Closed-loop current transducer with magnetic core

Abstract

A closed-loop electric current transducer (2) for measuring the current flowing in one or more primary conductors extending through a primary conductor passage (16) of the current transducer, the current transducer comprising a magnetic core arrangement (4), a compensation coil (14) wound around the magnetic core arrangement (4), and a magnetic field detector (18). The magnetic core arrangement comprises an outer closed magnetic core (6) and an inner split magnetic core (8) comprising an airgap (10), the split magnetic core separately formed from the closed magnetic core and positioned adjacent and along a radially inner periphery of the closed magnetic core, the magnetic field detector (18) being positioned within said airgap (10).

Description

[0001] P3079PC00

[0002] CLOSED-LOOP CURRENT TRANSDUCER WITH MAGNETIC CORE

[0003] The present invention relates to a closed-loop current transducer with a magnetic core surrounding a central passage configured for receiving one or more primary conductors for measuring a current flowing in said one or more conductors.

[0004] A well-known type of current transducer comprises a magnetic core with an airgap in which a magnetic field detector, such as a Hall effect sensor, is positioned to measure the magnetic field generated by the current flowing in the primary conductor. Transducers of the closed loop type typically have a compensation coil wound around the magnetic core, the compensation coil connected in a feedback circuit loop to the magnetic field detector, configured to generate a secondary current seeking to cancel the magnetic field generated by the primary current.

[0005] Certain current sensing applications require a very high current measurement range, that may span across hundreds of amperes, for instance from less than 1 ampere to 800 amperes or more, for applications such as battery management for electric vehicles. State for the art closed-loop current sensors may typically have, over a full scale of 800 amperes, an offset uncertainty between 1 and 1.5 amperes, around 50% of which is contributed by the magnetic offset of the magnetic core. It would be desirable to reduce the offset uncertainty as much as possible. Moreover, for accurate measurement it is important to reduce the effects of external magnetic fields on the measurement output. The sensor in many applications should also function with a high frequency bandwidth for instance up to 200 kHz or more.

[0006] It is known to provide magnetic cores with a closed ring and a partial airgap formed by an indent in the ring, for instance as described in DE3705450A1. The closed ring core portion carries a substantial portion of the magnetic field generated by the current flowing in the primary conductor, and is coupled in transformer mode to the secondary conductor wound around the magnetic core. A minor portion of the magnetic field passes through the partial airgap and is thus picked up by the magnetic field detector within the airgap which is used in a feedback loop to drive the compensation coil seeking to cancel the magnetic field generated by the primary conductor. The current flowing in the compensation coil thus provides a measurement signal of the primary current as perse well known.

[0007] One of the problems of such arrangements is that the magnetic field in the airgap, for low primary currents, becomes very small, thus lowering the closed-loop gain and thus accuracy of the transducer. The formation of the partial indent within the same magnetic core materialP3079PC00

[0008] as the full ring creates a magnetic field imbalance that is difficult to accurately measure, especially over varying temperatures and frequencies, such that is it is very difficult to calibrate for the effects of the change in current imbalance. Moreover, this problem is exacerbated by the effects of external magnetic fields.

[0009] It is also known to provide shielding arrangements around magnetic cores for instance as described in DE10003638A1 to reduce the impact of external fields. Such shields provided around the magnetic core and compensation winding have an important drawback whereby the magnetic field generated by the primary current is compensated in the magnetic core but not in the shield. The shield carries a high magnetic flux, which is proportional to the primary current, which in turn affects the magnetic field picked up by the magnetic field detector, thus leading to inaccurate measurement.

[0010] In view of the foregoing, it is an object of the invention to provide a current transducer having a high current measurement range, in particular for applications that may span from a few amperes or less than 1 ampere, to a few hundred amperes, for instance up to 500 amperes or more, which is accurate, in particular has a low offset uncertainty.

[0011] It is advantageous to provide a closed-loop current transducer that can operate over a high frequency bandwidth, for instance for measurement of currents from 0 Hz to 100 kHz or more.

[0012] It is advantageous to provide a closed-loop current transducer that is economical to produce.

[0013] It is advantageous to provide a closed-loop current transducer that is compact and robust.

[0014] It is advantageous to provide a closed-loop current transducer that has a low sensitivity to external magnetic fields.

[0015] It is advantageous to provide a closed-loop current transducer that may be operated in harsh environments, for instance for implementation in electric vehicles.

[0016] Objects of the invention have been achieved by providing the current transducer according to claim 1.

[0017] Disclosed herein is a closed-loop electric current transducer for measuring the current flowing in one or more primary conductors extending through a primary conductor passage ofP3079PC00

[0018] the current transducer, the current transducer comprising a magnetic core arrangement, a compensation coil wound around the magnetic core arrangement, and a magnetic field detector.

[0019] The magnetic core arrangement comprises an outer closed magnetic core and an inner split magnetic core comprising an airgap, the split magnetic core separately formed from the closed magnetic core and positioned adjacent and along a radially inner periphery of the closed magnetic core, the magnetic field detector being positioned within said airgap.

[0020] In an advantageous embodiment, the inner split magnetic core has a thickness in a radial direction less than a thickness in a radial direction of the outer closed magnetic core.

[0021] In an advantageous embodiment, the radial thickness of the split magnetic core is less than 50% of the radial thickness (r) of the outer closed magnetic core.

[0022] In an advantageous embodiment, the outer closed magnetic core has a circular annular shape and the split magnetic core has a shape conforming to the said inner periphery of the outer closed magnetic core.

[0023] In an advantageous embodiment, the current transducer further comprises a magnetic core cover forming a coil support surrounding the magnetic core arrangement on and around which windings of the compensation coil are wound.

[0024] In an advantageous embodiment, the current transducer further comprises a positioning bridge mounted across ends of the split magnetic core forming the airgap, the positioning bridge fixed, clamped, bonded or welded to said ends of the split magnetic core to fix and define the airgap length.

[0025] In an advantageous embodiment, the split magnetic core has bent ends forming the airgap therebetween. The ends may be bent substantially orthogonally or at an angle inclined with respect to the radial direction. Alternatively, the split magnetic core may have overlapping ends that overlap substantially tangentially, thus forming the airgap between the overlapping ends.

[0026] In an advantageous embodiment, the transducer comprises an insulating housing element surrounding a portion of the magnetic core arrangement with the airgap and magnetic field detector.P3079PC00

[0027] In an advantageous embodiment, the magnetic field detector comprises a Hall effect sensor provided as an ASIC (Application Specific Integrated Circuit) having electrical terminals projecting in an axial direction and connected to a circuit board of an electronic circuit, terminal ends of the compensation coil also being connected to the circuit board.

[0028] In advantageous embodiments, the split magnetic core is formed of a bent strip of soft magnetic alloy selected from any one or more of Nickel iron alloys, with nickel content from 45 to 85 %, preferably an alloy with 75 to 82 % nickel, e.g. known commercially as Permalloy, or Mu-Metal, as well as cores made from amorphous soft magnetic alloys, e.g. Cobalt based amorphous alloys, or cores made of nanocrystalline soft magnetic alloys, e.g iron based nanocrystalline alloys.

[0029] In advantageous embodiments, the outer closed magnetic core is made of a magnetic material selected from any one or more of Soft Iron, Electric steel, Cobalt based magnetic alloy, Silicon Iron, Grain Oriented Silicon Iron, Nanocrystalline alloy, iron powder cores e.g. iron carbonyl, or metal powder soft magnetic cores, or sintered ferrites.

[0030] In an embodiment, the current transducer may further comprise a second closed magnetic core mounted adjacent and along an inner radial periphery of the split magnetic core.

[0031] Further objects and advantageous features of the invention will be apparent from the claims, from the detailed description, and annexed drawings, in which:

[0032] Figure 1a is a perspective view of a current transducer according to an embodiment of the invention;

[0033] Figure 1 b is a cross-sectional view orthogonal to a center axis of the transducer of figure 1 a;

[0034] Figure 1c is a cross-sectional view of the transducer of figure 1a in a plane containing the center axis;

[0035] Figure 1d is a perspective view of a magnetic core arrangement and magnetic field detector of the transducer of figures to 1 a to 1 c;

[0036] Figure 2a is another perspective view of a current transducer according to an embodiment of the invention;P3079PC00

[0037] Figure 2b is a perspective partial cross-sectional view of the transducer of figure 2a;

[0038] Figures 3a and 3b are schematic illustrations of magnetic core arrangements according to first and second variants of a current transducer according to embodiments of the invention;

[0039] Figure 3c is a close-up schematic view of a portion of the magnetic core arrangement at the airgap, showing a positioning bridge spanning the airgap according to an embodiment of the invention.

[0040] Referring to the figures, an electrical current transducer 2 comprises a magnetic core arrangement 4, a compensation coil 14 wound around the magnetic core arrangement 4, and a magnetic field detector 18.

[0041] The current transducer may further be provided with an electronic circuit 24 including a circuit board 26 and circuit components 28 mounted on the circuit board. The circuit board 26 is connected to terminals of the magnetic field detector 18 supplying power to the magnetic field detector and receiving measurement signals from the magnetic field detector, and is configured for further connection to external circuitry for outputting the measurement signal and for receiving power for the electronic circuit and magnetic field detector. The electronic circuit 24 may further be electrically connected to the terminal ends of the compensation coil.

[0042] The electrical current transducer 2 comprises a primary conductor passage 16 through which one or more primary conductors carrying a current to be measured extend, as per se well known in the art. The primary conductor may also have one or more turns wound around the magnetic core arrangement or simply comprise a conductor that passes through without being wound around the magnetic core arrangement.

[0043] The magnetic core arrangement 4 comprises an outer closed magnetic core 6 made of a magnetically permeable material. In a preferred embodiment, the outer closed magnetic core may be in the form of a circular annular core. The magnetic core arrangement 4 further comprises an inner split magnetic core 8 having at least one airgap 10, The inner split magnetic core 8 is positioned radially inwardly from the outer closed magnetic core 6.

[0044] It may be noted that although the illustrated magnetic core arrangement has a circular annular shape, it is possible for the magnetic core arrangement to have other shapes that surround a central passage, for instance a polygonal shape such as a square or hexagonalP3079PC00

[0045] shape, or a non-axisymmetric shape such as an elliptical or oval shape, without departing from the scope of the invention.

[0046] The split magnetic core 8 positioned radially inwardly from the closed magnetic core 6 has a shape that conforms to an inner peripheral surface of the outer closed magnetic core 6. The split magnetic core 8 may be in contact with the inner surface of the closed magnetic core 6 or may be separated therefrom by a small spacing having a radial depth preferably less than a radial thickness of the closed magnetic core 6.

[0047] The split magnetic core 8 with the airgap 10 may advantageously be made of a high performance soft magnetic material with a low coercivity, high permeability, and low saturation flux density, such as a permalloy, mumetal, or cobalt based amorphous soft magnetic alloy. Preferably, the selected soft magnetic material has a coercive field below 1 A / m preferably below 0,5 A / m ideally below 0,4 A / m, and a saturation flux density Bs of about 0,78 T.

[0048] The airgap, in a first variant, may be made by bending the strip of soft magnetic alloy forming the split magnetic core 8 orthogonally, as illustrated in figures 1d and 3a. In another variant, as illustrated in figure 3b, the airgap may be formed by overlapping ends of the strip of material forming the split magnetic core 8.

[0049] The outer closed magnetic core 6 may be made of various per se known soft magnetic materials, such as a stacked laminated core, or an integrally formed ferrite core, or in preferred embodiments by a wound tape forming a concentrically layered wound magnetic core. In the latter variant, the spiral / concentrically wound tape may for instance be made of a tape of grain-oriented silicon steel, for instance with a saturation flux density greater than 1.8 T. It may be noted that the formation of magnetic cores of ferrite, stacked laminated sheets, or wound tape, is perse known.

[0050] The current transducer may further comprise a positioning bridge 20 mounted around the magnetic core arrangement 4 at the location of the airgap 10, the positioning bridge 20 comprising one or more elements that clamp, fix or attach to the split magnetic core 8 either side of the airgap 10 in order to securely hold the airgap in a rigid position, defining accurately the length of the airgap.

[0051] The positioning bridge may be made of a non-magnetic metal, for instance an austenitic stainless-steel compatible with the annealing of nickel-iron soft magnetic materials. TheP3079PC00

[0052] positioning bridge may be spot welded or clamped to ends of the split magnetic core 8 adjacent the airgap 10. The positioning bridge 20 also facilitates the assembly of the inner split magnetic core 8 to the outer closed magnetic core 6. In figure 3c, the positioning bridge is schematically illustrated as mounted on an outer radial side of the split magnetic core, and in the illustrated embodiment may be bonded or welded at points 21 of the core adjacent the airgap 10 and on opposite sides of the airgap.

[0053] The airgap formed by tangentially overlapping ends of the split magnetic core (figure 3b) or by orthogonally bent ends of the split magnetic core (figure 3a) allows to have an airgap of a large size compared to a thickness in a radial direction of the split magnetic core material.

[0054] The outer closed magnetic core functions on the one hand to provide magnetic material with sufficient cross-section for the operation of the transducer in transformer mode, and at the same time the concentric arrangement of the outer closed magnetic core 6 around the inner split magnetic core 8 provides a magnetic shielding function to reduce the effects of magnetic fields created for instance by neighboring primary conductors.

[0055] In a variant, the magnetic core arrangement may comprise a second closed annular core mounted adjacent and along an inner radial periphery of the split magnetic core, as schematically illustrated by the dotted line 6b in figure 3b. This inner closed annular core may be provided primarily to reduce the effects of off-centered primary conductors and prevent localized saturation of the split magnetic core.

[0056] The inner split magnetic core 8 is separately formed from the outer closed magnetic core 6 and can thus be made of a different material from the closed magnetic core 6. The material separation between the inner split magnetic core 8 and outer closed magnetic core allows to better control the magnetic flux flowing within the split magnetic core 8 and picked up by the magnetic field detector 18 positioned within the airgap 10 thereof.

[0057] The magnetic core arrangement 4 may be surrounded by a magnetic core cover that forms a coil support 12 around which the windings of the compensation coil 14 are wound. The coil support 12 may be made for instance of a polymer that is overmolded or mounted around the magnetic core arrangement 4 and serves to avoid direct contact of the coil windings on the magnetic core arrangement, forming a protective base for the winding of the coil to avoid damage to the coil and short circuits between windings.P3079PC00

[0058] The magnetic field detector 18 may be surrounded by a detector housing portion 22 that may also be secured to the circuit board 26 of the electronic circuit 24. The detector housing portion 22 forms a protective cover and positioning element for the magnetic field detector 18.

[0059] The electric current transducer illustrated in the figures may further comprise an optional outer housing or tape or other insulating cover (not shown) depending on the application, whereby this outer housing may also be provided in external devices in which the current transducer is installed.P3079PC00

[0060] List of references used

[0061] Closed-loop current transducer 2

[0062] optional outer housing I tape I cover (not shown) magnetic core arrangement 4

[0063] closed magnetic core

[0064] outer closed magnetic core 6 inner closed magnetic core 6b split magnetic core 8

[0065] airgap 10

[0066] magnetic core cover I coil support 12 compensation coil 14

[0067] primary conductor passage 16

[0068] magnetic field detector 18

[0069] positioning bridge 20

[0070] weld or bonding points 21

[0071] detector housing element 22

[0072] electronic circuit 24

[0073] circuit board 26

[0074] circuit components 28

[0075] radial thickness of the outer core r

[0076] radial direction R

[0077] axial direction A

Claims

P3079PC00Claims1. A closed-loop electric current transducer (2) for measuring the current flowing in one or more primary conductors extending through a primary conductor passage (16) of the current transducer, the current transducer comprising a magnetic core arrangement (4), a compensation coil (14) wound around the magnetic core arrangement (4), and a magnetic field detector (18), wherein the magnetic core arrangement comprises an outer closed magnetic core (6) and an inner split magnetic core (8) comprising an airgap (10), the split magnetic core separately formed from the closed magnetic core and positioned adjacent and along a radially inner periphery of the closed magnetic core, the magnetic field detector (18) being positioned within said airgap (10).

2. The current transducer of claim 1 wherein the inner split magnetic core (8) has a thickness in a radial direction less than a thickness in a radial direction of the outer closed magnetic core (6).

3. The current transducer of the preceding claim wherein the radial thickness of the split magnetic core is less than 50% of the radial thickness (r) of the outer closed magnetic core.

4. The current transducer of any preceding claim wherein the outer closed magnetic core has a circular annular shape and the split magnetic core has a shape conforming to the said inner periphery of the outer closed magnetic core.

5. The current transducer of any preceding claim further comprising a magnetic core cover forming a coil support (12) surrounding the magnetic core arrangement (4) on and around which windings of the compensation coil (14) are wound.

6. The current transducer of any preceding claim further comprising a positioning bridge (20) mounted across ends of the split magnetic core (8) forming the airgap (10), the positioning bridge fixed, clamped, bonded or welded to said ends of the split magnetic core to fix and define the airgap length.

7. The current transducer of any preceding claim wherein the split magnetic core has bent ends forming the airgap therebetween.

8. The current transducer of any preceding claim wherein the transducer comprises an insulating housing element (22) surrounding a portion of the magnetic core arrangement with the airgap and magnetic field detector.P3079PC009. The current transducer of any preceding claims 1 to 6 wherein the split magnetic core has overlapping ends that overlap substantially tangentially, thus forming the airgap between the overlapping ends.

10. The current transducer of any preceding claim wherein the magnetic field detector comprises a Hall effect sensor provided as an ASIC (Application Specific Integrated Circuit) having electrical terminals projecting in an axial direction and connected to a circuit board (26) of an electronic circuit, terminal ends of the compensation coil also being connected to the circuit board (26).

11. The current transducer of any preceding claim wherein the split magnetic core is formed of a bent strip of soft magnetic alloy selected from any one or more of Nickel iron alloys, with nickel content from 45 to 85 %, or an amorphous soft magnetic alloy, for instance a cobalt based amorphous alloy, or an iron based nanocrystalline soft magnetic alloys.

12. The current transducer of any preceding claim wherein the outer closed magnetic core is made of a magnetic material selected from any one or more of Soft Iron, Electric steel, Cobalt based magnetic alloy, Silicon Iron, Grain Oriented Silicon Iron, Nanocrystalline alloy, iron carbonyl, metal powder soft magnetic cores, or sintered ferrite.

13. The current transducer of any preceding claim further comprising a second closed magnetic core (6b) mounted adjacent and along an inner radial periphery of the split magnetic core (8).

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