Abstract:
A circuit breaker rotary contact arrangement is disclosed in which the ends of the line and load straps supporting the fixed contacts are hook-shaped to control the angle of the repulsive force exhibited between the fixed contacts and the movable contacts arranged at the opposing ends of the rotary contact arm. The fixed contacts face outwardly away from the central pivot of the contact arm such that a horizontal component of the popping force acts away from the center of rotation keeping the contact arm in tension for avoiding a buckling effect allowing contact arms with smaller cross sectional area to be used to increase contact arm mobility and reduce the cost.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to circuit breakers, and, more particularly, to circuit breakers having a rotary contact arm arrangement. 
     U.S. Pat. No. 4,616,198 entitled “Contact Arrangement for a Current Limiting Circuit Breaker” describes the early use of a first and second pair of circuit breaker contacts arranged in series to substantially reduce the amount of current let-through upon the occurrence of an overcurrent condition. 
     When the contact pairs are arranged upon one movable contact arm such as described within U.S. Pat. No. 4,910,485 entitled “Multiple Circuit Breaker with Double Break Rotary Contact”, some means must be provided to insure that the opposing contact pairs exhibit the same contact pressure to reduce contact wear and erosion. 
     One arrangement for providing uniform contact wear is described within U.S. Pat. 4,649,247 entitled “Contact Assembly for Low-voltage Circuit Breakers with a Two-Arm Contact Lever”. This arrangement includes an elongate slot formed perpendicular to the contact travel to provide uniform contact closure force on both pairs of contacts. 
     State of the art circuit breakers employing a rotary contact arrangement employ a rotor assembly and pair of powerful expansion springs to maintain contact between the rotor assembly and the rotary contact arm as well as to maintain good electrical connection between the contacts. The added compression forces provided by the powerful expansion springs must be overcome when the contacts become separated by the so-called “popping force” of magnetic repulsion that occurs upon over-current conditions to momentarily separate the circuit breaker contacts within the protected circuit before the circuit breaker operating mechanism has time to respond. The thickness of the moveable contact arm as well as the size of the contact springs has heretofore been increased to proportionately increase the overcurrent level at which the popping force causes the contacts to become separated. However, increased thickness and size decreases contact arm mobility and increases the cost. 
     SUMMARY OF THE INVENTION 
     In an exemplary embodiment of the invention, a movable contact arm arrangement for rotary contact circuit breakers comprises a movable contact arm having a central pivot point adapted to be pivotally connected within a circuit breaker interior. A first movable contact is arranged at first end of the contact arm and a second movable contact is arranged at a second end of the contact arm. A line strap arranged at the first end of the contact arm has first end portion with a first fixed contact connected thereto and arranged opposite the first movable contact. A second end portion of the line strap is adapted for connection within an electric circuit. The line strap has a hook-shaped configuration so that an outer face of the first fixed contact faces away from the central pivot point of the contact arm and is further arranged at a non-zero degree angle relative to the second end portion of the line strap. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a front perspective view of a circuit breaker interior depicting a rotary contact arrangement; 
     FIG. 2 is an enlarged front plan view of the prior art rotary contact arrangement within the rotary contact arrangement of FIG. 1; 
     FIG. 3 is an enlarged front plan view of another prior art rotary contact arrangement; 
     FIG. 4 is an enlarged front plan view of a rotary contact arrangement of the present invention; and, 
     FIGS. 5A and 5B compare the contact gaps created in the arrangements for FIG.  3  and FIG. 4, respectively, upon rotation of the contact arm. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The rotor assembly  10  in the circuit breaker interior assembly is depicted in FIG. 1 intermediate the line strap  12  and load strap  16  and the associated arc chutes  26 A,  26 B. Although a single rotor assembly is shown, it is understood that a separate rotor assembly is employed within each pole of a multi-pole circuit breaker and operates in a similar manner. Electrical transport through the circuit breaker interior proceeds from the line strap  12  to the associated fixed contact  20 B to the movable contact  22 B connected to one end of the movable contact arm  24 . The current transfers then to the opposite movable and fixed contacts  22 A,  20 A to the associated load strap  16 . The movable contact arm  24  moves a central pivot  30  in unison with the rotor  28  which connects with the circuit breaker operating mechanism (not shown) by means of the levers  32 A,  32 B to move the movable contacts  22 A,  22 B between OPEN, CLOSED and TRIPPED positions. The central pivot  30  responds to the rotational movement of the rotor  28  to effect the contact closing and opening function. The extended pin  34  provides attachment of the rotor  28  with the circuit breaker operating handle (not shown) to allow manual intervention for opening and closing the circuit breaker contacts. 
     The contact arm  24  is shown in FIG. 2 intermediate the line and load straps  12 ,  16  to depict the positional relationship between the fixed and movable contacts  20 A,  20 B,  22 A,  22 B. The popping force, which is proportional to the square of the current, is normal to the surface of the contacts  20 A,  20 B. The contacts can pop (separate) when the moment due to popping force can overcome the contact pressure induced by the rotor spring force. The line of force B acting through the contacts  20 A,  22 A is shown in phantom. Plane A, also shown in phantom, passes through the pivot  30  and is parallel to end portions  14  and  18  of line and load straps  12  and  16 , respectively. It is further noted that the contacts are positioned parallel to the plane A and that the line and load straps each define a pair of adjacent 90 degree angles  38  and  40 . 
     The popping force, defined earlier, is a factor of the moment defined by the length of the movable contact arm  24  from the axis of rotation, defined by pivot  30 , multiplied by the sine of the angle  36  defined between the reference lines A and B. With the angle  36  equal to 90 degrees, as is shown in FIG. 2, the sine of the angle is equal to one resulting in a maximum popping force that must be overcome to prevent contact popping at correspondingly low over-current values. 
     Turning now to FIG. 3, an alternate contact arm arrangement of the prior art is shown. The movable contact arm  52  intermediate the line and load straps  42 ,  48  depict the positional relationship between the fixed and movable contacts  20 A,  20 B,  22 A,  22 B. The line of force C acting through the contacts  20 A,  22 A is shown in phantom. The plane A, also shown in phantom, passes through the pivot  30  and is parallel to end portions  44  and  50  of the line and load straps  42  and  48 . The line and load straps  42  and  48  each define a single acute angle  46  to angle the fixed contacts  20 B and  20 A towards the contact arm  52 . Thus, an angle  56  is defined between the line of force C and the plane A. With the angle  56  equal to 45 degrees, for example, the sine of the angle is less than one (approximately 0.707), resulting in almost a third less the value of the popping force associated with the Prior Art arrangement shown earlier in FIG.  1 . However, as further shown in FIG. 3, the popping force F, when broken down into horizontal and vertical components Fsin φ and Fcos φ, respectively, demonstrates a horizontal component Fsin φ which acts towards the center of rotation  30  of the arm  52  (where the angle φ is defined as the angle between the popping force F, along the line of force C, and the vertical component of the popping force F, i.e. Fcos φ, along a line perpendicular to plane A). A buckling effect is thus created, due to the Fsin φ component of repulsion forces acting towards the center of rotation  30 . Therefore, contact arm  52  must be designed with increased cross-sectional area to withstand this buckling effect which in turn results in decreased contact arm mobility and increased cost. 
     According to an embodiment of the present invention, FIG. 4 shows a contact arm  60  having a first end  62  and a second end  64 . The contact arm  60  further includes a central section  59 , a first connecting arm  61  extending angularly from one comer of the central section  59 , and a second connecting arm  63  extending angularly from a diagonally opposite corner fo the central section  59 . Again, the positional relationship between the fixed and movable contacts  20 A,  20 B,  22 A,  22 B is shown. The present invention reduces the moment created by the popping force by inclining the contacts at an angle. The line of force D acting through the contacts  20 A,  22 A is shown in phantom. The plane A, also shown in phantom, passes through the pivot  30  and is parallel to second end portions  76  and  88  of the line and load straps  66  and  82 . 
     As shown, the line and load straps  66  and  82  each define a pair of adjacent acute angles  78  and  80  to angle an outer face of the fixed contacts  20 B and  20 A away from the center of the contact arm  60 . That is, an acute angle  78  is formed between first end portion  68  and portion  70 , and another acute angle  80  is formed between portion  70  and portion  84  of line strap  66 . Likewise, an acute angle  78  is formed between first end portion  84  and portion  86 , and another acute angle  80  is formed between portion  86  and second end portion  88  of load strap  82 . Thus, an angle  90  is defined between the line of force D and the plane A. With the angle  90  equal to 135 degrees, for example, the sine of the angle is less than one (approximately 0.707), resulting in almost a third less the value of the popping force associated with the Prior Art arrangement shown earlier in FIG.  1 . Reduction of the moment due to popping force indicates increased popping level at which the contacts pop. The present invention increases the amount of overcurrent that can pass through the contact arm before contact popping occurs, which causes contact erosion. If the moment of the force required to pop the contact is less, then popping of the contacts can be minimized thus reducing the erosion of the contact. The angle  90  can be altered for optimal results in each application. Although the line and load straps  66  and  82  are shown with acute angles  78  and  80 , it should be noted that the line and load straps could be formed in a continuous curve such that the fixed contacts  20 B and  20 A still face in the same direction as shown. 
     Advantageously, the popping force F of this embodiment, when broken down into horizontal and vertical components Fsin φ and Fcos φ, respectively, demonstrates a horizontal component Fsin φ which acts away from the center of rotation  30  of the arm  60 , keeping the contact arm  60  in tension. By using this design, the buckling effect created in the embodiment shown in FIG. 3 can be avoided. Therefore, contact arms with smaller cross sectional area can be used to increase contact arm mobility, and also reduce the cost. Lighter contact springs (not shown) can also be employed. 
     A further advantage to the embodiment of FIG. 4 is demonstrated by a comparison of FIGS. 5A and 5B. FIGS. 5A and 5B show contact arms  52  and  60 , respectively, each rotated counterclockwise an equal number of degrees. As can be seen, however, the distance d 1  between movable contact  22 A and fixed contact  20 A of FIG. 5A is less than the distance d 2  between movable contact  22 A and fixed contact  20 A of FIG.  5 B. Thus, the contact gap d 2  of FIG. 5B is greater than the contact gap d 1  of FIG. 5A per degree rotation, thereby enabling interruption at higher voltage stresses in the embodiment of FIG.  4 . 
     A simple and effective arrangement has herein been described for controlling the popping force within rotary contact circuit breakers for improved overall circuit breaker performance and lower costs. 
     While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.