Patent Publication Number: US-2015069021-A1

Title: Apparatus and method for reducing electrical arcing in a circuit breaker while transitioning to a closed circuit condition

Description:
This application claims benefit of the Sep. 11, 2013 filing date of U.S. provisional application 61/876,246, which is incorporated by reference herein. 
    
    
     FIELD OF THE INVENTION 
     Disclosed embodiments are related to circuit breakers, and, more particularly, to apparatus and method for reducing formation of electrical arcing in a circuit breaker while transitioning to a closed circuit condition. 
     BACKGROUND OF THE INVENTION 
     Circuit breakers are commonly used to protect electrical circuits or systems from certain abnormal conditions. In thermo-magnetic circuit breakers, typically, an electrical contact assembly, such as involving a stationary contact and a movable contact, is used to open and close the circuit. Circuit breakers can develop relatively high forces to drive the movable contact during a circuit closing event, and hence high impact can occur between such electrical contacts. This high impact can create a bouncing condition in the movable contact, and during this condition, momentary separations may be formed between such contacts, which can lead to the formation of electrical arcing between the contacts. This electrical arcing can cause damage to the contacts and could potentially lead to contact welding. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is explained in the following description in view of the drawings that show: 
         FIG. 1  is a schematic of an apparatus, such as a circuit breaker, that can benefit from aspects of the present invention. 
         FIGS. 2 and 3  are respective schematics for conceptualizing aspects of the present invention. 
         FIGS. 4 and 5  are schematics respectively illustrating a non-limiting embodiment of an apparatus embodying aspects of the present invention. 
         FIGS. 6 and 7  are plots useful for comparing a typical response of a prior art circuit breaker involving a bouncing condition ( FIG. 6 ) relative to a non-limiting example response ( FIG. 7 ) of a circuit breaker embodying aspects of the present invention. 
         FIG. 8  is flow chart of a method embodying aspects of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The inventor of the present invention proposes an innovative contact structure that cost-effectively and reliably is effective for reducing formation of electrical arcing in a circuit breaker while transitioning to a closed circuit condition. Typically, an operating mechanism in the circuit breaker can drive electrical contacts at relatively high speeds and hence can produce a relatively high impact in the electrical contacts, which can lead to a bouncing condition where momentary air gaps may be formed between the electrical contacts, which in turn can lead to the formation of electrical arcing between such electrical contacts. 
       FIG. 1  is a schematic of an apparatus  10 , such as a circuit breaker, that can benefit from aspects of the present invention. In one non-limiting embodiment, an electrical contact assembly  12  comprises a stationary contact  14 , which may be electrically connected to a power line  16 , and a movable contact  18  that may be disposed at an end of a contact arm  20 , which may be electrically connected, such as through a load terminal  19  and a load connector  21 , to an electrical load (not shown). Movable contact  18  by way of contact arm  20  is responsive to an operating mechanism  22  including an operating spring  24  to move towards the stationary contact and initiate a closed circuit condition. As used herein, the term stationary contact does not preclude transient motion that can occur while transitioning to or from a closed circuit condition. The description below elaborates details in connection with an oscillatory element  30 , such as an oscillatory spring, operatively connected to stationary contact  14 . As will be appreciated from the description below, oscillatory element  30  may be broadly conceptualized as a shock absorber. As used herein the phrase “configured to” embraces the concept that the feature preceding the phrase “configured to” is intentionally and specifically designed or made to act or function in a specific way and should not be construed to mean that the feature just has a capability or suitability to act or function in the specified way, unless so indicated. 
       FIGS. 2 and 3  are respective schematics for conceptualizing aspects of the present invention. In one non-limiting embodiment, oscillatory element  30  is disposed opposite a side  15  of stationary contact  14  that makes contact with movable contact  18  during the closed circuit condition. It will be appreciated that aspects of the present invention are not limited to any particular type of oscillatory spring. Non-limiting examples of oscillatory springs may be a compression spring, a leaf spring, a clip spring, an extension spring, an elastomeric spring, etc. 
       FIG. 2  illustrates an open circuit condition prior to transitioning to a closed circuit condition, where a single-headed arrow  26  represents movement of movable contact  18  towards stationary contact  14  to initiate the closed circuit condition. As may be appreciated in  FIG. 3 , in one non-limiting embodiment, oscillatory element  30  is configured to provide a transient response effective to adapt shock energy resulting from impact of movable contact  18  with stationary contact  14  to joint oscillatory motion (schematically represented by a twin-headed arrow  28 ) of stationary contact  14  and movable contact  18 , so that stationary contact  14  and movable contact  18  remain interconnected to one another during the joint oscillatory motion, thus reducing formation of electrical arcing during the closed circuit condition. 
     As will be appreciated by those skilled in the art, to achieve the desired transient response, certain parameters of oscillatory element  30 , such as its spring rate, mass, etc., are chosen based on relevant parameters of the structures involved, such as the mass of movable contact  18  and other structures connected to movable contact  18 , e.g., contact arm  20 , etc., the closing speed of movable contact  18 , the spring rate of operating spring  24 , the mass of stationary contact  14 , etc. Broadly, in a given application, oscillatory element  30  can be tailored to achieve the desired transient response based on relevant parameters of the structures involved in the given application. 
       FIGS. 4 and 5  are schematics respectively illustrating a non-limiting embodiment of an apparatus  10  embodying aspects of the present invention.  FIG. 4  illustrates an open circuit condition prior to transitioning to a closed circuit condition, and  FIG. 5  illustrates a closed circuit in a steady state condition subsequent to the transient response. That is, subsequent to the joint oscillatory motion discussed above in the context of  FIG. 3 . 
     In this embodiment, oscillatory element  30 , such as a spring clip, is disposed in a pocket  32  formed between an inner wall  34  of a housing  36  of the apparatus and a contact terminal  38  connected to stationary contact  14 . Spring clip  30  may comprise a first leg  40  affixed to contact terminal  38  and a second leg  42  connected to first leg  40  through a conjoining section  44  of spring clip  30 . Second leg  42  of spring clip  30  is affixed to inner wall  34  of housing  36  of the apparatus. In this embodiment, a mechanical stop  46  is disposed opposite the side of stationary contact  14  that makes contact with movable contact  18 . 
     As may be appreciated in  FIG. 5 , mechanical stop  46  is configured to limit travel of stationary contact  14  during the closed circuit condition. This travel occurs in response to an urging force (labeled F 1 ) applied by operating mechanism  22  against stationary contact  14  through movable contact  18  during the steady state condition, subsequent to the transient response. Thus, it will be appreciated that after completion of the joint oscillatory motion of stationary contact  14  and movable contact  18 , oscillatory element  30  is overcome by urging force F 1  and thus, in the steady state condition—and opposite to the joint contact engagement achieved during the transient response—the role played by oscillatory spring  30  between stationary contact  14  and movable contact  18  is practically nil. That is, during the steady state condition, subsequent to the transient response, oscillatory element  30  is practically inactive for purposes of engagement of the stationary contact and the movable contact to one another. This is opposite to certain prior art contact structures that rely on biasing forces from a biasing spring positioned behind the stationary contact to enhance the contact between the electrical contacts. 
     Thus, in one non-limiting embodiment, oscillatory element  30  may be characterized as comprising an active mode (analogous to a shock absorber) during the transient response (e.g., the joint oscillatory motion) that occurs while transitioning to the closed circuit condition for purposes of engagement of the stationary contact with the movable contact. Oscillatory element  30  may be further characterized as, during the steady state condition subsequent to the joint oscillatory motion, comprising an inactive mode for purposes of engagement of the stationary contact with the movable contact. 
       FIGS. 6 and 7  are plots depicting respective voltage traces useful for comparing a typical response of a prior art circuit breaker involving a contact structure susceptible to a bouncing condition ( FIG. 6 ) relative to a non-limiting example response ( FIG. 7 ) of a circuit breaker embodying aspects of the present invention.  FIG. 6  illustrates a voltage trace indicative of a bouncing condition lasting approximately 1 millisecond and comprising two bouncing events where electrical arcing may be formed. 
       FIG. 7  illustrates a voltage trace indicative of a joint oscillatory motion of the stationary contact and the movable contact lasting approximately ⅓ of the total bouncing time discussed in the context of  FIG. 6 . In this case, the stationary contact and the movable contact remain interconnected to one another during the joint oscillatory motion, and thus in accordance with aspect of the present invention the disclosed apparatus is effective for reducing formation of electrical arcing while transitioning to the closed circuit condition. 
       FIG. 8  is flow chart of a method embodying aspects of the present invention. Subsequent to start step  60 , step  62  allows disposing an oscillatory spring opposite a side of a stationary electrical contact that makes contact with a movable electrical contact during a closed circuit condition. Step  64  allows arranging a transient response by way of the oscillatory spring effective to adapt shock energy resulting from impact of the movable contact with the stationary contact to joint oscillatory motion of the stationary contact and the movable contact. The stationary contact and the movable contact remain interconnected to one another during the joint oscillatory motion, thus reducing formation of electrical arcing while transitioning to the closed circuit condition. Step  66  allows disposing a mechanical stop opposite the side of the stationary contact that makes contact with the movable contact. Step  68  allows arranging the mechanical stop to limit travel of the stationary contact during the closed circuit condition. 
     The travel of the stationary contact occurs in response to an urging force applied by an operating mechanism against the stationary contact during a steady state condition subsequent to the dynamic transient response. In the steady state condition, the oscillatory spring is overcome by the urging force and consequently—opposite to the joint contact engagement achieved during the transient response—the role played by the oscillatory spring for engaging the stationary contact and the movable contact to one another is practically nil; thus practically inactivating the oscillatory element during the steady state condition regarding engagement of the stationary contact with the movable contact. Thus, as illustrated in step  70 , prior to stop step  72 , in one non-limiting embodiment, this overcoming of the oscillatory spring by the urging force applied by the operating mechanism, in effect, during the steady state condition, allows nullifying an effect of the oscillatory spring regarding engagement of the stationary contact with the movable contact. 
     While various embodiments of the present invention have been shown and described herein, it will be apparent that such embodiments are provided by way of example only. Numerous variations, changes and substitutions may be made without departing from the invention herein. Accordingly, it is intended that the invention be limited only by the spirit and scope of the appended claims.