US2240824A - Electrical resistor of thermoflex material - Google Patents

Description

y c. FIALBAN ETAL 2,240,824

ELECTRICAL RESISTOR 0F THERMOFLEX MATERIAL Filed Dec. 6, I957 2 Sheets-Sheet 1 mill] y 6, 1941- C. F. ALBAN ETAL ,2 0,824

ELECTRICAL nssxswon OF THERMOFLEX MATERIAL Filed Dec. 6, 1937 2 Sheets-Sheet 2 LM H INVENTORS iarence 7'7 422mm @5642?? 7E. jfood/ F ATIiORNEYfi BY m Patented May 6, 1941 4 UNlTED STATES PATENT OFFICE ELECTRICAL RESISTOR F THERMOFLEX MATERIAL Clarence F. Alban and Stanley R. Hood, Detroit, Mich, asslgnors to W. M. Chace Company, Detroit, Mich a corporation of Michigan Application December 6, 1937, Serial No. 1783M 8 @laims.

This invention relates an electrical resistor oi thermcflezr material.

As herein used the term thermofiex material denotes any composite material consisting of two or more laminae of metals or metal alloys, one lamina of which has a different coemcient of expansion than the other so that upon a change or temperature the metal will bend or flex due to the differential expansion oi the said laminae. These thermofiex metals are commonly called thermostatic bilnetal, thermostatic trimetal, and so on up the line depending upon how many laminae there are.

The known metal alloys suitable for use in thermoflex metal are limited in number and the physical and chemical laws governing their behavior are known. For example, to obtain the best presently available themmfiex materials or thermostatic bimetals having highest cceficient of deflection, it is customary with materials having approximately equal moduli of elasticity to make the high expanding lamina and low expanding lamina of apprcsimately equal thickness. We have discovered that t physical law applies when more than two laminae are used to fabricate a given thern'iofiex material. in this invention it is necessary to classify the alloys or metals used into high or low expanding components as the situation may require. After this classification has been made, we have found it good practice to maize the sum of the thicknesses of the low and high expanding laminae approximately equal.

We have also discovered that in the now of electrical current through thermofiex metal that the laminae form a parallel circuit rather than a series circuit. This being true, the reciprocal cuit formed by thermofiex metal is the sum of the reciprocals of the individual resistances of the several laminae.

The use of thermofiex material as a resistor in an electrical circuit is not new. For example, circuit interrupters as well as circuitbreakers of bimetallic thermoflex material are known. However, the application of themioflex material as a resistor in an electrical circuit has been handicapped because the known metal alloys suitable for-use in thermoflex material are limited in number. Since the known metal alloys suitable for use in thermoflex material are limited in-number, the used these thermo- 'of the equivalent resistance of the parallel cirflex materials as electrical resistors is dependent upon the electrical resistivity of any standard length and section (for example, circular mil foot) of these materials. The known metal alloys suitable for use in thermcflex material have very diilerent resistivities measured in ohms per circular mil foot at a given temperature. "3y variation of analysis of these alloys, theresistivity of a standard length and section of thermofiex material can be varied within limits, but this impairs the defiectivity of the material.

Bimetal can be made having'the low expanding lamina of invar (36% nickel, 64% iron) and the high expanding lamina of pure nickel. The dificulty with this bimetal is that its coefficient of deflection'is much less than the best known alloys suitable for use in bimetal such for example, as an alloy for the low expanding lamina consisting of invar (36% nickel, 64% iron) and the high expanding side consisting of 22% nickel, 4%% chromium and the remainder substantially iron. It will he noted that this secand mentioned thermoilex material is the same as the one above-mentioned except that the high expanding lamina has a different composition. The difference in electrical resistivity per circular mil foot of these two bimetals is roughly four hundred ohms, but even by variation of analysis it has been found that at most only a few alloys suitable for use in thermoiiex material can be obtained. The bimetals made from these few alloys have different resistivities per circular mil foot, but when arranged in order from the lowest to the highest resistivity per circular mil foot, it is found that there is a considerable hiatus between the resistivities of most thermoiiex materials. Therefore it is impossible, using a standard length and section of thermoflex material, to fabricate a set of resistors having resistivities covering a desired range. For example, if it were required to fabricate a set of thermoflex elements for use in electrical circuits having amperage ratings of 10, 15, 20, 25, 30, 35, 40, 45, and 50, it would have been heretofore impossible because of the above-mentioned facts pertaining to thermofiex material. Therefore, it is the object of this invention to overcome the above-mentioned limitation in the use of thermoflex material as an electrical resistor and to produce a. thermoflex material which will permit the fabrication of electrical resistors having any desired resistivity or amperage rating per given standard length and standard cross section.

Three constant conditions must be taken into consideration whenever thermoflex material is being made to be used as a resistor in an electrical circuit: namely, (1) the low expansion lamina of the thermoilex material should approximate one-hali' the total thickness of the thermoflex material; (2) the compositions of alloys suitable for laminae in thermofiex material are fairly well ilxed and limited; (3) the thermoflex material conducts currents as a parallel circuit.

This object has been achieved by incorporating in our new thermoflex material, either as an intermediate or outer lamina or laminae, a lamina or laminae of a metal or metal alloy having high electrical conductivity. Among these metals may be mentioned nickel. copper cobalt, silver, gold, aluminum, platinum, mam ganese, iron, chromium, molybdenum, tungsten and alloys containing these or other metals having a high electrical conductivity. As set forth in Handbook of Chemistry and Physics, 19th Ed. by Charles D. Hodgman, edition oi September 1934, pages 1337 through 1343, the resistivity of each of the above metals measured in ohms per cubic centimeter at a temperature of 20 C. is as follows:

Nickel 7.8 X 10* Copper 1.69 X 10- Cobalt 9.7 X 10- Silver 1.63 010 Gold 2.44 x Aluminum 2.83 xiiH Platinum 10. X 10- Manganese 5. =iil- Iron 10. x10 Chromium 2.6 )(zlil Molybdenum 5.7 -l() Tungsten "c- 5.6 X 10-- All oi the above metals have a high electrical conductivity within the meaning of the term as herein used, that is, a low electrical resistivity. Hence, as used herein the phrase "metal or metal alloy having high electrical conductivity means any metal or metal alloy having an electrical resistivity measured as above and equivalent to the electrical resistivity of any of the above listed metals. According to the above-mentioned Handbook of Chemistry and Physics, the electrical resistivity measured as above of the following nickel steel alloys is as follows:

Invar (35% nickel) 8l l0" 10% nickel, 41% cobalt 29X 10- nickel, .l% cobalt 559x10 These alloys for the purposes oi this description are considered as having a low electrical conductivity. Although no hard and last line of division can be drawn between high and low electrical conductivity, the above-mentioned metals are considered as'having high electrical conductivity whereas the above-mentioned nickel-steel alloys are considered as having a low electrical conductivity. Preferably these metals should be pure or have but a very small or inconsequential amount oi impurities therein. This lamina of high conducting metal functions primarily as an electrical conductor and the other laminae function primarily to cause the flexing of the material incident to temperature change.

Since the resistivity oi each of these pure metsis is very easily computed and the resistance (or any given length varies inversely with the cross sectional area of this pure metal lamina, and having discovered that the thermohcx ma.- tcrial conducts current as a parallel circuit, the resistivity of the thermoflcx ateriai is varied by varying the thicknessot e pure'or high conducting metal lamina. Further, since the low expanding lamina or component of the thermoilex metal must always at least approximate one-half of the sum of the thicknesses of all oi" the laminae, therefore the sum of the thicknesses of the high expanding lamina. and the high conducting metal lamina is computed to approximately equal the thickness of the low expanding side. By increasing the thickness of the high conducting metal lamina relative to the thicknesses of the other laminae the resistance of the material per circular mil foot is decreased and by decreasing the thickness of the high conducting metal lamina the resistance of the thermofiex element per circular mil foot is increased.

By "way of description rather than limitation there will now be described amethod ior fabrieating an electrical resistor such as a circuit breaker element having any given resistivity. When it is proposed to use thermofiex n'laterial as a circuit breaker element, particularly ior circult breakers having a low amperage rating, say from in to 100 amperes, such as are suitable for domestic use, or having an amperage rating still lower such as are suitable for use in the electrical circuit of an automotive vehicle, then the-re must be taken into consideration a iourth constant condition,'that is, that these breaker elements are manufactured in standard sizes. Hence, a breaker element having any given amperage rating, say '10 amperes, has to be made the same site as a circuit breaker element having another given amperage rating either more, such 100 amperes, or less, say 1 ampere.

In the drawings:

Fig. l is a sectional view through a circuit breaker having a tripping or breaker element of thermoflex material showing the position oi the elements when the circuit is completed and the current is flowing through the circuit.

Fig. 2 is a view similar to Fig. 1 showing the circuit breaker 'in tripped position and the circult broken.

F188. 3 and 4 show circuit breaker elements of standard size but having different resistivi ties.

Figs. 5, 6, 7 and 8 show electrical resistors of thermoilex material each showing the several different laminae arranged in difierent positions one relative to the other.

Referring to Fig. 1 the circuit breaker comprises a stationary electrically conducting contact i, a contact bar 2 pivoted as at 1 upon the plunger 4, and a handle 5 pivoted as at 8 upon the case "I which encloses the circuit breaker mechanism. A compression spring 1 is seated in a socket 9 in the base of the case 1 and engages the contact bar as at ID to provide direct pressure upon the stationary contact i and the movable contact ll carried by the contact bar 2. The spring 8 also acts through the contact bar 2 to provide a minor pressure upon the thermoiiex element II;

The contact her is also connected to the thermode: clement ll by a flexible lead II which is -and is fixed at one end as at 22 to the mounting bracket 0. Whcnthe circuit breaker is in on" position, as shown in Pig. 1, and current at or below the amperage rating or. the tripping or element N is flowing through the' cir- 2,240,824 v cult, then contacts I and H are engaged. The

current atthis time is flowing through contacts I, II, contact bar 2, lead ll, thermofiex element 20 and bracket 23. Whenever the current flowing through this circuit is stepped up, e. g., when the circuit is shorted or overloaded, the temperature of the thermoflex tripping or breaker element 20 increases, when the overload reaches about 50% of the rated amperage, depending, of course, on the time, thenthe rise in temperature causes the thermoflex tripping element 20 to flex as indicated in Fig. 2. This releases the contact bar 2 and permits the spring 8 to pivot the contact bar 2 to the position shown 7 in Fig. 2, thus disengaging or rupturing contactsi and M. This breaks the circuit between contacts l and H and current ceases to flow through the circuit. At this time the handle 5 is shifted from the on position shown in Fig. l to ofi or tripped position shown in Fig. 2. llVhen the cause of the short circuit or overload has been removed the circuit breaker may he reset by returning the handle 5 to on position. If the overload or short circuit remains in the circuit, then upon moving the handle to on position to again reset the circuit breaker, it will again overload the thermoiiex tripping element and cause the circuit breaker to trip as above described.

In Fig. 3 there is shown a circuit breaker element of standard size. This element 2d consists of a low expansion lamina l2, high expansion lamina l3 and an intermediate lamina M of high conductivity. When the lamina i2 is an alloy of nickel, 8% cobalt and 62% iron having a thickness of .532 inch and the high conductivity lamina it has a thickness of .265 inch and consists of approximately pure nickel and the high expanding lamina i3 is an alloy consisting of 22% nickel, 2%70 chromium and the remainder substantially iron and a thickness of .265 inch, then the resistivity of this trimetal element is 150 ohms per circular mil foot at 80 F. This element has a high coefilcient of deflection. Itis' not commercially practical to obtain 100% pure metal. 'Therefore the high conducting lamina it is preferably made from any of the above-mentioned high conducting metals but with a predetermined small amount of impurities therein such that the electrical resistivity can be controlled. Thus by the addition of small amounts of impurities to obtain the predetermined allowable amount of impurities a high conducting lamina H of the same resistivity can always be obtained.

To produce a circuit breaker element having a resistivity of 200 ohms per circular mil foot at 80 F., then, for the reason above defined, lamina i2 will have the same thickness of .530 inch but lamina I4 is decreased to .100 inch and the thickness of lamina i3 is increased to .430

inch.

To obtain higher resistivity per circular mil foot then it is proposed to use a thermoflex metal breaker element as shown in Fig. 4 in whichthe low expanding lamina I5 consists of 36% nickel and 64% iron and the high conducting lamina I6 consists of substantially pure nickel and the high expanding lamina l'l consists of 22% nickel and 4 4% chromium and the remainder substantially iron. To obtain a resistivity of 300 ohms per circular mil foot at 80 F., the thickness of lamina I5 is .530inch; that of lamina I 6, .085 inch; and lamina H, .445 inch. To obtain a resistivity of this thermofiex metal of 350 ohms per circular mil foot at 80 F., then lamina l5 remains the same thickness but lamina i6 is reduced to .055 inch and lamina i1 is increased in thickness to .475 inch. To obtain a resistivity of 400 ohms per circular mil foot at 80 F., the thickness of lamina I6 is decreased to .032 inch and the thickness of lamina I1 is increased to .498 inch. After the composite welded ingots are prepared as described by the foregoing examples, they are rolled by standard procedure to required thickness.

From the above it will be noted that the thickness of the low expanding lamina is .530 inch which is equal to the sum of the thicknesses of the other two laminae. In fabricating breaker elements of a standard size the thermoflex metal having the higher resistivity per circular mil foot is used for those breaker elements having a lower armgerage rating, and the thermofiex metal having the lower resistivities are used for breaker elements having a higher amperage rat- From the above it is evident that by gradually decreasing the thickness of the high conducting lamina W and correspondingly increasing the thickness of the high expanding lamina that one can obtain a thermofiex material having any resistivity desired within predetermined limits, for descriptive purposes shown as a trimetal, and having a high fiexivity corresponding very favorably with the flexivity of the best thermofiex metals now known and produced. Thus, within a given range of amperage ratings there can be obtained for a standard size electrical resistor a set of resistors having increased amperage ratings in whatever progression desired.

In the resistor shown in Fig. 5 the two outer laminae 52d consist of a high conducting metal which can have a high coefficient of expansion such as nickel or a high electrical conducting metal having a low coeilicient of expansion such as chromium, molybdenum or tungsten. Of the two intermediate laminae 2i and (1 22, 2! has'a higher coemcient of expansion than I22. In such case lamina 26 can be made of a nickel-steel alloy containing 22% nickel and 4 /4 chromium, and lamina E22 can consist of invar. For purposes of description, this type could be fabricated from a standard type of thermofiex, it only being necessary to plate or weld on high electrical conducting layers as laminae I20. It is, of course, understood that laminae I20 are not necessarily of similar materials or of exactly equal thickness. Thus, laminae 20, under most conditions, would be very thin and have little effect on the deflection properties of the thermoflex.

In the electrical resistor shown in Fig. 6 the 1 5 lamina 23 has a high coeflicient of expansion and mediate the laminae 26 and 21.

can consist of a nickel-steel alloy comprising 22% nickel, 4 4% chromium. Lamina 24 can consist of a high electrical conducting metal or metal alloy having a low coeilicient of expansion such as chromium, molybdenum or tungsten. The low coeflicient lamina 25 can be invar. Here again the total thickness of laminae 24 and 25 equals that of lamina 23, that (is, A equals B.

In Fig. '7 the lamina 26 has a low coefficient of lamina 21 has a high coefficient of expansion and I can consist of the nickel-steel alloy having 22% nickel and 4 /4% chromium. The two high conducting laminae 28 and 29, are positioned inter- The lamina 28 has a high electrical conductivity but a low coefllcient of expansion and can be made from chromilun, molybdenum or tungsten. The lamina 29 has a high electrical conductivity and a high coeflicient of expansion and can be made from nickel or copper, for example. Here again the total thickness of the laminae 2B and 28 equals that of 21 and 29, that is, A equals B.

'lIn Fig. 8 the electrical resistor has outer laminae 30 and 3! each having the same thickness, A equals B. Lamina 30 can be invar and the lamina 3! a nickel-steel alloy containing 22% nickel. l /4% chromium which has a high coelllcient of expansion. The intermediate lamina 32 has a high electrical conductivity and can be made from a metal such as nickel or copper having a high coefllcient of expansion or from a metal such'es chromium, molybdenum or tungsten having a low coefficient of expansion. In this form preferably the lamina 32 should be as thin as possible commensurate with the desired conductivity of the resistor because maximum defiectivity is obtained when the total thickness of the high expanding lamina or laminae will approximately equal the total thickness of the low expanding lamina or laminae.

The terms high coefficient of expansion and low coefllcient oi expansion are herein used in their relative sense. As is well-known in order to obtain any deflectivity in thermoflex material the one lamina must have a lower coefficient of expansion than the other. A given metal alloy or metal may be used as the high expanding lamina in one thermoflex material and the same given metal alloy or metal may be used as the low expanding lamina in another thermofldx material. In the first case the lamina will be referred to as the high expanding lamina" whereas in the second case this lamina from the same metal or metal alloy will be referred to as the low expanding lamina.

We claim:

1. An electrical resistor of thermoflex material having a high temperature coeiiicient of deflection, a predetermined cross section and length and electrical resistance consisting of a plurality of joined metallic laminae including a metallic lamina having a high thermal coeflicient of expansion, a metallic lamina having a low thermal coefllcient of expansion and at least'one metallic lamina having a high electrical conductivity and a cross sectional area correlated with respect to the cross sectional areas and electrical resistances of said above mentioned laminae to obtain said predetermined resistance whereby the deflection rate is maintained substantially the same throughout a wide range of cross sectional areas for said high conducting lamina.

2. An electrical resistor of thermoflex material having a high temperature coefliclent of deflection, a predetermined cross section and length and electrical resistance consisting of a plurality of joined metallic laminae including a metallic lamina having a high thermal coemclent of expansion, a metallic lamina having a low thermal coemcient of expansion and at least one metallic lamina having a high electrical conductivity, and a cross sectional area correlated with. respect to the cross sectional areas and electrical resistances of said above mentioned laminae to obtain said predetermined resistance whereby the deflection rate is maintained substantially the same throughout a wide range of cross sectional areas for said high conducting lamlna, the thickness of the metallic lamina having a low thermal cofllclent of expansion being approximately equal to the sum of the thicknesses of the lamina having a high coeflicient of expansion and the lamina having a high electrical conductivity.

3. An electrical resistor oi thermoflex material having a high temperature coefilcient ol' deflection, a predetermined cross section and length and electrical resistance consisting of a plurality of joined laminae including a lamina of an alloy having a high thermal coefficient ofexpansion from the group of alloys consisting of iron-nickel alloy, iron-nickel-chromium alloy, and iron-nickel-manganese alloy, a lamina of a nickel-iron alloy having a higher nickel content than the above mentioned laminae and a low thermal coefllcient of expansion and a metallic lamina having a high electrical conductivity and a cross sectional area correlated with respect to the cross sectional areas and electrical resistances of said above mentioned laminae to obtain said predetermined resistance whereby the deflection rate is maintained substantially the same throughout a wide range of cross sectional areas for said high conducting lamina.

4. An electrical resistor of thermoflex material, a predetermined cross section and length and electrical resistance consisting of a plurality of joined laminae including a lamina of an alloy having a high thermal coeiilcientof expansion from the group of alloys consisting 01 ironnickel alloy, iron-nickel-chromium alloy, and iron-nickel-manganese alloy, a lamina comprising a nickel-iron alloy having a low thermal coefficient of expansion, and an intermediate metallic lamina having a high electrical conductivity and a cross sectional area correlated with respect to the cross sectional areas and electrical resistances of said above mentioned laminae to obtain said predetermined resistance whereby the deflection rate is maintained substantially the same throughout a wide range of cross sectional areas for said high conducting lamina, the ratio of the nickel to the iron content in each of the first two mentioned laminae being such that the thermoflex material has a high ilexivity.

5. An electrical resistor of thermofiex material having a high temperature coefllcient of deflection, a predetermined cross section and length and electrical resistance consisting of a plurality of Joined metallic laminae including a metallic lamina having a high thermal coelihcientof expansion and a low electrical conductivity, a metallic lamina having a low thermal coefficient of expansion and a low electrical conductivity and at least one metallic lamina positioned between the above-mentioned laminae having a high electrical conductivity and a coefficient of expansion intermediate to that of one of the above laminae and a cross sectional area correlated with respect to the cross sectional areas and electrical resistances of said above mentioned laminae to obtain said predetermined resistance whereby the deflection rate is maintalned substantially the same throughout a wide range of cross sectional areas for said high conducting lamina, the total thickness 01' the high expanding lamina or laminae being approximate- 1y equal to that 01' the low expanding lamina or laminae.

' 6. An electrical resistor of thermoflex material having a high temperature coefllclent or deflection, a predetermined cross section and length and electrical resistance consisting of a plurality of joined metallic laminae including a metallic lamina having a high thermal ccefficient or expansion, a metallic 3 lamina having a low thermal coemcient or expansion and at least one lamina of nickel having a high electrical conductivity and a cross sectional area correlated with respect to the cross sectional areas and electrical resistances of said above mentioned laminae to obtain said predetermined resistance whereby the deflection rate is maintained substantially the same throughout a wide range of cross sectional areas for said high conducting lamina.

7. An electrical resistor of thermoflex material having a high temperature coefllcient of defiection, a predetermined cross section and length and electrical resistance consisting'of a plurality of joined metallic laminae including a metallic laminahaving a high thermal coefficient of expansion, a metallic lamina'having a low thermal coefficient of expansion and at least one lamina of copperhaving-a high electrical conductivity and a cross sectional area correlated with respectto the cross sectional areas and electrical resistances of said above mentioned laminae to obtain said predetermined resistance whereby the deflection rate is maintained substantially the same throughout a wide range of cross sectional areas for said high conducting lamina.

8. An electrical resistor of thermoflex mate'- rial having a high temperature coeificient of deflection, a predetermined cross section and length and electrical resistance consisting of a plurality of joined metallic laminae including a metallic lamina having a high thermal coeflicient of expansion, a metallic lamina having a low thermal coeflicient of expansion and at least one lamina of iron having a high electrical conductivity and a cross sectional area correlated with respect to the cross sectional areas and electrical resistances of said above mentioned laminae to obtain said predetermined resistance whereby the deflection rate is maintained substantially the same throughout a wide range of cross sectional areas for said high conducting lamina.

CLARENCE F. ALBAN. STANLEY R. HOOD.

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