US2088420A - Thermal release device - Google Patents

Description

July 27, 1937. KOCH 2,088,420

THERMAL RELEASE DEVI CE Filed April 11, 1936 Fig.5 v Fig.4.

Birnetallic 37 :30 -3l 32 ITWVG'HGOT": f 5 3/ Kurt Koch,

by Wag/oz 49 Hi Attm n e y.

Patented July 27, 1937 PATENT OFFICE THERMAL RELEASE DEVICE Kurt Koch, Berlin-Eichwalde, Germany, assignor to General Electric Company, a corporation of New York Application April 11,

1936, Serial No. 73,900

In Germany July 15, 1935 10 Claims.

My invention relates to thermal release devices, more particulariy'to current carrying thermal elements of the type commonly applied to thermal circuit breakers or relays, and has for an object the provision of an inexpensive and improved thermal element of this character.

Current carrying thermal elements, such for example as bimetallic elements, when applied to relays or control devices for providing automatic response to predetermined current conditions, are commonly given such a configuration that unequal current densities exist .in different portions of the element. Under normal current con ditions, or upon the flow of relatively light overload currents, the thermal capacity and the thermal conductivity of the element are ordinarily sufiicient to provide a substantially uniform heating in all parts of the thermal element. It has been found, however, that upon the flow of heavy overload currents such as short-circuit currents, the regions of high current density are quickly heated to excessive temperatures. These portions of the thermal element may, in fact, reach a red or a white heat and the thermal element may be damaged before the'heat can be conducted away from these portions by twisting of the white hot metal due to the electrodynamic forces exerted on the thermal element by currents of such magnitude.

Although not limited thereto, my invention is particularly applicable to U-shaped bimetallic elements having a slot extending inwardly from one edge to form the U.. In such elements, the dangerous regions of high current density exist adjacent the base of the slot, especially in the portions of the thermal element adjacent the slot angles. This is due to the fact that the current tends to concentrate along the inner edges of the slot. When the legs of the U-shaped thermal element are secured to suitable terminals, the current flowing through the element follows the path of lowest resistance between the terminals, that is, the shortest path, and this shortest path is, of course, along the inner edges of the slot.

It is a. further object of my invention, therefore, to provide-a thermal element in which such excessive concentration of current and excessive heating of certain portions of the element is prevented even under short circuit current conditions.

In carrying out my invention in one form, I provide a thermal element having a configuration such that regions of high current density would normally exist therein and conductor means for bridging over said regions to prevent excessive heating thereof under heavy overload conditions.

For a more complete understanding of my invention, reference may now be had to the drawing, in which Fig. 1 is a somewhat schematic perspective view of a thermal device embodying my invention applied to'a suitable circuit breaker; Fig. 2 is a detail view of the thermal device shown in' Fig. 1; Figs. 3 and 4 are plan and side views respectively of a modified form of thermal device embodying my invention; and Fig. 5 is a fragmentary horizontal sectional View taken along the center line of the pin 3! of Fig. 3.

Referring now to Figs. l and 2 of the drawing, I have shown my invention in one form as applied to a bimetallic thermal element 10 arranged to control the operation of a circuit breaker II. The circuit breaker H is of the type described and claimed in a copending application of Winfield A. Atwood, Serial No. 667,- 855, filed April 25, 1933, entitled fCircuit breakers, which application is assigned to the same assignee as the present invention. In the present application, only so much of the circuit breaker is shown as is necessary for an explanation of my invention.

As shown, the bimetallic element I0 is provided with a slot l2 extending inwardly from one edge sogas to provide a U-shaped member, the legs of the U-shaped member being secured to suitable terminal members [3 and I4. It will thus be seen that the thermal element H], which controls the operation of the circuit breaker II in a manner. to be described hereinafter, is connected in circuit with the contacts of thecircuit breaker so that the thermal element is heated by the current flowing through the circuit breaker. The current path through the circuit breaker may be traced from the terminal l3 upwardly through one leg of the thermal element across the transverse portion or yoke of the thermal element downwardly through the other leg of the thermal element through the terminal member M, a suitable flexible conductor l5, a movable contact IB, a co-operating stationary contact ll, to the other terminal I8 of the circuit breaker.

' As described above, the current flowing through the thermal element I0 follows the shortest path between the terminal members l3 and I4 and accordingly is concentrated adjacent the closed end or base of the slot l2, especially adjacent the slot angles indicated by the reference numerals the flow of current therethrough is substantially uniform throughout the legs and the yoke portion of the thermal element. As described above, however, upon the occurrence of short circuit conditions, the concentration of current adjacent the base 0! the slot I2 is such as normally to produce excessive heating at these portions of the thermal element. In accordance with my invention, I, therefore, provide conductor means comprising a bar 2| formed of high electrical conductivity material which is secured to the thermal element ll adjacent the base of the slot l2, as shown, so as to provide a current conducting bridge between the legs of the thermal element. Thus, the bridging conductor 2| forms a low resistance current path adjacent the base of the slot I2 and by-passes a large part of the current flowing in the legs of the thermal element so as to prevent this excessive concentration of current and excessive heating adiacent the base 01 the slot.

When applied to a circuit breaker oi the type shown, the conducting bar 2| may also serve as a latch, the lower edge of the bar 2| being arranged to engage a latch flnger 22 when the thermal element is in the position shown. As is more fully described in the above referred to Atwood application, the circuit breaker II is provided with an overcenter spring (not shown) which is operatively connected to a pair of switch operating links 23 connected together by a cross portion 24, the links 23 being arranged to operate the movable contact l6 between the open and closed positions upon movement of the overcenter spring. A suitable tripping spring 25, normally under compression, biases a tripping member 26 and an extending portion 21 carried thereby in a direction to separate the contacts. Normally, however, the tripping member 26 is held against movement by engagement of the latch flnger 22 with the under side of the con ducting bar 2|.

It will be understood, 0! course, that when the thermal element i0 is heated, the free end of the thermal element carrying the conducting bar 2| moves in a counter-clockwise direction, as viewed in Fig. 1, to disengage the bar 2| from the latch finger 22 and permit the tripping member 28 to move under the influence oi the tripping spring 25. The conducting bar 2| may of course, be given any suitable shape and is connected to the thermal element II in good electrical conducting relation therewith so as to provide a low current path between the legs of the U-shaped thermal element.

In Figs. 3 and 4, I have shown a modified form of thermal element embodying my invention comprising a U-shaped bimetallic element 28 having the base portion of the slot 29 enlarged, as shown, to provide a substantially circular aperture 30. Arranged in the aperture 30 I provide a suitable pin or rivet 3| which may be secured in position by overturning the ends thereot, as shown best in Pig. 4. The diameter oi the pin or rivet 3| is sumciently large so as intimately to engage the edges or the aperture 3. so that the pin 3|, which is iormed of high.

electrical conductivity material, such as copper,

forms a low resistance path between the legs of the thermal element in shunt circuit relation to the yoke of the thermal element. Although I have shown a solid rivet or pin ll, it will, of

course, be apparent that a hollow rivet or screw may equally well be employed so long as the material of the rivet has a sufilciently high electrical conductivity to provide a low resistance path between the legs of the thermal element.

I have also found that in certain cases, especially in connection with very thin bimetallic elements, it is advantageous to provide the conducting member and the engaging portions of the thermal element with a coating of highly conducting metal. Thus, in Fig. 5 a coating 32 of highly conducting material is shown applied to the engaging surfaces of the pin 3| and the element 28. This coating may be applied in any well known manner, such, for example, as by spraying or by electroplating.

While I have shown a particular embodiment of my invention, it will be understood, of course, that I do notwish 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.

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

1. A thermal device comprising a current carrying bimetallic element having a configuration such that regions of high current density would normally exist therein under heavy overload conditions, and conducting means fixedly secured to said element for bridging over said regions, the conductivity of said conducting means being so related to the conductivity of said regions that the current flowing in said regions is insuflicient to cause excessive heating of said regions of said element under said heavy overload conditions.

2. A'thermal device comprising a current carrying thermal element having a configuration such that regions oi high current density would normally exist therein upon the flow of heavy overload currents therethrough, and conducting means permanently secured to said element at said regions for bridging over said regions, the conductivity of said conducting means being so related to the conductivity of said regions that the current flowing in said regions is insufficient to cause excessive heating of said element upon the flow 01' said heavy overload currents therethrough.

3. A thermal device comprising a current carrying thermal element having a slot extending inwardly from one edge to form a plurality of current carrying legs connected by a transverse portion, and conducting means permanently connecting said legs immediately adjacent said transverse portion in shunt circuit relation with said transverse portion, the conductivity of said conducting means being so related to the conductivity of said transverse portion that the current flowing through said transverse portion is insufilcient to cause excessive heating of said element by concentration of current in said element adjacent the closed end of said slot upon the flow of heavy overload currents in said element.

4. A thermal device comprising a current carrying bimetallic element having a slot extending inwardly from one edge to form a pair of current carrying legs connected by a transverse portion of said element, and a conducting bar permanently secured to said legs immediately adjacent the closed end 01 said slot to shunt said transverse portion, the conductivity of said conducting bar being so related to the conductivity 01 said transverse portion that the current flow- 7s ing through said transverse portion is insufficient to cause excessive heating of said element by concentration of current in said transverse portion adjacent the end of said slot upon the flow of heavy overload currents in said element.

5. A thermal device comprising a U-shaped current carrying element, and conducting means permanently'connected to the legs of said U- shaped member adjacent the yoke portion thereof for shunting said yoke portion, the conductivity of said conducting means being so related-to the conductivity of said yoke'portion as to prevent excessive concentration of current in the inner edge of said yoke upon the flow of heavy overload currents in said element.

6. A thermal device comprising a U-shaped thermal element, means for connecting the legs of said element in an electric circuit so that current flowing in said circuit passes through one of said legs, then through the yoke portion of said element,- and then through the other of said legs in series, and a conductor permanently secured to said legs in shunt circuit relation with said yoke portion immediately adjacent said yoke, the conductivity of said conductor being so related to the conductivity of said yoke portion that the current flowing through said yoke portion is insufficient to cause excessive concentration of current at the inner edge of said yoke upon the flow of heavy overload currents in said circuit.

7. A thermal device comprising a current carrying thermal element having a slot extending inwardly from one edge to form a U-shaped element, the base of said slot being enlarged to provide a substantially circular aperture, and a pin of conducting material extending through said aperture, said pin being of a diameter to fit tightly in said aperture so as to provide a conducting bridge between the legs of said U-shaped element and thereby prevent excessive concentration of current in said thermal element adjacent the base of said slot upon the flow or heavy overload currents in said element.

8. A thermal device comprising a current carrying thermal element having a slot extending inwardly from one edge to form a U-shaped element, the base of said slot being enlarged to provide a substantially circular aperture, and a rivet formed of conducting material tightly fitting in said aperture so as to provide a conducting bridge between the legs of said U-shaped element and thereby prevent excessive concentration of current in said thermal element adjacent the base of said slot upon the flow of heavy overload currents in said element.

9. A thermal device comprising a current carrying bimetallic element having a slot extending inwardly from one edge to form a U-shaped element, the base of said slot being enlarged to provide a substantially circular aperture, and a conducting pin'extending through said aperture, said pin and the edges of said aperture being provided with a coating of highly conducting material so as to provide a low resistance connection between the legs of said U-shaped element and thereby prevent excessive concentration of current in said element adjacent the base of said slot upon the flow of heavy overload currents in said element.

10. A thermal device comprising a U-shaped current carrying bimetallic element, a conductor fixedly secured to the legs of said element adjacent the yoke portion thereof for electrically connecting said legs in shunt to said yoke, the

, engaging portions of said element and said conductor being provided with a coating of highly conducting material so as to provide a low resistance connection between the legs of said U- shaped element and thereby prevent excessive concentration of current in the inner edge of said yoke upon the flow of heavy overload cur rents in said element.

. KURT KOCH.

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