US2050879A - Thermal release device - Google Patents

Description

Aug. 11', 1936.

H. V. ERBEN THERMAL RELEASE DEVICE Filed March 3,v 1954 m m w rE n w. m Vf VWXCM bww@ H H H Uu Patented Aug'. 11, 1936 UNITED STATES THERMAL umass nEvrcE Henry V..Erben, Philadelphia, Pa., assigner to General Electric Company, a corporation of New York Application March 3,

9 Claims.

My invention relates to thermal release devices for circuit breakers and has for an object the provision of a simple, inexpensive thermal release having improved operating characteristics.

My invention is particularly applicable to circuit breakers of the type which are automatically operated to the open circuit position in response to a predetermined overload current. Heretofore it has .been common practice to restrain a switch opening member in a predetermined position by a latching member secured to a bimetal thermal element. Since the switch opening member is strongly biased to the open circuit position a considerable amount of friction must be overcome by the thermal element in unlatching the circuit breaker. By providing polished metal surfaces the friction is substantially decreased. However, the polished surface is quickly damaged by undue temperature such for example as is produced by the flow of overload current through the circuit breaker.

In carrying out my invention inV one form thereof, I provide a thermal release comprising two upright bimetallic members connected together by means of a shunt formed of a homogeneous strip of high electrical conductivity material and a' latching member formed of steel or the like. The heating of the steel member is minimized since substantially all of the current flows through the high conductivity material, such for example as copper, the resultant heating of the steel member being practically negligible. Besides securing improved operation of the thermal release a substantial economy is effected in manufacture since there is no waste of the bimetallic material, such for example as ordinarily occurs when special forms of thermal elements are stamped from stock. In accordance with my invention, the bimetallic material can be produced in strips and cut to desired lengths without anyr waste whatsoever.

For a more complete understanding of my invention, reference should now be had to the draw-A ing wherein I have shown in perspective in IFig. 1 a thermal release device constructed in accordance with my invention, while in Fig. 2 I have shown in perspective a modification of my invention.

Referring to the drawing, I have shown my invention in one form as applied to a circuit breaker I2 of the type described in a copending application of Winfield A. Atwood, Serial No. 667,855, filed April 25, 19,33, entitled Circuit breakers and assigned to the same assignee as the present invention. In the present application only so much 1934, Serial No. 713,898

of the circuit breakerisshown as is necessary for the explanation of my invention.

An over-centerv spring (not shown) is operatively connected to a pair of switch operating links I3 connected together by a cross-portion 5 i3d to operate a movable contact I4 between openv and closed circuit positions with respect to a sta.-` tionary contact I5. An opening spring I6 normally under compression biases a tripping member I8 and a downwardly extending projection I8a 10 carried thereby in a direction to separate the contacts. Normally, however, the tripping mem-A ber I8, provided with a latching member 20, is held against movement by reason of the engagement of the latching member 20 with a steel latching 15- member or bar 2|, secured to two rectangular bimetallic thermal elements 22 and 24 formed of strip material. Between the latching member or bar 2| and the upper or free ends 25 and 26 of the thermal elements there is provided a crossbar 28 formed of high electrical conductivity material, such for example as copper. The copper crossbar 28 and the steel latching member 2I are securely fastened tothe bimetallic elements 22 and 24 as by rivets 2i! and 30 formed of steel or 2'5 yMonel metal, while the lower ends of the bimetallic elements 22 and 24 are xedly secured to a pair of rugged stationary terminal lugs 32 and 34 as by steel rivets 35 and 36.

In order to minimize the friction between the 30 latching member 20 andthe steel latching bar 2|,v the surfaces 20a and Ia are highly polished. These polished surfaces are protected from overheating since substantially all of the current flows through the high conductivity material 28, the current flow causing a minimum amount of heating due to the low resistance of the material V28 Referring to the bimetallic members 22 and 24, it will be understood that the current flows' from the terminal 32 through the bimetallic elementV 24, and divides at its upper end so that substantially all of the current flows through the high conductivity material 28 to the bimetallic element 22 and thence by a flexible conductor 40 to the switch contacts I4 and I5. 'I'he bimetallic elements of relatively high resistance material are heated in proportion to the magnitude oi.' the current. The bimetallic elements 22 and 24 are arranged so that they bend in a counter-clockwise direction, as viewed in Fig. l, in response to an increase in temperature due to the unequal expansion of the metals forming the bimetallic elements, as will be understood by those skilled in the art. Any suitable metals having satisfactory electrical and heat resistance characteristics and having temperature coeilicients of expansion which are suiciently dissimilar may be used, such as nickel-steel and nickel-copper alloys.

As each bimetallic element is heated by the flow of a predetermined current it bends in a counter-clockwise direction, thus carrying the latching bar 2| and the crossbar 28 away from the latching member 20 and releasing it. Since all of the bimetal extends in a vertical direction, as shown substantially every portion of it is eiiiciently used to produce deflection of the latchf ing bar 2|.

As soon as the latching member 20 is released the vcompression springv i6 rotates the downwardly extending projection |8a into engagement with the cross-portion 13a to operate the movable contact I4 to the open circuit position.

In U-shaped bimetallic elements the cross member formed of bimetallic material and connecting the two vertical portions serves no useful purpose in the deflection of the element, but tends to twist the vertical portions so as to inhibit the deilection of the element. Furthermore, it has been found that during overload conditions the current distribution through the U-shaped bi- .metallic elements is not uniform, the current tending to concentrate along the inner edges of the bimetallic elements. The .reason for the nonuniform now of current is that the rivets securing the lower ends of the U-shaped elements to their stationary terminal lugs produce the greatest contact pressure in the portion of the material directly adjacent the rivets. The current path of low resistance is, therefore, the shortest distance between the rivets, this current path of course being along the inner edge of the U-shaped ele ments.

In accordance with my invention, the fastening means for the bimetal elements are arranged so that the path of lowest electrical resistance is through the central portion of the bimetallic elements and the crossbar. Although any suitable securing means can be employed, for example spot Welding, I have shown the rivets 30 and 35 of the bimetallic element 24 located in the center of the element. Similarly the rivets 29 and 36 of the bimetallic element 22 are located in the center of the element. Consequently, the respective rivets 29, 30, 35, and 36 produce a greater contact pressure between the portion of the elements directly adjacent the rivets so that the current entering a bimetallic element through one riveted fastening means flows by way oi' the lowest resistance/path `to the other riveted or welded fastening means.

ySince the riveted fastening means are located in.

the centers of the elements 22 and 24 and the crossbar 28, there is no tendency for the current to divide unevenly or to concentrate along the edges of the elements because the lowest resistance paths are along straight lines extendingbetween rivets and through the central portion of the bimetallic elements and the crossbar.- In actual operation of my invention, the bimetal elements were uniformly heated, although under the same overload conditions the U-shaped bimetal elements of the type heretofore used were burned by the concentration of current along the inner edges of the elements. It has further been found that the time required for my thermal elements to produce a given deflection for a given amount of current flowing through them is substantially less than with the U-shaped or Y-shaped thermal elements.

In 'case a magnetic trip is to be provided for the circuit breaker, a current responsive coil 45 can be mounted adjacent the latching member 2| and the crossbar 28 so as to produce an attractive effort on the steel latching member 2| in a direction to release the latching member 20, the coil being connected across a shunt 48 connected in circuit with the circuit breaker contacts. For applications of my invention to thermal releases having a low current rating, for example 15 amperes, I prefer to use only a steel crossbar connecting the upper 'ends of the bimetallic elements. As shown in Fig. 2, a pair of bimetallic elements 50 and 5| supported from the terminals 52 and 53 have their upper ends connected together by a steel crossbar 54, riveted connections being used, a latching portion 54a being polished to minimize friction. For low current ratings the steel crossbar 54 does not become heated sumciently to injure the polished surface. The steel crossbar 54 can be used as the armature of a magnetic trip' in the same manner as described in connection with the coil 45 of Fig. 1.

It will be understood that the current flows from the lug 53, the rivet 56 which secures the bimetallic element 5| to the lug, through the bimetallic element 5|, through the rivet 51 which secures the upper end of the element 5| to the crossbar 54, thence through the crossbar 54 to the rivet 58 which secures the other end of the crossbar 54 to the bimetallic element 50, and through the bimetallic element 50 and the rivet 59 which secures the fixed end of the bimetal element 50 to the stationary lug 52. As I have stated in connection with Fig. 1, the respective rivets 56 and 59, inclusive, produce a greater contact pressure between the portion of the conductors directly adjacent the rivets so that the lowest resistance path is between the rivets and since the rivets are located in the center of the bimetallic elements there is no tendency for current to divide unevenly or to concentrate along the edges of the bimetal elements.

While I have shown a particular embodiment of my invention, it will be understood, of course, that I do not wish to be limited thereto since many modifications may be made, and I, therefore, contemplate by the appended claims to cover any such modifications as fall within the true spirit and scope of my invention.

IWhat I claim as new and desire to secure by Letters Patent of the United States is:

1. A current responsive thermal release device comprising, a plurality of stationary terminal lugs, a bimetallic thermal element for each lug, means iixedly securing at one end said thermal elements to their respective terminal lugs, a homogeneous crossbar formed of high electrical conductivity material, means connecting said crossbar to the free ends of said elements to form a'. current path between said elements, and a metallic latching member formed of low electrical conductivity material supported from said elements in electrical conducting relation with said elements and said crossbar, the heating of said latching member incident to current ilow being minimized by said high conductivity crossbar.

2. A current responsive thermal release device comprising, a plurality of bimetallic thermal elements, means iixedly securing corresponding 70 tween said elements having a lower electrical 75 resistance than the electrical resistance of said elements, and a latching member of relatively low electrical conductivity supported by said elments and operable by the deflection of said elements to and from a latching position, said low conductivity material preventing undue heating of said latching member.

3. A current responsive thermal release device ior a circuit breaker having a tripping member one end of which is provided with a polished latching surface comprising, a plurality of bimetallic thermal elements, means iixedly mounting corresponding ends of said elements, a homogeneous crossbar formed of high electrical conducting material, means securing said cross-bar to the opposite free ends of said bimetallic elements to form a low resistance current path between said elements, a metallic latching member provided with a polished surface for engaging said polished surface of said tripping member, means mounting said latching member on said crossbar in electrical conducting relation therewith for movement with said bimetallic elements, said polished surfaces being located intermediate the ends of said free ends of said bimetallic elements, said high conductivity material shunting said latching member and thereby minimizing heating of said polished surfaces.

4. A current responsive thermal release device for a circuit breaker having a tripping member a portion of which is provided with a polished latching surface comprising, a pair of bimetallic thermal elements, means iixedly mounting corresponding ends of said elements, a homogeneous crossbar formed of high electrical conductivity material, a magnetizable latching bar formed of low electrical conductivity material, means securing said cross-bar in intimate electrical engagement with the opposite free ends of said bimetallic elements to form a Icurrent path between said eleinents having a lower electrical resistance than the' resistance of said elements, said means also securing said latching bar against said crossbar, said latching bar being provided with a polished surface intermediate said bimetallic elements for engagement with said polished latching surface of said tripping member the heating of said surfaces due to the flow of current through said magnetizable bar being minimized by said high conductivity crossbar, and means responsive to current flow through said circuit breaker for producing a magnetic attractive force on said magnetizable bar to release said tripping member irrespective ot said bimetallic elements.

5. A current responsive thermal release device comprising, a pair of stationary terminal lugs spaced apart one from the other, a pair of thermal elements formed of strip bimetallic material, means fixedly securing at one end thereof a bimetallic element to said lugs, a homogeneous crossbar formed of high electrical conductivity material, a. steel latching member having a relatively low electrical conductivity, means seeming the ends of said crossbar and said latching member together in intimate electrical engagement, said means also securing said crossbar in intimate electrical engagement with the opposite free ends of said thermal elements to form a low resistance current path between said free ends of said thermal elements whereby the heating of said latching member due to current flow is minimized.

6. A current responsive thermal release device 5 comprising, a plurality of stationary terminal lugs, a bimetallic thermal element for each lug, a rivet for iixedly securing at one end` a thermall element to its terminal lug, a crossbar formed of electrically conductive material, rivets for coninecting said crossbar to the free ends of said elements to form a current path between said elements, said rivets being located midway between the edges of said elements so that the current path of minimum resistance lies directly between said rivets and through the center portion of said bimetal element whereby an even current distribution through said bimetal elements is insured.

7. A current responsive thermal release device comprising, a pair of stationary terminal lugs, a pair of bimetallic thermal elements, said elements being provided with apertures adjacent their ends and located midway between the edges of said elements, fastening means extending through said apertures for fixedly securing the respective corresponding ends of said bimetal elements to said lugs so as tc produce a maximum pressure of said elements against said lugs directly adjacent said aperture, a crossbar formed of electrically conductive material, fastening means extending through the apertures in the iree ends of said elements for securing said crossbar thereto and for producing a pressure for urging said free ends of said elements into intimate relation with said crossbar, the maximum pressure being applied to the elements adjacent said apertures.

8. A current responsive thermal release device comprising, a pair of stationary terminal lugs, a pair of bimetallic. thermal elements, a crossbar formed of electrically conductive material, fastening means for xedly securing corresponding ends of said bimetal elements to said lugs and for securing said crossbar to said opposite ends of said bimetal elements so that the lowest resistance current path is located through the centrai portions of said bimetallic elements and said bar whereby a uniform heating of said bimetal elements is assured -under all conditions.

9. A current responsive thermal release device comprising, a plurality of stationary terminal lugs, a bimetal thermal element for each lug, fastening means flxedly securing at one end said thermal .elements to their respective terminal lugs, a crossbar formed of high electrically conductive material, means connecting said crossbar to the free ends of said elements to form a current path between said elements, and a latching member formed of low electrically conductive material supported from said elements whereby the heat of said latching bar incident to current flow is minimized by said crossbar, said fastening means for said elements forming a current path of minimum resistance extending through the central portions of said bimetal elements and said crossbar.

HENRY V. ERBEN.

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