Patent Publication Number: US-9424975-B2

Title: Split core transformer with self-aligning cores

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional App. No. 61/869,344, filed Aug. 23, 2013. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to devices for sensing current in a conductor and, more particularly, to a split core current sensing transformer having core portions which self-align during assembly. 
     Allocation of power cost among members of a group of users, protection of circuits from overload and/or monitoring continued operation and/or malfunctioning of a remote circuit or device are just a few exemplary reasons for monitoring the flow of electric current in a conductor. Current monitoring is frequently performed with a sensing or current transformer (CT), typically comprising a coil of wire wrapped around the cross-section of a magnetically permeable core which, in turn, encircles a conductor in which the current is to be measured. An alternating current flowing in the conductor, the primary winding of the transformer, magnetizes the core inducing a current in the coil of wire, the secondary winding, which is substantially proportional to the current in the conductor and the ratio of the number of coils in the transformer&#39;s primary winding to the number of coils in the secondary winding. 
     Sensing transformers may have either a solid core or a split core. A solid core is typically a toroid of magnetically permeable material which encircles the conductor in which the current will be sensed. A disadvantage of a solid core sensing transformer is the requirement that the conductor be disconnected when installing the encircling toroidal core on the conductor. Where the conductor to be monitored has already been connected, a sensing transformer with a split core is often used to facilitate installation. Cota, U.S. Pat. No. 5,502,374, discloses a split core transformer comprising a pair of hinged housing halves each enclosing half of a toroidal transformer core. The transformer can be installed on a conductor by pivoting the free ends of the housing/core portions away from each other; positioning the conductor to be monitored in the center of one of the portions; and closing and latching the core halves around the conductor. Bernklau, U.S. Patent Publication No. 2009/0115403, discloses another split core transformer comprising hinged C-shaped or U-shaped transformer core portions. While a hinged split core transformer can be installed without disconnecting the conductor in which the current is to be monitored, sensing transformers are commonly installed in enclosures, such as, a motor starter enclosure, where there is insufficient room to open the hinged portions and maneuver the conductor into position. Bruno, U.S. Pat. No. 7,312,686, discloses a split core current transformer comprising separable core portions. While the disassembled transformer requires no more space than the assembled transformer, it can be difficult to align the core portions when reassembling the core, particularly, in the crowded confines of an enclosure for electrical equipment. 
     What is desired, therefore, is a split core sensing transformer including core portions which can be conveniently assembled in a limited or crowded space. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an isometric view of a split core sensing transformer with separated and rotated transformer portions. 
         FIG. 2  is an isometric view of the split core sensing transformer of  FIG. 1  with joined transformer portions. 
         FIG. 3  is an elevation view of the split core sensing transformer of  FIG. 1  with separated and rotated transformer portions. 
         FIG. 4  is an elevation view of the split core sensing transformer of  FIG. 1  with joined transformer portions. 
         FIG. 5  is a cutaway view of the split core transformer of  FIG. 4 . 
         FIG. 6  is an isometric view of a housing for a sensing transformer which comprises a C-shaped core portion. 
         FIG. 7  is an elevation view of a first portion of the transformer of  FIG. 1 . 
         FIG. 8  is an end view of the first transformer portion of  FIG. 7 . 
         FIG. 9  is an opposite side elevation view of the first transformer portion of  FIG. 7 . 
         FIG. 10  is a section view of a first section of the guide pin of the first transformer portion of  FIGS. 7-9  taken along line A-A. 
         FIG. 11  is a section view of a second section of the guide pin of the first transformer portion of  FIGS. 7-9  taken along line B-B. 
         FIG. 12  is a section view of a third section of the guide pin of the first transformer portion of  FIGS. 7-9  taken along line C-C. 
         FIG. 13  is a section view of the split core transformer of  FIG. 3  taken along line D-D. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring in detail to the drawings where similar parts are identified by like reference numerals, and, more particularly to  FIGS. 1-6 , a split core sensing transformer  20  comprises, generally, a first transformer portion  22  and a second transformer portion  24  which are separable and joinable by relative translation and rotation. 
     The first transformer portion  22  includes a first magnetically permeable core portion  30  which is contained in a first core housing  32 . The first core housing  32  includes an elongate first portion  34  which encloses a substantial portion of the beam shaped first core portion  30 . The first core housing includes portions defining apertures  36 ,  38  through which end portions  40 ,  42  of the first core portion  30  are exposed. The centers of the apertures  36 ,  38  define a longitudinal axis  44  of the first core portion  30  and the elongate portion  34  of the first core housing  32  which encloses the first core portion. Although it might comprise other materials, preferably, the first core housing comprises a resilient, insulating plastic. 
     The second transformer portion  24  comprises, generally, a U-shaped, second magnetically permeable core portion  52  which is contained in a second U-shaped core housing  50  which also comprises, preferably, a resilient, insulating plastic material. Referring to  FIG. 2 , to sense current in a conductor  54 , the conductor is passed through a central opening  56  in the transformer which is formed when the end portions  40 ,  42  of the first core portion  30  are joined with the end portions  58 ,  60  of the U-shaped, second core portion  52 . An alternating current in the conductor will induce an expanding and collapsing magnetic field in the encircling core portions  30  and  52  which will, in turn, induce an electric current and voltage in the wire of a secondary winding  62  which is wound on a bobbin  64  and which encircles the cross-section of one of the core portions. The ratio of the current induced in the secondary winding of the sensing transformer to the current flowing in the conductor  54  is substantially proportional to the ratio of the number of turns in the primary winding to the number of turns in the secondary winding. The number of turns in the primary winding is commonly one as the conductor is commonly passed through central opening of sensing transformer only once. To provide access to the portion of the central opening  56  defined by the U-shaped second transformer portion  24 , the joined transformer portions  22 ,  24  may be separated by relative translation and/or rotation as illustrated in  FIG. 1  even to the point of detachment from each other as illustrated by transformer portions  152 ,  154  in  FIG. 6 . 
     While the exemplary sensing transformer  20  comprises a beam shaped first core portion and a U-shaped second core portion, split core sensing transformers commonly include two U-shaped core portions or a C-shaped core portion in combination with a second C-shaped core portion or a U-shaped core portion and can comprise plural core portions of one or more other shapes which when brought into contact with each other can be arranged to encircle a conductor. For example, referring to  FIG. 6 , the split core transformer housing  150  includes a first housing portion  152  arranged to enclose a C-shaped core portion and a second housing portion  154  arranged to enclose a U-shaped core portion. 
     While disconnecting the conductor to be monitored is unnecessary when installing it in the central opening of a split core sensing transformer, sensing transformers are often installed in small and/or crowded enclosures where there may be insufficient room to open the sections of a hinged split core or where the open hinged core portion may block access to the conductor, a portion of the sensing transformer or other equipment in the enclosure. The portions of some split core transformers are separable facilitating installation of the transformer in spaces which are only a little larger than the space occupied by the assembled transformer but aligning the portions during reassembly may be difficult, particularly, in a confined or crowded space. The inventor concluded that if the portions of a sensing transformer could be rotated relative to each other about an axis offset from the axis defined by the end portions of one of the transformer core portions, the available space around the transformer could be utilized more effectively and obstacles could be avoided and if the core portions of a sensing transformer self-aligned as the transformer cores were joined, following installation of the conductor, installation of the sensing transformer, including reassembly of separated core portions, would be facilitated, particularly, in crowded or close environments. 
     The first core housing  32  includes a portion defining an elongate guide pin  46  that projects substantially normal to the longitudinal axis  44  of the elongate portion  34  of first core housing  32  which houses the first core portion  30 . Referring also to  FIGS. 7-12 , the guide pin  46  has a surface defined by the respective surfaces of plural cylindric sections taken normal to and spaced along the pin&#39;s longitudinal axis  47 . The surfaces of the cylindric sections preferably comprise arcuate surfaces of varying lengths of one or more sectors of varying radius and, where appropriate, surfaces that connect the arcuate surface portions of sectors of differing radii. Referring to  FIG. 10 , cylindric sections, exemplified by section  102 , spaced along a first length  82  of the guide pin  46 , proximate the connection of the guide pin to the portion  34  of the first transformer housing enclosing the first transformer core portion  30  have a surface defined by the arcuate surface  104  of a first sector having a larger radius and the arcuate surface  106  of a second sector of smaller radius. The transition between the surface  104  of first sector and the surface  106  of the second sector defines a portion of a directing element  108 , an enlarged portion of the guide pin  46 , bounded by closely spaced, parallel portions of a directing surface  110  which project approximately normal to the surface of the guide pin and extend longitudinally for the first length  82  of the guide pin. Referring to  FIG. 11 , as exemplified by the cylindric section  109 , the surface of cylindric segments taken along a second length  84  of the guide pin comprise a surface portion  104  of the larger radius sector radius and a surface portion  106  of the smaller radius sector but the relative lengths of the respective sector surfaces vary defining portions of the directing surface  110  extending from the ends of the respective first lengths of the directing surfaces and spirally diverging around the pin  46  to an intersection  86  on the side of the pin opposite the parallel first lengths of the directing surface  110 . The guide pin  46  includes a third length  88 , distal of the second length  84 , where the surfaces of plural cylindric segments comprise the arcuate surfaces  106  of circles of the smaller radius. Over a fourth length  90  of the guide pin  46 , cylindric sections comprising alternating sectors of the larger radius and the smaller radius form the surfaces  104  of plural triangular projecting surface portions  112  which are spaced around the circumference of the guide pin. 
     The second core housing  50  includes a portion defining an elongate guide pin socket  70  to slidingly receive the guide pin  46  of the first core housing  32 . When the guide pin  46  is inserted into the guide pin socket  70 , the projecting triangular raised surface portions  112  slidingly contact the inner surface of the socket providing initial guidance to the translation of the guide pin and second transformer portion  22 . Referring also to  FIG. 13 , the portion of the second core housing defining the guide pin socket  70  also defines a second directing element  72 , a tab or block, projecting from the inner surface of the socket toward the center of the socket. As the guide pin  46  translates into the guide pin socket  70 , the second directing element  72  slidingly engages the directing surface  110  of the first directing element  108 , the larger portion of the guide pin, and urges the first transformer portion  22  to rotate relative to the second transformer portion, if necessary, to align the exposed end portions  40  and  42  of the first core portion  30  with the respective end portions  60  and  58  of the second core portion  52  and to maintain alignment of the end portions of the first and second core portions as the second directing element enters the narrowly spaced, parallel portions of the directing surface proximate the housing portion  34 . The larger cross-section of the pin  46  proximate the housing portion  34  also controls the direction of translation of the first transformer portion  22  as the transformer portions approach contact. 
     The first core housing  32  includes projecting lips  74  which at least partially surround the apertures  36 ,  38  through which end portions  40 ,  42  of the first core portion  30  are exposed. Similarly, projecting lips  76 ,  78  of the second core housing  50  at least partially surround each of the exposed ends  58 ,  60  of the second core portion  52 . The lips  74  are arranged to intermesh with the lips  76 ,  78  as the first core portion  30  engages the second core portion  52  to secure the joined transformer portions against separation by rotation and to extend a surface path length to satisfy creepage and clearance requirements. 
     To assure contact between the end portions  40 ,  42  of the first core portion  30  and the end portions  58 ,  60  of the second core portion  52  when the transformer portions are joined, one or more resilient members  118  bearing on the second core portion and a partition  128  secured within the second core housing  50  urge the end portions  58  and  60  of the second core portion  52  toward the first core portion  30 . Alternatively or additionally, the first portion  30  could be urged toward the second core portion by a resilient member acting between the top the first core portion and an inner surface of the first core housing  32 . Preferably, the first core portion  30  is spaced from the inner wall of the first core housing  32  by a centrally located fulcrum  33  which equalizes the forces of contact with the second core portion and permits movement of the end portions of the first core portion to achieve the best contact with ends of the second core portion. 
     When the first and second core portions are brought into contact, a surface  136  of a triangular raised surface portion  112  moves past a surface  134  of a locking element  130  projecting toward the center  73  the guide pin socket  70 . The resilient material of the second housing portion  50  defines a spring portion  132  which urges the locking element  130  toward the center of the guide pin interlocking respective surfaces  134  of the locking element and surface  136  of one of the triangular raised surface portions  112  to automatically lock the transformer core portions in the joined position. 
     Alternatively or additionally, as illustrated in  FIG. 6 , the first  152  and second  154  core housings could define a latch assembly comprising a first engaging element  158  cantilevered from one of the core housings and a fixed second engaging element  160 , for example, spaced blocks, projecting from the other core housing. As the core portions are brought into contact, a sloping portion  156  of the first engaging element contacts the second engaging element elastically deforming the first engaging element. As the core portions contact, interlocking surfaces  162  of the first engaging element  156  are resiliently urged into engagement with surfaces  164  of the fixed engaging element(s)  160  to lock the housing portions against separation. 
     A circuit board  129  is suspended in the second core housing  50  or in a configurable detachable end cap  51 . The circuit board  129  supports elements of an electronic circuit which typically conditions the output of the secondary winding  62  and commonly responds in some way to the electric current induced in the winding. For example, the exemplary sensing transformer  20  includes one or more capacitors  120  attached to the circuit board for filtering the signal induced in the secondary winding  62 , one or more trimpots  122  for adjusting the sensing circuit for the effect of variations in the characteristics of the detector circuit&#39;s components and plural light emitting diodes (LEDs)  126  to indicate the functioning and/or malfunctioning of the sensing transformer and/or a detector circuit. A lead  124  conducts the output of the sensing transformer and/or detector circuit to remote equipment. By way of examples only, Cota, U.S. Pat. No. 5,502,374, and Bernklau, U.S. Patent Publication No. 2009/0115403, incorporated herein by this reference, disclose exemplary circuit schematics comprising sensing transformers, for, respectively, a current sensor and a low threshold current switch which are exemplary of circuits which might be incorporated on the circuit board. 
     To gain access to the central aperture of the split core sensing transformer  20  to install a conductor  54  for monitoring, the first transformer portion  22  can be moved in translation relative to the second transformer portion  24  by releasing the interlocking surfaces  136 ,  134  of the latch assembly and sliding the guide pin  46  longitudinally in the guide pin socket  70  to disengage the lips  74  of the first transformer portion  22  from the intermeshing lips  76 ,  78  of the second transformer portion  24 . Continued translation for a distance equal to the first length  82  of the guide pin, releases the second directing element  72  from the narrowly spaced, parallel portions of the directing surface  110  releasing the transformer portions for relative rotation. Continued separation of the transformer portions  22 ,  24  allows increasing amounts of rotation about the longitudinal axis  73  of the guide pin socket  70  which is offset from the side of the second transformer core portion  52  facilitating access to the central part of the second core housing  50 . When the second transformer portion  22  is separated from the first transformer portion  24  by a distance equal to the sum of the first length  82  and the second length  84 , the transformer portions are free to rotate fully relative to each other. Further translation will withdraw the guide pin  46  from the guide pin socket  70 . Space around the sensing transformer can be utilized more effectively because the transformer portions can be rotated relative to each other to avoid obstacles on either side of the transformer and can be separated, if necessary, to minimize the area occupied by the transformer during installation of the conductor that is to be monitored. 
     When the conductor which is to be monitored  54  has been placed in the center portion of the U-shaped second transformer portion  24 , the guide pin  46  is inserted in the socket  70  if the transformer portions have been separated. Slidingly engaging the surfaces  104  of the triangular elements  112  of the guide pin with the wall of the guide pin socket  70  controls the direction in which the first transformer portion  22  translates relative to the second transformer portion. As the transformers portion are urged toward the joined position, the surface  110  of the first directing element  108  engages the second directing element  72  and relative rotation of the transformer portions  22 ,  24  to align the end portions  40 ,  42 , of the first core portion  30  with the end portions  58 ,  60  of the second core portion  52  will be urged, if necessary, as the guide pin continues to translate in the socket. The sliding engagement of the surface of the first directing element  108  with the wall of the guide pin socket further directs the relative translation of the transformer portions. Further, translation of the transformer portions  22 ,  24  toward the closed position, engages the intermeshing lip portions  74 ,  76 ,  78  further restricting relative movement of the transformer portions. As the end portions of the first  30  and second  52  core portions contact the resilient elements  118  are compressed and surfaces  134 ,  136  of the latch elements  112  and  130  engage and interlock as a result of the urging of the spring portion  132  securing the transformer portions  22 ,  24  and the transformer core portions  30 ,  52  against separation. 
     Relative translation and rotation of portions of a split core sensing transformer about an axis offset from the core portions makes utilization of the space around the transformer more effective and self alignment the transformer core portions during joining facilitates use of the transformer in crowded or close environments. 
     The detailed description, above, sets forth numerous specific details to provide a thorough understanding of the present invention. However, those skilled in the art will appreciate that the present invention may be practiced without these specific details. In other instances, well known methods, procedures, components, and circuitry have not been described in detail to avoid obscuring the present invention. 
     All the references cited herein are incorporated by reference. 
     The terms and expressions that have been employed in the foregoing specification are used as terms of description and not of limitation, and there is no intention, in the use of such terms and expressions, of excluding equivalents of the features shown and described or portions thereof, it being recognized that the scope of the invention is defined and limited only by the claims that follow.