US3514559A

US3514559A - Vacuum type circuit interrupter

Info

Publication number: US3514559A
Application number: US626293A
Authority: US
Inventors: John W Ranheim
Original assignee: McGraw Edison Co
Current assignee: Cooper Industries LLC
Priority date: 1967-03-27
Filing date: 1967-03-27
Publication date: 1970-05-26
Anticipated expiration: 1987-05-26

Description

y 6, 1970 J. w. RANHEIM 3,514,559

VACUUM TYPE CIRCUIT INTERRUPTER Filed March 27. 1967 2 Sheets-Sheet 1 IN V EN TOR.

{5/727 2% Ran/2494772 EM Wm r/fttorney 5, 970 J. w. RANHEIM 3,514,559

VACUUM TYPE CIRCUIT INTERRUPTEF.

Filed March 27, 1967 2 Sheets-Sheet 2 m ZPB n5 1 r ,7 H V W a f /)k\ 30 INVENTOR. ag/m 2M Ran/765272 United States Patent 3,514,559 VACUUM TYPE CIRCUIT INTERRUPTER John W. Ranheim, Oak Creek, Wis., assignor to McGraw- Edison Company, Milwaukee, Wis., a corporation of Delaware FiledMar. 27, 1967, Ser. No. 626,293 Int. Cl. H01h 9/30, 1/02 US. Cl. 200-444 8 Claims ABSTRACT OF THE DISCLOSURE A vacuum circuit interrupter comprising an evacuated insulated housing having a pair of conductive electrodes which are constructed and arranged to be moved into and out. of engagement and circuit making and breaking regions on the electrodes composed of a porous refractory such as W B for example, impregnated with a nonrefractory metal having high thermal and electrical conductivity wherein the refractory is readily wet by the nonrefractory when the latter is in its liquid state.

BACKGROUND OF THE INVENTION Vacuum circuit interrupters generally include an evacuated insulating housing and circuit making and breaking contacts constructed and arranged to be moved into and out of engagement. In addition to the ability to interrupt fault currents and to withstand impulse crestvoltages across is open contacts, vacuum circuit interrupters must be able to close against high momentary currents without producing objectionable welds between its contacts. Such welding is associated with the fluidizing of the metallic contact material as a result of the heat pro duced during arcing. This liquid contact material freezes after the contacts are brought into high pressure engagement, thereby forming a weld.

Oneprior art method for avoiding contact welds was to fabricate the contacts out of a refractory metal such as tungsten. Such contacts were not wholly satisfactory, however, because their are interrupting ability was relatively limited as a result of the low thermal conductivity of this material. Another prior art contact comprised a porous matrix of a refractory metal, such as tungsten, and a nonrefractory metal impregnant having high thermal and electrical conductivity, such as copper. Contacts of. this type were also satisfactory only for the interruption of relatively low currents. At high current values the copper would tend to evaporate from the arcing surfaces so that, after a few interruptions, there remained a region of relatively pure tungsten. This region would then heat to a relatively high temperature and emit are sustaining electrons by thermionic emission.

Ina further prior art attempt to solve the welding problem, the vacuum interrupter contacts were fabricated of an alloy of copper-bismuth. Although this material is readily melted during arcing, it is weak physically so that any resulting welds can easily be broken by the switch opening apparatus. These contacts were not wholly satisfactory, however, because they are easily eroded during arcing and, therefore, their useful life is relatively limited. In addition, a large amount of liquid contact material is lost by such contacts during arcing and this material tendslto condense on the internal surfaces of the interrupter, to form sharp projections and other irregularities which tend to reduce its dielectric properties.

SUMMARY OF THE INVENTION Int general terms the invention comprises a vacuum circuit interrupter in which at least one of its engageable contact portions comprises a porous refractory including 3,514,559 Patented May 26, 1970 BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a side elevational view, partly in section, of a vacuum circuit interrupter illustrating how the contacts, according to the instant invention, may be employed;

FIG. 2 is a view taken along lines 22 of FIG. 1;

FIG. 3 is a view taken along lines 3-3 of FIG. 2;

FIGS. 4 and 5 illustrate the method of determining the degree of wetta'bility of a first metal in its solid state and a second metal in its molten state;

FIG. 6 illustrates the physical relationship between the components of the contacts according to the instant invention; and

FIG. 7 illustrates an alternate contact configuration in which the contact material according to the invention may be employed.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows a vacuum-type circuit interrupter 10 having an open-ended outer envelope 11 of any suitable electrical insulating material, such as a ceramic, and upper and

lower end caps

12 and 13, respectively, each of which is fixedly secured at its outer periphery to one of the open ends of the envelope 11 to form a vacuum chamber 14. A fixed electrode, or contact rod, 15 is sealed to the upper end cap 12 and extends downwardly therefrom in a substantially coaxial relation to the envelope 11. A second electrode or contact rod 16, extends upwardly through an aperture 17 formed in the lower end cap 13 and is movable longitudinally into and out of engagement with the fixed contact rod 15 by means which are not shown but which are well known in the art. A bellows 18, or its equivalent structure, is connected in a vacuum tight relation at each of its opposite ends to the contact rod 16 and to the lower end cap 13 in surrounding relation to the aperture 17.

The

contact rods

15 and 16 are axially aligned within the envelope 11 and respectively carry arcing contacts 19 and 20 at their inner ends. The arcing contacts 19 and 20 may be generally disc-shaped and are disposed in an opposed relation relative to each other. An arc shield 21 may be affixed to the envelope 11 in surrounding relation relative to the contacts 19 and 20 to prevent contact material, which may be vaporized during an are interruption, from depositing on the inner surface of the envelope 11 and thereby to provide a short-circuit path around the

electrodes

15 and 16. In addition a second, generally cup-shaped shield 25, is aflixed to the rod 16 for protecting the bellows 1-8.

The arcing contact 20 is preferably a mirror image of contact 19, and accordingly, only contact 19 will be discussed in detail. Referring now to FIGS. 2 and 3, the arcing contact 19 is shown to be generally disc-shaped and to be affixed at its upper surface to the contact rod 15 in any suitable manner, such as by brazing. The lower surface of the contact 19 is provided with a generally circular recess 22 for receiving a relatively flat, circular disc-shaped member 23 which is secured in the recess in a manner which will be discussed hereinbelow. The thickness of the member 23 is greater than the depth of the recess 22 so that it will be elevated relative to the remainder of the contact 19 and thereby will engage a corresponding member 23 of the lower contact 20.

A generally frustoconical support 26 is afiixed to the back of each of the contacts 19 and 20 to provide rigidity for said contacts.

The contact making and breaking regions 23, according to the invention, comprise a porous body, or matrix, composed of a refractory impregnated with a nonrefractory metal having high thermal and electrical conductivity wherein the nonrefractory tends to wet the refractory. In addition, it is preferable that the solubility of the refractory in the liquid state nonrefractory metal be substantially limited so that the porous matrix of the refractory matrix will be retained after repeated arcings.

It is preferred that the refractory comprise a compound composed of a metal and a metaloid, and it is also preferable that the oxides of the rnetalloid be solid at the normal operating temperatures of the interrupter so that gases will not be liberated during arcing which would tend to destroy the vacuum. Two metalloids which form a refractory compound with certain metals and which exhibit these characteristics are boron and silicon. These elements are also effective getters so that when compounds of boron and a metal are employed in vacuum interrupters, it is possible to eliminate several of the expensive degassing operations employed in the fabrication of certain prior art vacuum interrupter contacts, such as those composed of copper and copper-bismuth.

The nonrefractory metal having high thermal and electrical conductivity and which impregnates the refractory compound matrix may consist, according to the invention, of copper, silver, aluminum or any high thermal and electrical conductivity mixture thereof.

Certain compounds of boron and a metallic are unsatisfactory for use in circuit interrupter contacts because they have relatively low melting points, such as NiB, which melts at approximately 1020" C. Preferably the melting point of the compound should be in excess of 1750 C.

To illustrate certain refractory compounds of a metalloid and a metallic, reference is made to Table I wherein the melting points of certain illustrative compounds which may be used in circuit interrupters of the invention are given.

TABLE I.MELTING POINTS OF SOME ILLUS- TRATIVE REFRACTORY COMPOUNDS Approximate Compound: melting point C.)

crB 1850 MoSi 2050 Mo B 2100 MoB 2100 VB 2100 MoB 2100 WSi 2165 CeB 2190 ThB 2195 LaB 2210 SrB 2235 BaB 2270 UB 236 5 W B 2770 WB 2900 TaB 2900 NbB 2900 W13 2900 vB 2900 wB 2920 W B 2980 H113 3100 ZrB 3040 Certain of these compounds, which were tested and found to be highly satisfactory for high current applications, are ZrB W B, WB and W 8 Other compounds, such as CrB Mo B, MoB and MoB having a lower melting point, would be satisfactory at lower current applications.

Boron is known to form several other compounds with certain of these metals, such as ZrB, ZrB and ZrB For the sake of brevity, all of these compounds have not been listed.

The wetting action of a liquid metal on another material in its solid state has been discussed in the literature. This phenomenon is reported in terms of the angle that a droplet of the molten metal makes with a planar surface of the solid state metal as illustrated in FIGS. 4 and 5. In FIG. 4 a droplet of copper at 1400 C. is shown to make an angle of 36 with ZrB and in FIG. 5 a droplet of copper at 1400 C. is shown to make an angle of 154 with TiB Thus, because the molten droplet of copper wets ZrB it spreads out to form a relatively small angle with the planar surface of the refractory. On the other hand, the molten copper droplet does not readily wet TiB so that it tends to assume a spherical shape whereby a large angle is formed with the planar TiB surface. The angles formed between molten cop-per droplets of some illustrative boron compounds as reported in the literature are set forth in Table II.

TAB LE II Angle formed by drop of pure molten Cu on planar surface of com- Compound pound (deg) It can be seen from Table II that the ability of molten copper to wet certain of the refractory compounds increases substantially as the temperature is increased from 1100 C., which is slightly in excess of the melting point of copper, to 1400-1500 C. The ability of molten copper to wet certain other compounds, however, such as TiB is not appreciably increased by such an increase in temperature. This, while TiB is sufficiently refractory, having a melting point of 2980 0., its use in the vacuum interrupter contacts according to the invention is not preferred because it is not readily wet by molten copper.

The ability of molten copper to wet certain refractory materials may be enhanced by the addition to the copper of certain wetting agents, such as Ni. Such wetting agents should preferably be added only in amounts which will not adversely effect the thermal and electrical conductivity of the copper. It may be noted that it is well known in the prior art that the metals and metalloids which form the compounds illustrated in Table I are tightly bonded together so that very little of the boron or silicon metalloids in solid form will be free to join with nickel to form the undesirable refractory compounds such as NiB. Also, it is known that the evaporated gaseous silicon or 'boron which combines in the atmosphere within the arc shield 21 with nickel or oxygen will most likely deposit in solid form on the arc shield or non-contacting surfaces of contacts 19 and 20 because such surfaces are relatively large in comparison with the engaging portions of the contacts.

While the examples in the foregoing description have all involvedthe ability of molten copper to wet various refractories, those skilled in the art will appreciate that this will have application to silver and aluminum as well.

The arcing contact 23 is shown in FIG. 6 to comprise a porous body, or matrix, 27 of the refractory compound impregnated with a nonrefractory high thermal and electrical conductivity metal 28.

One illustrative process for forming the circuit making and breaking contact 23 and involving the refractory compound W B and copper will now be discussed. A mixture of powdered W B and powdered copper is placed, in a die having the shape of the current making and breaking contacts 23, and a pressure of approximately 50,000l00,000 p.s.i. is applied, although lower pressures are acceptable if the mold is suitably heated. This compresses the powdered mixture into a relatively solid mass. The compressed member is then placed into a vacuum furnace and heated at a temperature which is in excess of 1083" C., the melting point of copper, although a temperature in the order of 1700 C. is preferred. This thoroughly wets the refractory with copper and tends to cause the refractory particles to coalesce and grow together:to form a matrix. The liquefied copper tends to fill the spaces between the W B matrix, although some copper is lost by evaporation.

Thecontact is then placed in a mold containing molten copper and heated under vacuum at a temperature in excess of the melting point of copper, but preferably at 1600-l700 C., for a period of sufficient length to thoroughly degas the copper. The temperature is then reduced to approximately 1250 C. for a period of time sufficient to fill any remaining voids with copper. Because the specific gravity of the refractory is substantially greater than that of the pure copper, the matrix member impregnated with copper does not tend to float in the molten copper. When the mold is cooled, a. composite contact member is then provided which consists of the contact making and breaking member 23 bonded to a backing of purse copper 19. The copper and the copperimpregnated refractory may then be machined to the desired shape shown in FIGS. 2 and 3. Small quantities of an impurity, such as a few percentage or less of silver or zirconium may be added to the copper to improve its machinability;

This process insures that the spaces in the W B matrix are filled with copper and also eliminates the necessity for thereafter brazing, or otherwise joining, the contact making and breaking electrode 23 to the copper body 19..

While the foregoing process has been discussed with respect to W B it is intended that the same process would be employed with various other higher melting point compounds such as W B, WB, TaB NbB VB HfB and ZrB The proportion of powdered refractory and copper 1n the original mixture should be such that there will be substantially continuous contact between the refractory particles. This result has been obtained with about equal portions by volume of powdered copper and W 3 If the coppen portion exceeds about 60% by volume, then a continuousmatrix may not be obtained. If the proportion of copper falls below about 15% by volume, then the desired degree of thermal and electrical conductivity may not be achieved. The preferred grain sizes are about 350 mesh or less for both the copper and the refractory.

The desired results are also possible if mixtures of various powdered refractories are employed, such as W B and ZrZB In addition, some minor proportion of a refractory metal such as tungsten may also be employed, although with less satisfactory results.

According ,to another embodiment of the invention illustrated in FIG. 7, the contact making and breaking material composed of the matrix of the refractory compound impregnated with a nonrefractory high thermal and electrical conductivity metal covers the entire arcing face: of the contact 30. Here the circuit making and breaking contact comprises a thin lamination 31 of the arcing material according to the invention and a relatively thicker base 32l0f high thermal conductivity metal, such as copper.

The contacts, according to the instant invention, provide most of the advantages of refractory contacts and of high. thermal and electrical conductivity nonrefractory contacts while eliminating most of the disadvantages thereof. Consider, for example, a contact composed of a matrix of W 3 impregnated with copper. Because the refractory W B is readily wet by liquid copper, very little copper is lost during arcing even though some of the copper may become liquid. As a result, the matrix remains impregnated with copper so that, even after repeated arcings, there is a relatively high thermal and electrical conductivity at the contacting faces of the contacts and, in addition, heat generated during arcing will be rapidly conducted away from the arcing surfaces. Because of the relatively high melting point of the refractory compound, very little metal will be melted during arcing so that there will be little tendency for contacts to weld upon re-engagement. In addition, because of the refractory nature of the matrix, there will be very little contact erosion even after repeated arcing so that a relatively long contact life is realized. Also, because the solubility of W B in liquid copper is relatively low, the matrix will not dissolve to any great extent in any copper which may become liquid during arcing. Further, the current chopping level of contacts comprising a W B matrix impregnated with copper compares favorably with other vacuum interrupter contact materials such as an alloy of copper-bismuth which has a current chopping level averaging between 4 and 5 amperes when the contact configurations shown herein are used.

While only a few embodiments and examples of the instant invention have been illustrated and described, these are only intended to he illustrative.

I claim:

1. A vacuum circuit interrupter comprising an evacuated envelope, a pair of contact members having engageable portions and being constructed and arranged for movement into and out of engagement, whereby a current interrupting arc is struck between said engageable portions, the improvement wherein at least one of said engageable portions comprises a porous refractory material including a compound consisting of a refractory metal and a metalloid taken from the group consisting of boron and silicon, and an impregnant consisting of a nonrefractory high thermal and electrical conductivity material, wherein the refractory material is readily wet by the nonrefractory material when the latter is in its liquid state.

2. The interrupter set forth in claim 1 wherein the oxides of said metalloid are solid at the normal operating temperatures of the interrupter.

3. The circuit interrupter set forth in claim 1 wherein said metal is taken from the group consisting of Ba, Ce, Cr, Hf, La, Mo, Nb, Sr, Ta, Th, W, U, V and Zr.

4. The circuit interrupter set forth in claim 1 wherein said metal is taken from the group consisting of Mo, Cr and W.

-5. The circuit interrupter set forth in claim 1 wherein said metal comprises tungsten and said metalloid comprises boron.

6. A vacuum circuit interrupter comprising an evacuated envelope, a pair of contact members having engageable portions and being constructed and arranged for movement into and out of engagement, whereby a current interrupting arc is struck between said engageable portions, the improvement wherein at least one of said engageable portions comprises a porous refractory matrix including a compound of boron and a metal taken from the group consisting of Mo, Cr, Zr, Hf and W impregnated with a nonrefractory high thermal and electrical conductivity metal.

7. The circuit interrupter set forth in claim 6 wherein said nonrefractory consists essentially of a metal taken from the group consisting of copper, silver and high thermal and electrical conductivity mixtures thereof.

8. A vacuum circuit interrupter comprising an evacuated envelope, a pair of contact members having engageable portions and being constructed and arranged for movement into and out of engagement, whereby a current 7 interrupting arc is struck between said engageable por- OTHER REFERENCES tions, the improvement wherein at least one of said en- Hacklfs chemical Dictionary; by Julius Grant; third gageable Portia? matrix induding a edition; copyright, 1944, by the McGraw-Hill Book Comcompound of zlrcomum and boron impregnated with a pany, Inc; p. 529 pertinent nonrefractory high thermal and electrical conductivity metal- ROBERT s. MACON, Primary Examiner References Cited UNITED STATES PATENTS