US2975255A - Vacuum circuit interrupters - Google Patents

Description

March 14, 1961 J. M. LAFFERTY 2,975,255

VACUUM CIRCUIT INTERRUPTERS Filed July 24, 1958 @IX/ll/l in van tor.-

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//s Attorney United States Patent VACUUM CIRCUIT INTERRUPTERS James M. Lafferty, Schenectady, N.Y., assignor to General Electric Company, a corporation of New York Filed July 24, 1958, Sel. No. 750,614

6 Claims. (Cl. 200-144) The present invention relates generally to vacuum type electric circuit interrupters uniquely adapted for use in inductive circuits wherein the current interrupted is of low magnitude.

The phenomena of electric current interruption in atmosphere or higher pressures, on one hand and in high vacuum on the other hand are extremely complex and basically different in nature. So also, are the problems which must be overcome in producing operative devices to perform either function. Thus, for example, one greatproblem in non-vacuum circuit interrupters is the deterioration of the contacts thereof due to the formation of oxides of the contact metal and other chemical com pounds due to the presence of atmospheric air, a protective gaseous atmosphere or an oil bath. Any compound so formed is readily separated from the interrupter contacts. Such a continuous formation and removal of non-metallic compounds rapidly causes the interrupter contacts to erode and deteriorate.

In the vacuum interrupter, on the other hand, the complete absence of gases or liquids in contact with the interrupter contacts virtually precludes the formation of contact-eroding oxides and compounds.

Other problems, however, not generally considered or significant in the non-vacuum circuit interrupter are of prime importance in the construction of vacuum circuit interrupters. Vacuum interrupters designed to interrupt alternating currents operate substantially as follows:

When an arc is struck between the electrodes of a vacuum interrupter, as for example, by the opening of a pair of switch contacts maintained in a vacuum of 10" mm. of mercury pressure or less, the arc vaporizes some of the electrode material. If the contacts have been properly processed to remove therefrom all occluded and sorbed gases, only ionized metallic vapor is present in the arc. The are continues to exist until the alternaaing current sought to be interrupted falls to a natural zero value, as normally occurs once each alternation. When this occurs, the arc is distinguished, and the ionized metallic particles rapidly diffuse to the cold vacuum chamber walls where they are cooled and de-ionized. When the interrupted current seeks to rise again, the are usually cannot be reestablished because of the high dielectric strength of the vacuum separating the interrupter contacts. Thus, in an alternating current vacuum circuit interrupter, the current is usually completely extinguished at the first naturally occurring instantaneous zero value of current.

The foregoing occurs satisfactorily in vacuum circuit interrupters operating currents of high current ratings, namely of 500 amperes or greater.v For lower values of alternating current, the phenomenon generally de nominated as chopping occurs. .At these low alternating current values, the arc, rather than persisting until the first natural current zero, is abruptlyv extinguished at some low current value and falls instantaneouslyfrom that value to zero. The value of current at which instantaneous. arc extinction occurs is denominated the.

chopping current of the device. Typical chopping currents of prior art vacuum interrupters may be from 10 to 40 amperes. While chopping in vacuum interrupters associated with capacitive circuits may be tolerated, it is unacceptable with inductive loads because of voltage surges induced therein by the high rate of change of current time (di/dt) when an interrupter chops. This may be seen from the relationship:

L n e where V=the surge voltage induced by chopping 1 =the chopping current.

L=the equivalent inductance of the circuit C=the equivalent capacitance of the circuit,

and surge impedances of inductive devices commonly have values of tens of thouands ohms.

For most industrial inductive circuit loads, it is necessary to reduce the chopping current level of a vacuum circuit interrupter to a value of below 4 amperes and, in most instances to a value of below 2 amperes.

Accordingly, it is an object of the present invention to provide vacuum circuit interrupters suitable for inter- 4 rupting low value alternating currents in inductive circuits.

tion thereof to a value of several amperes.

Still another object of the invention is to provide vacuum circuit interrupters having arc electrodes which minimize chopping and have hardness and brittleness characteristics which avoid permanent welding together of the contacts or shattering thereof.

In accord with the present invention a vacuum circuit interrupter is provided in the form of an evacuable chamber capable of maintaining a vacuum of less than 10* mm. of mercury, which chamber contains a pair of opposed arc-electrodes adapted to be the terminal points of an electric are carrying the current to be interrupted. The portions of the arc electrodes which serve as such terminal points are substantially free of occluded and sorbed gases and are formed primarily of an alloy of antimony, bismuth or mixtures thereof, the remainder being copper, silver or mixtures thereof. While the electrodes may be constructed substantially entirely of the above class of alloys, these alloys may, in one embodiment, be utilized as an impregnating agent to fill the alternating current illustrating the effect of the chopping phenomenon.

In Fig. 1, an interrupter chamber 10 comprises a wall member 11 which may be cylindrical in shape and is constructed of a suitable insulating material, having at the ends thereof a pair of metallic end members 12 and 13 closing the volume therein to form an interrupter chamber. Suitable seals 14 are provided between casing 11 and end members 12 and 13 to render the interrupter chamber vacuum tight.

A further object of the invention is to provide a vacuum circuit interrupter capable of reducing the value ofinstantaneous alternating current changes upon interrup-' Located within chamber are a pair of separable contacts or are electrodes and 16 shown in their closed circuit or engaged position. Upper contact 15 is a stationary contact suitably attached electrically and mechanically to a conducting rod 17 which, at its upper end, is united electrically and mechanically with end member 12. Lower contact 18, mounted upon, and electrically united with a suitable conducting rod 18, is movable and is connected through bellows or an equivalent vacuum tight member premitting reciprocating motion. Terminal mounting rod 18 projects through a suitable orifice in end member 13, and suitable actuating means may be connected thereto to cause a reciprocating motion of rod 18 to cause contact 16 to enter into engagement with, and be removed out of engagement with, contact 15. For certain types of interrupters, as for example, vacuum fuses and lighting arresters, electrode 16 need not be movable but may be spaced apart from electrode 15 a suitable distance. The electrical circuit which is sought to be interrupted by the interrupter device may be completed by making suitable connections to contact terminal 21, electrically and mechanically mounted upon end member 12, and terminal 22, electrically and mechanically mounted upon rod 18. A suitable insulator shield, such as metallic cylindrical member 23, capped with an arc-preventing ferrule 24, is interposed between electrodes 15-16 and insulator 11 to prevent the latter from becoming coated with metal and becoming electrically short-circuited.

The volume within interrupter chamber 10 is suitably evacuated through an exhaust tubulation (not shown) during the final assembly thereof. For proper operation of the interrupter as a vacuum-type interrupter of alternating currents, the pressure within chamber 10 must be maintained at a pressure at least 10 mm. of mercury, but is preferably maintained within the range 10- to 10'' mm. of mercury. The foregoing requirement is essential for the operation of the devices as vacuum interrupters of alternating currents. This requirement is necessary because, in order that the current-carrying arc struck between electrodes 15 and 16 be extinguished at the first occurring current zero value, there must be substantially no ionizable gas present within chamber 10. The occurrence of ionization may be substantially prevented if the possible breakdown paths between electrodes 15 and 16, or their respective supports, are small with respect to the mean free path of an electron within the atmosphere obtained within the device. This mean free path is designated as the statistical distance which an electron may travel without colliding with agas molecule at a given pressure. These conditions may be established within the devices of the present invention only when the pressure within interrupter 10 chamber is below 10- mm. of mercury and preferably below 10- mm. of

mercury.

In Fig. 2 of the drawing there is shown, in graphical form, an illustration of the choppingphenomena. In Fig. 2 the instantaneous value of a sinusoidal alternating current, sought to be interrupted by a vacuum circuit interrupter, is plotted for one-half cycle. As the current depicted by curve A rises from instantaneous value of zero, the contacts, as for example, contacts 15 and 16 in Fig. 1, are separated, causing the establishment of an arc discharge therebetween. This arc discharge is sustained exclusively by the metal evaporated from contacts 15 and 16 by the heat generated at the contact surfaces by the are. The terminal points of the are are known respectively as cathode and anode spots. As a matter of practice, most of this evaporation occurs at the cathode, or negatively maintained electrode. It is, however, difficult to predetermine which electrode is negative ata particular instant when an alternating current circuit is interrupted. In Fig. 2,; the value of current flowing in the arc follows its natural course along the sinusoid of curve A and, for high current arcs (those above 500 amperes),

follows the dotted line until a zero value is reached. At this instant, the arc is extinguished and the energized metallic ions between the electrodes rapidly diffuse to the cold walls of members 12, 13 and 23 where they are cooled and deionized. The are remains extinguished because, when a high voltage builds up between contacts 15 and 16, the arc is not reestablished, due to the high dielectric strength of the vacuum separatingthe contacts.

In the operation of vauum circuit interrupters wherein the current sought to be interrupted is of a relatively low value (below 500 amperes) the instantaneous current value does not follow the dotted line continuously to a zero value but, rather, at some low current value, denominated by I and occurring at time 0, the arc is abruptly and prematurely extinguished. This results in an instantaneous change of current from a value of 1,, to zero. The value I is referred to herein as the chopping current value for a particular device. As will be readily appreciated, this almost instantaneous change of current from I to zero results in a high rate of change of current with time (di/dt) and may result in the production of extremely high surge voltages in inductive loads which may be connected thereto. These surges may cause the breakdown of insulation and are generally highly injurious to electrical equipment.

In the copending application of T. H. Lee and I. D. Cobine, S.N. 750,784 filed concurrently herewith, it is disclosed that chopping of low level alternating currents in vacuum circuit interrupters may be caused by an unstable condition resulting from a preponderance of are: constricting magnetic pressure over opposing vapor pressure in the region of one of the electrodes between which the arc is struck. As a remedy to the chopping problem, it is shown therein that vacuum circuit interrupters may be constructed utilizing arc-electrodes comprising particular high vapor pressure materials so that sufiicient vapor pressure is available to counterbalance the arc-constricting magnetic pressure.

Although the contact materials contemplated by the aforementioned Lee and Cobine application are quite satisfactory in providing low values of chopping current in the interruption of low current alternating currents, certain of these materials in elemental form tend to be relatively soft or to have low melting points. As the result of these characteristics, certain of the pure metals may be readily eroded and melted and, when melted by a relatively high temperature are, tend to weld and stick. Additionally, certain low melting point electrodes have an undue tendency to weld under the heating effect of momentary currents which flow before the arc-electrodes are separated and an arc is struck.

In accord with present inventionat least one of electrodes 15 and 16 and, in some instances both of these electrodes, are composed of alloys, one constituent of which is selected from the group consisting of bismuth, antimony and mixtures thereof while the major con stituent is selected from the group consisting of copper, silver and mixtures thereof. The advantages gained by the alloys of the aforementioned constituents are the attainment of higher melting points, low thermal conductivity, and an unexpected but not excessive degree of hardness and brittleness, highly desirable in vacuum circuit interrupters. As used herein, the melting point of an alloy composition is used to identify that temperature atwhich a body of the alloy composition becomes entirely molten.

Insofar as these characteristics are concerned, the characteristics may be obtained in satisfactory fashion if copper-bismuth alloys are prepared in the range of 10 s 10 to 15% by weight antimony, the remainder being silver. If silver-bismuth is the alloy system chosen, the alloy may be from to 20% by weight of bismuth, the remainder being silver.

If, in all of the alloy systems utilized, the percentage of the high vapor pressure component (bismuth or antimony) is below 10% or lower, the contribution ofthis constituent to supply a quantity of metallic vapor to prevent extinction of the arc, and consequent chopping because of vapor starvation, is insufiicient. If, on the other hand, the quantity of the high vapor pressure material exceeds the maximum stated values, the advantages of hardness and higher melting points are not obtained.

As an illustration of the advantages in the attainment of high melting points by utilizing the specific alloys set forth herein over utilizing elemental high-vapor pressure metals it is to be noted that while bismuth has a melting point of 271 an alloy of 10% bismuth and 90% copper exhibits a melting point of approximately 980 C. and an alloy of 35% bismuth and 80% copper has a melting point of 860 C. Likewise, an alloy of 10% bismuth and 90% silver exhibits a melting point of approximately 840 C. and an alloy of 20% bismuth and 80% silver has a melting point of 825 0., both as compared with the melting point of 271 C. for pure bismuth. Additionally, whereas antimony in its elemental form has a melting point of 630 C., an alloy of 10% antimony and 90% copper has a melting point of approximately 935 C. while an alloy of 20% antomony and 80% copper has a melting point of-870 C. Likewise, an alloy of 10% antimony and 90% silver has a melting point of approximately 835 C. and an alloy of antimony and 85% silver has a melting point of 800 0., all of which are compared with a melting point of 630 C. for elemental antimony.

That the melting point of vacuum alternating current are interrupters is important in preventing excessive melting of the contacts and, more important, in preventing the contacts from welding together, may readily be seen from the fact that the cathode spot of a vacuum arc interrupter, such as is utilized to interrupt currents of several hundred amperes magnitude at 600 volts, often has a temperature of from 2500 K. to 3500 K.

A further unexpected advantage obtained in vacuum arc interrupters utilized for the interruption of alternating current in accord with the present invention, by uitlizing the disclosed alloys of silver or copper with antimony or bismuth, is the highly desirable intermediate brittleness of the alloys formed by the ranges disclosed herein. As is well known in the art, welding together of the contacts of a vacuum switch is a serious problem. If a weld between two contacts composed of relatively ductile elements such as bismuth does tend to form, the weld may withstand the initial shock of an attempt to separate the contacts, so that a permanent juncture therebetween is formed. The alloys of this application, however, are all quite brittle and, should an initial welding action tend to take place, this brittleness, with its attendant lack of tensile strength, allows for such a weld to be broken so that the contacts do not permanently remain joined together The brittleness of the alloys of the present invention does not approach the point, as does that of some elemental high-vapor pressure materials, where shattering of the contact becomes a problem As a condition precedent to satisfactory operation of any vacuum switch it is necessary that the electrodes between which an arc is to exist momentarily be substantially free of occluded and sorbed gases This requires that the electrodes be previously conditioned in order to remove from them all sorbed and occluded gases. For this reason, among others, prior art vacuum circuit interrupters have not heretofore been constructed utilizing any of the high vapor pressure constituents of the alloys disclosed and claimed herein. Rendering a material completely free of occluded and sorbed gases is most effectively done by outgasing and baking the material at 6 extremely high temperatures. Thus, operative vacuum circuit interrupters have in the past utilized refractory meta-ls as the electrodes thereof, since these materials may be heated to temperatures in the range of 2000' C. without appreciably evaporating or melting. Obviously, such treatment cannot be utilized upon the contact of the alloy independently as for example, by repeated. arc meltings in a vacuum. The constituents thereof are then once again melted in vacuum, mixed and cast in the form desired as an electrode or contact.

It is, of course, necessary to provide a standard for occluded and sorbed gases for vacuum interrupter contacts in order that the interrupters be operative.

the devices of the present invention, contacts, when placed in a vacuumized test chamber a few liters in volume and subsequently deeply eroded by a repetitive arcing, as for example, with a voltage of commercial power at a current of 100 amperes or more, the pressure level in the container, a few cycles after arcing, does not rise substantially from its initial value in the absence of get-ters and pumps even if the initial pressure is the order of 10* mm. of mercury. Analytically, this requirement may be stated in the relationship that the contact mate-" rial must contain less than 10- atomic parts of all gases. Since repeated arc erosion in accord with such tests consumes a substantial portion of the contact material, it provides a satisfactory test for the determination whether the arc electrodes are substantially free of all occluded and sorbed gases since even the infinitesimal presence of such would cause a substantial rise in the presence of the test chamber. As a specific example of arc-electrode material utilized in accord with the present invention, contacts of an alloy of 20 weight percent bismuth, copper, approximately /2" in diameter and substantially free of all occluded and sorbed gases successfully interrupted currents of 1500 amperes at 15000 volts repeat-' edly. In low-current tests of the same contact material the chopping level was held to below 1 ampere and the contacts did not weld or stick.

After the arc-electrodes of the interrupter of Fig. 1

have been prepared in accord with the foregoing procedures, the entire assembly is assembled, connected to.

a suitable vacuum pump and evacuated. While evacuated to a pressure of at least lO- mm. of mercury, or less, the entire device is suitably raised to a temperature of at least 500 C. in order to bake out all of the components thereof to free them from surface adsorbed gases so that an operative pressure may be maintained therein even under high temperature conditions caused by repeated arcing between the electrodes thereof. Suitably the device may be evacuated for approximately 10 hours at a pressure l0 mm. of mercury to satisfactory perform this function.

Although it is an object of the present invention to provide harder, intermediately brittle and higher melting point, high vapor pressure contact electrodes for vacuum circuit interrupters, and the alloys of the present invention serve this object, the mechanical characteristics of the alloys alone need not be relied upon for resistance to wear in the fabrication of vacuum circuit interrupters in accord with the present invention. Thus, electrodes 15 and 16 of the device of Fig. 1, in addition to being composed entirely or substantially entirely of the disclosed and claimed alloys, may comprise a refractory However, it has been found that, once the- Accordingly, the elec-- Such a standard is provided by a requirement that, for use in:

matrix or, for example a porous mass of tungsten or molybdenum; into which the high-vapor pressure, highmelting point alloysdisclosed hereinbefore may be added by'infiltration' by melting in intimate contact in high vacuum. In such an arrangement, the mechanical strength of the porous refractory matrix adds greatly to, the utility of the interrupter and greatly prolongs its life.

As is mentioned hereinbefore, presently available evidence indicates that chopping, the premature extinction of an alternating current are before a normally occurring current zero, isprimarily a phenomena which occurs at the cathode electrode. Accordingly, when suitable precautions are taken so that a given electrode may be madetheinstantaneous cathode when an alternating current is interrupted, in order that the advantages of the present invention be obtained, it is only necessary that one ofthe alloys disclosed herein for use as electrodes for vacuum circuit interrupter-s, be utilized for the cathode electrode. Accordingly, it is clearly within the scope of the present invention that operative vacuum circuit interrupters which exhibit low values of chopping currents, without undesirable welding or sticking disadvantages, may be constructed wherein only one of the arc electrodes is fabricated of the alloys disclosed and claimed herein. It is, however, contemplated in a preferred embodiment of the invention that both electrodes be constructed of these materials since, in general,-it is difficult' to predetermine which electrode will be the instantaneous cathode at the instant the circuit is inter rupted and it may not be feasible to provide means for establishing such certainty.

While the invention has been disclosed herein" with respect to certain embodiments thereof, it is apparent that many modifications and changes will readily occur to'those skilled in the art. Accordingly, by the appended claims I intend to cover such modifications and changes as fall'within true spirit and scope of the present invention.

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

1. A vacuum alternating current circuit interrupter comprising; an evacuable envelope evacuated to a'pressure lower than 10- mm. of mercury; a pair of electrical terminals adapted for connection in an alternating current circuit; a pair of electrodes located within said envelope, connected in circuit between said terminals and disposed in spaced-apart relationship during circuit interrupting operation to allow for the establishment of a. circuit interrupting arc therebetween; each of said electrodeshaving'a-region on which the respective electrodes spots for a low current are may be established, one of said electrode regions com-prising an alloy selected from the group consisting of 10 to 35% by weight of bismuth the remainder being copper, 10 to by weight of antimony the remainder being copper, 10 to 15% by 2. The circuit interrupter of claim 1 wherein the electrode region is an alloy of approximately 20% by weight of bismuth, the remainder being copper.

3. A vacuum alternating current circuit interrupter comprising; an evacuable envelope evacuated to a pres"- sure lower than 10* mm. of mercury; a pair: of electrical terminals adapted for connection in an alternating current circuit; a pair of electrodes located within said envelope, connected in circuit between said terminals anddisposed in spaced apart relationship during circuit interrupting operation to allow for the establishment of a circuit interrupting arc therebetween; one of said'elec trodes comprising an alloy selected from the group consisting of 10 to 35% by weight of bismuth the remainder being copper, 10 to 20% by weight of antimony the remainder being copper, 10 to 15% by weight antimony the remainder being silver, and 10 to 20% by weight of bismuth the remainder being silver; said electrodes being substantially free of all occluded and sorbed gases.

4. The circuit interrupter ofclaim 3 wherein the electrode alloy consists essentially of approximately 20% by weight of bismuth the remainder being copper.

5. A vacuum alternating current circuit interrupter comprising; an evacuable envelope evacuatedto a pressure lower than 10' mm. of mercury; a pair of electrical terminals, adapted for connection in an'alternating current circuit; a pair of electrodes located within said envelope, connected in circuit between said terminals and disposed in spaced-apart relationship, during circuit interrupting operation to allow for the establishmentof a circuit interrupting are therebetween; one of said electrodes comprising a porous refractory body the interstices of which are substantially filled with an impreg- 20% by weight of bismuth the remainder being copper. 7

References Cited in the file of this patent UNITED STATES PATENTS Millikan et al. Dec. 9, 1930 Ruben May 14, 1940

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