BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention is directed to a trip unit and a circuit breaker incorporating such a trip unit that is calibrated by adjusting a gap between a magnetic armature and magnetic yoke to which the armature is attracted by a selectable level of current in the protected circuit. The gap is adjusted without affecting the adjustable spring force that sets the current level at which the circuit breaker trips. 
     2. Background Information 
     Many circuit breakers utilize a magnetic trip unit to provide an “instantaneous” response to overcurrent or short circuit conditions. In one common type of magnetic trip unit, a magnetic yoke positioned around the load conductor focuses the magnetic field induced by the load current to attract an armature, which as it moves toward the magnetic yoke actuates the operating mechanism that opens the circuit breaker&#39;s separable contacts. The armature is biased away from the magnetic yoke by a spring. The spring force, and the gap between the armature and the magnetic yoke, affect the current at which the circuit breaker is tripped open. 
     The level of load current at which the circuit breaker trips varies with the feeder being protected and preference of the user. In order to accommodate a range of trip currents with a single trip unit, it is common to make the trip unit adjustable. A common range of adjustability of the trip current is five to ten times the rated current of the breaker. Typically, this adjustment is made in the spring force. 
     The trip unit must be calibrated at both the high and low end of trip currents to assure that the response is within tolerance, for example, plus or minus 20%. Calibration is effected by adjusting the gap between the spring biased armature and the magnetic yoke. As the magnetic yoke is fixed in position, the armature is moved, closer to the magnetic yoke to adjust the trip current downward, and away to increase the trip current. However, moving the armature in such a trip unit also changes the spring force, but in the opposite sense. Thus, while moving the armature closer to the magnetic yoke decreases the gap, tending to lower the trip current, it stretches the spring more, which increases the spring force, and therefore, the current needed to trip the breaker. In breakers with higher current ratings, the reduction in trip current produced by shortening the gap between the armature and the magnetic yoke is greater than the increase in trip current resulting from the associated stretching of the spring, so that the trip unit can be calibrated. However, in breakers with lower current ratings, the increase in trip current caused by the stretching of the spring is greater than the reduction caused by the shortening of the gap and, hence, the unit cannot be calibrated. 
     SUMMARY OF THE INVENTION 
     The present invention permits the gap between the armature and the magnetic yoke in a magnetic trip unit to be adjusted without affecting the bias force applied by the spring, thereby making it possible to calibrate such magnetic trip units regardless of the current rating. Thus, in accordance with one aspect of the invention, an adjustable magnetic trip unit for interrupting a load current through a circuit breaker comprises: a magnetic pole in which a magnetic field is generated by the load current; and an armature assembly. The armature assembly comprises a bracket supported for movement toward and away from the magnetic pole, a spring biasing the bracket to a position spaced from the magnetic pole, an armature, and a mount selectively positioning the armature on the bracket to adjust a gap between the armature and the magnetic pole. The adjustable magnetic trip unit can include in the armature assembly an adjustment mechanism selectively adjusting the bias applied by the spring to the bracket, whereby the load current, at which the magnetic field in the magnetic pole overcomes the bias applied by the spring and pulls the armature to the yoke, can be adjusted. 
     The mount for the armature can comprise a hinge connection and an adjustment member setting a hinge angle between the armature and the bracket. This adjustment member can comprise a threaded rod having a neck at one end. Either the armature or the bracket can have a tapped hole in which the rod is threaded while the other has a slot capturing the neck of the threaded rod. 
     The bracket can be elongated with a pivot member on at a first end, a first part of the hinge connection adjacent the first end and either the tapped hole or the slot adjacent the second end. In this configuration, the armature can comprise an armature paddle, an extension extending from the armature paddle and having a free end with the second part of the hinge connection adjacent the free end and with the other of either the tapped hole or the slot on the extension between the free end and the armature paddle. The bracket can have a T-shape at the first end forming the pivot member. Furthermore, the bracket can have a main body with an integral tab extending along each side toward the first end and offset from the main body to form the T-shape of the first end and configured to form the first part of the hinge connection. In this embodiment, the second part of the hinge connection on the extension of the armature comprises a T-shaped free end, and the tabs on the side edges of the bracket are configured as hooks forming the first part of the hinge connection on which the T-shaped free end of the armature extension seats. 
     The invention also embraces a circuit breaker incorporating this adjustable magnetic trip unit. 
    
    
     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 sectional elevation view of a trip unit incorporating the invention with the remaining portion of the circuit breaker in which the trip unit is installed shown schematically. 
     FIG. 2 is an isometric view with some parts removed for clarity of the operative portions of the trip unit shown in FIG.  1 . 
     FIG. 3 is an isometric view of a bracket which forms part of a preferred embodiment of an adjustable magnetic trip unit incorporated in the trip unit of FIGS. 1 and 2. 
     FIG. 4 is an isometric view of an armature member which also forms part of the adjustable magnetic trip assembly in accordance with the preferred embodiment of the invention. 
     FIG. 5 is an isometric view of the preferred embodiment of the adjustable magnetic trip unit which incorporates the bracket of FIG.  3  and the armature member of FIG.  4 . 
     FIG. 6 is a sectional elevation view of the trip unit illustrated in FIG. 1 but shown with the adjustable magnetic trip unit shown in an exaggerated adjusted position. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 illustrates a circuit breaker  1  with a molded housing  3  (shown schematically) containing separable contacts  5  including a fixed contact  7  affixed to a line conductor  9  and a movable contact  11  mounted on a pivoted contact arm  13  connected to a load conductor  15 . The contact arm  13  is actuated by an operating mechanism  17  to open and close the separable contacts  5 . The operating mechanism  17  can be operated manually by a handle (not shown) or automatically by a thermal/magnetic trip unit  19 . The exemplary circuit breaker  1  is a three pole breaker so that there are three sets of separable contacts  5 , one for each pole, but all operated by a single operating mechanism  17 , in a manner which is well known 
     Referring to both FIGS. 1 and 2, the thermal magnetic trip unit  19  has three poles  21   a ,  21   b , and  21   c . Each pole of the trip unit  19  includes a section of the load conductor  15  which is bent into an inverted U  15   u  leading to a horizontally extending terminal section  15   t . Each pole  21   a ,  21   b , and  21   c  has a bimetal  23  secured to the load conductor at  15   u  by screws  25 . Calibrating screws  26  are screwed through the upper ends of the bimetals  23 . A trip bar  27  extending across all three poles is pivotally mounted for rotation about a horizontal axis  29 . As is well known, persistent overload current through the load conductor  15  of any of the poles  21   a ,  21   b , and  21   c  heats up the associated bimetal  23  causing the free (upper) end to bend, in a clockwise direction in FIG. 1 until it contacts and rotates the trip bar  27  in a clockwise direction. This rotation of the trip bar actuates the operating mechanism  17  in a well known manner to open the separable contacts  5 . The current/time at which the thermal trip occurs can be adjusted for all three poles simultaneously by axial movement of the trip bar  27  in a known manner by a thermal trip adjustment knob (not shown). The individual bimetal  23  for each pole is calibrated by the associated calibration screw  26 , also in a known manner. 
     An instantaneous or magnetic trip function is provided in the thermal/magnetic trip unit  19  by an adjustable magnetic trip device  31  associated with each of the poles  21   a ,  21   b , and  21   c . Each adjustable magnetic trip device  31  includes a magnetic pole formed by a magnetically permeable U-shaped yoke  33  captured in the U-shaped section  15   u  of the load conductor  15 , and an armature assembly  35 . Turning to FIGS. 3-5, in addition to FIG. 2, the armature assembly  35  includes a bracket  37  shown separately in FIG. 3, bias spring  39 , an armature  41  shown separately in FIG. 4, and a mount  43  mounting the armature  41  on the bracket  37 . The bracket  37  is elongated and is T-shaped at a first or upper end  45  to form a pivot member  47  that seats in a saddle  49  molded into the casing  51  of the thermal/magnetic trip unit  19 . A loop  53  formed in the bracket  37  by stamping is engaged by one end of the bias spring  39 . The other end of the bias spring  39 , which is a helical tension spring in the exemplary armature assembly, engages a hook  55  projecting laterally from the horizontally extending magnetic trip adjustment bar  57 . The bias spring  39  biases the bracket, and therefore the armature assembly  35  counterclockwise as viewed in FIG. 1 against the stop  59  formed by the wall of the casing  51 . 
     The exemplary armature  41 , as shown in FIG. 4, comprises an armature paddle  61  and an extension  63  extending from the armature paddle and having a free end  65 . The mount  43  which mounts the armature  41  on the bracket  37 , is a hinge connection  67  in the exemplary armature assembly  35 . The first part of the hinge connection  67  is formed adjacent the first end  45  of the bracket  37  by integral tabs  69  extending toward the first end but offset from the bracket main body  71  and configured as a hook  73 . The second part of the hinge connection  67  is formed by arms  66  extending laterally from the free end  65  of the armature extension  63  that seat in the hooks  73  on the bracket  37 . 
     The mount  43  mounting the armature  41  on the bracket  37  further includes an adjustment member  75 , which in the exemplary embodiment is a threaded rod. This threaded rod  75  engages a tapped hole  77  adjacent a second end  79  of the bracket  37 . A neck  81  adjacent one end of the threaded rod  75  is received in a slot  83  in a keyhole opening  85  in the extension  63  on the armature  41 . Alternatively, the tapped hole can be provided in the armature extension  63  with the keyhole slot in the bracket. Other adjustment members for setting the angle between the armature and the bracket can also be used. 
     As shown in FIG. 1, the spring  39  biases the armature assembly counterclockwise so that the bracket  37  seats against the stop  59  formed by the casing  51  setting a gap  87  between the armature paddle  61  and the U-shaped magnetic yoke  33 . High overcurrents flowing through the separable contacts  5  and therefore the load conductor  15 , such as could be produced by a short circuit, generate a magnetic field which is focused by the yoke  33  to attract the armature paddle  61  clockwise toward the yoke. Before reaching the yoke  33 , the armature  41  engages an arm  89  on the lower end of the trip bar  27  thereby rotating the trip bar clockwise to actuate the operating mechanism  17  and open the separable contact  5 . The magnitude of the load current at which the armature  41  is attracted to the yoke  33  is set by the bias spring  39 . This trip current is simultaneously set for all three poles  21   a ,  21   b , and  21   c  by rotation of the magnetic trip adjustment bar  57  through a single adjusting knob (not shown). The magnetic trip function is separately calibrated for each pole by rotating the threaded rod  75  to pivot the armature  41  relative to the bracket  37 . FIG. 6 illustrates an exaggerated adjustment of the armature relative to the bracket while FIG. 1 illustrates full adjustment in the opposite direction. 
     It can be appreciated from FIGS. 1 and 6 that the magnetic trip for each individual pole can be calibrated by adjustment of the associated threaded rod  75  without having any effect on the trip level set by the spring  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.
The magnetic trip device of a circuit breaker has a magnetic pole in which a magnetic field is generated by load current, and an armature assembly that includes: a bracket supported for pivotal movement relative to the magnetic pole; a spring biasing the bracket to a position spaced from the magnetic pole; an armature hinged on the bracket; and an adjusting screw for adjusting a gap between the armature and the magnetic pole for calibrating the trip device without affecting the spring bias, which can be separately and independently adjusted to select the load current at which the magnetic trip is initiated.