US3319028A - Springless time lag fuse for motor circuits - Google Patents

Description

May 9, 1967 I SPRINGLESS TIME LAG FUSE FOR MOTOR CIRCUITS Filed Aug. 5, 1965 Inventor: Frederik J KOZCICkCI y MAW y.

F. J. KOZACKA 3,319,028

United States Patent Ofiice 3,319,028 Patented May 9, 1967 3,319,028 SPRINGLESS TIME LAG FUSE FOR MOTOR 5 Claims. (Cl. 200-120) This is a continuation-in-part of my copending patent application Ser. No. 414,630, filed on Nov. 30, 1964, now US. Patent No. 3,261,952, issued Juyl 19, 1966, for Time-Lag Fuse With Ribbon Fuse Link Having Two Systems of Bends.

It is one object of this invention to provide electric fuses incorporating the teachings of US. Patent 3,123,- 693, issued Mar. 4, 1964, for Time-Lag Fuses of the Blade Contact Type, and in US. Patent 3,143,615, issued Aug. 4, 1964, for Springless Time-Lag Fuses for Motor Circuits, yet having significant advantages over the fuses disclosed in these two patents.

If fuses according to the two above patents are filled with an arc-quenching filler having a lower thermal conductivity than quartz sand, their time-lag at about five times the rated current is sufficiently long to preclude blowing at the occurrence of relatively high motor starting inrush currents. The use of such arc-quenching fillers limits, however, the interrupting capacity of the fuse structure. Substitution, in such fuses, of quartz sand for an arc-quenching filler having a relatively lower thermal conductivity results in an increase of the rated current of the fuse structures, and in a reduction of the time-lag thereof, thus limiting their applicability in motor circuits, or making it altogether impossible to use such fuse structures in motor circuits.

It is, therefore, an object of this invention to provide electric time-lag fuses incorporating the teachings of the two aforementioned patents, and having all the advantages of the fuses described and claimed in the two aforementioned patents, yet capable of achieving even in the presence of quartz sand as arc-quenching medium the maximum degree of time-lag ever required in motor circults.

Another object of the invention is to provide time-lag fuses which generate high arc-voltages due to the presence of a silici-ous arc-quenching filler having a high thermal conductivity, particularly quartz sand, and which fuses have a high interrupting capacity and much longer time-lags than any comparable prior art fuses.

Another object of this invention is to provide fuse structures making it possible to control and increase time-lag to a greater extent than had been possible heretofore, and to effect such a control and increase of timelag even in the presence of quartz sand as arc-quenching filler.

Another object of this invention is to provide electric fuses having a fuse link structure including a link-severing overlay of a metal having a relatively low fusing point operative on occurrence of relatively small protracted overload currents to sever the fuse link structure by a metallurgical reaction, wherein the fuse link structure is designed to effectively interrupt high short-circuit currents when submersed in an appropriate arc-quenching mediumand wherein the fuse link structure is designed to achieve any time-lag required for application of the fuses in motor circuits having high motor starting inrush currents,

Another object of this invention is to provide electric fuses capable of achieving any time-lag required for application of the same in motor circuits having high motor starting inrush currents without resorting to spring-biased solder joints for controlling small protracted overload currents, without breaking the continuity of the fuse link structure between the fuse terminals as required when spring-biased solder joints are being used for controlling small protracted overload currents, and without resorting to arc-quenching media having a smaller arc-quenching action than quartz sand.

In the fuses of the two aforementioned US. Patents 3,123,693 and 3,143,615 long time-lags are achieved mainly by folding the center portion of a fuse link transversely to define a duct extending in a direction longitudinally of the casing of the fuse, thus limiting heat exchange between the fuse link and ambient space and adjacent structures. According to the present invention the connector tabs of the fuse link are folded longitudinally to assume substantially zig-zag configuration, this kind of folding being in addition to the aforementioned transverse folding of the center portion of the fuse link. As a result of this kind of composite folding of the fuse link in transverse and longitudinal directions, heat losses through heat transfer are further reduced, resulting in a relatively heavy fuse link structure, or one involving a relatively large mass, to achieve a given current rating. Consequently the lag-times of such fuses are particularly long.

Fuse structures of the kind disclosed and claimed in my aforementioned copending patent application Ser. N 0. 414,630 are particularly indicated where the arc-voltagegenerating center portion of the fuse link is significantly shorter than the length of the casing, or fuse tube, and where the radial clearance between the fuse link and the casing, or fuse tube, is relatively small. The present invention is particularly intended for instances where the radial clearance between the fuse link and the casing, or fuse tube, is relatively large and where the distance between the arc-voltage-generating center portion of the fuse link and the terminal elements of the fuse may be relatively limited.

For a better understanding of the invention reference may be had to the accompanying drawings wherein- FIG. 1 is in part a side elevation and in part a section along -11 of FIG. 2 and illustrates a first embodiment of this invention;

FIG. 2 is in part a top plan view and in part a longitudinal section along 2- of the fuse structure of FIG. 1;

FIG. 3 is in part a side elevation and in part a section along 33 of FIG. 4 and illustrates a second embodiment of this invention; and

FIG. 4 is in part a longitudinal section of the fuse structure of FIG. 3 taken along 4-4 thereof and in part a top plan view of that structure.

Referring now to the drawings, and more particularly to FIGS. 1 and 2 thereof, numeral 1 has been applied to indicate a casing of insulating material such as, for instance, a synthetic-resin-glass cl0th laminate. Both ends of casing 1 are closed by terminal caps 2. The inside of casing 1 is filled with a pulverulent arc-quenching filler 3, preferably granular quartz or quartz sand. Casing 1 houses a ribbon fuse link 4 of current-limiting metal. The term current-limiting metal is used in this context as generic term including silver and copper, and excluding other metals. Fuse link 4 comprises a center portion generally designated by the reference character 4a and connector tabs designated by the reference character 4b. Center portion 4a is perforated or, to be more specific, it has five transverse lines 5 of perforations. Center portion 4a is formed by sheet metal bent substantially to the shape of a duct which is square in cross-section and extends in a direction longitudinally of casing 1. The four surfaces of center portion 4a are relatively narrowly spaced, resulting in mutual heating thereof. The cross-sectional area and the width of the center portion 4a of fuse link 4 have maximum values at the points situated between the lines 5 of perforations, and the crosssectional area and the width of the center portions 4a of fuse link 4 have minimum values at the points where the transverse lines 5 of perforations are located. Fuse link 4 is provided with a link-severing overlay 6 of a metal having a relatively low fusing point, or a fusing point less than that of the base metal, or current-limiting metal. Overlay 6 may, for instance, be of tin, or indium. The cross-sectional area and the width of each pair of connector tabs 4b is less than the maximum cross-sectional area and the maximum width of center portion 4a of which connector tab-s 4b form axial extensions and integral parts. On the other hand, the cross-sectional area and the width of each pair of connector tabs 4b are larger than the minimum cross-sectional area and the minimum width of each center portion 4a. The term minimum width of each center portion 4a is used in this context to denote the total width of each portion 4a, its. the aggregate width of its four sides minus the aggregate width of all the perforations in one of the lines 5 of circular perforations. Each connector tab 4b is folded at two spaced points 7, 8. Points 7 have a predetermined spacing from the median plane A-A of center portion 4a and points 8 have a predetermined spacing from the median plane A-A of center portion 4a, the former spacing being less than the latter spacing. Each connector tab 4b forms at the point '8 thereof a backward loop toward the median plane A-A of center portion 4a, and the terminal cap 2 immediately adjacent thereto. The axially outer ends of connector tabs 4b may be bent around the axially outer rims of easing 1 and sandwiched between the outer surface of casing 1 and the inner surface of caps 2. The axially outer ends of connector tabs 4b are conductively connected to caps 2 by appropriate solder joints (not shown). It will be apparent from FIG; 2 that the two loops formed by each connector tab 4b extending in a direction substantially longitudinally of easing 1. Thus connector tabs 4b have generally a zigzag geometry, each constituent loop thereof being substantially in the shape of a U whose constituent portions extend in a direction longitudinally of easing 1. The portions of connector tabs 4b immediately adjacent bent 7 thereof overlap center portion 4a and are sandwiched between center portion 4a and housing 1.

In FIGS. 3 and 4 the same reference characters as in FIGS. 1 and 2, however with a prime added, have been applied to designate like parts. Thus reference character 1' has been applied to designate a tubular casing of insulating material closed at the ends thereof by a pair of metal caps 2'. Casing 1' is filled with a pulverulent arcquenching filler 3', preferably quartz sand. Fuse link 4 comprises a perforated axially inner center portion 4a and axially outer connector tabs 4b. Center portion 4a is folded transversely to form a multilateral prism extending in a direction longitudinally of casing 1'. Center portion 4a has a minimum cross-sectional area and a minimum width at the points where its five transverse lines 5' of circular perforations are located. At the points between transverse lines 5 of perforations the width and the cross-sectional area of the center portion 4a of fuse link 4' are at maxima. Reference character at the point 7 thereof a forward loop toward 6' has been applied to indicate a link-severing overlay of a metal having a relatively low fusing point, e.g. tin or indium, or appropriate alloys thereof. In the structure of FIGS. 3 and 4 (as well as in that of FIGS. 1 and 2)' the center portion 4a of fuse link 4' is bent into the shape of a four-sided prism which is square in cross-section, and overlay 6 extends transversely across all four sides of the prism. The connector tabs 4b arranged on opposite sides of center portion 4a are bent around the rim of casing 1 to the outer surface thereof and overlapped by terminal caps 2'. Solder joints (not shown) are formed at the points where caps 2 engage connector tabs 4b. The two connector tabs 4b on each of the axially outer ends of center portion 4a have an aggregate cross-sectional area, and an aggregate width, less than the maximum cross-sectional area, and less than the maximum width, of center portion 4a, but larger than the mini-mum cross-sectional area, and the minimum width, of the center portion 4a. Each connector tab 4b is folded at two points 7', 8'. The spacing of point 7' from the median plane'A-A of center portion 4a is less than that of point 8'. Each connector tab 4b forms at its point '8' a backward loop toward the median A-A' of center portion 4a, and each connector tag 4b forms at point 7' a forward loop toward the terminal cap 2 immediately adjacent thereto.

In the structure of FIGS. 1 to 4, in response to major fault currents, fusion occurs at the five serially related points of minimum cross-sectional area defined by the five transverse lines 5 and 5, respectively, of circular perforations. The are voltage generated at these points is sufliciently high to cause interruption of the faulted circuit prior to the next natural current zero. The aforementioned relations of the cross-sectional areas and the width of parts 4a, 4a and 4b, 4b precludes fusion of parts 4b, 4b rather than of parts 4a, 4a on occasion of major fault currents.

In case of relatively small overload currents of inadmissible duration the fuse links 4 or -4' are severed by a metallurgical reaction at the point of narrowest crosssection formed by the transverse center lines 5 and 5', respectively, of perforations. The geometry of the particular kind of link-severing overlay shown in FIGS. 1 to 4, inclusive, has been disclosed and claimed in US. Patent 2,988,620 to F. I. Kozacka, Time-Lag Fuses, June 13, 1961, assigned to the same assignee as the pres ent invention, and reference may be had to the aforementioned patent for additional information regarding the geometry of the link-severing overlay and its relation to the geometry of the fuse link proper.

The transverse and the longitudinal bends in the fuse links 4 and 4' minimize heat exchange, or heat dissipation or, in other words, they increase the thermal efficiency of the fuse structure, and hence require the fuse links to be made of relatively thick and wide sheet metal to achieve a given current carrying capacity, or current rating. This is conductive to long time-lags, or delay times.

The time-current characteristic of a fuse structure as shown in FIGS. 1 and 2 is substantially the same as that shown in the aforementioned US. Patent 3,123,693 illustrating the performance of the kind of fuses disclosed and claimed in that patent. While the link configuration disclosed in US. Patent 3,123,693, in order to achieve with it such a time-current curve, requires an arc-quenching filler having a smaller thermal conductivity than quartz sand and generating relatively small arc-voltages, the structures of FIGS. 1-4 allow to achieve such a timecurrent curve in the presence of quartz sand as arequenching medium. The fuse link structures of FIGS. 1-4 also allow to increase time-lag above and beyond that possible with fuses according to US. Patent 3,123,- 693 if these fuse link structures are combined with a pulverulent arc-quenching filler having a smaller thermal conductivity than quartz sand.

While, in accordance with the patent statutes, I have disclosed the specific details of two embodiments of the invention, it is to-be understood that these details are merely illustrative, and that many variations thereof may be made without departing from the spirit and scope of the invention. It is my desire, therefore, that the language of the accompanying claims be interpreted as broadly as possible, and that it be limited only as required by the prior state of the art.

I claim as my invention:

1. An electric fuse comprising in combination: a tubular casing of insulating material; electroconductive terminal elements arranged at the ends of said casing; a pulverulent arc-quenching filler inside of said casing; a ribbon fuse link of a current-limiting metal inside said casing, submersed in said filler, conductively interconnecting said terminal elements, said fuse link including a perforated center portion having a predetermined maximum cross-sectional area and a predetermined minimum cross sectional area, said center portion being folded transversely to define a duct extending in a direction longitudinally of said casing; a link severing overlay of a metal having a fusing point lower than the fusing point of said current-limiting metal supported by said center portion of said fuse link; said fuse link further including a pair of connector tabs at each of the axially outer ends of said center portion, said pair of connector tabs having a substantially smaller cross-sectional area than said maximum cross-sectional area of said center portion and having a larger cross-sectional area than said minimum cross-sectional area of said center portion, each of said pair of connector tabs being folded twice in a direction substantially longitudinally of said casing to form a pair of loops both extending substantially in a direction longitudinally of said casing; and each of said pair of connector tabs including a portion extending from one of the axially outer ends of said center portion toward the median plane of said center portion and overlapping a portion of said center portion.

2. An electric time-lag fuse comprising in combination: a tubular casing of insulating material; a pair of electroconductive terminal elements each arranged at one of the ends of said casing; a quartz arc-quenching filler inside of said casing; a ribbon fuse link of a current-limiting metal arranged inside said casing, submersed in said filler, conductively interconnecting said pair of terminal elements, said fuse link including a perforated center portion defining a plurality of serially related points having a predetermined maximum cross-sectional area and a plurality of serially related points having a predetermined minimum cross-sectional area, said center portion being folded transversely to define a duct extending in a direction longitudinally of said casing; a link-severing overlay of a metal having a relatively low fusing point supported by said center portion; said fuse link further including a pair of connector tabs at each of the axially outer ends of said center portion and having a cross-sectional area smaller than said maximum cross-sectional area of said center portion and larger than said minimum cross-sectional area of said center portion, each of said pair of connector tabs being folded at a first point thereof having a predetermined axial spacing from the median plane of said center portion to form a backward loop toward said median plane of said center portion situated inside a space laterally bounded by said center portion and said casing and overlapping a portion of said center portion; and each of said pair of connector tabs being further folded at a second point thereof having a smaller axial spacing from said median plane of said center portion than said first point to form a forward loop toward one of said pair of terminal elements immediately adjacent thereto.

3. An electric fuse comprising in combination: a tubular casing of insulating material, electroconductive terminal elements arranged at the ends of said casing; a

pulverulent arc-quenching filler inside said casing; a ribbon fuse link of a current-limiting metal inside said casing, submersed in said filler, conductively interconnecting said terminal elements, said fuse link including a perforated center portion defining a plurality of serially related points having a predetermined maximum crosssectional area and defining a plurality of serially related points having a predetermined minimum cross-sectional area; a link-severing overlay of a metal having a lower fusing point than said current-limiting metal supported by said center portion; said fuse link further including a pair of connector tabs at each of the axially outer ends thereof having a substantially smaller cross-sectional area than said maximum cross-sectional area of said center portion and having a larger cross-sectional area than said minimum cross-sectional area of said center portion, each of said pair of connector tabs being folded longitudinally to assume a zig-zag configuration involving a first loop toward said center portion of said fuse link and overlapping a portion of said center portion and a second loop away from said center portion of said fuse link and arranged radially outwardly from said first loop.

4. An electric fuse comprising in combination: a tubular casing of insulating material; a pair of terminal caps closing the ends of said casing; a granular quartz filler inside said casing; a ribbon fuse link of a currentlimiting metal inside said casing, submersed in said filler, conductively interconnecting the inner surfaces of said pair of terminal caps, said fuse link including a perforated substantially tubular center portion defining a plurality of serially related points having a predetermined maximum cross-sectional area and defining a plurality of serially related points having a predetermined minimum cross-sectional area; a link-severing overlay of a metal having a lower fusing point than said current-limiting metal supported by said center portion; said fuse link further including a pair of connector tabs at each end thereof having ends sandwiched between said casing and one of said pair of terminal caps, each of said pair of connector tabs having a substantially smaller cross-sectional area than said maximum cross-sectional area of said center portion and each of said pair of connector tabs having a larger cross-sectional area than said minimum cross-sectional area of said center portion, and each of said pair of connector tabs being folded twice in a direction substantially longitudinally of said casing to form a pair of loops both extending substantially in a direction longitudinally of said casing, said pair of loops being arranged in regions of said casing radially outwardly from said center portion of said fuse link.

5. An electric fuse comprising in combination: a tubular casing of insulating material; a pair of terminal caps closing the ends of said casing, a granular quartz filler inside said casing; a ribbon fuse link of .a current-limiting metal inside said casing, submersed in said filler, conductively interconnecting said pair of terminal caps, said fuse link including a center portion folded transversely to form a multilateral prism extending in a direction longitudinally of said casing, said center portion defining a plurality of serially related points having a predetermined maximum cross-sectional area and defining a plurality of serially related points having a predetermined minimum cross-sectional area; a link-severing overlay of a metal having a lower fusing point than said currentlimiting metal supported by said center portion; said fuse link further including a pair of connector tabs at each end of said center portion thereof bent around the rims of said casing to the outer surface there-of and overlapped .by said pair of terminal caps, said pair of connector tabs having an aggregate cross-sectional area less than said maximum cross-sectional area of said center portion and said pair of connector tabs having an aggregate cross-sectional area exceeding said minimum crosssectional area of said center portion, each of said pair of connector tabs being folded at a first point thereof having a predetermined axial spacing from the median plane of said center portion and forming a backward loop toward said median plane of said center portion overlapping a portion of said center portion, and each of said pair of connector tabs being further folded at a second point thereof situated between one of the ends of said center portion and said median plane of said center portion and forminga forward loop toward one of said pair of terminal caps immediately adjacent thereto.

No references cited.

5 BERNARD A. GILHEANY, Primary Examiner.

H. GILSON, Assistant Examiner.

Claims (1)

1. AN ELECTRIC FUSE COMPRISING IN COMBINATION: A TUBULAR CASING OF INSULATING MATERIAL; ELECTROCONDUCTIVE TERMINAL ELEMENTS ARRANGED AT THE ENDS OF SAID CASING; A PULVERULENT ARC-QUENCHING FILLER INSIDE OF SAID CASING; A RIBBON FUSE LINK OF A CURRENT-LIMITING METAL INSIDE SAID CASING, SUBMERSED IN SAID FILLER, CONDUCTIVELY INTERCONNECTING SAID TERMINAL ELEMENTS, SAID FUSE LINK INCLUDING A PERFORATED CENTER PORTION HAVING A PREDETERMINED MAXIMUM CROSS-SECTIONAL AREA AND A PREDETERMINED MINIMUM CROSSSECTIONAL AREA, SAID CENTER PORTION BEING FOLDED TRANSVERSELY TO DEFINE A DUCT EXTENDING IN A DIRECTION LONGITUDINALLY OF SAID CASING; A LINK SEVERING OVERLAY OF A METAL HAVING A FUSING POINT LOWER THAN THE FUSING POINT OF SAID CURRENT-LIMITING METAL SUPPORTED BY SAID CENTER PORTION OF SAID FUSE LINK; SAID FUSE LINK FURTHER INCLUDING A PAIR OF CONNECTOR TABS AT EACH OF THE AXIALLY OUTER ENDS OF SAID CENTER PORTION, SAID PAIR OF CONNECTOR TABS HAVING A SUBSTANTIALLY SMALLER CROSS-SECTIONAL AREA THAN SAID MAXIMUM CROSS-SECTIONAL AREA OF SAID CENTER PORTION AND HAVING A LARGER CROSS-SECTIONAL AREA THAN SAID MINIMUM CROSS-SECTIONAL AREA OF SAID CENTER PORTION, EACH OF SAID PAIR OF CONNECTOR TABS BEING FOLDED TWICE IN A DIRECTION SUBSTANTIALLY LONGITUDINALLY OF SAID CASING TO FORM A PAIR OF LOOPS BOTH EXTENDING SUBSTANTIALLY IN A DIRECTION LONGITUDINALLY OF SAID CASING; AND EACH OF SAID PAIR OF CONNECTOR TABS INCLUDING A PORTION EXTENDING FROM ONE OF THE AXIALLY OUTER ENDS OF SAID CENTER PORTION TOWARD THE MEDIAN PLANE OF SAID CENTER PORTION AND OVERLAPPING A PORTION OF SAID CENTER PORTION.

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