Patent Publication Number: US-6667675-B2

Title: Adjustable magnetic trip assembly for circuit breaker

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a trip assembly for a circuit breaker which responds to the magnetic forces generated by overcurrents. More particularly, it relates to an arrangement for the user of the circuit breaker to easily adjust the level of overcurrent at which the magnetic trip assembly responds. 
     2. Background Information 
     A common type of circuit breaker trip unit which responds with an instantaneous trip to overcurrents such as those caused by a short circuit utilizes a magnetic solenoid. The current in the protected circuit is conducted through the coil windings generating attraction forces between a stationary and a movable armature. A spring generates a bias force opposing the attraction force applied to the movable armature. When the current magnitude achieves a level at which the magnetic force exceeds the spring biasing force, the moving armature pulls into the stationary armature. This mechanical motion is used to trip the circuit breaker. Commonly, this occurs at a discreet value of current. 
     It is an object of the present invention to provide an arrangement for easy adjustment of the magnetic tripping current in the field. 
     SUMMARY OF THE INVENTION 
     In accordance with the invention, an adjustable magnetic trip unit for a circuit breaker comprises a trip solenoid having a coil and a plunger movable relative to the coil. A first end of the plunger is extendable from a first end of the coil to a trip position when the coil is energized by a current of at least of a selected amplitude. The second end of the plunger extends from the second end of the coil. A spring assembly includes a spring engaging the second end of the plunger to apply a bias force to the plunger setting the selected amplitude of current, and an adjustment mechanism adjusting the bias force, and therefore, the selected amplitude of the current. The spring can be a torsion spring having a first end engaging the second end of the plunger and a second end wound by the adjustment mechanism to adjust the bias force. This adjustment mechanism includes an indexer setting discrete positions to which the second end of the torsion spring is wound to provide a plurality of discrete values of the selected amplitude of current. The adjustment mechanism also includes an adjustment knob for setting the indexer to the plurality of discrete positions and a coupler coupling the adjustment knob to the second end of the torsion spring. The coupler comprises a first bevel gear engaging the second end of the torsion spring, a second bevel gear engaging the first bevel gear, and a shaft connecting the second bevel gear to the adjustment knob for rotation by the adjustment knob. The torsion spring has an axis coincident with the pivot axis of the first bevel gear which is substantially perpendicular to the plunger. The shaft has an axis which is orthogonal to both the plunger and the axis of the torsion spring. 
     The adjustable magnetic trip unit is adapted to be mounted in a circuit breaker housing having a seat. The indexer is mounted on the shaft and has a peripheral cam surface engagable with the seat in the plurality of discrete rotational positions. The indexer is axially displaceable between a locked position engaging the seat and an unlocked position free of the seat in which the indexer, and therefore the shaft, can be rotated between the plurality of discrete positions. A second spring biases the indexer to the locked position. The indexer is movable against the biasing provided by the second spring by depression of the adjustment knob. The adjustment mechanism can include a gap adjuster engaging the second end of the plunger and driven by the first bevel gear to adjust the position to which the plunger is biased by the torsion spring. 
     In addition, the invention is directed to a circuit breaker adjustable magnetic trip assembly which includes a housing, a trip solenoid having a coil energized by load current and a plunger movable to a trip position in response to load current above a selected amplitude, a torsion spring having a first end engaging the plunger to apply a bias force to the plunger setting the selected amplitude of the load current at which the plunger moves to the trip position and an adjustment mechanism. The adjustment mechanism comprises a first bevel gear engaging a second end of the torsion spring, a second bevel gear engaging the first bevel gear, a shaft engaging the second bevel gear, an adjustment knob mounted on the shaft, and an indexer setting a plurality of discrete rotational positions of the adjustment knob. The housing has a seat and the indexer selectively engages the seat at the plurality of discrete rotational positions. The indexer has a peripheral cam surface with a plurality of flats and a seat in the housing has complimentary flat surfaces against which the indexer seats at the plurality of discrete rotational positions. The indexer is mounted for axial movement between a locked position in which it engages the seat and an unlocked position in which it and the adjustment knob are free to rotate. The adjustment mechanism further includes a bias spring biasing the indexer axially to the locked position. The housing can be a molded housing having a first slot forming the seat and an axially adjacent second slot with which the indexer is aligned in the unlocked position. The second slot is sized to allow rotation of the indexer. The housing has a third slot adjacent the second slot in which the second bevel gear is mounted for rotation. The adjustment knob and the indexer are axially movable relative to the second bevel gear which is restrained from axial movement by the third slot. 
     The adjustment mechanism further includes a gap adjuster adjusting an unactuated position to which the plunger is biased by the torsion spring. This gap adjuster can comprise an adjustment member coupled to and moved by the first bevel gear. The plunger can have a first inclined surface and the adjustment member can be a slide having gear teeth engaging the first bevel gear and a complimentary second inclined surface engaging the first inclined surface. The plunger can have a first end extendable from the first end of the coil to a trip position and a second end extending from a second end of the coil and having a flange with a first axially facing surface engaged by the first end of the torsion spring and a second axially facing surface bearing against the first inclined surface. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A full understanding of the invention can be gained from the following description of the preferred embodiments when read in conjunction with the accompanying drawings in which: 
     FIG. 1 is a fragmentary isometric view of a circuit breaker with the cover removed illustrating the adjustable magnetic trip unit of the invention. 
     FIG. 2 is a fragmentary cross-sectional view through a portion of the circuit breaker housing shown in FIG.  1 . 
     FIG. 3 is a fragmentary elevation view of a circuit breaker housing with the cover removed illustrating a second embodiment of the invention. 
     FIG. 4 is a partial elevation view of a multipole embodiment of the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 illustrates a circuit breaker adjustable magnetic trip assembly  1  which includes a molded circuit breaker housing  3  and an adjustable magnetic trip unit  5 . This adjustable magnetic trip unit  5  includes a trip solenoid  7  which has a coil  9  through which current in the protected circuit is routed. The coil  9  is wound on a nonmagnetic sleeve  11  within which a stationary armature  13  is mounted. A plunger is forming a movable armature is axially slideable within the stationary armature  13 . The sleeve  11  and therefore the trip solenoid  7  is fixed within the molded housing  3  by integral mounts  17  molded as part of the housing. The plunger  15  has a first end  19  that extends out of a first end  21  of the coil  9  and can be a separate nonmagnetic section. A circular flange  23  forms a second end of the plunger  15  that extends out of a second end  25  of the coil  9 . 
     A spring assembly  27  biases the flange  23  of the second end of the plunger  15  against an abutment surface  29  within the housing  3 . Current through the coil  9  generates magnetic forces which tend to draw the plunger  15  to the right as viewed in FIG. 1 so that the first end  19  extends out of the first end of the coil to a trip position in which it engages and trips the trip mechanism indicated generally at  31  in FIG.  1 . This only occurs when the magnetic attraction force generated by the current is sufficient to overcome the bias force applied by the spring assembly  27 . The spring assembly  27  allows the magnitude of the current at which this occurs to be selected. 
     The spring assembly  27  includes a spring  33  and an adjustment mechanism  35  for adjusting the spring force. This adjustment mechanism  35  includes an indexer  37  and an adjustment knob  39  which rotates the indexer. The spring  33  is a torsion spring having a first end  41  which bears against the flange  23  to bias the plunger  15  against the abutment surface  29 . A second end  43  of the torsion spring  33  is coupled to the adjustment mechanism  35  by a coupler  45  which includes a first or driven bevel gear  47  mounted in the housing  3  for rotation about an axis  49  perpendicular to the plunger  15 . The second end  43  of the torsion spring  33  is secured to this driven bevel gear  47  and rotated by it to adjust the bias force applied to the plunger  13 . The coupler  45  includes a second or driving bevel gear  51  engaging the first bevel gear  47 . A noncircular shaft  53  is axially slidable with respect to the second bevel gear  51  and is keyed to this second bevel gear  51  to rotate it with rotation of the shaft. The shaft  53  carries the indexer  37  and the adjustment knob  39 . 
     The circuit breaker housing  3  has a seat  55  in which the adjustment mechanism  35  is seated. This seat  55  defines a first slot  57  in which the indexer  37  is seated by a locking spring  59 . The indexer  37  is a disc with a peripheral camming surface  61  which can be formed by a number of flats. As best seen in FIG. 2, the slot  57  is sized so that the indexer can only be seated when it is rotated by the adjustment knob  39  to positions in which the flats  61  are aligned with the walls of the slot  57 . When seated in the slot  57 , the indexer  37  cannot be rotated. Below the slot  57  is a second slot  63  which is wider and allows the indexer  37  to be rotated between the plurality of discrete positions set by the flats  61 . A third slot  65  in the molded housing below the slot  63  mounts the second bevel gear  51  for rotation (see FIG.  1 ). 
     The locking spring  59  biases the indexer  37  upward to a locked position within the first slot  57  at one of the plurality of discrete positions. The bias force applied by the torsion spring  33  to the plunger  15  can be adjusted by depressing the adjustment knob  39  thereby moving the indexer axially downward out of the slot  57  into alignment with the second slot  63  in which it can be rotated to another one of the discrete rotational positions in which the flats  61  of the indexer align with the second slot. This rotation of the indexer  37  rotates the second bevel gear  51  which, in turn, rotates the first bevel gear  45  to adjust the bias force applied by the torsion spring  33  to the plunger  13  of the trip solenoid  7 . Release of the adjustment knob  39  allows the locking spring  59  to raise the indexer  37  into the locked position within the slot  57  in one of the discrete positions. 
     With the arrangement shown, the torsion spring  33  can be specified with many windings and a generous outside diameter resulting in a low spring constant. Low spring constants are beneficial in adjustment mechanisms because they are less sensitive to dimensional variation in assembly components. 
     In the arrangement described above, the air gap  67  between the flange  23  on the plunger or moving armature  15  and the stationary armature  13  is fixed. In situations where larger adjustment ranges are required than can be provided by adjusting the bias force applied by the torsion spring  33 , this air gap  67  can also be varied. Thus, as shown in FIG. 3, the adjustment mechanism  35  can include a gap adjuster  69 . The gap adjuster  69  includes an adjustment member in the form of a slide  71  that is coupled to and moved by the first bevel gear  47 . This coupling is accomplished by a rack  73  having gear case  75  which mesh with a pinion gear  77  fixed to the first bevel gear  47 . In this embodiment, the flange  23  on the end of the plunger  15  has a first axially facing surface  79  against which the first end  41  of the torsion spring  33  bears, and a second axially facing surface  81  having a first inclined surface  83 . The slide  71  has a second incline surface  85  complementary to and engaging the first inclined surface  83 . Now, when the adjustment knob  39  is depressed and rotated to adjust the bias force applied by the torsion spring, the slide is simultaneously translated to adjust the air gap  67 . The wrap of the torsion spring  33  is such that when the second bevel gear  47  is rotated clockwise to increase the bias force applied by the spring  33 , the air gap  67  is increased to also increase the current required to actuate the trip mechanism  31 . Similarly, relaxation of the torsion spring simultaneously reduces the air gap  67  to lower the trip current. As a result, the combination of the complementary adjustment of the spring bias and the air gap increases the range of load current over which the solenoid can be selected to respond. 
     The invention can be applied to multipole circuit breakers as shown in FIG.  4 . As shown there, the shaft  87  carrying the first bevel gear  47  is extended such that a separate torsion spring  33   1 - 33   3  can be provided for each pole  89   1 - 89   3  of, for instance, a three pole circuit breaker. With this arrangement, the actuating current in all of the multiple poles can be adjusted simultaneously by the rotation of the single adjustment knob  39 . 
     While specific embodiments of the invention have been described in detail, it will be appreciated by those skilled in the art that various modifications and alternatives to those details could be developed in light of the overall teachings of the disclosure. Accordingly, the particular arrangements disclosed are meant to be illustrative only and not limiting as to the scope of the invention which is to be given the full breadth of the claims appended and any and all equivalents thereof.