BRIEF DESCRIPTION OF THE DRAWINGS
The invention may best be understood by reference to the following description taken in conjunction with the accompanying drawings, in which:
FIG. 1 is a cross-sectional diagrammatic view of certain components of a circuit breaker having stacked bimetal elements according to an aspect of the present invention;
FIG. 2 is a side diagrammatic view of a bimetal assembly having a pigtail conductor attached to both bimetal elements at their respective free ends according to an aspect of the present invention;
FIG. 3 is an exploded diagrammatic perspective view of a small yoke attached to stacked bimetal elements wherein each bimetal is attached to a pigtail conductor that is wound around the yoke multiple times according to an aspect of the present invention;
FIG. 4 is a perspective diagrammatic view of a bimetal assembly in which one bimetal has a notch for receiving a tail end of a pigtail conductor from one direction according to an aspect of the present invention; and
FIG. 5 is a perspective diagrammatic view of part of a bimetal assembly in which each bimetal has a staggered notch at each respective free end thereof to permit attachment of the pigtail conductor from either direction according to an aspect of the present invention.
DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS
Although the invention will be described in connection with certain preferred embodiments, it will be understood that the invention is not limited to those particular embodiments. On the contrary, the invention is intended to include all alternatives, modifications and equivalent arrangements as may be included within the spirit and scope of the invention as defined by the appended claims.
Referring now to the drawings and initially to FIG. 1, there is shown a miniature circuit breaker 10 that includes a bimetal assembly 12 attached to a load terminal 25 at a load end of the bimetal assembly 12 and to a yoke 20 at a free end of the bimetal assembly 12 on a first side of the bimetal assembly 12. The other side of the bimetal assembly 12 at the free end is attached to a flexible pigtail conductor 22 such that both bimetals in the bimetal assembly 12 are directly heated by the attached pigtail conductor 22. These details are more clearly shown in the figures that follow. By “miniature,” it is meant that the overall width of the circuit breaker is about 1 inch or smaller, preferably about ⅜ inch. According to an aspect of the present invention, the miniature circuit breaker 10 is a half-size or tandem circuit breaker in which two poles fit into the same amount of available space without sacrificing the ampere rating, protection, or features compared to a standard miniature circuit breaker. For ease of illustration, some components of a circuit breaker are omitted or not described, however, these components, which may be found in the QO® or Homeline® miniature circuit breakers available from Square D Company, are not necessary for an understanding of aspects of the present invention. The miniature circuit breaker 10 has a relatively low amperage rating, from 10 A to 150 A.
The stacked arrangement of the bimetals according to aspects of the present invention allow the width of the bimetals to be reduced while maintaining the same overall cross-sectional area, compared to a single bimetal having a larger width, commensurate with the reduction and the same thickness. Bimetal flexibility is highly dependent upon thickness. The thicker the material, the less deflection will occur for a given temperature rise. The stacked arrangement allows the bimetals to retain their original thickness while maintaining the same overall cross-sectional area and the same flexibility compared to a single bimetal and producing the same force during bending movements to overcome the latch force during tripping.
The bimetal assembly 12 includes two bimetals having the same dimensions and composition. In a conventional miniature circuit breaker, a single bimetal is used and has a width typically on the order of ¼ inch. According to a specific aspect of the present invention, the two bimetals comprising the bimetal assembly 12 have a width of ⅛ inch and are attached by a sandwich weld at the load end where the load terminal 25 is connected. The bimetals are attached in a front-to-back arrangement such that major plane surfaces of the respective bimetal elements are adjacent and aligned with one another. The other end of the bimetal assembly 12 is not attached such that each bimetal is free to bend relative to one another, avoiding undesirable binding of the two bimetals during bending movements. Because both bimetals are attached to the pigtail conductor 22, current will flow through each bimetal equally, generating uniform heat and bending in the bimetals. The combined movement and forces from the bimetals are sufficient to consistently thermally trip the circuit breaker mechanism.
In a preferred aspect, the flexible pigtail conductor 22 is attached by welding it to both bimetals to allow an equal flow of current through both bimetals simultaneously and thereby cause a uniform bending response of the bimetal assembly 12 to overcurrents. If only one bimetal were attached to the pigtail conductor 22, the other indirectly heated bimetal would react more slowly and may even act against the directly heated bimetal. This competing and unbalanced arrangement may cause inconsistent tripping or even delayed tripping, which can be dangerous and is undesired. By contrast, when both bimetals are directly heated simultaneously, they react simultaneously and uniformly (assuming identical dimensions and substantially similar compositions), resulting in consistent tripping. In this respect, the width of the bimetals can be reduced along with the width of the miniature circuit breaker without sacrificing its performance characteristics.
Though the preferred aspect described above refers to a two-bimetal assembly, in other aspects, the bimetal assembly can comprise more than two bimetals, such as three or four. For example, in an aspect, four bimetals each having a width of 1/16 inch are stacked together, yielding the same force and bending movement as the above-described two-bimetal arrangement, such that they are attached at the load end to a load terminal and the other ends are free to move relative to the others. The pigtail conductor may be attached to all four bimetals or to the two end bimetals, indirectly heating the interior bimetals. Attachment of the pigtail conductor is described in more detail in connection with FIGS. 4 and 5.
FIG. 2 illustrates the bimetal assembly 12 comprising a first bimetal 12a and a second bimetal 12b having respective free ends 14, 16 welded to tail ends of the pigtail conductor 22. The free ends 14, 16 of the bimetal assembly 12 are free to move relative to one another, to allow the bimetals 12a,b to bend together without binding or buckling. The connection of the tail ends 18a,b of the pigtail conductor 22 to each of the bimetals 12a,b, respectively, causes an equal amount of electrical current to flow to each bimetal 12a,b. As is known, each individual bimetal is composed of two dissimilar metals that expand under thermal stress at different rates, causing the bimetal to bend in a direction away from the high-expansion side of the bimetal. As both bimetals 12a,b undergo thermal deflection due to excessive electrical current in the pigtail conductor 22, they undergo uniform bending movements and exert uniform forces to trip the circuit breaker mechanism in a consistent manner. Preferably, the high-expansion side of the bimetal 12b faces the mechanism side of the circuit breaker (i.e., the left side as shown in FIG. 2), causing the yoke to be pulled toward the armature.
The yoke is not shown in FIG. 2 for ease of illustration, and the specific attachment of the tail ends 18a,b of the pigtail conductor is but one of several possible attachment arrangements according to aspects of the present invention. The connection of the pigtail conductor 22 shown in FIG. 2 causes both bimetals 12a,b to be directly heated by electrical current passing through the pigtail conductor 22 simultaneously and equally.
A smaller yoke 33 (compared to the yoke 20 shown in FIG. 1) is shown in FIG. 3 in an exploded view that includes a bimetal assembly 32 and an armature 34. A pigtail conductor 36 is wound multiple times around the yoke 33 for increasing the magnetic field required to the yoke 33 toward the armature 34. Each double turn of the pigtail conductor 36 approximately doubles the magnetic field induced by the electrical current passing through the pigtail conductor 36. A load terminal 35 is welded to a first bimetal 32a that is also sandwich welded to a second bimetal 32b at the top or load end of the bimetal assembly 32. At the opposite or free end thereof, the bimetals 32a,b are free to move relative to one another. Respective tail ends of the pigtail conductor 36 are welded onto the respective free ends of the bimetals 32a,b such that an equal amount of current passes through each bimetal 32a,b. Preferably, the high-expansion side of the bimetal 32a,b face away from the load terminal 35 and toward the trip mechanism of the circuit breaker.
FIG. 4 illustrates a pigtail connection according to an aspect of the present invention in a manner that directly heats both bimetals 42a,b of a bimetal assembly 42. The pigtail connection shown in FIG. 4 also allows a pigtail conductor 46 to be fastened to the free ends of the bimetals 42a,b from one direction, simplifying the assembly process. It also allows the yoke to be attached to the opposite side of the bimetal assembly 42 without having to accommodate the pigtail conductor 46, yielding a more consistent assembly process.
As shown, a notch 43 is formed in the bimetal 42b to allow access by the pigtail conductor 46 to both bimetals 42a,b from one direction. The tail ends 48a,b of the pigtail conductor 46 can be fastened to both free ends of the bimetals 42a,b from one direction. A yoke, such as the yoke 20 in FIG. 1 or the yoke 33 in FIG. 3, is welded to the other side of the bimetal 42a. A load terminal 45 is welded to the bimetal 42a at its load end.
In FIG. 5, mirror-image notches 53a,b are formed on respective bimetals 52a,b as shown so that both bimetals 52a,b have the same cross-sectional profile at their free ends. These notches 53a,b also permit the tail ends 58a,b of the pigtail conductor 56 to be welded from either direction. On the side where the pigtail conductor 56 is not fastened, a yoke, such as the yoke 20 in FIG. 1 or the yoke 33 shown in FIG. 3, is welded at the free end of the bimetal 52b. A load terminal 55 is welded to the bimetal 52a at its load end.
In an embodiment in which four bimetals are used, such as described above, the notches may be staggered like those in FIG. 5 to permit the pigtail conductor tail ends to be fastened therein for each bimetal.
Words of degree such as “substantially” or “about” are used herein in the sense of “at, or nearly at, given the process, control, and material limitations inherent in the stated circumstances” and are used herein to keep the unscrupulous infringer from taking advantage of unqualified or absolute values stated for exemplary embodiments.
While particular embodiments and applications of the present invention have been illustrated and described, it is to be understood that the invention is not limited to the precise construction and compositions disclosed herein and that various modifications, changes, and variations may be apparent from the foregoing descriptions without departing from the spirit and scope of the invention as defined in the appended claims.
Patent Application
20080084266
