1. Field of the Invention
This invention is in the field of current transformers and, more particularly, spit core current transformers.
2. State of the Prior Art
Current transformers are common devices used for measuring AC current flow in electric wires or bus bars, typically, but not exclusively, in higher power installations and equipment. High power as used in this description is not intended to be limiting, but generally refers to electric power with voltages above twenty volts, as opposed to low voltage electronic circuits that operate with less than twenty volts. Essentially, a current transformer outputs a small current that is proportional to a larger current flowing in a high power electric wire or bus bar, and the use of a burden resistor on the output can provide a low voltage signal that is proportional to the current flowing in the high power electric wire or bus bar. Such small current or low voltage output signals from the current transformer can be used in a variety of instrumentation and control applications, including, for example, measuring and/or metering the amount of electric current that is generated or flowing to a load, or measuring and/or metering the amount of power that is used by a load.
A typical current transformer comprises a magnetic core, a primary winding (which may be the high power wire or bus bar), and a secondary coil wound around one or more sectors or sections of the magnetic core. Solid toroidal magnetic cores generally provide the best electrical performance for current transformers, i.e., outputting small current or voltage signals in direct proportion to, and in phase with, the current flowing in the high power primary wire or bus bar with minimal errors, and other solid (not split) core configurations, for example, square or rectangular loops are also quite good. For simplicity and convenience, the term “solid core” or adjective “solid-core” in this description includes any such toroidal, oval, square, rectangular, or other shaped solid (not split) magnetic core. However, to install a current transformer with a solid core onto a high power wire or bus bar, the high power or bus bar has to be inserted through the center hole or aperture of the solid core, which requires disconnecting the high power wire or bus bar from its high power circuit and inserting it through the solid core, and then reconnecting the high power wire or bus bar to the high power circuit.
Current transformers equipped with split magnetic cores, often called “split-core” current transformers, alleviate this inconvenience by enabling the core to be opened or disassembled for installation around a high power wire or bus bar and then closed or reassembled for operation without having to disconnect the high power wire or bus bar from its circuit. A typical split magnetic core may comprise two semicircular halves of a toroidal magnetic core, two C-shaped halves or other portions of a square or rectangular magnetic core, two U-shaped halves or other portions of an oval magnetic core, a U-shape magnetic core section with a closing-bar core section extending from one leg of the U-shape section to the other leg, and other core section configurations that can be opened or disassembled. However, a magnetic core that is split, so that it can be opened or disassembled, has unavoidable air gaps in the magnetic core, thus increasing the magnetic reluctance, which in turn decreases the permeability and causes higher excitation current, all of which increases the secondary coil output errors, particularly the phase angle error between the phase of the current in the high power wire or bus and the phase of the output current or voltage from the secondary winding. Consequently, while split-core current transformers are generally more convenient and easier to use than solid-core current transformers for many installations and circumstances, the electrical performance of split-core current transformers is not as good as comparable sized and shaped solid-core current transformers, assuming all other factors are constant, and typical split-core current transformers also draw more magnetizing current than solid-core transformers made with the same core material and of the same size. Also, while split-core current transformers alleviate the need to disconnect the high power wire or bus bar for installation, as explained above, they need bracketry and mechanisms to clamp or hold the spit-core components together upon installation on a high power wire or bus bar, which is more complicated than solid-core current transformers and can be somewhat cumbersome to use.
The foregoing examples of the related art and limitations related therewith are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings.
The accompanying drawings, which are incorporated herein and form a part of the specification, illustrate some, but not the only or exclusive, example embodiments and/or features. It is intended that the embodiments and figures disclosed herein are to be considered illustrative rather than limiting. In the drawings:
An example current transformer apparatus 10 is illustrated diagrammatically in
The example current transformer 10 shown in
Referring now primarily to
Secondary windings 40, 42 are mounted on bobbins 44, 46, which are positioned around the respective legs 35, 37 of the U-shaped base core section 32. The secondary windings 40, 42 typically comprise insulated, electrically conductive wires 41, 43 wound on the respective bobbins 44, 46. The windings 40, 42 can be wired in series to function as a single secondary winding or in parallel. The number of turns of the wires 41, 43 on the spools 44, 46 depends on the design and can be varied or adjusted to optimize performance based on a number of criteria, including, for example core dimensions, desired voltage output, burden resistance, sensitivity to external load, phase angle error, ease of capacitive phase angle compensation, power dissipation, peak core flux, winding time, the cost of winding the wire, and other factors that are well-known to persons skilled in the art.
A printed circuit board (PCB) 50 is mounted in the bottom portion of the base module 14 under the yoke portion 39 of the U-core section 32. The printed circuit board 50 comprises electronic components for conditioning and processing the output of the secondary windings 40, 42, which is induced by the magnetic field in the core 30, into current measurement signals, including, for example, the burden resistor, adjustment components, and protection components. Small catches or other retainer structures (not seen in
The core 30, including the U-core section 32 and the closing-bar core section 34, can be made of any typical magnetic material, including, but not limited to, iron, grain oriented silicon steel, nickel alloys, or ferromagnetic ceramic material (e.g., Fe3O4 or BaFe12O19), which is commonly called ferrite. The combination U-core section 32 and closing bar 34 in the example current transformer 10 allows maximum space for vertical secondary windings 40, 42 on both of the legs 35, 37 of the U-core section 32 to minimize magnetic leakage, susceptibility to external magnetic fields, and magnetic saturation without resorting to use of secondary windings on the closing-bar core section 34 and yoke portion 39 of the core 30, which would increase manufacturing and assembly complexity and require the overall size of the base module 14 and cover module 16 to be larger, wider, and more bulky for a given core 30 size. These features also decrease sensitivity of the current measurement signal output to the location of the primary conductor W in the aperture 18 in relation to the core 30.
The interfaces 31, 33 of the U-core section 32 and closing-bar core section 34 are air gaps that increase the magnetic reluctance, decrease the permeability, and increase the leakage inductance (i.e., more of the magnetic flux flows through the air around the core 30 due to the higher reluctance of the path through the core), so the current transformer 10 requires a higher magnetizing (exciting) current than would a continuous core made of the same material and of comparable size and weight. Such higher magnetizing current requirement results in a larger phase angle error and lower accuracy than would occur in a solid core made of the same material and of comparable size and weight, but the advantage of being able to open the split core 30 for inserting a primary conductor W outweigh those disadvantages for many applications. Moreover, some of these disadvantages can be mitigated. For example, the core interfaces of the split core, such as these interfaces 31, 33 in the example current transformer 10, are typically shaped or machined (e.g., flat) to minimize the air gap and enhance magnetic coupling across the interfaces 31, 33 and reduce leakage inductance. Even so, inevitable slight misalignments and manufacturing variations, tolerances, and other imperfections can cause increased magnetic reluctance and leakage at the interfaces 31, 33. To further address and further mitigate this problem, the closing-bar core section 34 in the example current transformer 10 is over-sized to be longer than the distance between the respective outer edges of the tops 36, 38 of the U-core legs 35, 37, as best seen in
Current transformers are sometimes used on bare (uninsulated) bus bar primary conductors W. Therefore, they have to be constructed in a manner that isolates a user from the voltage in a bus bar primary conductor W positioned in the aperture 18, including any high voltage spikes that might occur in a bus bar or other primary conductor W in the aperture. As can be seen in
In the example current transformer 10, the bobbins 44, 46 are shaped to provide additional insulative shrouding for the core 30 to increase clearance and creepage distances. Referring first to the bobbin 44 in
Similarly, the bobbin 46 includes a sleeve section 90 made of electrically insulative material, around which the secondary winding 42 is wound, and a top flange 92 and bottom flange 94 between which the secondary winding 42 is wound. The sleeve section 90 receives and surrounds the other leg 37 of the U-core section 32 with insulative material to insulate the U-core 30 from the secondary winding 42. Also, the sleeve 90 has a top extension 96 made of electrically insulating material that extends above the top flange 92 to or near the upper end 38 of the U-core leg 37 and a bottom shroud 98 made of electrically insulating material that extends below the bottom flange 94 to or near the bottom of the U-core leg 37. The bottom shroud 98 also has a channel portion 100 that extends laterally along the top and sides of the yoke portion 39 of the U-core 30. The top extension 96 also has an auxiliary flange 102 extending outwardly from the top extension 96 a distance above the top flange 92. The top flange 92, top extension 96, and auxiliary flange 102 are all made of electrically insulative material and increase the creepage distance between the core 30 and the secondary winding 42 above the secondary winding 42. Likewise, the bottom shroud 98, including the laterally extending channel portion 100, are made of electrically insulative material and increase the creepage distance between the core 30 and the secondary winding 42 below the secondary winding 42. An electrically insulative sheet 112 is wrapped around the yoke portion 39 of the U-core section 32 to provide additional creepage distances.
As best seen in
The housing 60 of the base module 14 is also made in a manner to enhance safety isolation without the need for potting the interior and electrical components or sonic welding of casings in order to meet safety isolation requirements, which is an advantage for manufacturing and assembling. Such potting and sonic welding can also affect the accuracy of current transformers, so conventional current transformers that require potting and/or sonic welding have to be verified for accuracy again after the potting and/or sonic welding, which adds another manufacturing process step and has the potential of causing quality control rejections of finished devices. In contrast to such conventional current transformer manufacturing issues, the main housing section 60 of example current transformer 10 is made as a unitary, hollow, component that receives and mounts the entire, unitary assembly of the U-core section 32, bobbins 44, 46 with secondary windings 40, 42, and printed circuit board 50, which was described above, through an open bottom 118. Therefore, there are no side seams in the main housing section 60 that that have to be sonic welded in order to provide the required clearance and creepage distances.
The open bottom 118 of the main housing 60 allows access to the printed circuit board 50 for calibration after the entire current transformer 10, including the cover module 16, is assembled, except for the bottom panel 56. Once calibrated, the only remaining assembly step is to snap the bottom panel 56 into place to close the bottom opening 118 of the main housing 60, which is a simple operation that sets a pair of resilient snap dogs 113, 115, at opposite ends of the panel 56 to engage ridges, 117, 119, respectively, at the bottom of the main housing 60, which does not affect the calibration. The bottom panel 56 also has sidewalls 120, 122 that extend into the main housing 60 far enough, when the bottom panel 56 is snapped into place, to surround the printed circuit board 50 and sides of the yoke portion 39 of the U-core section 32, which provides a large creepage at the bottom of the base module 14 for safety isolation. Additional catches 123 in the center portions of the sidewalls 120, 122 of the bottom panel 56 engage mating protrusions or other catch features in the main housing 60 (not visible in the drawings, but understandable by persons skilled in the art) enhance secure attachment of the bottom panel 56 to the main housing 60.
The current measurement signals from the printed circuit board 50 are output via lead wires 125, which extend through a duct 127 in a side, e.g., the back side 129, of the main housing 60, as best seen in
The top walls 124, 126 of the main housing 60 also close the top of the main housing 60, except for windows 128, 130 that are sized and shaped to allow protrusion of the top ends 36, 38 of the U-core legs 135, 137 for contact with the closing-bar core section 34 in the cover module 16, as explained above. The upper ends of the extensions 76, 96 also protrude through the windows 128, 130 around the legs 135, 137 with the auxiliary flanges 82, 102 positioned just under the top walls 124, 126, which also helps to maintain a large creepage distance.
The closing-bar core section 34 is nested in the cover module 16, which comprises a cover housing 140 that is pivotally attached to the main housing 60 of the base module 14 by the hinge 20, which can be any structure or combination of components that provides a pivotal or hinged attachment. The cover housing 140 has an open top 142 and a closed bottom 144, except for windows 146, 148, which allow protrusion of the top ends 36, 38 of the U-core section 32 into the cover module 16 to contact the closing-bar core section 34 at the interfaces 31, 33 explained above. A cap panel 150 snaps into place on the cover housing 140 to close the open top 142 with a pair of springs 152, 154 mounted between the cap panel 150 and the closing-bar core section 34 to apply a bias force against the closing-bar core section 34 toward the bottom 144 of the cover housing 140. Therefore, when the cover module 16 is closed onto the base module 14, the top ends 36, 38 of the U-core section 32 protrude into the cover module 16 to contact and interface with the closing-bar core section 34. The springs 152, 154 in the cover module 16 bear on the closing-bar core section 34 in a yieldable manner to allow some adjustment of the position of the closing-bar core section 34 to accommodate the protrusion of the top ends 36, 38 of the U-core section 32 into the cover module 16 while maintaining the closing-bar core section 34 in snug contact with the contacting interfaces 31, 33 of the U-core section 32 to minimize the air gap between the closing-bar core section 34 and the U-core section 32, thereby maximizing the core 30 permeability for enhanced current transformer 10 performance.
As mentioned above, a latch mechanism 22 latches the cover module 16 to the base module 14 when the cover module 16 is closed onto the base module 14. In the example current transformer 10, the latch mechanism comprises two squeeze latches 160, 162 on opposite sides of the cover housing 140. As best seen in
In contrast, some of the other state-of-the-art split-core current transformers have latches that protrude significantly from adjacent exterior surfaces. Still others protrude little, if any, when latched, but they protrude significantly when unlatched and opened. Such protruding latch components in those types of state-of-the-art split-core current transformers can be very awkward and inhibiting when trying to maneuver the open current transformer around or onto a high power conductor in a tight space, for example, in a switch box, fuse box, or other electrical service panel where there are other wires or obstacles in close proximity. Such protrusion of a latch component causes at least two serious problems: (i) It makes the current transformer more difficult to install, because it becomes bulkier and harder to feed the cover housing between two closely spaced conductors, for example, in an electrical service panel; and (ii) There is a risk of breaking off such extended or protruding latch components during installation or removal. Therefore, by integrating the latches 160, 162 into the cover housing 140 as explained above, such problems with protruding latch components are eliminated.
Some of the other state-of-the-art split-core current transformers have screw fasteners that require turning for fastening one portion of the device in closed mode to another portion, and some other state-of-the-art split-core current transformers have latches that require getting a fingernail or thin object into a slot or under a ledge to pry the latch open. Those and other maneuvers that are almost impossible to perform with gloved hands are not needed for unlatching and opening the latch mechanism 22 with the latches 160, 162 of the example current transformer 10, which can be opened by squeezing as described above.
To close, the cover module 16 can simply be pivoted about the hinge 20 (
Since this latch mechanism with the resilient latches 160, 162 utilizes essentially no space in the interior of the main housing and very little space in the cover module 16, as described above, it is an important packaging feature that contributes to the compactness and overall small size of the current transformer 10, even though the closing-bar core section 34 in the cover module 16 needs and occupies a large space in the cover module 16.
In another example embodiment (not shown) the latch mechanism can have only one squeeze latch similar to either of the squeeze latches 160, 162 described above, but located on the end of the cover housing 140 that is opposite the hinge 20. Such a single squeeze latch may have a resilient extension of the cover housing 140 and comprises a dog on its distal end that engages a catch in the main housing 60 to latch the cover module 16 to the base module 14 in a releasable manner in much that same configuration and manner as described above for the squeeze latch 160 with the dog 164 that engages the catch 166. Also, such a single squeeze latch can be molded in a unitary manner with the cover housing 140 so that no assembly of the latch to the cover housing 140 is required, and the resilient extension can be substantially flush with the adjacent exterior surfaces of the cover housing 140 as also described above so that no latch parts, whether latched or unlatched, protrude outwardly from the body housing 60 or the cover housing 140 enough to snag or bind with external obstacles in tight spaces.
In the example current transformer 10 shown in
The over-sizing of the closing-bar core section 234 is easily accomplished when using ferrite magnetic material for the deep U-core section 32 and the shallow U-core section 234, because ferrite can be molded and sintered in just about any shape and size core sections desired. The over-size ratios described above for the I-core section 34 are applicable for the shallow U-core section 234.
A shallow U-core section 234 for the closing-bar core section similar to that shown in
The foregoing description is considered as illustrative of the principles of the invention. Furthermore, since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and process shown and described above. Accordingly, resort may be made to all suitable modifications and equivalents that fall within the scope of the invention. The words “comprise,” “comprises,” “comprising,” “include,” “including,” and “includes” when used in this specification are intended to specify the presence of stated features, integers, components, or steps, but they do not preclude the presence or addition of one or more other features, integers, components, steps, or groups thereof. Also, directional references used herein, such as top, bottom, above, and below, are for convenience in describing relationships of components and parts as they appear in the drawings, but are not intended to imply that the current transformer 10 or any variation has to be used in the orientation shown in the drawings or that those features, parts, or components have to be in those orientations in real use. On the contrary, the current transformer 10 and alternatives can be, and are often, used in different orientations, including right side up, upside down, and other orientations.