Toroidal current transformer assembly and method

Abstract: A current transformer assembly comprises a toroidal core and a conductor having a body portion and a mounting portion. The conductor's body portion extends through the aperture of the toroidal core. The mounting portion is positioned for a mechanical and electrical connection between an external conductor and the current transformer. A connection is made between the body portion and the mounting portion wherein mechanical engagement is sufficient to prevent unintended separation of the body portion from the mounting portion and electrical coupling is sufficient to permit the flow of current between the body portion and the mounting portion.

Description: BACKGROUND OF THE INVENTION This invention relates to a current transformer assembly and, in particular, to a current transformer including a toroidal core with a winding and a circular aperture extending therethrough. FIELD OF THE INVENTION Current transformer assemblies are often positioned between the line side of a trip unit of a circuit breaker and the load side in order to monitor the current therebetween. One type of current transformer assembly generally includes a doughnut-shaped toroidal core with a winding of wire positioned on the core, wherein the core generally includes a central, circular aperture. In a current transformer assembly that is used as a trip unit, the conductor extends through the aperture of the toroidal core and the conductor is connected between the line side and load side. Conventional current transformers often use a custom one-piece die-cast bus component to form the conductor that extends through the toroidal core. Such a bus component requires custom tooling to form the die with which it is produced. Custom tooling can be quite expensive especially when the quantity of a particular bus conductor to be manufactured is small and the cost of the tooling cannot be spread out over a large quantity of parts to reduce the per piece tooling cost. Other conductors have been formed by a one-piece metal stamping. Such a stamping also requires tooling which can be quite expensive for short production runs. Also, a one-piece metal stamping formed from a sheet or plate of conductive material cannot be any larger in thickness than the aperture of the toroidal core. Accordingly, the stamped conductor's cross-sectional area is significantly smaller than the area of the core's aperture because it would have a square or rectangular cross sectional shape that would necessarily create significant gaps between the outer surface of the conductor and the inner surface of the toroidal core's circular aperture. This results in a conductor having a cross-section smaller than it needs to be. It is also recognized that toroidal cores are not very easy to secure in conjunction with other circuit elements. This is true on the one hand because they are relatively heavy and, on the other hand, because they do not allow a direct, mechanical fastening to other circuit components since the toroidal core is completely surrounded by a fragile wire winding. Also, any significant relative motion between the toroidal core and other circuit elements should be avoided in order to eliminate damage to the electrical connections between the circuit elements. U.S. Patent No. 4,754,250, issued to Albert Duin, describes a holding device for toroidal cores. The Duin patent describes conventional toroidal cores clamped between two plates. According to Duin, such a construction has a disadvantageous effect by exerting a high pressure on the winding of the toroidal core itself, thereby damaging the insulation of the individual windings. Instead, Duin suggests a holding arrangement wherein a wound toroidal core is positioned over a vertically-oriented centering arbor of a carrier plate. Despite such attempts to mount a toroidal core, there remains a need for a low-cost current transformer assembly wherein the toroidal core is securely mounted mechanically and electrically in order to reduce any possible damage to the core and its windings. US-A-2 929 037 discloses a high voltage bar type current transformer comprising: a relatively lower voltage through type current transformer defining a passage; first and second spaced insulating bushings removably mounted at opposite ends of said passage, each said bushing defining an aperture, each said bushing further having an end portion extending into said passage for alignment therewith and a flange in abutment with an exterior surface of said through type current transformer for limiting the extension of said end portion into said passage; an elongated insulated conductive member removably disposed in said passage and through each said aperture; and means removably mounted on the ends of said elongated member for maintaining said member and bushings in a fixed relation with said through type current transformer and for connecting a current carrying conductor in series with said member. SUMMARY OF THE INVENTION According to a first aspect of the present invention there is provided a current transformer assembly comprising: a toroidal core with a winding positioned thereon and a central aperture; a conductor comprising a body portion and a mounting portion positioned adjacent to said body portion; said body portion of said conductor extending through said aperture of said toroidal core, said body portion having a cross-sectional area approaching that of said aperture; said mounting portion of said conductor being positioned for mechanical and electrical connection between an external conductor and said body portion; and means for connecting said body portion to said mounting portion, said means providing mechanical engagement sufficient to prevent unintended separation of said body portion from said mounting portion, said means further providing electrical coupling to permit the flow of current between said body portion and said mounting portion, said body portion having an end portion of reduced thickness that extends through a recess and adjacent counterbore in said mounting portion, said end portion having a head in said counterbore that engages the counterbore for engagement of said mounting portion and said body portion. Preferably, said conductor comprises plural mounting portions. Preferably, said aperture of said toroidal core is substantially circular. The body portion of said conductor may then have a substantially circular cross-sectional shape, an outer diameter of said body portion approaching an inner diameter of said aperture of said toroidal core. Preferably, said mounting portion is formed from a conductive sheet. According to a second aspect of the present invention there is provided a method for assembling a current transformer having a toroidal core with a central, circular aperture, said method comprising the steps of: (a) providing a conductor body portion having a substantially circular cross-sectional shape that is smaller in diameter than the diameter of said aperture, said body portion having an end portion of reduced thickness; (b) providing a conductor mounting portion having a recess and adjacent counterbore; (c) positioning said conductor body portion to extend through said aperture of said toroidal core, and positioning said end portion to extend through said recess and counterbore; and (d) deforming said end portion into engagement with said counterbore thereby engaging said conductor mounting portion to provide a mechanical connection sufficient to prevent unintended separation of said conductor body portion from said conductor mounting portion and to provide an electrical connection sufficient to permit the flow of current between said conductor body portion and said conductor mounting portion, thereby providing a continuous conductor as well as a means for mounting said current transformer assembly with respect to external conductors. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an exploded perspective view of a current transformer assembly. Although not in accordance with the present invention, the current transformer of FIG. 1 is useful for understanding the present invention. FIG. 2 is a cross-sectional side view of the current transformer assembly shown in FIG. 1. FIG. 3A is a cross-sectional side view of a cut-away portion of a first embodiment of a current transformer assembly according to this invention, before components of the conductor are fastened. FIG. 3B is a cross-sectional side view of a cut-away portion of the current transformer assembly shown in FIG. 3A subsequent to fastening of the conductor components. FIG. 4 is a cross-sectional side view of a cut-away portion of a second embodiment of a current transformer assembly according to this invention subsequent to fastening of the conductor components. DETAILED DESCRIPTION OF THE INVENTION Generally speaking, the current transformer assembly according to this invention provides a conductor assembly that provides both a low-cost electrical conductor for extension through the aperture of the toroidal core as well as a means for mounting the toroidal core with respect to other circuit components. This dual function of the conductor assembly provides a low-cost current transformer assembly that is inexpensive and easy to produce while providing optimal performance. Referring to FIGS. 1 and 2, a current transformer assembly is generally designated by the numeral "10". It includes a substantially doughnut-shaped toroidal core 12 that has a winding positioned on it in the usual manner. It also includes a central, substantially circular aperture that extends all the way through the toroidal core. Such toroidal cores are well known in the art. The remaining components shown in FIGS. 1 and 2 together combine to form the bus-like conductor. A body portion 14 of the conductor is provided with female threads such as straight threads that extend into its end surfaces. The female threads can be machined into blind holes provided in the body portion 14 along the body's axis. Alternatively, the female threads can be formed in a tubular body portion 14 that has a hollow inner surface into which the female threads can be easily machined. Body portion 14 of the conductor is most preferably formed from inexpensive copper rod of standard, stock diameter that is preferably silver plated for maximum conductivity. Body portion 14 is easily manufactured simply by cutting a piece of standard copper rod to the desired length (depending upon the thickness of the toroidal core and windings assembly) and by machining the female threads in its ends. Although body portion 14 is preferably formed from rod stock, it can also be formed from standard tubular stock, as described earlier, by cutting the tube to length and by adding the female threads at its ends using conventional machining processes. The conductor also includes a load side mounting portion 16 as well as a line side mounting portion 18, which are substantially mirror images of one another. Mounting portions 16 and 18 are preferably formed from standard copper plate or sheet. The mounting portion components can simply be manufactured by stamping shapes from stock sheet having a stock thickness, drilling the appropriate mounting holes (including recesses 20 and 22 and mounting recesses 28 and 30), and finally by bending the stamped sheet into the configuration shown in FIGS. 1 and 2. The mounting portions 16 and 18 are also preferably plated with a conductive coating such as a silver plating. Mounting portions 16 and 18 each have a portion such as a vertical leg that extends along the sides of toroidal core 12 in a radial direction relative to the axis of the core's aperture. The vertical legs are sufficiently tall to extend to a position that is radially spaced with respect to an outer diameter of the toroidal core. This structure permits the use of mounting portions 16 and 18 to mount toroidal core 12 on a surface in such a way as to avoid contact between the outer diameter of the core (and its windings) and the mounting surface. Mounting portions 16 and 18 each also include a portion such as a horizontal leg that preferably extends at a right angle with respect to the vertical leg. The mounting recesses 28 and 30 are positioned in the horizontal legs to facilitate electrical and mechanical connection between the mounting portions and other circuit components such as a line bus on one side and a load bus on the other. Recesses 28 and 30 are preferably elongated slots to permit alignment with mating components. In order to make a connection between the mounting portions 16 and 18 and the body portion 14 of the conductor, a load side screw fastener 24 and a line side screw fastener 26 are engaged in the female threads at both ends of body portion 14; more specifically, load side screw fastener 24 is inserted through recess 20 in load side mounting portion 16 for threaded connection to the body portion 14. Similarly, line side screw fastener 26 is inserted through recess 22 in line side mounting portion 18 for threaded connection to the female threads in body portion 14. Mounting recess 28 in load side mounting portion 16 and mounting recess 30 in line side mounting portion 18 are provided so that each of the mounting portions 16 and 18 can be securely mounted to any desired surface such as a line side bus or a load side bus, as described earlier. It will be appreciated that the conductor of the current transformer assembly is formed by connection between the load side mounting portion 16, load side screw fastener 24, body portion 14, line side screw fastener 26, and line side mountain portion 18. Such assembly creates a securely connected and high quality electrical conductor. It also at the same time provides a sturdy mounting structure for the toroidal core 12 in such a way as to protect the various windings around the toroidal core from damage. These benefits are achieved at a low-cost by using standard components and stock material such as standard-sized copper sheet, stock copper rod or tube, and off-the-shelf fasteners. Although a wide variety of fastener materials can be used, the screw fasteners 24 and 26 are preferably silver plated or otherwise plated metallic screws. Each of the conductor components is preferably plated with silver or another conductive material to ensure adequate electrical connection. Referring now to FIGS. 3A and 3B, a first embodiment of a current transformer assembly according to this invention will now be described. The current transformer assembly is generally designated by the numeral "100". The primary difference between current transformer assembly 100 and current transformer assembly 10 is the manner in which the conductor components are connected. As an alternative to the screw fastener used to connect mounting portions 16 and 18 to the opposite ends of body portion 14 in current transformer assembly 10 the mounting portions of this first embodiment are connected by means of a mechanical rivet. Referring specifically to FIG. 3A, a body portion 114 is provided with a reduced diameter portion 115 at both ends (only one end is shown). The load side mounting portion 116 is provided with a recess 120 as well as a counterbore 121 adjacent to the recess 120 and sharing an axis with recess 120. The reduced diameter portion 115 of body portion 114 is positioned to extend through the recess 120 and counterbore 121 of the load side mounting portion 116. The size of counterbore 120 is selected to correspond to the size of the reduced diameter portion 115 for reasons that will become clear later with reference to FIG. 3B. As illustrated in FIG. 3B, a spin-riveting procedure is performed in order to provide secure mechanical and electrical engagement between body portion 114 and load side mounting portion 116. More specifically, the reduced diameter portion 115 of body portion 114 is deformed toward the outer surface of the load side mounting portion 116 so that the end portion of the body portion 114 fills the counterbore region 121 formed in mounting portion 116. This procedure creates a rivet head 117 that substantially fills the counterbore 121 and leaves the rivet head substantially flush with the outer surface of the load side mounting portion 116. Referring now to FIG. 4, a second embodiment of a current transformer assembly "200" will now be described. This embodiment is similar to the one illustrated in FIGS. 3A and 3B in that a riveting procedure is performed in order to provide secure mechanical and electrical engagement between the body portion and mounting portion of the conductor. Current transformer assembly 200 differs in the configuration of the rivet joint. More specifically, the load side mounting portion 216 is provided with a recess 220 as well as an angled counterbore 221. The end portion 215 of body portion 214 is positioned to extend through the recess 220 and counterbore 221 of the load side mounting portion 216. The end portion 215 of body portion 214 is deformed so that it fills the counterbore region 221. This procedure creates a rivet head 217 that is substantially flush with the outer surface of the load side mounting portion 216. Also, although not shown, it is contemplated that the current transformer assembly may be encapsulated in a so-called "trip case". For example, such a trip case might be provided with a thermoset plastic base of heat-resistant material and/or a thermoplastic cover in order to encapsulate and protect the components of the assembly from external damage. If so, horizontal legs of a mounting portion of the conductor can extend from the encapsulated trip case for mounting the trip case and for electrical connection. Also, it is contemplated that it may be preferred to plate every component of the conductor with silver or another conductive material to maximize the stability of the electrical connections. Although two mounting portions are illustrated in the drawings, only one may be required depending on the configuration and positioning of external circuit components to which the assembly is intended to be connected. Also, the shape of the mounting portions is not important to the invention.

Claims: A current transformer assembly (100 or 200) comprising: a toroidal core (12) with a winding positioned thereon and a central aperture; a conductor comprising a body portion (114 or 214) and a mounting portion (116 or 216) positioned adjacent to said body portion (114 or 214); said body portion (114 or 214) of said conductor extending through said aperture of said toroidal core (12), said body portion (114 or 214) having a cross-sectional area approaching that of said aperture; said mounting portion (116 or 216) of said conductor being positioned for mechanical and electrical connection between an external conductor and said body portion (114 or 214); and means (115, 120, 121, 117 or 215, 220, 221, 217) for connecting said body portion (114 or 214) to said mounting portion (116 or 216), said means (115, 120, 121, 117 or 215, 220, 221, 217) providing mechanical engagement sufficient to prevent unintended separation of said body portion (114 or 214) from said mounting portion (116 or 216), said means (115, 120, 121, 117 or 215, 220, 221, 217) further providing electrical coupling to permit the flow of current between said body portion (114 or 214) and said mounting portion (116 or 216), said body portion (114 or 214) having an end portion (115 or 215) of reduced thickness that extends through a recess (120 or 220) and adjacent counterbore (121 or 221) in said mounting portion (116 or 216), said end portion (115 or 215) having a head (117 or 217) in said counterbore (121 or 221) that engages the counterbore (121 or 221) for engagement of said mounting portion (116 or 216) and said body portion (114 or 214). The current transformer assembly (100 or 200) defined in claim 1, said conductor comprising plural mounting portions (116 or 216). The current transformer assembly (100 or 200) defined in claim 1, wherein said aperture of said toroidal core (12) is substantially circular. The current transformer assembly (100 or 200) defined in claim 3, wherein said body portion (114 or 214) of said conductor has a substantially circular cross-sectional shape and wherein an outer diameter of said body portion (114 or 214) approaches an inner diameter of said aperture of said toroidal core (12). The current transformer assembly (100 or 200) defined in claim 1, wherein said mounting portion (116 or 216) is formed from a conductive sheet. The current transformer assembly (100 or 200) defined in claim 1, wherein an end surface of said body portion (114 or 214) of said conductor is positioned against a surface of said mounting portion (116 or 216) of said conductor. A method for assembling a current transformer (100 or 200) having a toroidal core (12) with a central, circular aperture, said method comprising the steps of: (a) providing a conductor body portion (114 or 214) having a substantially circular cross-sectional shape that is smaller in diameter than the diameter of said aperture, said body portion (114 or 214) having an end portion (115 or 215) of reduced thickness; (b) providing a conductor mounting portion (116 or 216) having a recess (120 or 220) and adjacent counterbore (121 or 221); (c) positioning said conductor body portion (114 or 214) to extend through said aperture of said toroidal core (12), and positioning said end portion (115 or 215) to extend through said recess (120 or 220) and counterbore (121 or 221); and (d) deforming said end portion (115 or 215) into engagement with said counterbore (121 or 221) thereby engaging said conductor mounting portion (116 or 216) to provide a mechanical connection sufficient to prevent unintended separation of said conductor body portion (114 or 214) from said conductor mounting portion (116 or 216) and to provide an electrical connection sufficient to permit the flow of current between said conductor body portion (114 or 214) and said conductor mounting portion (116 or 216), thereby providing a continuous conductor as well as a means for mounting said current transformer assembly (100 or 200) with respect to external conductors.

Inventors: TITUS SOLOMON R, TITUS, SOLOMON R.
Applicants: SIEMENS ENERGY & AUTOMAT, SIEMENS ENERGY & AUTOMATION, INC.
Application year: 1999
Main classification: H01F38
IPC codes: H01F38