuum interrupter art. Tellurium, at high concentrations above about 5 percent, was avoided as a current carrying component because of the high resistivity of pure Tellurium, 4 X micro-Ohm-cm, but the material of this invention has a resistivity between 150200 micro- Ohm-cm, which is comparable with pure Bismuth, 120 micro-Ohm-cm. Also, contacts of Copper Telluride material is brittle and will not weld when engagement between contacts has been made.

The material Copper Telluride for the contacts herein set forth is preferably comprised of 50 percent to 52 percent wt Tellurium with the balance Copper. A contact as set forth of an alloy having a compound of a very high percentage of a nonmetallic material alloyed with an equal or lesser amount of metallic ingredient such as Copper results in a contact having a uniform chopping level which is maintained during the lifetime of the interrupter providing arc stability. An advantage of the contact of the material set forth, besides the fact that the resulting contact material has a relatively high melting temperature about 857 C., is that it can be easily joined with Copper; and if so combined, the resistivity of the resulting composition is comparable with that of Bismuth. Also, the contact of the described material Copper Telluride exhibits antiweld characteristics since the Copper Telluride material is brittle and, therefore, will not weld.

EXPLANATION OF THE CAPTIONS UTILIZED IN FIGS. 2 AND 3 FIG. 2 The dependence of the average arc lifetime on the arcing current for the various test materials:

G) o Tungsten, G9 ACopper, +ABismuth coated Copper, VACopper 2 percent wt Bismuth, X Copper 2 percent wt Tellurium, and 0% Copper Telluride, at the surface layer and after 100 operations of 250 amp., 0.5 sec. arc erosions, respectively.

FIG. 3 the change of the average arc lifetime with are erosion for Copper 2 percent wt Bismuth and for Copper Telluride. The number of arcs shown above represent d.c. arcs of 250 amp. and 0.5 sec. duration. The average arc lifetime was measured at the test currents 01.25 amp. and 695 amp. for Copper 2 percent wt Bismuth. The average arc lifetime was measured at the test currents 01.25 amp. and amp. for Copper Telluride.

The embodiments of the invention in which an exclusive property of privilege is claimed are defined as follows:

1. In a vacuum type circuit interrupter having an evacuated sealed envelope;

a pair of contacts supported by said envelope for relative movement into and out of engagement;

at least one of said contacts having cirucit making and breaking regions formed of Copper Telluride wherein the Tellurium concentration is at least 50 percent by weight and not more than 52 percent by weight, and the balance Copper.

2. A vacuum type circuit interrupter according to claim 1 wherein said Copper Telluride is an alloy of two elements, one of which is a metal and the other being a nonmetal or semiconductor to provide a homogeneous single phase alloy of Copper Telluride with definite crystallographic structure and an electrical resistivity of no more than 200 micro-Ohm-cm and a chopping level of less than one amp.

3. A vacuum type circuit interrupter according to claim I wherein said Copper Telluride material is a single phase alloy exhibiting a homogeneous arc stability character with an average chopping current less than one amp.

Claims (3)

1. IN A VACUUM TYPE CIRCUIT INTERRUPTER HAVING AN EVACUATED SEALED ENVELOPE; A PAIR OF CONTACTS SUPPORTED BY SAID ENVELOPE FOR RELATIVE MOVEMENT INTO AND OUT OF ENGAGEMENT; AT LEAST ONE OF SAID CONTACTS HAVING CIRCUIT MAKING AND BREAKING REGIONS FORMED OF COPPER TELLURIDE WHEREIN THE TELLURIUM CONCENTRATION IS AT LEAST 50 PERCENT BY WEIGHT AND NOT MORE THAN 52 PERCENT BY WEIGHT, AND THE BALANCE COPPER.

2. A vacuum type circuit interrupter according to claim 1 wherein said Copper Telluride is an alloy of two elements, one of which is a metal and the other being a nonmetal or semiconductor to provide a homogeneous single phase alloy of Copper Telluride with definite crystallographic structure and an electrical resistivity of no more than 200 micro-Ohm-cm and a chopping level of less than one amp.

3. A vacuum type circuit interrupter according to claim 1 wherein said Copper Telluride material is a single phase alloy exhibiting a homogeneous arc stability character with an average chopping current less than one amp.

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