Abstract:
A wire-wound bobbin serves as the core of a current transformer. A magnetic material flux path is disposed on the core that encircles the core coincident with the coils of the bobbin at least once and radially encircles the cross section of the core at lest once. The magnetic flux path comprises an assembly of distinct magnetic members positioned on the core and held in place by tape or the like.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to current transformers which are utilized to detect the presence of a current in a conductor and to produce an analog signal proportional to the magnitude of the current. A continuous goal in the development of current transformers is to reduce the physical size thereof while at the same time increase the current range which may be sensed and lower the cost of manufacture of the current transformer. This invention is related to these goals. 
     SUMMARY OF THE INVENTION 
     The invention provides a wound core having a central opening adapted to receive the electrical conductor through the opening. Current flowing through the conductor will generate a magnetic flux having a cylindrical pattern around the conductor. However, such pattern will be coincident with the core windings and will cut no turns of the core, and therefore no transformer action will take place. The invention provides a magnetic path on the core which coincides with the winding of the core to completely encircle the conductor at least once and extends radially to completely encircle a cross section of the winding at least once. Flux in the magnetic path induced by current in the conductor will cut all the turns of the coil to produce an induced secondary current. The invention preferably provides a wire-wound bobbin core. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an isometric view of the current transformer of this invention with an electrical wire conductor extending coaxially therethrough; 
     FIG. 2 is a partial cross section of the current transformer of this invention taken along the line 2--2 in FIG. 1; 
     FIG. 3 is a top view of the current transformer shown in FIG. 1; 
     FIG. 4 is a side elevation view of the current transformer shown in FIG. 3; 
     FIG. 5 is a bottom view of the current transformer as shown in FIG. 3; 
     FIG. 6 is an isometric view of members making up the magnetic path of the current transformer shown in FIGS. 1-5; 
     FIG. 7 is an isometric outline view of the current transformer of FIGS. 1-6 with a magnetic flux path illustrated thereon; 
     FIG. 8 is a top view of an alternate embodiment of the current transformer of this invention; 
     FIG. 9 is a side elevation view of the current transformer shown in FIG. 8; 
     FIG. 10 is a bottom view of the current transformer shown in FIG. 8; 
     FIG. 11 is an exploded isometric view of members making up the magnetic path for the alternate version current transformer shown in FIGS. 8-10; and 
     FIG. 12 is an isometric outline view of the current transformer of FIGS. 8-11 with the magnetic flux path of the magnetic elements shown in FIG. 11 illustrated thereon. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The current transformer 2 of this invention is isometrically shown in FIG. 1 wherein a length of insulated wire conductor 4 extends axially through a central opening 6 of current transformer 2. Referring also to FIG. 2, current transformer 2 preferably comprises a wire-wound bobbin core 7 which consists of a plastic or the like insulating bobbin 8 which is wound with multiple turns of electrical conducting wire 10 such as transformer wire or the like. It is to be understood that the wound conductor may be a flat ribbon, or may be wire wound on a form other than a bobbin However, a wound-wire bobbin has a greater fill-factor than other forms of winding, providing a very high &#34;turn&#34; count, and is less expensive to wind than other forms. A rubber or vinyl insulating tape 12 is wound around the outer perimeter of coil 10 and bobbin 8 to retain the coil wire 10 in place on the bobbin and to provide protection and electrical insulation for the coil. Opposite ends 10a and 10b (FIG. 1) of wire 10 are brought out of the coil and bobbin 8 at a single location for connection to means for measuring the analog output of the coiled wire 10. When conductor 4 is disposed through the central opening 6 of the current sensor 2, current i flowing in conductor 4 in either direction according to arrows 14 or 14&#39; will induce a flux β in a circular (cylindrical) pattern around conductor 4 as represented by arrows 16 or 16&#39;, respectively. It will be appreciated that arrows 16 or 16&#39; are in the same direction and plane as the coils of wire 10 of the core and therefore the flux β creates no transformer action because it cuts no conductor coils. 
     According to this invention, a closed magnetic material flux path is provided on the wire-wound core to direct the flux β in a path that will completely encircle conductor 4 and will radially completely encircle the coils of wire 10. The magnetic material path comprises a plurality of magnetic members which are arranged in a predetermined abutting order. To enhance the magnetic characteristics, each member is preferably made of three laminations, but for ease of discussion will be referred to herein as a single member. To fabricate the members in three laminations, it is preferable that the sheets of magnetic material be laminated in the three layers first, bonded together with an adhesive (e.g. Permabond 240 made by Permabond International of Englewood, N.J.), and then cut as a laminated assembly from the bonded sheets so as to maintain a reasonable degree of precision for all laminations at the butt contact surfaces with other members. Referring also to FIGS. 3-6, the magnetic members comprise a substantially circular inner lamination 18 having a gap 18a, a substantially circular outer lamination 20 having a gap 20a, and top and bottom laminations 22 and 24, respectively. Top and bottom laminations 22 and 24 are essentially identical arcuate segments of less than 180° length and are oppositely disposed as seen in FIGS. 3 and 5. It will also be noticed in FIGS. 3 and 5 that the inner and outer laminations 18 and 20 are disposed to have their respective gaps 18a and 20a arranged in diametrically opposed relationship with the ends of top and bottom laminations 22 and 24 terminating adjacent the respective gaps 18a and 20a. A piece of tape 26 encircles outer lamination 24 to provide electrical insulation for the lamination and to hold the lamination to the core 7. Similarly, a piece of tape 28 lines the interior of inner lamination 18 to provide an insulated central opening 6 for the current transformer 2 of this invention. 
     Referring particularly to FIGS. 6 and 7, the magnetic flux path established by the magnetic members 18-24 is shown on the outline drawing of the current transformer in FIG. 7. The flux path extends within the outer lamination 20 to top lamination 22, radially inward along top lamination 22 to inner lamination 18, along the inner lamination to bottom lamination 24, outwardly along bottom lamination 24 to outer lamination 20, thereby making a complete loop around the central opening 6 and a complete, generally radially extending loop around the coils of wire 10. This flux path is illustrated as P1 on the coil in FIG. 7. 
     An alternate version 2&#39; of the current sensor of this invention is shown in FIGS. 8-12 wherein a modification of the magnetic members provides two turns of the flux path around the coils of wire 10. As seen best in FIG. 11, inner and outer generally circular laminations 32 and 34, respectively, comprise separate arcuate strips which, when positioned to the bobbin wound core 7 have two diametrically opposed gaps each, 32a and 34a, respectively The top and bottom laminations each comprise two identical members 36, 38 and 40, 42, respectively. As in the foregoing description, each magnetic member preferably comprises three separate laminations as shown, but will be referred to as a single member in the description. In essence, all four members 36, 38, 40 and 42 are identical but have been given different numbers to facilitate the description. However, only the structural features of lamination member 36 will be described, using alpha subscripts which apply equally to the members 38, 40 and 42. Member 36 comprises an outer arcuate segment 36a which is less than 180° in length, and an inner arcuate segment 36b which is also less than 180° in length. The inner and outer arcuate segments are rotationally offset 90° and are interconnected by a web 36c which extends approximately, but less than 90° in arcuate length. When top lamination members 36 and 38 are positioned on the top of the bobbin wound core 7, the adjacent ends of respective outer arcuate segments 36a and 38a form a pair of diametrically opposed gaps g1 as do the respective ends of inner arcuate segments 36b and 38b, the latter gaps g2 being offset 90° from the first mentioned gaps g1. As seen in FIGS. 8 and 10, the inner laminations 34 are disposed within the center opening of the wire wound core 7, and the outer laminations 32 are disposed around the periphery of the core. The outer laminations 32 abut the outer arcuate segment 36a, 38a, 40a and 42a of the respective top and bottom laminations 36-42. Similarly, the inner laminations 34 abut the respective inner arcuate segments 36b, 38b, 40b, and 42b, with the gaps 34a aligned with the gaps g2 in the inner arcuate segments of top and bottom laminations and the gaps 32a being aligned with the gaps g1 in outer arcuate segments of the top and bottom laminations. 
     This alternate magnetic material flux path forms two complete loops around the coils of wire 10 and one complete loop around the conductor 4. This flux path is illustrated on the isometric view of core 7&#39; shown in FIG. 12, wherein the path extends along one of the outer laminations 32 to top lamination outer arcuate segment 38a, inwardly through web 38c to inner arcuate portion 38b, downwardly through one of the inner laminations 34 to arcuate portion 42b, outwardly through web 42c to outer portion 42a into an oppositely disposed outer lamination 32, upwardly to outer arcuate portion 36a of top lamination 36, inwardly through web portion 36c to inner web portion 36b, downwardly through inner lamination 34 to inner arcuate portion 40b, outwardly through web portion 40c to outer arcuate portion 40a and back into the first mentioned outer lamination 32. Thus the flux path makes one complete loop around central opening 6 and a conductor 4 disposed therethrough and two complete loops radially around the coils of wire 10&#39;  of core 7&#39;. 
     The foregoing has described an improved current transformer for use as a current sensor to detect and measure current flowing in a wire conductor. The overall size and cost of the current transformer is greatly reduced by providing a wire-wound bobbin as a core and redirecting magnetic flux from a circular path coincident with the coils of wire to a radial path that encircles the coils of wire. Improved sensitivity can be obtained by an alternate version of magnetic members which create a magnetic path that encircles the coils of wire 10 twice in series. It is to be understood that this invention is susceptible of various modifications without departing from the scope of the appended claims. For example, although the magnetic path has been described as comprising several discrete magnetic members, it is contemplated that a flexible magnetic material could be wound upon the core, or that magnetic material could be deposited in a prescribed path on the core by various spraying or coating methods.