US2841671A - Circuit breaker - Google Patents

Description

July 1, 1958 J. F. MARQUIS CIRCUIT BREAKER Filed May 28, 1957 INVENTOR.

United States Patent ClRQUlT BREAKER John F. Marquis, St. Charies, Ill., assignor to Littelfuse Incorporated, Des Plaines, 111., a corporation of Illinois Application May 28, 1957, Serial No. 662,026 Claims. (Cl. 200-113) This invention relates to circuit breakers which utilize snap-acting bimetallic elements as the actuating means therefor.

In circuit breakers of this type, the current being controlled usually passes through a bimetallic element which has been initially deformed or prestressed into a dished shape. The bimetallic element includes superimposed strips of metal having substantially different thermal coefiicients of expansion. Under normal temperature conditions, the metal having the highest thermal coeflicient of expansion is on the concave side of the deformed bimetallic element. Heat is generated in the bimetallic element by the current passing therethrough, and, as the temperature of the element progressively increases, the high expanding side thereof tends to flatten out and eventually reaches a snapping point when the temperature reaches a point indicating a current flow requiring a circuit interruption. The bimetallic element carries a movable contact which, in the normal dished condition of the bimetallic element, makes contact with a stationary contact. When the control temperature is reached, the

bimetallic element snaps into an oppositely dished condition, and the movable contact is pulled away from the stationary contact to thereby break the circuit. With the cessation of current flow, the temperature of the bimetallic element drops, and, when the reverse snapping temperature is reached, the bimetallic element snaps back into a circuit closing position if the circuit breaker is of the usual self-resetting type. If the circuit breaker is of the manual resetting type, then a manual reset control must be operated to return the bimetallic element to a circuit closing position.

Circuit breakers of the type being described may be utilized in a variety of ways. For example, they may be used as a flasher switch for interrupting a circuit leading to the lights on an advertising sign or on an automobile signal light, or they may be used as a protective device for limting excessive current flow in a circuit. Where it is used as a protective device, it may be desirable to provide a significant time lag between the time that the excessive current begins to flow in the circuit and the time the circuit breaker opens. This time lag is of importance where the circuit breaker is used in situations where the current flow may normally exceed only momentarily a given control limit, such as occurs when an electric motor is turned on, and where a circuit interruption is, therefore, unnecessary except where the excessive current is prolonged beyond a safe time interval.

Among the objects of the present invention are to provide an improved circuit breaker of the bimetallic type which provides an appreciable time lag between the initiation or a current which, it maintained, would cause the switch to snap, and the instant the bimetallic element actually snaps into a circuit opening position, to provide such a circuit breaker which requires a high ambient temperature to operate the same whereby the circuit breaker may be utilized in environments having relatively high ambient temperatures, such as in motor housings, and

(:4 to provide such a switch which is rugged in construction, foolproof in operation and which may be inexpensively manufactured.

In accordance with the most preferred form of the invention, the switch blade of the circuit breaker is made of a dished bimetallic element of preferably elongated rectangular shape and havin a wide H-shaped cutout or slot centered therein, so that the opposite legs of the H are parallel to the short sides of the bimetallic element. The legs of the H-shaped cutout or slot are spaced apart a substantial amount so that the metal material between the legs of the slot contributes substantially to the snap-acting characteristics of the bimetallic element. Moreover, the legs of the H slot extend to points contiguous to but spaced from the margins of the bimetallic element, to provide a number of spaced-apart points where the current path through the bimetallic element are substantially narrowed. Most preferably, the ratio of the distances from the margin of the bimetallic element to the web of the H-slot, on the one hand, and to the end of the legs of the H-slot, on the other hand, is at least in the neighborhood of two to one, so that the heat developed by current flowing between the ends of the bimetallic element is concentrated at the ends of the slot legs. The resistance per unit path length at the ends of the slot legs will be twice that of the metal between the legs where the above-mentioned ratio is two to one. Since, for a given current, the heat loss through a resistance in which the current is flowing is proportional to the value of the resistance, the heat developed per unit path length in the metal at the slot legs will, in the above example, be two times that in the metal between the legs.

An electrical contact is secured to one end of the bimetallic element at a point outside of one of the slot legs. A connecting terminal means is secured at a corresponding position at the other end of the bimetallic element. This terminal means may be a stationary ter minal post suitably connected to the bimetallic element, or, it could be another contact. A contact mounting base made of insulating material is provided which supports one or more stationary contacts which, under normal circuit operating conditions, makes engagement with the movable contact or contacts carried by the bimetallic element. With this construction, two parallel current paths are provided between the terminal or contact points on the bimetallic element, which paths include the substantially narrowed path portions afforded by the H-slot. With the wide l-l-slot centered in the dished bimetallic element, the material between the legs of the wide I slot becomes a dominating influence in the control of the snap action characteristics of the bimetallic element.

For zero or low current values, the temperature of the bimetallic element may be predominantly affected by the ambient temperature. This ambient temperature uniformly heats the entire bimetallic element including the substantial metal portions between the widely spaced H- slot legs, which, under normal ambient temperature conditions, provides appreciable stiffness requiring a high ambient temperature condition to raise the bimetallic element to the snapping temperature. If the current flowing through the bimetallic element should then momentarily increase to an abnormally high value, as for example when a motor is initially turned on, the narrow current path points at the ends of the slot legs will immediately heat up to a substantial temperature which would ordinarily cause the bimetallic element to snap into a circuit opening position, once the metal portions between the slot legs are heated to a high temperature also. Since the latter metal portions between the slot legs have a relatively low resistance, only a relatively small amount of heat is developed by the current flowing therein. The heat concentrated at the ends of the legs will gradually heat said metal portions between the legs of the H-slot. When the current flowing through the narrow current path points exceeds the control limit for the'delay period, the metal between the slot legs will reach a temperature which will cause the bimetallic element to snap into a circuit opening position. Thus, this arrangement provides a time delay circuit breaker which has a high ambient temperature operating point by the simple expedient of a wide H-shaped slot with relatively r long legs providing a substantialconcentration of heat at the ends of the legs of the slot.

Other objects, advantages and features of the invention will become apparent upon making reference to the specification to follow, the claims and the drawings wherein:

Fig. 1 is an exploded view ofthe parts making up the most preferred form of the invention; 7

Fig. 2 is a central longitudinal vertical sectional View through the circuit breaker of the invention;

Fig. 3 is a horizontal sectional view of the circuit breaker, taken along section line 33 in Fig. 2;

Fig. 4 is a horizontal sectional view of the circuit breaker, taken along section line 4-4 in Fig. 2; and

Fig. 5 is an enlarged transverse vertical sectional view of the circuit breaker, taken along section line 5-5 in Fig. 2.

One form of circuit breaker constructed in accordance with the present invention is generally designated at 10, and includes a generally rectangular housing 12 having a base portion 14 made of insulating material and a metal cover 16, and an elongated, rectangular, dished bimetallic snap-acting element 18 mounted within the housing 12. A metal terminal post 20 and a stationary contact 22 are secured in spaced-apart relation upon the base portion 14 of the housing by terminal studs 24 and 26 and nuts 28 and 30. The base portion 14 has recesses 32 and 34 which receive inwardly directed nibs 36 and 33 formed in the cover bottom to hold the cover and base portion of the housing together.

The dish-shaped bimetallic element 18 is supported at one end from the top of the terminal post 20 by welding, riveting or otherwise suitably securing the bimetallic element thereto, with the normally concave side of the bimetallic element facing the base portion 14 of the housing. In the embodiment illustrated in the draw ings, the terminal post is formed with a reduced end it) (Fig. l), which is passed through a hole 42' at one end of the bimetallic element and then turned over the top of the bimetallic element to lock it to the post. The bimetallic element is made of two elongated rectangular superimposed metal strips 50 and 52, secured together as by welding and deformed in any suitable way into a segmental spherical shape. Under normal temperature conditions, the metal strip 52 on the convex of the bimetallic strip has a relatively low thermal coefficient of expansion and the metal strip Stl on the concave side thereof has a relatively high thermal coeflicient of expansion. For example, the metal strip 52 may be made or" lnvar or the like and the other metal strip 59 may be formed of brass, copper, nickel, chromium or iron alloys, or'the like. The distal end of the bimetallic element carries a contact 53 riveted or otherwise suitably secured to the bimetallic element on the concave or high expanding side of the bimetallic element. Under normal temperature conditions, the contact 53 is in engagement with the stationary contact 22.

In accordance with the most preferred form of the invention, the bimetallic element 18 is provided with a wide generally H-shaped cutout or slot 54 centered therein between the terminal post 2% and the movable con tact 523. Both the legs 54'54' and the web 54" of the H-slot are relatively narrow so that a minimum amount of material is removed from the bimetallic element. Moreover, the legs 54-54 of the slot are appreciably spaced apart so that the metal portions between the legs have a predominating efiect on the snapacting characteristics of the bimetallic element. This is because the innermost or most depressed portion of a dished shaped bimetallic element normally has the greatest effect over the stiffness of the element. The ends of the slot legs extend to points contiguous to but spaced from the long sides of the bimetallic element, so as to provide narrow current paths at the ends thereof, identified as points P1, P2, P3 and P4, relative to the widths of the current paths between the slot legs. The ratio of the distance d2 from the margin at each long side of the bimetallic'strip to the H-slot web 54", to the distance d1 from the corresponding margin to the end of each adjacent slot leg, is preferably at least in the neighborhood of 2/1 so that the resistance ratio thereat is at least 1/2 and the heat ratio thereat is at least 1/2 (the heat developed per unit length is proportional to the value of the resistance for a given current). With this construction, it can be seen that the heat developed by the flow of currentv between the movable contact 53 and the terminal post 20 is concentrated at the narrow current path points P1, P2, P3 and P4. Moreover, since the formation of the H-slot has removed only a relatively small amount of the material from the center of the bimetallic element, the rigidityof the same is aflected only to a small degree, so that the element may be made to operate at a relatively high ambient temperature snapping level.

The bimetallic element is designed so that at abnormal or low current levels, the heat developed by the fiow of current through the bimetallic element is insufficient to raise the temperature of the snap-acting control portions of the bimetallic element, that is the region of the bimetallic element between the slot legs, to the snapping point. When the current initially exceeds the control value for which the bimetallic element is designed, the temperature at the narrow path points P1, P2, P3 and P4 rises to a point which would eventually cause the bimetallic element to snap into an oppositely dished condition if the current remained at this level. Since the heat developed by the flow of current through the much wider path portion between the slot legs is relatively small, the temperature of this portion of the bimetallic element will not be very high at the beginning of the high current condition. However, since heat flows from points of high temperature to points of lower temperature, the snap-acting control portion of the bimetallic element between the slot legs will gradually increase until it reaches a point where the bimetallic element will snap to the oppositely dished shape shown in dotted lines in Fig. 2. When this occurs, the distal end of the bimetallic element is raised to bring the movable contact 53 to a position spaced substantially from the stationary contact 22, thereby opening the circuit in which the circuit breaker is connected. When the current is interrupted, the temperature of the bimetallic element drops until'a point is reached where the element snaps back into' a circuit closing position shown in solid lines in Fig. 2. Of course, if the ambient element increases to the snapping point, this will also cause the bimetallic element to snap into a circuit opening position, even though the current passing through the bimetallic element has not reached a value which itself would cause the eventual snapping of the bimetallic element. No appreciable time delay in the action of the bimetallic element would occur here because the control temperature then almost immediately exists over the entire bimetallic element.

As above explained, although the embodiment of the invention illustrated is of the self-resetting type, the principles of thepresent invention may be applied equally to a switch which requires a manual reset.

The present invention thus provides a'n exceedingly simple and economical bimetallic, delay acting circuit breaker adapted for operation at very high ambient temperature conditions.

It should be understood that numerous modifications may be made of the most preferred form of the invention above described without deviating from the broader aspects of the invention.

I claim as my invention:

1. A circuit breaker comprising a thin, flexible, snapacting bimetallic switch blade element formed by two superimposed and secured together metal strips having substantially diiferent thermal coefficients of expansion, said bimetallic element having a first dished shape below a given control temperature, with the higher expanding metal on the concave side thereof, and being adapted to snap into an oppositely dished shape above said temperature, said bimetallic element having a contact and terminal means spaced from said contact, said bimetallic element having an H-shaped electrical insulating portion in the central region thereof between said contact and terminal means, the opposite legs of the l i-shaped insulating portion extending contiguous to but being spaced from at least one margin thereof and forming at least one current path between said contact and terminal means having points located at the ends of the legs of the H where the current path narrows to a degree that the resistance per unit path length is at least in the neighborhood of twice that of the adjacent central portion of the bimetallic element between the legs of the H, so that the heat developed per unit path length at the narrow path points is at least in the neighborhood of twice that developed in said adjacent central portion of said bimetallic element, said latter portion of said bimetallic element contributing substantially to the snap-action characteristics of the bimetallic element and receiving suficient heat from said narrow path points under excessive current flow that the snapping temperature thereof is reached after a time delay from the instant of time the excessive current begins to flow, and a stationary contact supported externally of said bimetallic element and positioned to make firm contact with said contact on said bimetallic element when the latter is in said first dished shape and which is separated from said latter contact when said bimetallic element is in its oppositely dished shape.

2. A circuit breaker comprising a thin, flexible, snapacting bimetallic switch blade element formed by two superimposed and secured together metal strips having substantially difierent thermal coefiicients of expansion, said bimetallic element having a first dished shape below a given control temperature, with the higher expanding metal on the concave side thereof, and being adapted to snap into an oppositely dished shape above said temperature, said bimetallic element having a contact adjacent to one side thereof and terminal means on the opposite side thereof, said bimetallic element having a wide H- shaped cut-out portion in the central region the cot be tween said contact and terminal means, the opposite legs of the H extending contiguous to but being spaced from the opposite margins thereof, the cutout portion forming parallel current paths between said movable contact and terminal means each having points located at the ends of the legs of the H where the current path narrows to a degree that the resistance per unit path length is at least in the neighborhood of twice that of the adjacent central portion of the bimetallic element between the legs of the H so that the heat developed per unit path length at the narrow path points is at least in the neighborhood of twice that developed in said adjacent central portion of said bimetallic element, said latter portion of said himetallic element contributing substantially to the snap action characteristics of the bimetallic element and receiving sufiicient heat from said narrow path points under excessive current flow that the snapping temperature thereof is reached after a time delay from the instant of time the excessive current begins to flow, and a stationary contact supported externally of said bimetallic 6 element and positioned to make firm contact with said contact on said bimetallic element when the latter is in said first dished shape and which is separated from said latter contact when said bimetallic element is in its oppositely dished shape.

3. A circuit breaker comprising a thin, flexible, elongated, rectangular snap-acting bimetallic switch blade element formed by two superimposed and secured together metal strips having substantially difierent thermal coefiicients of expansion, said bimetallic element having a first dished shape below a given control temperature, with the higher expanding metal on the concave side thereof, and being adapted to snap into an oppositely dished shape above said temperature, said bimetallic element having a contact at one end thereof and terminal means at the opposite end thereof, said bimetallic element having a wide H-shaped cutout portion in the central region thereof, the opposite legs of the H being generally parallel to the short sides of said bimetallic element, the cutout portion forming parallel paths between said movable contact and terminal means each having points located at the ends of the legs of the 5-! where the current path narrows to a degree that the resistance per unit path length is at least in the neighborhood of twice that of the adjacent central portion of the bimetallic element between the legs of the H, so that the heat developed per unit path at the narrow path points is at least in the neighborhood of twice that developed in said adjacent central portion of said bimetallic element, said latter portion of said bimetallic element contributing substantially to the snap action characteristics of the bimetallic element and receiving sufiicient heat from said narrow path points under excessive current flow that the snapping temperature thereof is reached after a time delay from the instant of time the excessive current begins to flow, and a stationary contact supported externally of said bimetallic element and positioned to make firm contact with said contact on said bimetallic element when the latter is in said first dished shape and which is separated from said latter contact when said bimetallic element is in its oppositely dished shape.

4. A circuit breaker comprising a thin, flexible, snapacting bimetallic switch blade element formed by two superimposed and secured together metal strips having substantially different thermal coefiicients of expansion, said bimetallic element having a first dished shape below a given control temperature, with the higher expanding metal on the concave side thereof, and being adapted to snap into an oppositely dished shape above said temperature, said bimetallic element having a contact and terminal means thereon for making electrical connection to the bimetallic element, said bimetallic element having an H-shaped electrical insulating portion in the central re" gions thereof, wherein the opposite legs of the H extend '3 contiguous to but are spaced from opposite margins thereof, the insulating portion being located between said movable contact and terminal means forming parallel current paths between said movable contact and terminal means each having points located at the ends of the legs of the H where the current path narrows to a degree that the resistance per unit path length is at least in the neighborhood of twice that of the adjacent central portion of the bimetallic element between the legs of the H, so that the heat developed per unit path length at the narrow path points is at least in the neighborhood of twice that developed in said adjacent central portion of said bimetallic element, said latter portion of said bimetallic element contributing substantially to the snap action characteristics of the bimetallic element and receiving sufiicient heat from said narrow path points under excessive current flow that the snapping temperature thereof is reached after a time delay from the instant of time the excessive current begins to flow, and a stationary contact supported externally of said bimetallic element and positioned to make firm contact with said contact on said bimetallic ele- 7 a ment when the latter is in said first dished shape and which is separated from said latter contact when said bimetallic element is in its oppositely dished shape.

5. A circuit breaker comprising a thin, flexible, snapacting bimetallic switch blade element formed by two superimposed and secured together metal strips having substantially difilerent thermal coeflicients of expansion, said bimetallic element having a first dished shape below a given control temperature, with the higher expanding metal on the concave side thereof, and being adapted to snap into an oppositely dished shape above said temperature, said bimetallic element having a contact and terminal means thereon for making electrical connection to the bimetallic element, said bimetallic element having an H-shaped cut-out portion in the central regions thereof, wherein the opposite legs of the H extend contiguous to but are spaced from opposite margins thereof, the cut-out portion being located between said movable contact and terminal means and forming parallel current paths between said movable contact and'terminal means each having points located at the ends of the legs of the H where the current path narrows to a degree that the resistance per unit path length is at least in the neighborhood of twice that of the adjacent central portion of the bimetallic element between the legs of the H, so that the heat developed per unit path length at the narrow path points is at least in the neighborhood of twice that developed in said adjacent central portion of said bimetallic element, said latter portion of said bimetallic element contributing substantially to the snap action characteristics of the bimetallic element and receiving suflicient heat from said narrow path points under excessive current flow that the snapping temperature thereof is reached after a time delay from the instant of time the excessive current begins to flow, and a stationary contact supported externally of said bimetallic element and positioned to make firm contact with said contact on said bimetallic element when the latter is in said first dished shape and which is separated from said latter contact when said bimetallic element is in its oppositely dished shape.

References Cited in the file of this patent UNITED STATES PATENTS 2,266,537 Elmer Dec. 16, 1941 2,707,216 Farison Apr. 26, 1955 2,709,732 Davis May 31, 1955

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