IES20100423A2 - A current sensor assembly - Google Patents

Abstract

A coil assembly for a current sensor has a pair of flexible substrates of insulating material disposed face to face and joined along opposite edges, and a plurality of conductive tracks extending across each substrate transversely between the opposite edges. The ends of the tracks on one substrate are electrically connected to the ends of the tracks on the other substrate, and the flexible substrates are arranged to recieve a body of material (preferably magnetic material) sandwiched between the substrates to provide a current sensor. The conductive tracks are arranged in such a way as to form a continuous winding around the body to be recieved between the substrates. <Figure 4>

Description

A Current Sensor Assembly Field of the Invention IE 1 0 0423 This invention relates to a current sensor of the type comprising a current transformer, and a coil assembly for such a current sensor.

Background to the Invention A current transformer (CT) has long been one of the standard methods of measuring alternating current. A CT is designed to provide a current in its secondary winding proportional to the current in its primary winding. A CT also isolates the measuring instrument from what may be very high voltage in the primary circuit. CTs are commonly used in metering and for protection and measurement in the electrical power industry.

A current transformer has a primary winding, a magnetic core, and a secondary winding. An alternating current flowing in the primary winding produces a magnetic field in the core, which then induces current flow in the secondary winding circuit. A primary objective of current transformer design is to ensure that the primary and secondary windings are efficiently coupled, so that the secondary current bears an accurate relationship to the primary current.

A common design of CT, Figure 1, comprises a length of wire 10 wrapped many times around an annular magnetic core 12 surrounding a conductor 14 through which flows the current I being measured. The CT's primary winding therefore IE 1 0 0 4 2 3 consists of a single turn of the conductor 14, with the secondary winding 16 consisting typically of a multiple number of turns of the wire 10.

A known type of current transformer has a magnetic core comprising a flexible metal strip (e.g. silicon steel) which can be formed into a closed loop around the primary conductor. This allows the CT to be fixed around the primary conductor without the need to break the primary conductor circuit, thereby allowing the CT to be introduced onto a live conductor.

However, the manufacture of such a device is difficult, and requires specialised winding equipment substantially different from the standard coil winding equipment. Furthermore, limitations in the accuracy and repeatability of the winding equipment can lead to significant variation in characteristics in the CTs produced. Also, the manufacturing process can be expensive due to the high labour costs involved.

In other coil designs, such as a Rogowski coil, the magnetic core can be omitted and an air core is employed.

It is an object of the invention to provide an improved flexible CT-type current sensor.

Summary of the Invention The invention provides a coil assembly for a current sensor comprising two flexible substrates of insulating material disposed face to face and joined along opposite edges, and a plurality of conductive tracks extending across each J|1OO423 substrate transversely between the opposite edges, wherein the ends of the tracks on one substrate are electrically connected to the ends of the tracks on the other substrate, wherein the flexible substrates are arranged to receive a body of material sandwiched between the substrates, and wherein the conductive tracks are arranged in such a way as to form a continuous winding around the body to be received between the substrates.

The invention further provides a current sensor comprising the flexible coil assembly and a body of material sandwiched between the substrates.

Preferably, the current sensor comprises a flexible body of material. This may be formed from any suitable flexible material.

Further, preferably, the body of material is a magnetic body. Thus, while the coil may employ an inert body to give it shape and to provide coil spacing (so that it has in effect an air core), the preferred embodiment employs a flexible body of magnetic material, for example silicon steel, Preferably, the substrates and body are provided around a conductor whose current is to be sensed such that the tracks are at least approximately parallel to the conductor.

Preferably, along at least one edge the ends of the tracks are connected through via holes in the substrates.

IE 1 ΰ 0Λ2:3 If 1 0 04 23 The two facing substrates may be formed from a single flexible sheet by folding, or from two flexible sheets secured along opposite edges.

Where the facing substrates are formed from a single flexible sheet by folding along the folded edge, the tracks extend continuously across the fold from one substrate to the other.

Brief Description of the Drawings An embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1, previously described, shows a prior art CT-type current sensor.

Figures 2 and 3 are plan views of first and second flexible 20 PCB substrates used in the manufacture of the embodiment.

Figure 4 is a plan view of the substrates of Figures 2 and 3 joined edge to edge.

Figure 5 is an end view of the structure of Figure 4.

Figures 6 and 7 show how a ferromagnetic strip is inserted between the PCB substrates.

Figure 8 shows how the structure of Figure 7 is bent round to form the completed current sensor.

Description of an Embodiment of the Invention j| ο Ο 4 23 The drawings show successive stages in the manufacture of a CT-type current sensor according to the invention, The manufacture starts with two substantially rectangular, equal-sized, highly flexible sheets or substrates 20, 22 of electrically insulating material, Figures 2 and 3 respectively. Each substrate has a pair of opposite substantially parallel edges 24, 26 and a pair of opposite substantially parallel ends 28, 30.

A plurality of parallel conductive tracks 32, 34 are printed (or similarly formed) on the substrates 20, 22 respectively. On each substrate 20 or 22 the tracks 32 or 34 extend transversely across the substrate between the opposite edges 24, 26. The track pitch is substantially the same on each substrate. The tracks 34 on the substrate 22 may be substantially normal to the edges 24, 26 of that substrate, while the tracks 32 on the substrate 20 may be inclined at a very small angle to the normal so that the ends of the tracks 32 at the edge 24 are displaced by one track pitch period relative to the other ends of the tracks 32 at the edge 26. Each track 32, 34 terminates at one end in a respective conductive pad 36 and at the other end in a respective conductive pad 38.

The printed substrates 20, 22 may be manufactured using the technology used for making flexible printed multi-conductor cables for automobile electrics and other electrical equipment.

Next (Figures 4 and 5) the two substrates 20, 22 are disposed in register face-to-face with the tracks 32, 34 on IE 10 0423 the outside surfaces of the substrates, i.e. the tracks are separated by the thickness of both substrates (tracks 32 shown in broken-line outline). On the facing substrates the pads 36 on the substrate 20 are in register with the pads 36 on the substrate 22, and likewise the pads 38 on the substrate 20 are in register with the pads 38 on the substrate 22.

While in Figures 4 and 5 the substrates 20, 22 are arranged 10 with the tracks 32, 34 facing outwards, it will be understood that the substrates 20, 22 may be arranged in any suitable adjacent relationship, provided that the tracks 32, 34 are insulated from each other.

The substrates are joined along their opposite edges 24 and 26 by plated through-holes (vias) 40, Figure 5. The vias 40 contain electrically conductive material which connect each pad 36 on the substrate 20 with the corresponding aligned pad 36 on the substrate 22 and each pad 38 on the substrate 20 with the corresponding aligned pad 38 on the substrate 22. Thus a continuous electrical path is formed around the two substrates which can be regarded as a flat helix. Opposite ends of the path are connected respectively to terminals 42, 44 on the substrate 22.

The joining of the substrates 20, 22 may be strengthened by glue layers 48 (shown in dotted line outline in Figure 4) or a similar bonding mechanism disposed between the substrates in the areas of the vias 36, 38.

Next, the two substrates 20, 22 are deformed as shown in Figure 6, and a flexible strip 50 of ferromagnetic material is inserted into the gap between the substrates, as shown in cross section in Figure 7 (in Figure 7 the thickness of the strip 50 is - exaggerated). In a first embodiment, the strip 50 may extend substantially the full length of the substrates 20 and 22, i.e. substantially from one end 28 to the other end 30, or at least along the major part thereof between the tracks 32, 34, In a further embodiment, it will be understood that the strip 50 may be substantially longer than the length of the substrates 20, 22, such that the substrates 20, 22 are provided about a portion of the strip 50.

The net result is that a current transformer is created with the magnetic strip 50 between the substrates representing the magnetic core, and the continuous helical conductor formed by the tracks on the substrates, interconnected by the vias, representing a continuous coil winding around the core.

While the strip 50 of material is ferromagnetic in this embodiment, an alternative approach is to use a strip of non-magnetic material to provide similar structural characteristics but to employ an air core within the coil.

The sensor may normally be stocked flat, i.e. as shown in Figure 7. When selected for use, however, the entire structure can be wrapped round a conductor 52 whose current is to be sensed such that the tracks 32, 34 are at least approximately parallel to the conductor, Figure 8. The free ends of the structure may be joined together to form a continuous magnetic core ring. In the embodiment shown in Figure 8, the ends 28, 30 of the substrates overlap slightly. (It will be understood that in Figure 8 only a portion of the tracks 32, 34 are shown, for clarity IE 10 04 23 purposes.) However, in other embodiments the ends 28, 30 might not overlap, and in fact the sensor need not be wrapped fully around the conductor 52 (such an arrangement may result in a weaker output signal strength than the arrangement where the ends of the device overlap). The output from the sensor is available at the pads 42, 44 at the beginning and end points of the track representing the winding.

A layer of heat shrink tubing material may be provided about the sensor, which is then heated to seal the sensor. This may be done to provide improved protection of the sensor, and furthermore to provide extra rigidity and robustness.

In another embodiment, not shown, the two facing substrates 20, 22 are formed by folding a single substantially rectangular flexible sheet in half. In such a case the vias are only necessary along one edge, since along the folded edge the tracks can extend continuously across the fold from one substrate to the other.

In other embodiments the vias could be replaced by conductive clips which embrace the edges of the substrates.

Such clips would not only electrically connect the ends of the tracks, but may also serve to secure the edges of the substrates together.

The invention is not limited to the embodiments described herein which may be modified or varied without departing from the scope of the invention.

Claims (5)

Claims

1. A coil assembly for a current sensor comprising two flexible substrates of insulating material disposed face to 5 face and joined along opposite edges, and a plurality of conductive tracks extending across each substrate transversely between the opposite edges, wherein the ends of the tracks on one substrate are electrically connected to the ends of the tracks on the other substrate, wherein 10 the flexible substrates are arranged to receive a body of material sandwiched between the substrates, and wherein the conductive tracks are arranged in such a way as to form a continuous winding around the body to be received between the substrates.

2. A coil assembly as claimed in claim 1, wherein the plurality of conductive tracks extending across each substrate are parallel to one another. 20

3. A current sensor comprising the flexible coil assembly of any preceding claim and a body of material sandwiched between the substrates, the body being preferably a magnetic body. 25

4. , A current sensor as claimed in claim 3, wherein the body of material is a flexible body.

5. A current sensor as claimed in claim 3 or 4, wherein the substrates and body are provided in use around a 30 conductor whose current is to be sensed such that the tracks are at least approximately parallel to the conductor.