Patent Publication Number: US-6218920-B1

Title: Circuit breaker with adjustable magnetic trip unit

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to circuit breakers with a magnetic trip unit, and, more particularly, to circuit breakers with an adjustable magnetic trip unit. 
     Circuit breakers typically provide protection against the very high currents produced by short circuits. This type of protection is provided in many circuit breakers by a magnetic trip unit, which trips the circuit breaker&#39;s operating mechanism to open the circuit breaker&#39;s main current-carrying contacts upon a short circuit condition. 
     Modern magnetic trip units include a magnet yoke (anvil) disposed about a current carrying strap, an armature (lever) pivotally disposed near the anvil, and a spring arranged to bias the armature away from the magnet yoke. Upon the occurrence of a short circuit condition, very high currents pass through the strap. The increased current causes an increase in the magnetic field about the magnet yoke. The magnetic field acts to rapidly draw the armature towards the magnet yoke, against the bias of the spring. As the armature moves towards the yoke, the end of the armature contacts a trip lever, which is mechanically linked to the circuit breaker operating mechanism. Movement of the trip lever trips the operating mechanism, causing the main current-carrying contacts to open and stop the flow of electrical current to a protected circuit. 
     It is necessary for such magnetic trip units to be reliable. In addition, it is desired that magnetic trip units be adjustable, so that the breaker can be adjusted to trip at different levels of overcurrent. It is also desired that the magnetic trip units be compact. 
     BRIEF SUMMARY OF THE INVENTION 
     In an exemplary embodiment of the invention, a circuit breaker with adjustable magnetic trip unit includes a magnet yoke disposed proximate to an electrically conductive strap, and an armature pivotally disposed proximate to the magnet yoke. A trip shaft is configured to interact with a latching mechanism of the circuit breaker. The trip shaft has a cam extending therefrom, with the cam being arranged proximate to the armature. An adjusting bar is arranged to pivot around the trip shaft. The adjusting bar includes an adjusting arm extending therefrom. The adjusting arm contacts the armature for adjusting the distance between the magnet yoke and the armature. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an elevation view of a circuit breaker with a magnetic trip unit of the present invention; 
     FIG. 2 is an elevation view of the magnetic trip unit from the circuit breaker of FIG. 1; and 
     FIG. 3 is a perspective view of a multi-pole circuit breaker including the magnetic trip unit of FIG.  2 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A circuit breaker  1  equipped with the adjustable magnetic trip unit of the present invention is shown in FIG.  1 . The circuit breaker  1  has a rotary contact arm  2 , which is mounted on the axis  3  of a rotor  4  such that it can rotate. The rotor  4  itself is mounted in a terminal housing or cassette (not shown) and has two diametrically opposed satellite axes  5  and  6 , which are also rotated about the axis  3  when the rotor  4  rotates. The axis  5  is the point of engagement for a linkage  7 , which is connected to a latch  8 . The latch  8  is mounted, such that it can pivot, on an axis  10  positioned on the circuit breaker housing  9 . In the event of an overcurrent or short circuit condition, the latch  8  is released by a latching mechanism  11 , moving the contact arm  2  to the open position shown in FIG.  1 . 
     The latching mechanism  11  can be actuated by a trip lever  13  that pivots about an axis of rotation  12 . The other end of the trip lever  13  contacts a trip shaft  14 , which is mounted on an axis  15  supported by the circuit breaker housing  9 . Disposed on the trip shaft  14  is a cam  14   a , which can be pivoted clockwise in opposition to the force of a torsional spring  14   b  wound about the axis  15 . 
     Mounted to the circuit breaker housing  9  in the bottom region of the circuit breaker is a magnet yoke  16 , which encircles a current carrying strap  17  electrically connected to one of the contacts of the circuit breaker  1 . Arranged facing the magnet yoke is an armature element  18  in the form of a metallic lever, which is hinge-mounted by means of hinge pin sections  19  (see FIG. 3) to hinge knuckles (not shown) formed on the circuit breaker housing  9 . The armature  18  is also connected to strap  17  by a spring  20 , which biases the armature  18  in the clockwise direction, away from the magnet yoke  16 . In its upper region, armature  18  is equipped with a clip  21  rigidly mounted thereon, which can be brought into contact with the cam  14   a  by pivoting of the armature in a counter-clockwise direction. Movement of cam  14   a  by the armature  18  causes the trip shaft  14  to rotate about axis  15  and thereby actuate the latching mechanism  11  by means of the trip lever  13 . Once actuated, latching mechanism  11  releases latch  8  to initiate the tripping process in circuit breaker  1 . While the clip  21  is described herein as being mounted to armature  18 , the clip  21  can also be formed as one piece with the armature  18 , preferably of metal. 
     Referring now to FIG.  2  and FIG. 3, an adjusting bar  23  extends parallel to the axis  15  and is mounted on the axis  15 , by means of support arms  22 . The adjusting bar  23  has an adjusting arm  24  which is threadably engaged to an adjusting screw  25  for calibrating the trip unit. Adjusting bar  23  also includes a lever arm  26  which extends to a side of the adjusting bar  23  diametrically opposite adjusting arm  24 . The end of the lever arm  26  is in contact with a cam pin  27  of a rotary knob  28 , which is mounted in a hole in the upper wall of the circuit breaker housing  9  (FIG.  1 ). The surface of the rotary knob  28  is equipped with a slot  29  to make it possible to adjust the rotary knob  28  with the aid of a suitable tool, such as a screwdriver. 
     In the unactuated state of the magnet yoke  16 , which is to say when the contact arm  2  (FIG. 1) is closed and an overcurrent is not present, the adjusting screw  25  is in constant contact with an angled surface of the clip  21 . Contact between adjusting screw  25  and the angled surface of the clip  21  is ensured by a tensile force exerted by the spring  20  on the armature  18 . The force of the angled surface of the clip  21  on adjusting screw  25  biases the adjusting bar  23  in a clockwise direction about axis  15 , thus forcing lever arm  26  away from yoke  16  and against pin  27 . In this state, it is possible to change the tilt setting of the armature  18  either by extending (or retracting) adjusting screw  25  downward from (upward to) adjusting arm  24 , or by rotating the adjusting bar  23  about axis  15  by adjusting the rotary knob  28 . Thus, the distance L shown in FIG. 2 between the flap  18  and the magnet yoke  16  is adjusted, thereby setting the current at which the trip unit responds. 
     One advantage of the present invention is that an extremely reliable adjustment mechanism is guaranteed by the interaction of the adjusting bar  23 , which rotates around the axis  15  of the trip shaft  14 , and the rotary knob  28  that interacts therewith via cam pin  27 . Moreover, this mechanism is easy to produce and is compact in design. The tripping device of the present invention has only a few elements, which can be accommodated, in a space-saving manner, laterally in the switch. 
     The circuit breaker with adjustable magnetic trip unit shown in FIGS. 1,  2 , and  3  operates as follows. First, a person adjusting the circuit breaker I by turning rotary knob  28  sets the position of the adjusting bar  23  on the axis  15  and thus the distance between the armature  18  and the magnet yoke  16 , as shown in detail in FIG.  2 . Because of the relatively greater length of the lever arm  26  as compared to the adjustable arm  24 , the adjustment made by rotary knob  28  is fine. It must be noted here that a coarser adjustment of the distance L between the magnet yoke  16  and the flap  18  can be accomplished by turning the adjusting screw  25  during installation of the trip unit in the circuit breaker housing  9 . 
     In the case of a short circuit, an overcurrent naturally occurs, which flows through the current carrying strap  17 . This activates the magnet yoke  16  to the extent that when a specific current is exceeded, the magnetic force generated by the magnet yoke is sufficient to attract the armature  18  in opposition to the tensile force exerted by the spring  20 . Armature  18  pivots towards yoke  16 , and the cam  14   a  is pivoted clockwise in FIG. 1 (counter-clockwise in FIG. 2) by the clip  21  until the trip lever  13  is actuated. Actuation of the trip lever  13  then tilts the latching mechanism  11  such that it in turn can release the latch  8  for a pivoting motion, upward in FIG. 1, about the axis  10 . This motion is caused by a spring, which is not shown in detail in FIG.  1 . The motion of the linkage  7  that is coupled with the pivoting motion of the latch  8  brings about a rotation of the rotor  4  by means of the axis  5 , and thus finally a disconnection of the contact arm  2  from the current carrying straps. 
     As shown in FIG. 3, the trip unit can be arranged for use in a circuit breaker  1  having a plurality of breaker cassettes  30 , with each cassette  10  having its own contact arm  2  and rotor  4  arrangements. While only one cassette  30  is shown, it will be understood that one cassette  30  is used for each phase in the electrical distribution circuit. Adjusting bar  23  extends along the row of circuit breaker cassettes  30 , parallel to the axis  15  of the trip shaft  14 . Extending from adjusting bar  23  are several adjusting arms  24  corresponding to the number of circuit breaker cassettes  30 . Also formed on the adjusting bar  23  is one lever arm  26 , which is sufficient to rotate the adjusting bar  23  about axis  15  and, thus, pivot the armatures  18 . The tripping sensitivity in each circuit breaker cassette  30  can be adjusted separately by means of the screws  25  carried by each adjusting arm  24 . As a result, individual calibration of each circuit breaker cassette  30  can be undertaken independently of the adjustment of rotary knob  28 . 
     It will be understood that a person skilled in the art may make modifications to the preferred embodiment shown herein within the scope and intent of the claims. While the present invention has been described as carried out in a specific embodiment thereof, it is not intended to be limited thereby but is intended to cover the invention broadly within the scope and spirit of the claims.