US 3236976 A
Description (OCR text may contain errors)
Feb. 22, 1966 P. J. RAYNO FUSE DEVICE Filed June 22, 1961 United States Patent 3,236,976 FUSE DEVICE Paul J. Rayno, Hudson Falls, N.Y., assignor to General Electric Company, a corporation of New York Filed June 22, 1961, Ser. No. 118,822 5 Claims. (Cl. 200-135) The present invention relates to an electrical fuse device, and more particularly to a fuse device adapted for use in electrical capacitors.
The use of fuses in conjunction with capacitors for isolating one or more defective capacitor units from other units is already known in the art. While it is often advantageous to locate the fuse inside the wound capacitor roll, the known types of capacitor fuses have not proved satisfactory when so located, due to variation in characteristics and unreliability of prior art fuses in such location. One cause, for example, appears to be the variation in the tightness of the roll in which the fuse is incorporated, resulting in large changes in the fuse cooling rates. It appears that known types of fuses are unduly sensitive to changing or varied environment such as may be encountered in conventional capacitor designs and either must be isolated from it or the environment conditions must be made constant. These alternatives often are not feasible from a practical or economic standpoint in the commercial manufacture of capacitors. Aside from the above disadvantages of prior art fuses as applied specifically to capacitors, known types of fuse construction are often unsatisfactory due to the relatively long period required to blow the fuse at any level above thegiven rating. Known fuses, moreover, have the disadvantage that the current required to effectively blow the fuse is often unduly in excess of the carrying (rated) current, that is, the maximum current which the fuse can sustain before it begins to melt. In other words, such fuses have too high a ratio of blowing current to carrying current.
It is an object of the invention to provide an electrical fuse which avoids the above and other disadvantages of prior art fuses.
It is another object of the invention to provide an electrical fuse which hasa shortened blowing time, has an accurately predictable blowing current, has a lower ratio of blowing current to carrying current than prior art fuses, and is more reliable in operation than known types of fuses.
It is a further object of the invention to provide a fuse device, especially one adapted for electrical capacitor application, having reliable operation even under a wide range of environmental conditions, and which is compatible with conventional capacitor dielectric impregnants.
Other objects and advantages will become apparent from the following description and the appended claims.
With the above objects in view, the present invention relates to a fuse device which comprises an elongated electrically conducting metal member having a melting point of not less than about 600 C. divided into two portions separated by a gap, and a fusible member having a melting point not higher than about 400 C. connected at its ends to the ends of the separated conducting portions and conductively bridging the gap therebetween, the cross-sectional area of the fusible member at each end thereof being not less than the cross-sectional area of the metal member at the points of contact therewith.
In a preferred embodiment as used in capacitors, the fuse member is covered by a sheet of electrically insulating material, such as kraft paper or the equivalent, the insulating sheet having characteristics as more fully described hereinafter. In another preferred embodiment, the conducting member portions are tapered toward the ends of the fusible member.
The invention will be better understood from the following description taken in conjunction with the accompanying drawing, in which:
FIGURE 1 is a fragmentary perspective view of a fuse device of the present invention;
FIGURE 2 is a similar view of another embodiment of the invention;
FIGURE 3 is a similar view of still another embodiment of the present invention;
FIGURE 4 is a view of a partially unwound capacitor roll section, with portions broken away, incorporating a fuse device of the type shown in FIG. 2.
FIGURE 5 is an enlarged exploded view of the fuse device shown in FIGURE 4; and
FIGURE 6 is a view, partly broken away, of a capacitor assembly in which the fuse device of the invention may be embodied.
Referring now to the drawing, and particularly to FIG- URE 1, there is shown a fuse device constructed in accordance with the invention comprising an elongated electrical conductor 2 divided into portions 2a and 2b by a gap, and fuse link 3 bridging the gap. Fuse link 3 is electrically connected to the adjacent ends of conductor portion 2a, 212 by solder joints 3a, 3b or by any other suitable means which effectively electrically connect the parts. In accordance with the principles of the invention, fuse link 3 is preferably circular in cross-section, has a melting point not higherthan about 400 C., its crosssectional area at each end thereof is not less than the cross-sectional area of conductor 2 at the points of contact therewith and is preferably substantially equal thereto, and conductor 2 has a melting point not lower than about 600 C. For optimum results, the length of fuse link 3 should be between about 5 to 20 times its diameter, and within this range better results are generally obtained with shorter fuse links than longer ones, as will be explained hereinafter. Usually, also, there is an advantage in using shorter links for fuses to be used in air or free oil as compared to fuses used inside a capacitor roll.
Examples of low melting point fusible materials which may be used for fuse link 3 in practicing the invention are as follows:
Table] Composition: Melting point, C. 20% Bi% Sn 200 40% Pb60% Sn 186 33% Bi-67% Sn 166 50% Bi50% Pb 160 50% Bi27% Pb-l3% Sn10% Cd 72 Pb 327 100% Sn 232 100% Cd 321 Conductive materials of high melting point suitable for use as conductor 2 are as follows:
Table II Composition: Melting point, C. Copper 1083 Silver 960 Bronze 900 Brass 900 Aluminum 660 The ends of fuse link 3 may be soldered to conductors 2a, 2!) at joints 3a, 3b by use of a solder material of somewhat lower melting point than fuse link 3 itself. Such soldering may be achieved practically by initially coating fuse link 3 with such a solder material, as, for example, one composed of 33% bismuth and 67% tin which has a melting point of 166 C., and then soldering the parts by suitable application of heat at the ends of link 3 while in contact with ends of conductors 2a, 2b in accordance with known techniques. The specific solder material just described would be suitable for a fuse link melting at 186 C., and as will be understood, different solder alloys would be selected for use with different link alloys.
The principle of the present invention, in one of its aspects, departs from the prior art in that while the prior art finds it desirable to provide for heat conduction in a fuse away from the center of the fusible element and toward the terminals or conductors adjoining the fuse, the present invention in contrast provides for primary loss of heat from the fusible link through its surface to the ambient medium rather than from the link to the associated conductor. The prior art teaches away from the use of shorter as compared to longer fuse links because the shorter length of conventional fuses does not melt as quickly as the longer link under the same overload current conditions, the prior art concept being that with the same current, the rate of heat conduction to the terminals with a longer link is less than with a shorter link.
In the case of fuse devices of the present invention, however, the shorter the fuse link, the lower is the rate of heat loss from the link to the terminals, which is contrary to conventional fuse characteristics.
By virtue of the construction provided by this invention, the fuse has a number of advantages over known fuse designs. For one thing, the range of temperature between the maximum carrying current and the blowing current is reduced to a minimum, resulting in very reliable and accurately predictable operation of the fuse. Moreover, the amount of heat given off by the fuse and associated parts into the environment is markedly reduced and there is thereby avoided the consequent detrimental effects to the electrical device with which it is associated, as well as to the fuse itself.
The use of a low melting point fuse link in accordance with the invention contributes to the above-mentioned advantages, in that the fuse element melts at temperatures considerably below that of conventional high melting point fuse elements, and thus the amount of heat produced at blowing temperatures is quite small. This diminishes the effect of the surrounding environment which in the case of conventional fuse constructions has led to unreliable fuse operation.
In general, in practicing the invention, it is preferable that the conductors with which the fuse is in series he of thin, flat, strip configuration, a form which provides maximum heat-dissipating characteristics. For this component, copper and silver are preferred materials because of their excellent electrical and thermal conducting characteristics, and such conductors in strip form contribute to a low heating fuse.
FIGURE 2 shows another embodiment of the fuse device wherein the conductor portions 4a, 4b are tapered toward fuse link 5 which is soldered or otherwise secured to the peaks of the tapered ends, the other structural characteristics being as described in connection with FIGURE 1. Such tapered construction has the advantage of retarding heat flow from link 5 to conductor portions 4a, 4b, and thereby proportionately increases the percentage of heat flow from the link surface to the ambient medium as compared to the heat flow from link 5 to conductors 4a, 4b. This result is desirable because when less heat flow occurs to the conductors 4a, 4b, there is less influence of environmental conditions on the operation of the fuse and the fuse produces proportionately less heat for any given rating.
The tapered portions of conductors 4a, 4b preferably taper back for a distance at least equal to the width of the conductors and should be reduced in cross-section at the point of junction with fuse link 5 to the extent that resistance to electrical flow per unit of length at the narrowest point should be a value substantially equal to the electrical resistance per unit of length of the fuse link. When thus proportioned, fuse link 5 at high energy levels will open at the two points of junction, and at low overload blowing currents will open at the center of the fusible element.
FIGURE 3 shows another embodiment of the invention comprising a fuse device having a fuse link 6 bridging round conductors 7a, 7b. While such a configuration does not have the advantage of strip-shaped conductors as explained above, the FIGURE 3 form, which otherwise has the features and characteristics of the previously described fuses, is satisfactory for operation in a variety of applications. 1
FIGURE 4 shows a fuse device of the present lIIVfiIl tion as applied to an electrical capacitor. As illustrated, the capacitor comprises a rolled capacitor section 8 made up in conventional arrangement of wound alternate strips of metal foil 9 and 10 serving as electrodes with interposed strips of dielectric material 11 and 12 such as kraft paper, all interwound into a compact roll. Each alternate dielectric layer may be of a single sheet but is usually constituted by a plurality of sheets, and has a greater width than the electrode strips 9, 10. As shown, the dielectric layers 11, 12 project a suiiicient distance beyond the longitudinal edges of foils 9, 10 to prevent short circuiting between the electrodes which, in opera= tion, are of opposite polarity. In electrical contact with the respective foil electrodes 9, 10 are tap straps 13 and 14 which project from the ends of the roll. Tap strap 14 is provided with a fuse device in accordance with the invention and as will be seen by reference to FIGURE comprises tapered tap conductor portions 14a, 14b bridged by fuse link 15 with an electrically insulating sheet 16 enveloping the tap in the region of the use link and with the margins of sheet 16 at its open side suitably joined together, e.g., by adhesive material. Metal foil sheet 17 is folded about the inner end of the tap for making effective contact between the latter and electrode foil 10. Use of a flag such as sheet 17 in conjunction with tap 14 facilitates insertion of the tap into the roll during winding of the capacitor using automatic insertion equipment of known type. However, use of sheet 17 is not absolutely necessary, since tap 14 may be secured at its lower portion 14b directly to foil 10 by any suitable means, such as by welding, stitching, etc. As shown in FIGURE 4, tap 14 should be so arranged in roll 8 that portion 14a does not touch foil 10 and to this end insulating cover sheet 16 should extend well beyond the edge of foil 10.
The tap-fuse arrangement shown in FIGURE 4 and particularly the provision of cover sheet 16 are, as will be evident, necessary in the illustrated capacitor applica-' tion to prevent fuse link 15 from being electrically bypassed by the foil conductor which is in physical contact with the tap. While it might ordinarily have been considered that covering the fuse link with an electrically insulating material such as kraft paper would interfere with the desired thermal conditions in the fuse area, it has been found, however, that proper selection of the insulating cover will result in a material which will not significantly affect the heat flow from the fuse device. By properly dimensioning the surface area of insulating cover 16 and relating this factor to the thermal conductivity of its material, it is possible to provide a balance between the heat confined and the heat given off from the insulating cover. Increased surface area of the insulating cover increases thermal flow, while lower thermal conductivity of the cover material hinders heat. flow through it. The thickness of the sheet used will, of course, affect its thermal conductivity. In short, an in sulating cover is used wherein the thermal conductivity of the sheet balances its thermal resistance. The cover thus constitutes a thermal element which is half way between a thermal conductor and a thermal insulator and thereby contributes to a workable fuse dQV SZQ iQ I2 tection of a roll capacitor.
FIGURE 6 illustrates the fused capacitor roll section 8 inclosed in a sealed casing 18 containing a dielectric liquid 23 such as mineral oil or chlorinated diphenyl in which roll section 8 is immersed and which impregnates the dielectric sheets of the roll. Taps 13 and 14 are connected respectively to external terminals 19 and 20 which pass through insulating bushings 21, 22 mounted in cover 18a. The fuse device of the invention has been found to operate quite successfully in such capacitor assemblies.
By way of illustration, a capacitor fuse tap construction made in accordance with the invention had a fusible element of 60% tin-40% lead forming a round wire of .020" diameter and .25" length, the fusible element being butt soldered to tapered ends of tinned copper taps having a .160" width, .035" thickness and 2 /2" length. The fuse was covered with a gummed kraft paper sheet having unfolded dimensions of /2" width, .002" thickness and 1%" length.
The maximum continuous 60 cycle A.C. carrying current rating of the above fuse construction in uncirculated 25 C. air was found to be 8.8 amperes. The device functions in air, oil, or as part of a tap assembly of a capacitor, with currents of the .order of 20% above sustaining current levels blowing the fuse in less than five seconds, this being achieved without overheating the capacitor or charring the cover of the fuse tap.
The above fuse (Sample A) which had a link with length/diameter ratio of 12.5 to 1 was tested for performance characteristics with three other fuses which had the identical construction except that one, Sample B, had a link length of .0125" (ratio of 6.25 to 1) and a current rating of 10.5 amperes under the same conditions as with Sample A; another, Sample C, a link length of .0375 (ratio of 18.75 to 1) and current rating of 6.5 amperes; and another, Sample D, a link length of .750" (ratio of 37.5 to l), the latter being a typical length of fuse link used in conventional enclosed solder-wire link fuses, and having a current rating of 4.5 amperes. Measured performance of these fuses showed that they blew in three seconds with the following percent values of current load increase above continuous sustaining levels under the conditions shown:
It will be evident from the above data that the Sample D length of fuse link which is substantially above the 20 to 1 ratio limit required a considerably greater increase in current load in order to blow in the same time as the samples having length/diameter ratios within the ranges found to be optimum in accordance with the invention.
While the present invention has been described with reference to particular embodiments thereof, it will be understood that numerous modifications may be made by those skilled in the art without actually departing from the scope of the invention. Therefore, the appended claims are intended to cover all such equivalent variations as come within the true spirit and scope of the invention.
What I claim as new and desire to secure by Letters Patent of the United States is:
1. A fuse device comprising an elongated electrically conducting metal member having a melting point not less than about 600 C. divided intermediate its ends into two portions separated by a gap, and a fusible member having a melting point not more than about 400 C. connected at its ends to said separated portions of said conducting member and conductively bridging the gap therebetween, the cross-sectional area of said fusible member at each end thereof being at least equal to the cross-sectional area of said metal conducting member at the points of contact therewith, thereby insuring that the primary mode of heat dissipation is from said fusible member to the ambient rather than from said fusible member to said conducting member.
2. A fuse device comprising an elongated electrically conducting metal member having a melting point not less than about 600 C. divided intermediate its ends into two portions separated by a gap, and a fusible member having a melting point not more than about 400 C. connected at its ends to said separated portions of said conducting member and conductively bridging the gap therebetween, the cross-sectional area of said fusible member at each end thereof being not less than the cross-sectional area of said metal conducting member at the points of contact therewith thereby insuring that the primary mode of heat dissipation is from said fusible member to the ambient rather than through said conducting member, and electrically insulating material covering said fusible member and being of such dimensions that its thermal conductivity balances its thermal resistance.
3. A fuse device comprising an elongated electrically conducting metal member having a melting point not less than about 600 C. divided intermediate its ends into two portions separated by a gap, and a fusible member having a melting point not more than about 400 C. connected at its ends to said separated portions of said conducting member and conductively bridging the gap therebetween, the cross-sectional area of said fusible member being substantially circular and being approximately equal to the cross-sectional area of said metal conducting member at the points of contact therewith, the length of said fusible member being not more than about twenty times its diameter, thereby minimizing the percent values of current load increase above continuous sustaining levels before said fuse device blows.
4. A fuse device comprising an elongated strip-shaped electrically conducting metal member having a melting point of not less than about 600 C. and divided intermediate its ends into two portions separated by a gap, the ends of said portions adjacent the gap being tapered toward the gap, and a fusible member having a melting point not more than about 400 C. connected at its ends to the separated portions of said conducting member and conductively bridging the gap therebetween, the fusible member having a circular cross-section and its cross-sectional area being approximately equal to the cross-sectional area of the non-tapered portion of said metal conducting member at a point on the conducting member adjacent the tapered portion, the length of said fusible member being in the range of about five to about twenty times its diameter, thereby minimizing the percent values of current load increase above continuous sustaining levels before said fuse device blows.
5. A fuse device comprising an elongated electrically conducting metal member having a melting point not less than about 600 C. and divided intermediate its ends into two portions separated by a gap, a fusible member bridging said gap, said fusible member having a melting point not higher than about 400 C., and solder material having a melting point lower than that of said fusible member coating and joining the latter in electrical contact with the ends of said conducting portions, the cross-sectional area of the fusible member at each end thereof being not less than the cross-sectional area of said metal member at the points of contact therewith, thereby insuring that the primary mode of heat dissipation is from said fusible member to the ambient rather than through said conducting member.
(References on following page) 7 8 References Cited by the Examiner 2,815,414 12/ 1957 Iwantscheff et a1 200136 2,983,856 5/1961 Martin et a1 317-260 UNITED STATES PATENTS 713,831 11/1902 Badeau 200 135 3 fji PATENTS 776,660 12/1904 Glover 200-135 5 703,6 2 2 19 Germany- 12 ,704,341 3/1955 Stacy et a1. 317-256 JOHN F. BURNS, Primary Examiner.