US3546407A

US3546407A - Vacuum-type circuit interrupter

Info

Publication number: US3546407A
Application number: US660809A
Authority: US
Inventors: Harold N Schneider
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1967-08-15
Filing date: 1967-08-15
Publication date: 1970-12-08
Anticipated expiration: 1987-12-08
Also published as: GB1172190A

Description

United States Patent 3,014,]07 [2/1961 Cobine fit 8| 200/l44(.2) 3,082,307 3/1963 Greenwood 6! al 200/l44( 3,158,7l9 ll/l964 Polinko,.lr.et al. 200/l44(.2) 3,185,799 5/l965 Greenwood et al ZOO/ 3,3|9,l2l 5/]967 Lee 200/144(.2)X 3,441,698 4/]969 Sofianek ZOO/144(2) Primary ExaminerRobert S. Macon Attorneys.l. Wesley l-laubner, William Freedman, Frank L.

Neuhauser, Oscar B. Waddell and Melvin M. Goldenberg ABSTRACT: A vacuum-type circuit interrupter comprising magnetic means for forcing low current arcs from an arc-initiation region onto a separate first region made of a metal having a low effective current chopping level. High current arcs are forced in an opposite direction from the arc-initiation region onto a separate second region made of a nonrefractory metal well suited for high current interruptions.

VACUUM-TYPE CIRCUIT INTERRUPTER This invention relates to a vacuum-type circuit interrupter and, more particularly, to a vacuum-type circuit interrupter which is characterized by a low current-chopping level and by reduced evaporation of contact material under high current interrupting conditions.

It has been recognized that vacuum interrupters have a tendency to chop" during low current interruptions. By chopping" is meant forcing the current to zero abruptly and prematurely before a natural current zero is reached. The instantaneous current at which chopping occurs is referred to as the current-chopping level of the interrupter. Chopping is undesirable because it can generate across inductive devices overvoltages which usually vary in magnitude as a direct function of the current-chopping level of the interrupter. Accordingly, it is usually desirable to hold the current-chopping level as low as possible.

One solution to this problem is described and claimed in US. Pat. No. 2,975,256-Lee and Cobine, assigned to the assignee of the present invention, where it is proposed that the contactsbe formed of a high vapor pressure, low conductivity metal, such as bismuth, antimony, lead, tin, or a suitable alloy thereof. These high vapor pressure metals are capable of supplying a sufficient quantityof metal vapor to the arc during low current interruptions to prevent the occurence of chopping until very low instantaneous currents are reached,

e.g., a few amperes.

One problem encountered in using these high vapor pressure metals is that high current arcs tend to produce excessive ,vaporization of these metals. This results from the high temperature and the large area of the high current are terminal together withthe low boiling point of the high vapor pressure metal.

An object of my invention is to utilize a high vapor pressure metal to'prevent excessive chopping during low current interruptions without permitting excessive vaporization of the high vapor pressure metal to occur during high current interruptions.

Usually the high vapor pressure metals are dielectrically ratherweak in the sense that a relatively low voltage can v produce a sparkover between electrodes of these materials.

and breaking region on which arcs are initiated during a circuit interrupting operation. A separate first region of the contact is formed of a metal: that has a low effective chopping level relative to the metal of said circuit making and breaking regions. A separate second region of the contact is formed of a nonrefractory metal that iswell suited for high current inter ruptions. Magnetic means operable under low current interrupting conditions is provided; for forcing the terminal of low current arcs initiated on said circuit making and breaking re gion to move from said latter-region onto said first region. Acting in opposition to said magnetic means is arc-motivating means effective under high current interrupting conditions for forcing the terminal of high current arcs initiated on said circuit making and breaking region to move from said latter re gion onto said second region.ln a preferred form of my invention, said magnetic means comprises a plate of high permeability material imbedded in said contact beneath said first region, the plate being saturable at moderate currents to enable 5 said arc-motivating means to dominate over said magnetic means and drive high current arcs onto said second region.

For a better understanding of my invention, reference may be had to the following description taken in conjunction with the accompanying drawing, wherein;

FIG. 1 is a sectional view through a vacuum interrupter embodying one form of my invention.

FIG. 2 is a view takenalong the line 2-2 of FIG. 1.

Referring now to the interrupter of FIG. 1, there is shown a highly evacuated envelope comprising a casing 11 of a suitable insulating material, such as glass or alumina, and a pair of

metallic end caps

12 and 13, closing off the ends of the casing. Suitable seals 14 are provided between 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 awell-known manner to intercept arc-generated metallic vapors before they can reach the casing 11.

Located within the envelope 10 is a pair of

separable contacts

17 and 18, shown in their engaged or closed-circuit position. The upper contact l7'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 thecontacts and thus closes the interrupter. A typical gap length when the contacts are fully open is about one-half inch. The operating rod 18aprojects 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. This can be done by the zone-refining process of U.S; Pat. No. 3,234,35 l -I-lebb, assigned to the assignee of the present inven tion.

Although my invention is. not limited to any particular contact configuration, I prefer to use the contact configuration disclosed and claimed in my U.S. Pat. No. 2,949,520, assigned to the assignee of the present invention. Accordingly, each contact is of a disc 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 contact making and breaking portion area 30 surrounds this recess. These annular contact making and breaking portions 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 loopshaped path L, as is indicatedby the dotted lines of FIG. 1. High current flowing through this loop-shaped path has a magnetic effect which acts in a well-known manner to lengthen the loop. As a result, when the contacts are separated to form a high current are between the portions 30, the magnetic effect of the high current flowing through the path L will impel the arc radially outward.

As the arc terminals move toward the outer periphery of the

discs

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 disc. This circumferentially acting magnetic force is preferably produced by a series of slots 32 provided in the discs and extending from the outer periphery of the discs radially inward by generally spiral paths, as is shown in FIG. 2. These slots 32 correspond to similarly designated slots in my aforementioned 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 disc ro 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.

The annular contact making and breaking portions 30 are preferably formed as separate rings that are suitably brazed to the remainder of the

contacts

17 or 18 along their back surfaces 38. The rings 30 are formed of a material that has a high resistance to contact-welding and good dielectric strength properties, e.g., one of the materials disclosed and claimed in U.S. Pat. NO. 3,246,979, Lafferty et al., assigned to the assignee of the. present invention. A preferred material is a copper bismuth alloy containing .5 percent bismuth.

The body 1712 or 18b of each contact is made of a good conductivity metal which has good mechanical strength properties and is well suited for high current interruption. One example of such a metal is pure copper.

As pointed out in the introductory portion of this specification, I utilize a high vapor pressure metal to reduce the current-chopping level of the interrupter during low current interruptions. This high vapor pressure metal is located in the central recess 29 in each contact and is designated 40. The exposed surface 41 of the high vapor pressure metal is located well behind the front surface of the contact-making region 30. Beneath the high vapor pressure metal 40 is a plate 42 of a high permeability metal such as iron or low carbon steel. This plate 42 is completely imbedded within the contact material and is therefore unexposed to the arcs formed between the contacts.

When a low current are is formed between the contact-making rings 30 during a low current interruption, the magnetic field around the current and the are are distorted by the presence of the high permeability plate 42. This distortion of the magnetic field will force the low current are radially inward onto the high vapor pressure metal 40, where it will remain until a current zero is reached. This makes the high vapor pressure metal available to supply the necessary metallic vapor to prevent chopping at an unduly high current level. Even if the instantaneous current level at contact part is such that chopping will occur before the arc can be moved on to the high vapor pressure'material 40, then contact separation is small and any overvoltage will be self-limited by breakdown across the contacts. If contact separation is large, then the arc will have had time to be drawn onto the high vapor pressure chop-resisting material.

The plate 42 is designed to saturate when moderate values of current flow through the contact-making rings 30. Accordingly, during a high current interruption, the plate 42 will rapidly saturate and lose its ability to control the arc position. The magnetic effect on the high current are of current flowing through the loop-shaped path L will dominate and blow the arc radially outward. The thickness of plate 42 is so chosen as to give magnetic saturation at the desired current levels for transition from inward to outward motion of the arc. At low currents the radially outwardly acting magnetic effect of the current flowing through loop-shaped path I. is very low, and the magnetic effect of the high permeability plate is able to dominate.

Chopping appears to be a problem only for low current interruptions. The precise current level above which chopping ceases to be a problem varies from one material to the next and varies also with the type of circuit application involved. Typically, however, the thickness of the iron plate 42 is chosen so that only those arcs having a peak current less than several hundred amperes'are driven radially inward onto the high vapor pressure metal. Except for those arcs established just prior to current zero, all arcs with peak currents low enough to produce objectionable levels of chopping if interrupted on the material of the contact-making ring 30 or on the outer part of the contact are interrupted with their terminals on the chop-resisting material 40 at the center of the contact.

To assist in controlling the current level at which radially outward motion of the arc occurs, 1 provide a tube 50 of a high resistivity metal such as stainless steel which is imbedded in the contact in a position around its supporting rods 17a or l8a.The presence of this high resistivity tube 50 forces the effective current path to follow a more pronounced loop L. This assures that there will be a radially outward-acting force available to drive high current arcs off the contact-making ring 30 and onto the outer portion of the contact.

By forcing high current arcs radially outward and away from the high vapor pressure material 40, I can prevent these arcs from attaching to the central region 40 and can therefore prevent them from causing excessive vaporization of the high vapor pressure metal. In referring to high current arcs, I am referring to arcs of several thousand amperes or more. A typical current interrupting rating of the illustrated interrupter is 10,000 amperes, r.m.s.

The central region 40 may be of a material that is relatively weak dielectrically inasmuch as this region 40 is in a zone of low electric stress. This is the case because the annular button 30 projects appreciably beyond the exposed surface'of central region 40 toward the other contact. The high stress regions are immediately adjacent the forward surface of the two contactmaking rings 30. v

In myjoint US. Pat. No. 3,182,156, assigned to the assignee of the present invention, it is proposed that the contact-making ring 30 itself be made of a material capable of limiting chopping to the desired level. While this approach can satisfactorily limit chopping, it is subject to the disadvantage that the presence of the chop-resisting material in the high stress region can impair the dielectric strength of the interrupter. The chop-resisting materials are typically weaker dielectrically, and this combined with the high electric stress adjacent the ring 30 can lead to breakdown between the separable contacts at lower voltages than otherwise could be achieved. I am able to overcome this problem because my chop-resisting material is separate from the contact-making ring 30 and is located in a region of low electric stress, as

pointed out hereinabove, where it is less likely to permit a breakdown across the intercontact gap.

Since the regions 40 of chop-resisting material are divorced from any contact-making function, they require no resistance to contact wekling. Thus, I am able to use for this region 40 materials which would be unsuitable if antiwelding properties were needed. For example, I can use an alloy of constituents which are mutually soluble in the solid state, such as coppertin, or bronze.

In an additional embodiment of the invention, the high permeability plate 42 can be omitted and the chop-resisting material itself can be a high permeability metal. For example, the material 40 can be made of an alloy of iron and a secondary constituent such as copper or tin capable of supplying the vapor needed to limit chopping to the desired level. The high permeability of the material 40 results in low current arcs being forced radially inward inthe same manner as described hereinabove.

For high current interruptions, it is usually highly desirable that the arcing surfaces of the contact be free of refractory metals, such as tungsten. This is the case because refractory metals when exposed to a high current are thermionicallyemit for an extended period, and this interferes with dielectric recovery at current zero. But I can use a suitable refractory metal alloy, such as tungsten-tin or tungsten-copper for my material 40, because I prevent this material from being exposed to a high currentarc by driving all such arcs radially outward off of the contact-making ring 30. It is to be understood, however, that the contact-making ring 30 and the radially outer portion of the contact are still devoid of refractory metals on their arcing surfaces.

' While I have shown and described particular embodiments of my invention, it will beobvious to those skilled in the art that various changes and modifications may be made without departing from my invention in its broad aspects; and I, therefore, intend in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of my invention.

1 claim:

l. A vacuum-type circuit interrupter comprising:

a. a highly evacuated envelope;

b. separable contacts within said envelope;

c. one of said contacts having a circuit making and breaking region on which arcs are initiated during a circuit interrupting operation;

d. a separate first region of said contact being of a metal that has a low effective chopping level relative to the metal of said circuit making and breaking region;

e. a separate second region of said contact that is made of a nonrefractory metal well suited for high current interruptions;

e. said first region being located on one side of said circuit making and breaking region and said second region being located on an opposite side of said circuit making and breaking region from said .first region;

f. magnetic means of high permeability material on the same side of said circuit making and breaking region as said first region, operable under low-current interrupting conditions for forcing the terminal of low current arcs initiated on said circuit making and breaking region to move from said circuit making and breaking region onto said first region; and

g. and arc-motivating means opposing said magnetic means and effective under high-current interrupting conditions for forcing the terminal of high current arcs initiated on said circuit making and breaking regions to move therefrom onto said second region.

2. The circuit interrupter of claim 1 in which said magnetic means comprises a plate of high permeability material imbedded in said contact beneath said first region.

3. The circuit interrupter of claim 1 in which said magnetic means comprises a plate of high permeability material imbedded in said contact beneath said first region, said plate being saturable at moderate currents to enable said arcmotivating means to dominate over said magnetic means and drive high current arcs onto said second region.

4. The circuit interrupter of claim 1 in which said magnetic means is constituted by high permeability metal serving as a constituent of the metal of said first region.

5. The circuit interrupter of claim 1 in which the metal of said first region comprises a refractory metal constituent.

6. The circuit interrupter of claim 1 in which said first region is located in a zone of low electric stress compared to the electric stress present adjacent the forward surface of said circuit making and breaking region.

8. The circuit interrupter of claim 7 in which the material of i said first region is located in a position disposed behind the forward surface of said annular ring where the electric stress is low compared to that present adjacent said forward surface.