Abstract:
A method and magnetic trip unit for actuating a latching mechanism to trip a circuit breaker upon an overcurrent condition, the magnetic trip unit including: a first electrically conductive strap configured to conduct an electrical current; a first magnet yoke disposed proximate to the first electrically conductive strap; and a first armature pivotally disposed proximate to the first magnetic yoke in operable communication with the latching mechanism; the first armature providing a magnetic path having a reluctance to magnetic flux; and the reluctance is adjusted to prevent saturation of the magnetic flux when the current through the strap is a first number times a rated current of the circuit breaker and the reluctance is adjusted to promote saturation of magnetic flux when the current through the strap is a second number times the rated current of the circuit breaker, wherein the first number is a number smaller than the second number.

Description:
BACKGROUND OF INVENTION 
   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. 
   Currently, circuit breakers having a magnetic trip unit described above allow for adjusting the air gap distance between the magnet yoke and the armature to obtain different trip set points. The trip set point range offered by adjusting the distance between the magnet yoke and the armature is limited because a large trip set point range requires a large air gap adjustment range. Because available space is often limited, a smaller than desired adjustment range results. Furthermore, overcurrent protection at a low current trip setting (e.g., three times the rated current of the circuit breaker) is inhibited because the magnetically induced force acting on the armature isn&#39;t significant enough to trip the latch system. 
   Those skilled in the art will appreciate that the electrical load of a motor is characterized by a starting (run-up) current and a running current. The starting current averages about six times the full load current of the motor, but the peak of the first half cycle, the so-called “inrush” current, can reach values of up to twenty times the full load current. The lower overcurrent range for motor protection is commonly 3× the full load current of the motor. 
   It is necessary for such magnetic trip units to be reliable at a low overcurrent setting without altering the magnetically induced force acting on the armature at high overcurrent settings. In addition, it is desired that magnetic trip units offer a broader spectrum of overcurrent ranges (e.g., for use in motor protection), so that the breaker can offer a broader range to trip at different levels of overcurrent. It is also desired that the magnetic trip units be compact. 
   SUMMARY OF INVENTION 
   The above discussed and other drawbacks and deficiencies are overcome or alleviated by a magnetic trip unit for actuating a latching mechanism to trip a circuit breaker upon an overcurrent condition, the magnetic trip unit including: a first electrically conductive strap configured to conduct an electrical current; a first magnet yoke disposed proximate to the first electrically conductive strap; and a first armature pivotally disposed proximate to the first magnetic yoke in operable communication with the latching mechanism; the first armature providing a magnetic path having a reluctance to magnetic flux; and the reluctance is adjusted to prevent saturation of the magnetic flux when the current through the strap is a first number times a rated current of the circuit breaker and the reluctance is adjusted to promote saturation of magnetic flux when the current through the strap is a second number times the rated current of the circuit breaker, wherein the first number is a number smaller than the second number. 
   In an alternative embodiment, a method of increasing an induced magnetic force from a magnet yoke on a pivotally mounted armature of a trip unit in a circuit breaker at a low current without substantially altering the induced magnetic force acting on the armature at a high current, the method comprising: configuring the armature to provide a magnetic path having a reluctance to a magnetic flux; and adjusting the reluctance of the magnetic path to prevent saturation of the magnetic flux when a current through the trip unit is a first number times a rated current of the circuit breaker, and the magnetic path is generally saturated when the current through the circuit breaker is a second number times the rated current, wherein the first number is a number smaller than the second number. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
     Referring to the drawings wherein like elements are numbered alike in the several Figures: 
       FIG. 1  is an elevation view of a circuit breaker with a magnetic trip unit; 
       FIG. 2  is an elevation view of the magnetic trip unit from the circuit breaker of  FIG. 1 ; 
       FIG. 3  is a perspective view of a multi-pole circuit breaker including the magnetic trip unit of  FIG. 2 ; 
       FIG. 4  is a perspective view of an armature and yoke of the magnetic trip unit in  FIG. 3 ; 
       FIG. 5  is a perspective view of an alternative embodiment of the yoke shown in  FIG. 4 ; and 
       FIG. 6  is a graph illustrating the relationship between the induced force and gap distance of two different armature configurations shown in  FIGS. 4 and 5 . 
   

   DETAILED DESCRIPTION 
   A circuit breaker  1  equipped with an adjustable magnetic trip unit of the present disclosure is shown in  FIG. 1 . The circuit breaker  1  has a rotary contact arm  2 , which is mounted on an 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 rotational solenoid type magnetic assembly comprising a magnet yoke  16  and a biased armature  18 . Magnet yoke  16  encircles a current carrying strap  17  electrically connected to one of the contacts of the circuit breaker  1 . Arranged facing the magnet yoke is the armature  18  in the form of a metallic lever, which is hinge-mounted by means of hinge pin sections  19  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 . A top 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 armature  18  and the magnet yoke  16  is adjusted, thereby setting the current level at which the trip unit responds. 
   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  1  by turning rotary egg 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 gap L between the magnet yoke  16  and the armature  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  30  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 . 
   Referring to  FIG. 4 , a perspective view of an exemplary embodiment of armature  18  and yoke  16  of a magnetic trip unit assembly is illustrated. The magnet yoke  16  is shaped from a ferrous steel plate to define a backwall  40  having side arms  42 ,  44  extending generally perpendicularly from backwall  40  towards armature  18 . Each of side arms  42 ,  44  includes a flange  46 ,  48  extending generally perpendicularly therefrom to form a four-sided enclosure. Flanges  46  and  48  form an increased pole face area over that offered by side arms  42 ,  44 . Flanges  46 ,  48  further include a gap ‘z’ (pole face gap z) between edges  50 ,  52  of the flanges  46 ,  48 . 
   The armature  18  comprises of generally a flat metallic plate having a portion of material removed in the form of a rectangle  60 . Above rectangle  60  is a crossbeam component  62  of armature  18  that joins legs  64 ,  66 . Crossbeam  62  includes an aperture  68  formed therein for attaching one end of spring  20 . Clip  21  is formed at a top edge  70  of armature  18 . 
   Electrical current passing through strap  17  ( FIG. 2 ) induces magnetic flux in yoke  16  and armature  18 . Accordingly, a magnetic relationship exists between the length of the flanges  46  and  48  of the magnet yoke  16  and armature  18  that is dependent on gap L that separates the flanges  46 ,  48  from the armature  18  and gap z that separate edges  50 ,  52 . The magnetic flux generated within the flux concentrating magnet yoke  16  seeks the path of least magnetic reluctance. The path of least reluctance is the shorter of the gaps z or L. By maintaining the gap z greater than the gap L, the flux gathers between the flux concentrator magnet side arms  42 ,  44 , thereby driving the flux concentration within arms  42  and  44  to a high value. 
   Because of the high flux concentration within arms  42  and  44 , larger magnetic forces are generated at lower current levels resulting in more force generated at clip  21  to trip the latch mechanism (i.e., cam  14   a ). The added force is beneficial at low current trip settings (e.g., three times the rated current) where the low current is otherwise not enough to induce sufficient magnetic force on armature  18  to trip the latch system. For higher trip settings, however, this added force is not needed and may cause damage to the trip latch system. Therefore, the armature  18  is pivoted away from yoke  16 , thereby increasing gap L until it is greater than gap z. With gap L greater than gap z, yoke  16  shunts the magnetic flux from yoke  16  onto itself because the flux seeks the path of least magnetic reluctance. Accordingly, the magnetic force of yoke  16  on armature  18  is reduced. However, a further reduction in magnetic force may be needed. To achieve this reduction, an amount of material is removed from armature  18  such that armature  18  does not saturate at low current settings (e.g., having a maximum flux density of approximately 1.9 T (B MAX ) before saturation flux density (B SAT ) of steel at 2.0 T) and saturates at high current settings. Because the armature does not saturate at low current settings, armature  18  does not affect the increase of the magnetically induced force due to the increased pole face area of flanges  46 ,  48  acting on cross beam component  62  at low current settings. 
   More specifically, the reluctance of a magnetic circuit is analogous to the resistance of an electric circuit. Reluctance depends on the geometrical and material properties of the circuit that offer opposition to the presence of magnetic flux. Reluctance of a given part of a magnetic circuit is proportional to its length and inversely proportional to its cross-sectional area and a magnetic property of the given material called its permeability (μ). Iron, for example, has an extremely high permeability as compared to air so that it has a comparatively small reluctance, or it offers relatively little opposition to the presence of magnetic flux. Thus, it will be appreciated that opposition to an increase in magnetic flux and hence reaching saturation, is optionally controlled by selecting the length and cross-sectional area of the magnetic path or selecting a material with a permeability that is near saturation when the gap is small and approaches saturation as the gap increases. The magnetic path length is defined by the width of armature  18  and a cross section area  63  of cross beam  62 . Cross section area  63  of cross beam  62  is selected to obtain a reluctance that provides favorable magnetic properties at both small gaps L and large gaps L (i.e., first distances and second distances larger than first distances). 
   By setting the cross section area  63  based on low current requirements, armature  18  saturates at high current settings which results in a lower relative induced magnetic force. When an emanating magnetic field H permeates through a cross-section area of a medium (i.e., cross section area  63 ), it converts to magnetic flux density B according to the following formula: B magnetic flux density=μ H magnetic field where μ is the permeability of the medium. Flux density (B) is simply the total flux (φ) divided by the cross sectional area (A e ) of the part through which it flows—B=φ/A e  teslas 
   Initially, as current is increased the flux (φ) increases in proportion to it. At some point, however, further increases in current lead to progressively smaller increases in flux. Eventually, the armature  18  can make no further contribution to flux growth and any increase thereafter is limited to that provided by the permeability of free space (μ 0)—perhaps three orders of magnitude smaller. It will be appreciated that the missing material to form aperture  68  must be accounted for in the minimum cross sectional area (A e ) calculation for the flux density (B) in armature  18  cross beam component  62 . 
   Turning to  FIGS. 5 and 6 ,  FIG. 5  illustrates a yoke  16  without an increase in pole face area provided by flanges  46  and  48  extending from side arms  42  and  44 .  FIG. 6  illustrates the relationship between the induced force/torque and gap distance of the two different yoke configurations shown in  FIGS. 4 and 5 . In  FIG. 6  of the drawings, the force/torque versus gap graph  72  shows the different electromagnetic force levels recorded at different magnetic force levels (F m )(I×N ampere-turns) at different gap distances (L) utilizing two different yoke  16  configurations ( FIGS. 4 and 5 ). Based on the characteristic curves, one can easily see that the magnetically induced force/torque is substantially increased at small gap distances with a yoke  16  configured having inwardly facing flanges  46  and  48 , while at larger gap distances the magnetically induced force is basically unchanged between the two configurations. More specifically, curves  72 ,  76 , and  78  indicate a yoke with flanges  46  and  48 . Curves  84 ,  86 , and  88  indicate a yoke without flanges  46  and  48 . Curves  74  and  84  illustrate the torque at 3× the rated current, curves  76  and  86  illustrate the torque at 4.5× the rated current, and curves  78  and  88  illustrate the torque characteristic at 7.5× the rated current. In each case tested, the torque at a specific gap was substantially larger, about 10 N mm more, using a yoke with flanges  46 ,  48  ( FIG. 4 ) than a yoke  16  without flanges  46 ,  48  ( FIG. 5 ) at a first distance such as a low gap setting, i.e., about 2 mm. However, as the gap L increased, the resultant torque is substantially the same between the two yoke configurations. The reduction in the magnetically induced force at the high current settings (large gaps) due to saturation and due to the flux shunt yoke effect described above, allows a magnetic force that remains unchanged at a second distance such as a high current setting (large gap). 
   Thus, the above described yoke-armature system having a yoke with inwardly facing flanges with a gap z therebetween provides the necessary torque to trip the latch mechanism at small gaps (low current setting), while providing a torque that remains virtually unchanged at larger gaps (high current setting). 
   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.