US3821505A

US3821505A - Vacuum type electric circuit interrupting devices

Info

Publication number: US3821505A
Application number: US00361063A
Authority: US
Inventors: A Wood
Original assignee: English Electric Co Ltd
Current assignee: English Electric Co Ltd
Priority date: 1972-05-18
Filing date: 1973-05-17
Publication date: 1974-06-28
Anticipated expiration: 1991-06-28

Abstract

A vacuum interrupter having two contacts with co-operating contact-making parts each of which is constituted by a porous matrix of metal particles comprising chromium containing from 0.5 percent to 13.5 percent by weight carbon, or iron containing from 1.0 percent to 2.0 percent by weight carbon, metallurgically bonded together by compacting and heating under high vacuum, the interstices of the matrix being infiltrated under high vacuum with a metal which comprises copper or a copper alloy, and the infiltrated metal constituting between 10 percent and 40 percent of the volume of the infiltrated matrix. When the matrix metal is chromium the preferred range of the carbon content is 1 percent to 3 percent by weight of the matrix metal.

Description

United States Patent Wood June 28, 1974 VACUUM TYPE ELECTRIC CIRCUIT Primary Examin er-Robert S. Macon INTERRUPTING DEVICES Attorney, Agent, or Firml(irschstein, Kirschstein, Ot- [75] Inventor: Allan John Wood, Weeping Cross, tmger & Frank England [73] Assignee: The English Electric Company [57] Limited, London, England A vacuum interrupter having two contacts with co- [22] Filed; May 1973 operating contact-making parts each of which is constituted by a porous matrix of metal particles compris- FN- 3 ing chromium containing from 0.5 percent to 13.5 percent by weight carbon, or iron containing from 1.0

percent to 2.0 percent by weight carbon, metallurgi- [52] US. Cl 200/144 B, 200/166 C cally bonded together by compacting and heating [51] Int. ,Cl. HOlh 33/66 under high vacuum, the nt rsti es of th matrix eing [58] Field of Search 200/144 B, 166 C infiltrated under high vacuum with a metal which comprises copper or a copper alloy, andthe infiltrated metal constituting between 10 percent and 40 percent of the volume of the infiltrated matrix. When the ma- [56] References Cited Lrix metal is chromium the greferred rgnge of hthefcalron content is percent to ercent y we] to t e UNITED STATES PATENTS matrix metaL P g 2,975,256 3/l96l Lee et al. 200/144 B 355L622 12/1970 Takeuchi 200/166 C X 7 Claims, 2 Drawing Figures ingoccurs between contacts when they are separated to interrupt current, the mechanism of arc extinction in vacuum interrupters is somewhat different from the mechanism of arc extinction in other types of circuit interrupters in which the arcing occurs in a medium such as insulating gas or oil, and the tendency for the contacts to weld together is very much more severe. An object of this invention is to provide a vacuum interrupter having contacts with good anti-weld properties.

According to the present invention a vacuum interrupter comprises two contacts having co-operating contact-making parts each of which is constituted by a porous matrix of metal particles comprising chromium containing from 0.5- percent to 13.5 percent by weight carbon, or iron containing from 1.0 percent to 2.0 per-- cent by weight carbon, metallurgically bonded together by compacting and heating under high vacuum, the interstices of the matrix being infiltrated under high vacuum with a metal which comprises copper or a copper alloy, and the infiltrated metal constituting between percent and 40 percent of the volume of the infiltrated matrix.

When the matrix metal is chromium the preferred range of the carbon content is from 1 percent to 3 percent by weight of the matrix metal.

Preferably the matrix metal comprises a matrix of metal particles sintered together, the metal particles being of size not exceeding 250 microns. The infiltrated metal may comprise an alloy of copper and silver.

Alloys of copper suitable for the infiltrated metal may also include zirconium, tantalum or titanium, though only in small proportions; for example, the copper, alloy may consist of 99.7 percent copper and 0.3 percent zirconium by weight.

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

FIG. 1 is a vertical cross-section through a vacuum interrupter embodying the invention; and

FIG. 2 shows diagrammatically and to a very much larger scale a verticalcross-section through an arcing portion of one contact of the vacuum interrupter shown in FIG. 1.

Referring to FIG. 1, the vacuum interrupter comprises a pair of

end plates

11, 12 bonded in a vacuumtight manner respectively to

cylinders

13, 14 of insulating material. The cylinders 13, '14 are bonded to a flange 15 which is trapped between them, and carries a shield 16 of generally cylindrical form. 3

The vacuum circuit interrupter is provided with a pair of relatively

separable contacts

17, 18, the movable contact 17 being capable of movement by means of an actuator (not shown) towards and away from the fixed contact 18. The movable contact 17 has its contact stem 21 reciprocable in a bushing 19, and a flexible conductor is provided which is attached to the contact stem 21. A bellows device 20 is secured in a vacuum-tight manner to the contact stem 21 and to the base plate 12 to allow movement of the contact 17.

The contact stem 21 has a contact head 22 secured -to it. The latter is recessed at its centre to afford a flat annular face 23, which co-operates with a similar face 24 on the co-operating contact 18 when the contacts are moved into engagement. The fixed contact 118 has a stem 26, which is secured to the base plate 1111 and provides the other terminal of the circuit interrupter, and a head 27 which may conveniently be symmetrical with that of the movable contact 17.

In the case of a vacuum spark gap, the arrangement could be exactly as illustrated except that the elec' trodes would always be spaced apart and would not move relative to one-another, but nevertheless an arc may be drawn between the

faces

23, 24.

The

contact heads

22, 27 are manufactured by compacting commercially available chromium powder e.g. powder made by the aluminothermic process) of particle size not exceeding 250 microns, to which sufficient carbon has been added to bring the carbon content within the preferred range of 1 percent to 3 percent by weight, and then sintering the compact under high vacuum. In the normal aluminothermic process for making commercially available chromium powder a small amount of carbon enters the chromium from the reactants, usually less than 1 percent by weight,-so that the addition of carbon is necessary to obtain the required carbon content. Also there is a carbon reduction method for the production of chromium, in which the metal is reduced from the oxide by carbon in an electric furnace. Carbon can readily be added to yield chromium containing carbon within the preferred range of 1 percent to 3 percent by weight. The sintered compact is then infiltrated with molten copper under high vacuum and at a high temperature.

The copper occupies between 10 percent and 40 percent of the volume of the infiltrated matrix material, as determined by the porosity of the sintered compact, and hence as determined by the degree of compaction applied to the compact. If necessary, the infiltrated compact may be shaped by normal machining methods. The chromium powder on compaction yields a matrix metal of low ductility, so that the infiltrated matrix likewise exhibits a low ductility.

In place of chromium containing from 0.5 percent to l3.5 percent by weight carbon, the metal matrix may comprise iron containing from 1.0 percent to 2.0 percent by weight carbon. Moreover, the infiltrated metal may comprise alloys of copper with another metal of good electrical conductivity, for example silver.

Alloys of copper suitable for the infiltrated metal may also include zirconium, tantalum or titanium,

though only in small proportions; for example, the alloy may'comprise 99.7 percent copper and 0.3 percent zirconium by weight.

In FIG. 2 a typical microstructure of the contact material is shown, the scale marking representing 200 microns, and each chromium-carbon particle 30 is arrached to the neighbouring particles of chromiumvacuum interrupter it is believed that microdeformation occurs, resulting in a large number of well distributed contact points. It is considered that this condition would tend to result in low constriction resistance, good distribution of the FR loss at high current flow immediately prior to arcing, and little tendency to weld.

When such vacuum interrupter contacts breakwhich is effected at high velocity by means of well known per seit is thought that there is a high probability of multiple arcs being formed between the

faces

23, 24, giving good-distribution of the arc energy and consequently low and uniform erosion.

Even after arcing, it is thought that the size of asperities tends to be limited to that of the maximum dimensions of the particles of the matrix, so that the local electric field and consequent field emission for a given contact separation is low, and the breakdown voltage is higher and more predictable than for contacts of similar shapes made mainly of ductile materials.

Moreover, since the boiling point of the matrix material is below 3,000C, electron emission densities following a high-current arcing loop are reduced by several orders below that which occurs with a tungsten matrix when surface melting occurs, thus allowing a substantial improvement to be achieved in the recovery voltage performance.

Tests on contacts made in accordance with the invention from chromium-carbon compacts impregnated with 33 percent copper by volume have shown that arcs up to at least kA peak can be interrupted satisfactorily with low'and uniform contact erosion and no detectable welding prior to arcing.

As stated above the chromium-carbon constituent of the contact material may be replaced by iron with the required carbon content. The main properties of each such matrix are:

a. it is capable of being wetted by the impregnating metal during infiltration;

b. its melting point is higher than that of copper, and is preferably over l,200C, so that it is not melted by the infiltration process;

0. its melting point is lower than that of molybdenum;

d. its boiling point is not substantially greater than 3,000C; and e. it has a low ductility in relation to that of copper.

As stated above the copper constituent of the contact material may be replaced by a suitable copper alloy, for example an alloy of copper and silver.

The main properties of each such infiltration metal are:

l. it has high electrical conductivity in relation to that of the matrix metal; 2. its melting point is below that of the matrix metal,

and is below 1,200C;

3. its boiling point is not substantially greater than. and is preferably below that of the matrix metal; 4. it has high ductility relative to that of the matrix metal; and 5. it has low viscosity when molten so as to facilitate infiltration. Furthermore, the addition of carbon to the matrix metal in the specified amounts results in carbon precipitating in the grain boundaries, introducing weak links which are maintained in the particulate metal and in the final sintered compact. Since the breaking of a weld is initiated by the rupture of the matrix metal rather than the more ductile infiltrated metal, the presence of the carbon insignificantly improves the anti-weld properties of the electrodes.

1 claim:

1. A vacuum interrupter comprising two contacts having co-operating contact-making parts each of which is constituted by a porous matrix of metal particles comprising chromium containing from 0.5 percent to 13.5 percent by weight carbon, or iron containing from 1.0 percent to 2.0 percent by weight carbon, metallurgically bonded together by compacting and heating under high vacuum, the interstices of the matrix being infiltrated under high vacuum with a metal which comprises copper or a copper alloy, and the infiltrated metal constituting between 10 percent and 40 percent of the volume of the infiltrated matrix.

2. A vacuum interrupter as claimed in claim 1, wherein the matrix metal is chromium and the range of the carbon content is from 1.0 percent to 3.0 percent by weight of the matrix metal.

3. A vacuum interrupter as claimed in claim 1, wherein the particle size of the matrix particles throughout the matrix does not exceed 250 microns.

4. A vacuum interrupter as claimed in claim 1. wherein the infiltrated metal occupies 33 percent ofthe volume of the infiltrated matrix.

5. A vacuum interrupter as claimed in claim 1, wherein the infiltrated metal comprises an alloy of copper and silver.

6. A vacuum interrupter as claimed in claim 1, wherein the infiltrated metal comprises an alloy of copper with a metal chosen from the group zirconium, tantalum and titanium.

7. A vacuum interrupter as claimed in claim 1, wherein the infiltrated metal comprises an alloy consisting substantially of 99.7 percent copper and 0.3 percent zirconium by weight.

Claims

2. A vacuum interrupter as claimed in claim 1, wherein the matrix metal is chromium and the range of the carbon content is from 1.0 percent to 3.0 percent by weight of the matrix metal.
3. A vacuum interrupter as claimed in claim 1, wherein the particle size of the matrix particles throughout the matrix does not exceed 250 microns.
4. A vacuum interrupter as claimed in claim 1, wherein the infiltrated metal occupies 33 percent of the volume of the infiltrated matrix.
5. A vacuum interrupter as claimed in claim 1, wherein the infiltrated metal comprises an alloy of copper and silver.
6. A vacuum interrupter as claimed in claim 1, wherein the infiltrated metal comprises an alloy of copper with a metal chosen from the group zirconium, tantalum and titanium.
7. A vacuum interrupter as claimed in claim 1, wherein the infiltrated metal comprises an alloy consisting substantially of 99.7 percent copper and 0.3 percent zirconium by weight.