Abstract:
A divided armature for the trip mechanism of a circuit breaker especially useful for low trip current breakers allows for two independent adjustments: first of the magnetic air gap between the yoke and the armature and second of the clearance between the trip bar and the back plate of the armature. The divided armature allows the force of a return spring of the trip mechanism to be unchanged while adjusting the magnetic air gap to set the trip current point.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates generally to electromagnetic actuators and more specifically to actuators such as trip mechanisms found in circuit breakers, accessories of circuit breakers, relays, or actuators. 
         [0003]    2. Discussion of the Related Art 
         [0004]    Referring to  FIG. 1 , in a known armature-yoke system  11 , the input current in a conductor (not shown) within the yoke  13  creates a magnetic field in the yoke  13 , the armature  14  and the magnetic air gap (g) between them. This results in a magnetic torque that rotates the armature  14  towards the stationary yoke  13  and moves the trip bar  16 . The hammer  18  is then released and strikes a target device, e.g. a breaker latch release (not shown), as is understood by those in the art. 
         [0005]    The magnetic torque on the armature  14  is adjusted by turning a screw  20  to set the magnetic air gap (g). The smaller the magnetic air gap (g) the higher the magnetic torque. However, as the armature  14  moves closer to the yoke  13 , the force of the return spring  22 , attached to the bell crank  24  for resetting the armature  14 , also increases, thus counteracting the effect of the magnetic torque. The net result is a reduced sensitivity of the system to gap adjustment and a lower net torque on the armature  14 . This may not be desirable in applications where the input current is low. 
       SUMMARY OF THE INVENTION 
       [0006]    In one embodiment of the present invention a divided adjustable armature for the trip mechanism of a circuit breaker allows for two independent adjustments: first, of the magnetic air gap (g) between the yoke and the armature and second, of the clearance (c) between the trip bar and the back plate of the armature, thus allowing the mechanical spring force of the trip mechanism to be unchanged while adjusting the magnetic gap to set the trip current point. The performance of electromagnetic actuators can thus be enhanced by increasing their response to magnetic air gap adjustment. This allows a circuit breaker trip mechanism to use a reduced level of trip current or achieve a wide range of armature torque, or both. Thus, the present invention is especially useful for low trip current breakers. 
         [0007]    In a typically known magnetic tripping system, such as discussed above, the reduction in armature to yoke gap (g) is accompanied by an increase in the force of the mechanical spring  22  applied to the armature  14 , here through bell crank  24 , thus reducing the net torque applied to the armature  14  and resulting in a flat response. The present invention can increase the sensitivity of electromagnetic actuators to electric current and eliminate the flat spot found in the curve of trip current versus magnetic air gap for known tripping systems. 
         [0008]    Also in the known system, the clearance (c) between the armature  14  and the trip bar  16  changes, making the system response non-linear and calibration difficult. The present invention eliminates this interdependence by allowing adjustment of the magnetic air gap (g) without altering the clearance (c) or the tension of the armature return spring  22 . 
         [0009]    In one embodiment of the present invention a circuit breaker has a trip assembly with an armature electromagnetically attractable to a yoke, whereby the armature can be driven towards the yoke to release a trip bar. The trip assembly also has a return spring operably interacting with the armature for resetting the trip assembly. The armature of the trip assembly is divided, with a ferromagnetic front plate having a surface facing towards the yoke and a back plate adjustably settable in a fixed position relative to the front plate whereby the back plate can impinge on the trip bar to initiate the opening of a circuit. A first adjustment linkage is included for adjustably setting a magnetic air gap between the yoke and the front plate without material effect on the operating tension of the return spring. A second adjustment linkage for adjustably setting a relative position between the back plate and the trip bar is further included. 
         [0010]    In some embodiments of the invention the front plate and the back plate of the divided armature are kept rigidly attached together by means of a first screw and an anti-backlash set screw. The back plate to trip bar clearance can be adjusted with a second screw independently of the magnetic air gap. Thereby adjustment of the magnetic air gap via the first screw does not affect the armature return spring tension and adjustment of the magnetic air gap does not affect the clearance between the back-plate and the trip bar. Thus the present invention can provide higher sensitivity of the net armature torque to magnetic air gap adjustment, higher response of trip current to magnetic air gap adjustment, a higher range of tripping current adjustment, a very low end tripping current and a very linear response of tripping current to the magnetic air gap adjustment. 
         [0011]    In still other embodiments a circuit breaker according to the present invention may have a trip assembly with an armature electromagnetically attractable to a yoke, whereby the armature can be driven towards the yoke to release a trip bar, and with a return spring for resetting the trip assembly. The trip assembly can comprise a divided armature on a mounting plate included within the trip assembly, the divided sections being a ferromagnetic front plate having a surface facing towards the yoke and a back plate attached to the front plate opposite the surface facing toward the yoke, for impinging on a trip bar to initiate the opening of a circuit. A first adjustment screw can be included between the front plate and the back plate for adjustably setting a magnetic air gap between the yoke and the front plate; and a second adjustment screw can be included between the back plate and the mounting plate for adjustably setting a clearance between the back plate and the trip bar. 
         [0012]    Thus, an adjustment of the first screw will not materially affect the operating tension of the return spring. In some embodiments this circuit breaker may include an antibacklash set screw between the two armature pieces for fixing the distance therebetween. In some embodiments this circuit breaker may be arranged whereby the front plate threadably receives the first adjustment screw which is contained within the back plate for setting the clearance between the back plate and the front plate. In some embodiments this circuit breaker may be arranged whereby the second adjustment screw is threaded through the mounting plate and impinges on the back plate for setting the clearance between back plate and a trip bar. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1  illustrates a known tripping system according to the prior art. 
           [0014]      FIG. 2  illustrates a first embodiment of a tripping system according to the present invention. 
           [0015]      FIG. 3  illustrates an embodiment where the two armature sections are not hinged about the same pivot. 
           [0016]      FIG. 4  shows an alternate construction where the back plate is a spring element mounted inside a formed front plate of the armature. 
           [0017]      FIG. 5  is an alternate construction with the back plate/spring element mounted on the exterior of the front plate of the armature. 
           [0018]      FIG. 6  is an isometric view of the back plate and the front plate of  FIG. 5  separated. 
           [0019]      FIG. 7  shows an alternate means of connecting the back plate to the front plate of the armature. 
           [0020]      FIG. 8  illustrates an embodiment where the front plate and the back plate have been embodied as one flexure element. 
           [0021]      FIG. 9  shows an armature with a first way of retaining a pivot pin or boss for the armature. 
           [0022]      FIG. 10  shows an armature with an alternate way of retaining a pivot pin or boss. 
           [0023]      FIG. 11  shows in perspective an alternate construction of  FIG. 8  with the front plate having formed pole faces and a hinge comprising two coined corners on the back plate. 
           [0024]      FIGS. 12 and 13  show front and back perspective views, respectively, with the pivot located on the front plate. 
           [0025]      FIG. 14  is a variation of  FIG. 8  but with a coined pivot like  FIG. 11 . 
           [0026]      FIG. 15  shows an embodiment where the return spring acts directly on the armature. 
       
    
    
     DETAILED DESCRIPTION 
       [0027]    As seen in  FIG. 2 , a trip assembly  30  according to the present invention for a circuit breaker having a trip assembly, includes a divided armature  31  on a mounting plate  33  included within the trip assembly  30 . Two sections of the divided armature  31  are a ferromagnetic front plate  35  having a surface  37  facing towards the yoke  39  and a back plate  41  attached, or settable in a fixed position relative to, the front plate  35  opposite the surface  37  facing toward the yoke  39 . The back plate  41  can impinge on a trip bar  43  to initiate the opening of a circuit. A first adjustment linkage, represented by the first screw  45  between the front plate  35  and the back plate  41 , rotates for adjustably setting the distance between the two plates and thereby setting a magnetic air gap “g” between the yoke  39  and the front plate  35 . A second adjustment linkage, represented by screw  47  between the back plate  41  and the mounting plate  33 , rotates for adjustably setting a clearance “c” between the back plate  41  and the trip bar  43 . An adjustment of the first screw  45  does not materially affect the operating tension of the armature return spring  49  applied to the armature  31 , here through a bell crank  51  to which the return spring  49  is attached. 
         [0028]    Electric current flowing in a conductor (not shown) inside the yoke  39  creates a magnetic field that results in the ferromagnetic front plate  35  of the armature  31  being attracted towards the yoke  39 . The armature  31  carries the back plate  41  that eventually hits the trip bar  43 . Back plate  41  can be made of a nonmagnetic material. When the trip bar  43  has rotated sufficiently, the hammer  53  is released to strike a breaker delatching mechanism (not shown) as will be understood by those in the art. The return spring  49  returns the trip unit to its initial position through the bell crank  51  in contact with the back plate  41 . By adjusting the magnetic air gap (g), the armature torque and therefore the tripping current setting can be controlled. 
         [0029]    This adjustment is carried out by first loosening an antibacklash set screw  55  and then turning the first screw  45  in or out to vary the magnetic air gap (g). This change in magnetic air gap does not affect the trip bar clearance (c) or the tension of the return spring  49 . Consequently, the change in the magnetic torque is not offset by a change in the spring force. The result is a better system response and greater range of tripping current settings. The set screw  55  is then retightened to eliminate any backlash between the front plate  35  and the back plate  41 . 
         [0030]    Prior to performing the magnetic air gap adjustment, the trip bar clearance (c) is set by adjusting the second screw  47  anchored in the mounting plate  33  and extending towards the back plate  41 . An armature pivot  34  serves as a fixed base for the armature sub-assembly. The front plate  35 , the back plate  41  and the bell crank  51  are all hinged on the mounting plate  33 . The second screw  47  is threaded through the mounting plate  33 . The trip assembly housing  57  is typically the structure to which all the other parts are anchored. 
         [0031]    It will be appreciated that within the practice of the present invention many variations may occur, such as the set screw  55  can be replaced by another means to eliminate backlash between the front plate  35  and the backplate  41 . Further alternatives may include spring elements which can be used to perform the function of the backplate  41  and the set screw  55  and also keep the divided plates of the armature pre-loaded as further discussed below. In some embodiments the front plate and the back plate of the armature may be formed from a single piece flexure, as further discussed below. It will also be appreciated that the same principle of a divided armature can be applied to a system where the armature return spring acts directly on the backplate with the bell crank removed as seen in  FIGS. 15 and 16 . 
         [0032]    Referring to  FIG. 3 , in this embodiment, the two armature pieces are not hinged about the same pivot. Instead the back plate  59  pivots on a boss  58  of the mounting plate  60  and the front plate  61  pivots on a boss of the back plate  59  formed for this purpose. 
         [0033]    Referring to  FIG. 4  there is shown an alternate construction where the back plate  63  is a spring element mounted inside the front plate  65  of the armature thereby eliminating the need for the set screw  55  of  FIG. 2 .  FIG. 5  is an alternate construction whereby the spring element back plate  67  is mounted on the exterior of the front plate  69 . 
         [0034]      FIG. 6  is an isometric view of the back plate  67  and the front plate  69  of  FIG. 5  shown in a separated condition. The illustrated front plate  69  might be used with the arrangement of either  FIG. 4  or  FIG. 5 . 
         [0035]      FIG. 7  shows an alternate means of connecting the back plate to the armature whereby a spring element back plate  71  comprising a formed metal element is hinged about the same pivot pin  73  as the front plate  75  and makes contact with the front plate  75  through its spring tension at a bend in the back plate  71  serving as a fulcrum point  77 . The set screw  55  of  FIG. 1  is thus eliminated. It will be noted that a magnetic air gap adjustment screw, a mounting plate, and the clearance adjustment screw  47  are not shown in this figure for convenience of illustration but are normally present for operation. 
         [0036]    In  FIG. 8  the front plate  79  and the back plate  81  of a divided armature  83  have been formed from one flexure element. The front plate  79  may be flat without any formed pole faces.  FIGS. 9 and 10  show alternate means  82 ,  84  of retaining a pivot pin (not shown) within single piece armatures  83 ,  85 , respectively, by formed cut outs in the bight of the flexure bent to retain the pivot pin. 
         [0037]      FIG. 11  shows an alternate construction with a divided armature  87  formed from a single piece of metal and having at least one formed pole face  89  on the front plate  91 . The hinge consists of two coined corners  90 ,  92  on the back plate  93 .  FIGS. 12 and 13  show perspective views of similar constructions but with pivots  94 ,  96  located on the front plates  95 ,  97 , respectively. 
         [0038]      FIG. 14  shows a divided armature  99  formed from a single piece of metal and having coined pivots collectively  101  extending from the back plate  103 . This embodiment is similar to that of  FIG. 11  but without the formed pole faces. 
         [0039]      FIG. 15  shows an embodiment of the armature  107 , where a return spring  111  acts directly on the back plate  115 . A lanced or stamped and formed spring element  117  keeps the back plate  115  and the front plate  119  pre-loaded. 
         [0040]    This divided armature system can be applied to any device that is based on an electromagnetic actuation principle. This includes, but is not limited to, tripping systems and accessories of circuit breakers, relays, actuators. Having thus described a divided armature for an electromechanical actuator; it will be appreciated that many variations thereon will occur to the artisan upon an understanding of the present invention, which is therefore to be limited only by the appended claims.