Electrical current transducer with electrostatic shield

Abstract

Electrical current transducer to measure the current flowing in a primary conductor, the transducer including an outer casing, a magnetic core comprising a magnetic circuit gap, a magnetic field detector comprising a sensing portion positioned in the magnetic circuit gap, an insulating core housing, a secondary coil formed from a wire wound around a coil support portion of the insulating core housing, and an electrostatic shield. The electrostatic shield is positioned between secondary coil and a radially inner wall portion of the outer casing, the radially inner wall portion defining a passage configured to receive said primary conductor therethrough. The electrostatic shield comprises a skirt portion formed by a plurality of flexible strips extending axially from an edge of the shield, the flexible strips configured to elastically bias against a base wall portion of the outer casing.

Claims

1. Electrical current transducer to measure the current flowing in a primary conductor, the transducer including an outer casing, a magnetic core comprising a magnetic circuit gap, a magnetic field detector comprising a sensing portion positioned in the magnetic circuit gap, a core housing, a secondary coil formed from a wire wound around a coil support portion of the core housing, and an electrostatic shield positioned between the secondary coil and a radially inner wall portion of the outer casing, the radially inner wall portion defining a passage configured to receive said primary conductor therethrough, wherein the electrostatic shield comprises a skirt portion formed by a plurality of flexible strips extending axially from an edge of the electrostatic shield, the flexible strips configured to elastically bias against a base wall portion of the outer casing.

2. Current transducer according to claim 1, wherein the flexible strips are separated by slits, and wherein juxtaposed strips and slits are substantially evenly distributed around a circumference of the electrostatic shield.

3. Current transducer according to claim 1, wherein the electrostatic shield is formed from a thin flexible sheet, selected from a group consisting of stamped sheet metal and metallized flexible substrate, folded into a shape configured to surround the primary conductor passage.

4. Current transducer according to claim 1, wherein the electrostatic shield comprises a securing mechanism configured to hold the electrostatic shield in a closed bent configuration prior to mounting on to the core housing.

5. Current transducer according to claim 1, wherein the electrostatic shield comprises locking orifices cooperating with screen fixing protrusions of the core housing to position and hold the electrostatic shield to the core housing during manufacturing assembly.

6. Current transducer according to claim 5, wherein the locking orifices comprise a wide portion and a narrow portion, and the screen fixing protrusions comprise a profile having a wide head portion and a narrow stem portion, whereby the wide head portion is insertable through the wide portion of the locking orifices, and the narrow stem portion is slidable into the narrow portion of the locking orifices.

7. Current transducer according to claim 6, wherein the narrow portion of the locking orifices is oriented towards the skirt portion relative to the wide portion, such that the spring biasing force acting on the electrostatic shield when assembled in the outer casing tends to confirm the locking of the electrostatic shield to the core housing.

8. Current transducer according to claim 1, wherein the core housing comprises a coil terminal connection portion, said coil support portion, and at least one circuit board support portion.

9. Current transducer according to claim 8, wherein the coil support portion has a U-shaped cross-sectional profile comprising a base wall, a radially inner wall extending from the base wall and a radially outer wall extending from the base wall, the radially inner and outer walls having a height configured such that open end edges thereof extend slightly beyond an axial thickness of the magnetic core including any manufacturing tolerances such that the open end edges are folded inwards around corners of the magnetic core by the secondary coil wound around the core housing.

10. Current transducer according to claim 8, wherein the coil support portion comprises core latching elements in the form of latching shoulders configured to allow the magnetic core to be axially inserted in the core housing and to hold the core within the core housing during assembly.

11. Current transducer according to claim 8, wherein the coil support portion of the core housing comprises wire anti-slip elements configured to prevent slipping of a first winding layer of the secondary coil on the core housing during the winding process.

12. Current transducer according to claim 8, wherein the core housing comprises deflector ribs arranged in corners joining the coil support portion to the coil terminal connection portion and the circuit board support portion, the deflector ribs configured to avoid a build-up of windings adjacent side walls of the coil support portion and the coil terminal connection portion.

13. Current transducer according to claim 8, wherein the coil terminal connection portion comprises a magnetic sensor receiving cavity for lodging the magnetic field detector therein.

14. Current transducer according to claim 8, wherein the coil terminal connection portion comprises a circuit board resting edge and a circuit board latching element that extends over an edge of a circuit board, and wherein the circuit board support portion comprises a circuit board resting edge and the circuit board latching element configured to extend over an edge of a circuit board, to secure the magnetic core, the core housing, and the secondary winding assembly to the circuit board.

15. Current transducer according to claim 1, wherein the core housing is a single molded part.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIGS. 1a and 1 b are perspective views of a core housing of an electrical current transducer according to an embodiment of this invention;

(2) FIGS. 2a to 2d are respective sectional views of a core housing and magnetic core of an electrical current transducer according to an embodiment of this invention illustrating assembly steps;

(3) FIG. 2e is a perspective view of a portion of the core housing and magnetic core;

(4) FIGS. 3a and 3b are detail views of a portion of the core housing;

(5) FIGS. 4a to 4c are detailed perspective views illustrating winding of a coil around a portion of the core housing according to an embodiment of this invention;

(6) FIG. 4d is a detailed plan view of a portion of an electrical current transducer according to an embodiment of this invention;

(7) FIG. 5 is a detailed view of a portion of a core housing showing a coil terminal connection portion thereof;

(8) FIGS. 6a and 6b are perspective views of an electrical current transducer, with an outer housing removed, according to an embodiment of this invention;

(9) FIG. 6c is a view of a circuit board of the current transducer of FIGS. 6a, 6b;

(10) FIG. 6d is a detailed perspective view of a portion of the current transducer of FIG. 6a showing assembly of a magnetic field detector;

(11) FIG. 7a is a perspective view of an electrical current transducer with electrostatic shielding according to an embodiment of this invention, shown without housing;

(12) FIG. 7b is a perspective detailed view of a portion of the current transducer of FIG. 7a;

(13) FIG. 8a is an exploded perspective view of an electrical current transducer according to an embodiment of this invention;

(14) FIG. 8b is a top view of the transducer of FIG. 8a with a cover and circuit board removed;

(15) FIG. 9a is a perspective view of a casing base part and electrostatic shield of an electrical current transducer according to an embodiment of this invention;

(16) FIG. 9b is a perspective cross-sectional view of a portion of the casing base part and electrostatic shield of FIG. 9a;

(17) FIG. 10 is a perspective view of an embodiment of an electrostatic shield, prior to bending, of an electrical current transducer according to an embodiment of this invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

(18) Referring to the figures, an exemplary embodiment of an electrical current transducer 2 comprises an outer casing 3 comprising a base part 5 and a cover part 7, and mounted therein: a core housing 10, a magnetic core 6 mounted in the core housing 10, a secondary coil 4 mounted around the core housing and magnetic core, a magnetic field detector 8 inserted in a magnetic circuit gap of the magnetic core, and a circuit board 12 on which the core housing, magnetic core and secondary coil are mounted. The current transducer further comprises an electrostatic shield 14 to protect the magnetic core and secondary coil from high voltages in a primary conductor (not shown) carrying the current to be measured. The components mounted in the casing are surrounded by a dielectric potting material filled in the casing after assembly of the components therein.

(19) The magnetic field detector 8 may be in the form of an ASIC (Application Specific Integrated Circuit) comprising electrical terminals 16 for connection to electrical circuitry that supply power and process the measurement signals from the magnetic field detector. The magnetic field detector may be in the form of a Hall effect sensor, as is per se well known in the art, or may be based on other magnetic field sensing technologies such as magneto-resistive or fluxgate sensors that are also well known in the art. Hall cell magnetic field detectors are widely used in open-loop current sensing applications because of their low cost and good measurement performance in open-loop applications.

(20) As is per se well known in the art, the secondary coil acts as a compensation coil that is supplied with electrical current in a feedback loop connected to the magnetic field detector 8 that seeks to cancel the magnetic field generated by a primary conductor carrying the current to be measured, the primary conductor extending through a central passage 22 of the magnetic core 6. The magnetic field generated by the primary conductor circulates in the magnetic core 6 and is picked up by the magnetic field detector 8 positioned in the magnetic circuit gap of the magnetic core.

(21) The number of turns of the secondary coil is determined principally by the amplitude of the current to be measured in the primary conductor and the level of feedback current in the compensation coil that is desired. In order to reduce the amount of material used for the compensation coil, it is advantageous to have a tight, regular, compact winding with the smallest possible starting diameter. The starting diameter depends on the cross-sectional area of the magnetic core which is determined inter alia by the current measuring range, and by the thickness of the core housing surrounding the magnetic core.

(22) Certain magnetic cores are made by winding a strip of magnetic material to form a wound magnetic core which is particularly cost effective to produce. Such wound cores however have sharp corners and may have somewhat variable diameters and/or heights. It is thus advantageous to have a core housing 10 that can adjust for the tolerances in manufacturing of such magnetic cores, the core housing main function being to provide a protective insulating layer for the wire wound around the magnetic core.

(23) The magnetic core may also be made of a ferrite material, per se well known in the art, or of other materials such as stacked laminated sheets of magnetic material that are also per se well known in the art.

(24) Connection ends 20a, 20b of the secondary coil 4 are connected to electronic circuitry of the current transducer mounted on the circuit board 12 via wire end connection pads 60a, 60b to which the connection ends are soldered. Other known connection means between the coil and the electronic circuitry may be employed including welding, crimping, insulation displacement, clamping connection and the like. The circuit board 12 further comprises magnetic sensor connection pads 62 for connection of the magnetic field detector to the electronic circuitry of the current transducer and further comprises connection for grounding of the magnetic core and for grounding of the electrostatic shield 14.

(25) The circuit board 12 is provided with a central orifice aligned with the central passage 22 of the magnetic core in order to allow a primary conductor to extend therethrough. An edge of the circuit board defining the central orifice 70 comprises a support portion latch edge profile 68 and a connection portion latch edge profile 66 for connection of the core housing 10 to the circuit board as will be described in more detail further on. The circuit board may further comprise a wire end guide notch 72 used to guide the connection ends of the secondary coil for positioning the connection ends on the wire end connection pads 60a, 60b. The circuit board may further comprise a magnetic sensor receiving slot 64 to allow assembly of the magnetic sensor therethrough into the magnetic circuit gap of the magnetic core. The circuit board may further comprise connection pads 74 for connection to terminals of a connector and various electrical and electronic components (not shown) for filtering or processing the measurement signal and supply power to the magnetic field detector 8.

(26) The core housing 10 comprises a coil support portion 26 and a coil terminal connection portion 28. The coil support portion comprising a generally U shaped profile comprising a base wall 74a, a radially inner wall 74b and a radially outer wall 74c. The core housing 10 may advantageously be an integrally formed part, for instance an injection molded single piece plastic part. The radially inner wall 74b and radially outer wall 74c may advantageously comprise core latching elements 82 in the form of latching shoulders that hold the core within the housing by axially inserting and clipping the core into the housing during assembly. The base wall 74a may advantageously be provided with orifices 84 configured for forming the latch elements in a simple injection molding die tool to avoid the use of moving injector pins or parts within the die tool thus allowing a particularly cost effective manufacturing of the core housing.

(27) The inner and outer walls 74b, 74c have a height configured such that the open end edges 80b, 80c thereof extend slightly beyond the thickness of the magnetic core 6 including any manufacturing tolerances such that when the coil is wound around the magnetic core housing, the open end edges 80b, 80c are folded inwards around the corners of the magnetic core. This configuration advantageously allows the magnetic core housing to be formed of a single molded part in a simple economical molding process while nevertheless providing the important function of protecting the wire from damage around the corners of the magnetic core. The open ended single piece core housing is also advantageous in that it allows very simple axial assembly of the magnetic core within the housing, whereby the latching elements holding the two parts together prior to the winding operation.

(28) The core housing may further advantageously comprise wire anti-slip elements 76 to position accurately the first winding layer of the coil on the magnetic core housing and to prevent slipping of the coil during the winding process on the core housing that could lead to irregular and thus non optimal winding. Once the first layer of wire has been wound it forms a natural anti-slip layer, as does each successive layer. The tight regular winding of the first layer thus ensures that a compact winding is achieved. In a preferred embodiment, the wire anti-slip elements are provided on the open end edge 80c of the radially outer wall 74c and/or on the open end edge 80b of the radially inner wall. In variants, the wire anti-slip elements may alternatively or additionally be provided on a corner joining the base wall 74a with the radially outer wall 74c and/or the radially inner wall 74b. The anti-slips elements may be in the form of indents, which may be configured for the diameter of the wire to be wound around the core.

(29) The coil terminal connection portion 28 comprises a housing portion 36 defining a magnetic sensor receiving cavity 37. The housing portion 36 comprises a radially inner wall portion 36a, a radially outer wall portion 36b, a coil side wall portion 36c and a coil side wall portion 36d.

(30) The housing portion 36 may further comprise a pin holder cavity 34 to receive a pin tool 11 of a winding machine therein during the coil winding process. The pin tool 11 may be provided with a wire wrap channel 32 to anchor the starting end of the wire during the coil winding process. The coil terminal connection portion may further comprise a wire guide channel 40 to receive an end of the secondary coil wire therethrough for positioning and securing the wire end during the winding process. The housing portion 36 defines a circuit board resting edge 46 and a circuit board latching element 42 that extends over an edge of the circuit board to secure the magnetic core, core housing and winding assembly to the circuit board. The circuit board latching element may advantageously be provided as an extension of the radially inner wall portion 36a. A wire securing slot 38a in the housing portion 66 receives the connection end 20a therethrough which defines the wire starting end in the coil forming process, and once the coil has been fully formed the finishing end of the winding corresponding to connection end 20b is inserted through the wire securing slot 38d in the coil side wall portion 36d.

(31) The side wall portions 36c, 36d may further comprise deflector ribs 53 arranged in a corner joining the side wall portions to the base wall 74a, the function of the deflector ribs 53 being to avoid a build-up of windings adjacent the side wall portions thus ensuring a more uniform winding of the coil adjacent the side wall portions.

(32) In an embodiment, the magnetic sensor receiving cavity 37 is aligned with a magnetic sensor receiving slot 64 in the circuit board 12 to allow the magnetic field detector 8 to be assembled to the circuit board and into the air gap of the core after the circuit board has been assembled to the magnetic core and winding assembly (see FIG. 6d). In a variant, the magnetic field detector 8 may be pre-assembled to the circuit board prior to assembly of the circuit board 12 to the magnetic core and winding assembly 4, 6, 10. The magnetic sensor receiving cavity 37 may comprise guide rails 39 configured to guide insertion of the magnetic field detector and provide snug fitting of the detector in the cavity.

(33) The core housing may further advantageously comprise at least one circuit board support portion 30 mounted to the coil support portion 26, configured to provide a support leg for mounting of the core and winding assembly 4, 6, 10 to the circuit board. In a preferred embodiment, the circuit board support portion 30 is positioned opposite or approximately opposite the coil terminal connection portion 28 such that the mounting support of the core and winding assembly is substantially balanced. The circuit board support portion 30 comprises a housing portion comprising a radially inner wall portion 52a, a radially outer wall portion 52b, a coil side wall portion 52c, a coil side wall portion 52d, and circuit board positioning protrusions 56.

(34) The housing portion defines a circuit board resting edge 46 and a circuit board latching element 54 that extends over an edge of the circuit board to secure the magnetic core, core housing and winding assembly 4, 6, 10 to the circuit board 12. The circuit board latching element 54 may advantageously be provided as an extension of the radially inner wall portion 52a. In a preferred embodiment, at least one of the latching elements 42, 54 is elastically mounted, for instance in the form of a cantilever arm with a latching shoulder at the free end thereof, configured to enable the circuit board 12 to be clipped to the core housing 10.

(35) The side wall portions 36c, 36d may further comprise deflector ribs 53 arranged in a corner joining the side wall portions 52c, 52d to the base wall 74a, the function of the deflector ribs 53 being to avoid a build-up of windings adjacent the side wall portions thus ensuring a more uniform winding of the coil adjacent the side wall portions.

(36) The core housing 10 may further comprise screen fixing protrusions 44 configured to position and hold the electrostatic shield 14 to the core housing 10, in particular during manufacturing assembly of the components of the current transducer to improve positioning of the shield and simplify assembly operations. The screen fixing protrusions 44 may advantageously be provided on the radially inner wall portions 36a, 52a of the coil terminal connection and circuit board support portions 28, 30.

(37) The electrostatic shield may advantageously be formed from a thin flexible sheet, for instance stamped sheet metal, or a metallized flexible substrate, that may be folded into a closed shape configured to conform to the shape of the central passage 22 of the magnetic core or central orifice 70 of the circuit board. The electrostatic shield may comprise a securing mechanism, for instance in the form of a tab 25a and slit 25b configured to hold the shield in a closed bent configuration prior to locking on to the core housing. The electrostatic shield may however, in variants, be held in the bent closed position by other mechanical means or by welding, adhesive bonding, or soldering.

(38) The electrostatic shield 14 provides an electrical shielding between the primary conductor (not shown) carrying the current to be measured and the core and coil assembly. The electrostatic shield 14 comprises a skirt portion 15 formed by a plurality of flexible cantilever strips 17 separated by slits 19 extending axially from an edge of the shield, the cantilever strips distributed around the circumference of the shield. The cantilever sections may be bent out of the axial direction A into a curved shape to form flexible spring strips that bias against a base wall portion 5a of the outer casing 3 of the transducer. The main portion of the shield 14 is positioned adjacent a radially inner wall portion 5b of the outer casing 3. The flexible strips 17, in conjunction with the slits 19, allow potting material injected into the casing 3 to flow between the electrostatic shield 14 and the casing, in particular between the shield and the radially inner wall portion 5b of the casing. The potting material may flow into the space 9 between the shield and casing by flowing through the passages formed between the portion of flexible strips 17 not in contact with the base wall of the casing. In a variant, the shield may be provided with flow orifices (not shown) in addition to the flexible strips, to allow potting material to effectively fill in the gap between the electrostatic shield 14 and the casing radially inner wall portion 5b.

(39) The electrostatic shield may advantageously comprise locking orifices 21, for instance in the form of key-hole shaped orifices having a wide portion 21a and a narrow portion 21b. In this exemplary embodiment, the key-hole shaped orifices are configured to allow the screen fixing protrusions 44, which may for instance be provided with a substantially “T” shaped profile having a wide head portion 44a and narrow stem portion 44b, to insert through the wide portion 21a of the key-hole shaped orifice and slide such that the stem portion 44b slides into the narrow portion 21b of the key-hole and locks the shield to the core housing. The narrow portion 21b is advantageously oriented towards the skirt portion 15 relative to the wide portion 21a, such that the spring biasing force acting on the shield when assembled in the casing tends to confirm the locking of the electrostatic shield 14 to the core housing 10

(40) The flexible skirt portion 15 advantageously also provides a centering effect to centre the shield around the casing radially inner wall portion. The centering effect and even distribution of potting material in the space 9 between the shield and casing ensures a homogeneous and reliable electrical behavior of the region between the shield and primary conductor, thus leading to reliable and predictable control of partial discharges that may occur between the primary conductor and the shield. The precise positioning of the shield relative to the magnetic core and winding assembly also helps to ensure the accuracy and reliability of the electrical behavior of the transducer.

(41) The electrostatic shield 14 further comprises a grounding terminal 23 that may advantageously, in an embodiment, integrally extend from an edge of the shield for easy connection to a grounding circuit trace on the circuit board 12.

(42) Advantages of the invention include, inter alia:

(43) economical manufacturing, for instance due only one very thin and light plastic part for the core housing, no slider in the mold, no adhesive tape needed.

(44) protection against the magnetic core sharp edges of the enameled copper wire while winding

(45) fast and simple assembly by clipping the core housing on the magnetic core

(46) minimized copper quantity for the winding due to optimized section to be wound and the few (in a preferred embodiment only two) coil support portions on the core housing

(47) despite its coarse geometry, the magnetic core is perfectly positioned in the core housing

(48) homogeneous winding control thanks to winding wire anti-slip elements

(49) specific shapes enables to guide and protect winding wire all along the different process steps.

(50) magnetic core housing enables to put in reference precisely the magnetic core, circuit board and magnetic field detector ASIC

(51) flexible skirt on the shield enables to position more precisely electrostatic screen around primary hole of the casing and to ensure even potting material filling between the shield and housing.