US3678435A

US3678435A - Electrical resistor

Info

Publication number: US3678435A
Application number: US87925A
Authority: US
Inventors: Herbert M Pflanz
Original assignee: Allis Chalmers Corp
Current assignee: Allis Chalmers Corp
Priority date: 1970-11-09
Filing date: 1970-11-09
Publication date: 1972-07-18
Anticipated expiration: 1989-07-18
Also published as: SE367506B; IT944805B; CH549859A; CA934838A; FR2113683A5; NO129486B; DE2155698A1

Abstract

There are disclosed several embodiments of a resistor which comprises an electrical resistance element made of porous resistance material which is completely or partially embedded in a body of inelastic noncombustible electrical insulating material. In one embodiment, the resistance element in the form of a spirally wound ribbon is embedded in a body in the form of a hollow cylindrical insulator in which circuit breaker contacts are housed to provide a built-in resistor in circuit with the contacts. In another embodiment, the resistance element in spirally wound ribbon form is embedded in a body in the form of solid cylindrical insulator to provide a separate resistor. An electrical device, such as a static or solid state component, can be embedded in the solid body in circuit with the resistance element. In another embodiment, the resistance element in the form of a flat sinuous member is embedded in a body of insulating material in the form of a short solid cylindrical member which may be used independently as a resistor or stacked together with other similar units to provide a larger resistor.

Description

United States Patent Pflanz 1 July 18,1972

[54] ELECTRICAL RESISTOR [72] Inventor: Herbert M. Pflanz, Westwood, Mass.

[21] Appl. No.: 87,925

3,477,058 11/1969 Vedder ..338/238 Primary Examiner-E. A. Goldberg Attorney-Thomas F, Kirby, Lee H. Kaiser and Robert B. Benson ABSTRACT There are disclosed several embodimenm of a resistor which comprises an electrical resistance element made of porous re- 52 us. c1. ..338/263, 338/226, 338/269, Sistance material which is completely or Partially P q in 33 2 338/316, 75 "MG 1 a body of lnelastlc noncombustible electncal 1nsulat1ng [51] Int. Cl ..H0lc 1/02 matenalone embodlmen" the reslstalce element m the 58 Field of Search ..338/208, 210, 263, 269, 238, Mm a P 3""4 P P W the 338/320, 316; 75mm. 1, 222; 264/211; 24/1822, i f g uggnzincal l gulamguiirllttivxilufzsbficifi 1 23 C011 C are 0 0 provr e a with the contacts. in another embodiment, the resistance ele- [56] References Cited ment in spirally wound ribbon form is embedded in a body in the form of solid cylindrical insulator to provide a separate re- UNITED STATES PATENTS sistor. An electrical device, such as a static or solid state component, can be embedded in the solid body in circuit with the 2,287,460 6/1942 Wagenhals ..338/263 resistance element In another embodiment, the resistance 2835465 5/1958 Patterso 338/269 UX element in the form of a flat sinuous member is embedded in a 3,278,279 10/1966 Kraft ..75/DIG. 1 body ofinsulating material in the form ofa short Solid cylindri 3,285,825 1 1/1966 Jens ..75/DIG. l cal member which may be used independently as a resistor or 3 l 1/1966 stacked together with other similar units to provide a larger re- 3,344,385 9/1967 sistor. 3,398,233 8/1968 3,400,356 9/1968 Davis ..338/208 18 Claims, 7 Drawing figures 6i 5 70 66 km 56 60 j 1 5a 60 rH 1 1 x 1 n T s 6i \\\\-7Z\ O O \7:Z\\\\% I h & A- a ELECTRICAL RESISTOR BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates generally to electrical resistors. More particularly, it relates to high wattage electrical resistors which comprise a resistance element embedded in a body of insulating material and which may be employed with or built into electrical circuit interrupters or other electrical devices.

2. Description of the Prior Art Some electrical devices, including electrical circuit interrupters, employ high wattage resistors which perform an electrical function and are subject to rapid and intense heating when current flows therethrough. For example, some fluid (or air) blast type circuit breakers employ such resistors in circuit with separable electrical contacts for voltage grading, damping or other purposes. In such breakers, for example, each pair of separable contacts is enclosed in a gastight insulated housing, usually made of ceramic, and the associated resistor is mounted adjacent the housing as a separate component.

Typically, the prior art resistors take the form of a solid wire or ribbon which is spirally wound on a solid or hollow cylindrical ceramic body or a stack of blocks of resistance material, such as carbon, arranged in a tubular insulated housing. While such prior art resistors are generally satisfactory for their intended purpose, they are additional, large, space consuming, costly components in need of improvement.

Therefore, it would be advantageous in circuit breakers and in some other apparatus to provide the necessary resistor in the form of a built-in or integrally formed device which takes advantage of existing structural features of the apparatus. Furthermore, it would be advantageous to provide such a resistor in the form of a resistance element which is actually embedded (either fully or partially) in a body of electrical insulating material. However, prior efforts to embed or encapsulate resistance elements in electrical insulating materials have not always been completely successful. For example, thermal expansion of conventional resistance elements tends to cause breakage of the element, detachment of it from the associated insulating body, or cracking of the insulating body.

SUMMARY OF THE INVENTION The present invention provides improved resistors which comprise an electrical resistance element made of resistance material having a plurality or multiplicity of voids therein, such as porous metal, fiber metal or foam metal, which is partially or fully embedded in substantially rigid, nonexpandible, noncompressible, noncombustible electrical insulating materia], such as ceramic, PYROCERAM, RE-EX or the like. Heating of the resistance element caused by electric current flow therethrough does not result in a change in the physical dimensions of that portion of the resistance element which is trapped, clamped or placed under pressure by the insulating material because the resistance material expands inwardly, so to speak, into the voids in the material and this phenomena of inward expansion results in reduced physical and mechanical stresses being imposed on the surrounding body of insulating material.

The resistance element can take any form, for example, such as an elongated conductor, a ribbon, or a flat member, and can be arranged in the body of insulating material in any manner, for example, such as a wire passing straight through, a wire helically or spirally wound therein, or an embedded flat member. The body of insulating material with which the resistance element is associated can be a solid member in which electrical components, such as solid state or static devices, in circuit with the resistance element may be embedded. Or, the body can be a hollow member in which components, such as movable circuit breaker contacts, may be housed.

A resistor in accordance with the present invention may be in the form of an independent unit or one which is integrally formed with insulating bodies serving other functions in electrical apparatus. Furthermore, embodiments can be provided in the form of modular resistor units which can be used individually as resistors or combined to provide resistors of higher rating.

It is preferable in some applications that the resistance element and the body of insulating material have closely matching thermal expansion characteristics. It is also preferable in some applications that the resistance material of which the resistance element is made have the highest possible resistance so as to conserve space and materials, increase thermal performance, or reduce inductive effects.

OBJECTS OF THE INVENTION It is an object of the present invention to provide improved electrical resistors, particularly those wherein a resistance element must be compatible from a thermal expansion and contraction standpoint with a body of insulating material on which the element is supported or mounted.

Another object is to provide improved resistors wherein a resistance element made of material having a multiplicity of voids therein, such as porous, fibrous or foraminous resistance metal, is embedded in a body of electrical insulating material and wherein thermal expansion of the resistance element caused by current flow therethrough is inhibited because, due to the clamping pressure of the surrounding insulating material, inward expansion of the element into its voids takes place whereby minimum physical and mechanical stresses are imposed on the insulating material.

Another object is to provide resistors of the aforesaid character wherein electrical components in circuit with the resistance element are disposed in said body of insulating material, as by direct embedding in the case of solid state components, or by being housed in a cavity in said body of insulating material.

Another object is to provide resistors of the aforesaid character which comprise ribbon type resistance elements fully or partially embedded in a solid or hollow insulating body.

Another object is to provide improved resistors of the aforesaid character which are well suited for use with electrical apparatus, such as electric circuit interrupters, especially those of the fluid (or air) blast type.

Another object is to provide improved resistors of the aforesaid character for use with electrical apparatus wherein the resistance element is physically mounted on or integral with an insulating body which performs other functions in the apparatus besides merely supporting the resistance element.

Another specific object is to provide insulators of the aforesaid character wherein the resistance element is fully or partially embedded in a tubular insulating housing of a fluid blast circuit breaker which houses separable electrical contacts.

A more specific object is to provide modular resistors of the aforesaid character which comprise substantially flat resistance elements embedded in insulating bodies to provide resistor modules, which modules can be used separately as resistors or stacked together to provide larger resistors.

Other objects and advantages of the invention will hereinafter appear.

DESCRIPTION OF THE DRAWING The accompanying drawing illustrates several preferred embodiments of the invention but it is to be understood that the embodiments illustrated are susceptible of modifications with respect to details thereof without departing from the scope of the appended claims.

In the drawings:

FIG. 1 is a cross sectional view of one embodiment of the invention which comprises a porous resistance element fully embedded in the wall of a hollow insulating body such as a housing of an electric circuit breaker which has separable circuit breaker contacts therein;

FIG. 2 is a view taken along line IIII of FIG. 1;

FIG. 3 is a cross sectional view of another embodiment of the invention which comprises a resistance element in parallel circuit with an electrical component fully embedded in a solid insulating body;

FIG. 4 is a view taken along line IVIV of FIG. 3;

FIG. 5 is a cross sectional view of a portion of an embodiment in accordance with the invention which is applicable to the embodiments shown in FIGS. 1 and 3 and which shows a resistance element partially embedded in an insulating body;

FIG. 6 is a cross sectional view of another embodiment of the invention which comprises a plurality of modular units which are stacked together, each modular unit comprising an insulating body in which a resistance element is embedded; and

FIG. 7 is a view taken along lines VII-VII of FIG. 6 showing the configuration of the resistance elements.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIGS. 1 and 2 of the drawing, the numeral 10 designates a portion of electrical apparatus such as a fluid (gas or air) blast type electrical circuit interrupter of the type in which the present invention is advantageously employed. Portion 10, which is a combined resistor and contact housing, comprises a contact housing 12 in which are disposed electrical devices, such as a pair of spaced apart

stationary interrupter contacts

14 and 16 which are bridgeable by a movable bridging contact 18. In accordance with the invention, a resistance element is embedded in housing 12 and is in parallel circuit with or

shunts contacts

14 and 16. In operation, with

contacts

14 and 16 bridged by contact 18 and current flowing therethrough, separation of

contacts

14 and 18 by rightward movement of the latter causes full current flow through resistance element 20 to achieve a desired electrical effect and consequent heating of element 20. In a typical circuit breaker installation, resistance element 20 may reach temperatures on the order of 400 to 500 C, rising to their level from ambient in about 40 msec.

Housing 12 is a body of substantially inelastic non-combustible electrical insulating material such as ceramic, glass, PYROCERAM, Re-X or the like and takes the form of a hollow open ended cylinder having exterior skirts 22 and grooves 24. Housing 12 protects, supports and insulates the

contacts

14, 16 and 18 and channels or contains the insulating and are extinguishing gas which also effects contact operation. The

stationary contacts

14 and 16 are hollow cylindrical electrically conductive members having flanged

ends

26 and 28, respectively, which close off the ends of housing 12. Contact 16 is further provided with a cylinder 30 which cooperates with the piston-like end 32 of bridging contact 18. Contact 18 is understood to be reciprocably movable leftward and rightward in response to pressure changes to contact closed and contact open positions, respectively, but the exact details of the contact construction and operation do not form part of the present invention.

Resistance element 20 is in the form of an elongated conductor or ribbon which is spirally wound and completely embedded in the wall of housing 12, as by being cast therein, and is electrically connected at its opposite ends to

contacts

14 and 16 by

suitable connectors

34 and 36, respectively. If preferred, element 20 may be provided with convolutions 38 to increase its effective length without undue usage of space. In accordance with the present invention, resistance element 20 comprises resistance material which has a multiplicity of voids therein, such as porous metal, fiber metal or foam metal. Metals of this type are commercially available from many sources. Fiber metals, for example, are commercially available as F ELTMETAL from the Huyck Metals Company, 45 Woodmont Road, Milford, Conn. and comprise stainless steel fibers in various percentages of density per unit volume, Le. 20 percent, 45 percent, 62 percent. Normally, heating of a resistance element fabricated of solid metal or other solid resistance material caused by current flow therethrough causes expansion and an increase in the physical dimensions of the element. However, the voids in resistance material are not filled with insulating material and therefore permit inward expansion under the influence of heat of that portion of the resistance element which is entrapped, embedded, clamped, or placed under pressure by the surrounding insulating material while maintaining the heat absorbing boundary with the insulating material. As a result, the physical and mechanical stresses that would otherwise be placed on the frangible surrounding insulating material are substantially reduced and the likelihood of cracking or fracturing of the insulating material is similarly reduced. If preferred, these forces can be further reduced by selection of resistance material in accordance with the invention which is closely compatible from a coefficient of thermal expansion standpoint to the insulating material being selected for use. For example, Kovar metal (about 42 percent nickel and 58 percent iron) has a coefficient of thermal expansion which is similar to standard grade electrical ceramic. It is a feature of the present invention, however, that it does not require exact matching of the coefiicient of thermal expansion of the resistance material and the insulating material. Because of the intimate contact of the surface of the resistance material with the insulating material, heat transfer is excellent. This may even be improved by a large surface to cross section ratio of the porous resistance element. Therefore, advantage can be taken of the specific heat (or the ability to absorb energy) of the insulating material. In the embodiment shown in FIGS. 1 and 2 it is desirable that the resistance material have high electrical resistance, high density, and high specific heat (or, at least, that the product of these factors be as high as possible) to provide the shortest possible conductive path thereby conserving space and reducing inductive effects. In effect, a good interface increases artificially the product of resistivity, density and specific heat of the resistance material, because of the apparent increase of the specific heat or ability to absorb heat of the resistance material which is in contact with a higher specific heat material such as ceramic.

In the present invention a porous resistance material is in close contact with the insulating material. Since the pores are not filled, the resistance element can still expand inwardly under the influence of heat while maintaining the heat absorbing boundary with the ceramic. Such a design is therefore suitable for thermal transients.

FIG. 5 shows a modification which may be employed with the embodiment shown in FIGS. 1 and 2. In FIG. 5, a resistance element 40 identical to resistance element 20 hereinbefore described is shown partially embedded in a body of insulating material 41. The advantage of this construction is that part of element 40 is exposed to atmosphere and more rapidly dissipates heat than one which is fully embedded. However, element 40 is partially exposed and may not be suitable for use where shock hazards or electrical clearances are factors.

As is apparent, the embodiment shown in FIGS. 1 and 2, with or without the modification of FIG. 5, is well suited to effect conservation of space and materials in electrical apparatus such as an air blast circuit breaker which would otherwise require a separate resistor. However, other applications are suitable and other electrical devices besides circuit breaker contacts could be disposed within the hollow cavity in housing 12 and other electrical connections besides the parallel connection shown could be employed.

Referring to FIGS. 3 and 4, the numeral 42 designates an independently usable resistor which comprises a body 44 of insulating material in the form of a solid cylinder having electrically conductive end caps 45 and 46 which provide a means of mechanical support and electrical connection. A resistance element 48, identical to resistance element 20, is fully embedded in body 44 and has its opposite ends connected to end

caps

45 and 46. If preferred, element 48 may be partially embedded, as FIG. 5 shows, if its application so warrants. Furthermore, if preferred, an electrical device or component 50, such as a static state or solid state device, i.e., a capacitor or diode or the like, may be embedded by casting in body 44 and connected in circuit with resistance element 48, as by conductors or wires 52 and 54 which are connected to the device and to the end caps 45 and 46, respectively.

Referring to FIG. 6, the numeral 56 designates another independently usable resistor which comprises a plurality of modular resistor units 58 which are stacked end-to-end and physically secured together by suitable means such as adhesive 60 between adjacent units. Each modular unit 58 comprises a solid body 62 of insulating material, cylindrical in fonn and having flat end faces, in which is embedded a resistance element 64 having a substantially flat, sinuous shape to increase its resistance path. Each element 64 has integrally formed

connectors

66 and 68 at each end which extend from the sides of body 62 and enable electrical connections to be made to the element. As FIG. 6 shows, adjacent connectors 66 in resistor 56 are electrically connected by means of jumpers 70. As will beunderstood, each element 64 is made of resistance material of the same type as used in resistance element 20, hereinbefore described. lf preferred, each element 64 can be fabricated by stamping or punching from sheet stock and each is provided with integrally formed ears or tabs 72 which project from at least one surface of the element and serve as a supporting and positioning means whereby the element is held in place in a suitable casting mold while the insulating material is poured around it during manufacture of each modular unit. As F IG. 6 shows, each end module in resistor 56 has one of its

connectors

66 or 68 electrically connected to end

caps

74 or 76 which are secured to the ends of the stack of modules and provide a mounting means and electrical connection means for resistor 56. Resistor 56 can comprise as many modular units 58 as are necessary to provide a suitable resistor for any given application. Or, an individual modular unit 58 can be used as a complete resistor.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. In combination,

a body of electrical insulating material; and,

a continuous electrical resistance element at least partially embedded, in said body and comprising a plurality of intact metal fibers defining a unitary structure having a multiplicity of voids therein, said metal fiber structure being in intimate contact with said insulating body but the voids not being impregnated with the insulating material,

whereby the metal fiber structure can expand inwardly when heated by electrical current passing therethrough.

2. In combination,

a continuous metal fiber electrical resistance element of high resistivity having a multiplicity of intact metal fibers forming an integral structure having a multiplicity of voids and a density per unit volume of less than 100 percent, said resistance element being at least partially embedded on a body of electrical insulating material but not being impregnated with the insulating material,

whereby the integral metal fiber structure can expand inwardly when heated by electrical current passing therethrough so that mechanical stresses are not imparted to the body of electrical insulating material.

3. A combination according to claim 2 wherein the density per unit volume of said intact metal fiber integral structure is at least 20 percent.

4. A combination according to claim 1 wherein the specific heat of said body of electrical insulating material is higher than that of said intact metal fiber unitary structure.

5. A combination according to claim 1 wherein the density of said intact metal fiber unitary structure is relatively high.

6. A combination according to claim 1 wherein said intact metal fiber unitary structure has high resistivity.

7. A combination according to claim 1 wherein said intact metal fiber unitary structure evidences the characteristic that the product of the factors of electrical resistivity, density and specific heat is relatively high.

8. A combination according to claim 7 wherein said intact metal fiber unitary structure has a lar e surface to cross sectron ratio so that excellent heat trans er is obtained at the interface boundary of the intact metal fiber unitary structure with the body of electrical insulating material,

whereby the product of the factors of electrical resistivity,

density and specific heat are artificially increased.

9. A combination according to claim 1 wherein the intact metal fiber unitary structure has a coefi'icient of thermal expansion which is substantially similar to the coefficient of thermal expansion of said body of electrical insulating material.

10. A combination according to claim 1 wherein the specific heat of said intact metal fiber unitary structure is high.

11. A combination according to claim 1 wherein said intact metal fiber unitary structure is arranged in a helix in said body of electrical insulating material.

12. A combination according to claim 11 wherein said helix arranged intact metal fiber unitary structure is also corrugated.

13. A combination according to claim 1 wherein said voids in said intact metal fiber unitary structure are pores therein.

14. A combination according to claim 1 wherein said intact metal fiber unitary structure is foraminous.

15. A combination according to claim 1 wherein said intact metal fiber unitary structure is completely embedded in said body of electrical insulating material.

16. A combination according to claim 1, wherein said intact metal fiber unitary structure presents a configuration which in cross section has substantially greater width than depth so as to be relatively flat and being completely embedded in said body of electrical insulating material, said unitary structure having its ends extending externally of said body and having connector means thereon, said body and said intact metal fiber unitary structure cooperating to form a modular resistor unit.

17. A combination according to claim 1 wherein a plurality of said modular resistor units are secured together and the intact metal fiber unitary structures connected in series.

18. A combination according to claim 17 wherein adjacent units are joined together by adhesive.

(5/59) I RTKFKCAT F RREQTiQN Patent No. 3 4 5 I Dated July 18, 1972 Inventoits) M- It is certified. that error apbears in the ebov wideritified patent and what said Lat-.Lem Patent are he reby corrected as shown below:

Column 5, line 52, "on" should be in Column 6, line 51, "l" shoulg be l6 Signed and sealed this 2nd day of January 1973.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GOTTSCHALK Attesting Officer Commissioner of' Patents (5/59) I RTKFKCAT F RREQTiQN Patent No. 3 4 5 I Dated July 18, 1972 Inventoits) M- It is certified. that error apbears in the ebov wideritified patent and what said Lat-.Lem Patent are he reby corrected as shown below:

(SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GOTTSCHALK Attesting Officer Commissioner of' Patents I GP Patent 3 78.4 5 7 f Y J bated: July 18, 1972 v :f, P04 050 .v

' ill- 93 153 It is "Certified that error apiar i'fi the abic iv wdderitifiad patent and "that: said Lffiwrz: Patent are f xgggreb y corrected as shown below:

Column 5, line '52, "on' sh'ould be in Column 6, line, 5l, "1" shoulid; 'be l6 Signedahd sealed "this 2nd day of'Januai'y 197s.

(SEAL) Attest:

EDWARD M.FL'ETCHER,,JR. ROBERT GOTTSCHALK Commissioner of Patents Attesting Officer

Claims

1. In combination, a body of electrical insulating material; and, a continuous electrical resistance element at least partially embedded, in said body and comprising a plurality of intact metal fibers defining a unitary structure having a multiplicity of voids therein, said metal fiber structure being in intimate contact with said insulating body but the voids not being impregnated with the insulating material, whereby the metal fiber structure can expand inwardly when heated by electrical current passing therethrough.

2. In combination, a continuous metal fiber electrical resistance element of high resistivity having a multiplicity of intact metal fibers forming an integral structure having a multiplicity of voids and a density per unit volume of less than 100 percent, said resistance element being at least partially embedded on a body of electrical insulating material but not being impregnated with the insulating material, whereby the integral metal fiber structure can expand inwardly when heated by electrical current passing therethrough so that mechanical stresses are not imparted to the body of electrical insulating material.

8. A combination according to claim 7 wherein said intact metal fiber unitary structure has a large surface to cross section ratio so that excellent heat transfer is obtained at the interface boundary of the intact metal fiber unitary structure with the body of electrical insulating material, whereby the product of the factors of electrical resistivity, density and specific heat are artificially increased.

9. A combination according to claim 1 wherein the intact metal fiber unitary structure has a coefficient of thermal expansion which is substantially similar to the coefficient of thermal expansion of said body of electrical insulating material.