GB2088568A - A transducer for an alternating current measuring device - Google Patents

Abstract

A flexible former 10 has embedded therein a substantially non-extensible core such as a wire 12, extending along the whole length of the former, which is preferably cylindrical with the non-extensible element extending along the axis. A conductor 13 is helically wound around the former with evenly spaced turns to form a coil. The transducer thus forms a flexible element which can be bent around a current conductor to form one or more turns to constitute the coil of a current measuring device. <IMAGE>

Description

SPECIFICATION Alternating current measurement This invention relates to the measurement of alternating current, including low frequency currents.

It is well-known to measure an alternating current in a conductor by means of an iron-cored current transformer, usually in the form of a toroid arranged around the conductor in which the current is to be measured. Such devices can be large and heavy and cannot be used if there is a strong magnetic field due to any sources other than the current to be measured.

It is also known (see for example our Specification No. 2034487A) to use a toroidal conductor with a winding on a non-magnetic formed. Such a device forms a transducer which gives an output from the coil depending on the rate of change of current to be measured. Hence this output has to be applied to an integrator. In order to obtain high accuracy, the winding has to have evenly spaced turns, each enclosing the same area.

The present invention is directed to an improved form of current measuring device and of a transducer for such a device making use of a coil wound on a support for putting around a conductor carrying an alternating current.

According to one aspect of the present invention, a transducer for a current measuring device comprises a flexible former in which there is embedded at least one substantially non-extensible element extending along the whole length of the former, and a conductor helically wound around the former with evenly spaced turns to form a coil. The former, which is usually cylindrical, is flexible and thus may be bent round into a loop, e.g. to form a toroid, and hence this transducer can be bent around a currentcarrying conductor to form a coil for a current measuring device. The non-extensible element through the former serves to maintain the even spacing of the turns of the coil despite the bending of the former into the shape for example of a toroid.

A very particular advantage of this type of transducer however is that the former may be made sufficiently long to be put several times around a current carrying conductor. Ideally the two ends of the coil should be butted together leaving no resultant gap in the winding. However if the ends of the former are overlapped so that the ends of the winding lie alongside each other, there is only a very small loss of accuracy, due to the ends of the winding being displaced axially relative to each other; this loss of accuracy is more than compensated by the absence of a circumferential gap which is almost inevitably a result of attempting to abut the ends of a coil together. If the current carrying conductor, to which the transducer is applied, is straight in the region of the transducer, an axial displacement of the ends of the winding would cause no error at all.

It will be seen that using a flexible coil of this nature, it is readily possible to make measurements on conductors without having to breakthe conductor in order to instal the transducer. The flexible transducer coil can be wound around the conductor and the ends thereof then connected to the measuring apparatus. The flexible coil furthermore facilitates the making of measurements in awkward positions or on unusually shaped conductors, for example, for measuring current flow in the framework of electrical machinery. Such a transducer may be used for the measurement of very large currents.

It may be used to measure currents in conductors of very large diameter or on high voltage conductors where a large gap is needed between the conductor and the transducer.

The flexible material on which the coil is wound is preferably a resilient material and the winding is applied tightly so that the resilient material is compressed by the winding. Thus the diameter of the former is constricted. This helps to tighten the former onto the aforementioned non-extensible element so that the latter is gripped. As an alternative the former could be bonded to the non-extensible element in a previous process such as co-extrusion.

The former may be made for example of a natural or artificial rubber or of a resilient plastics material.

The aforementioned inextensible element conveniently is a single core axially located in the former.

The core may be made for example of glass fibre or a substantially inextensible plastics core. Very conveniently however a metal wire or braid is employed.

Such a metal wire or braid can act as a return electrical conductor from one end of the coil. This enables the tow leads carrying the output from the coil to be brought out at one end leaving the other end free and thereby facilitating the mounting of the transducer. It is also advantageous in that it reduces interference from conductors adjacent to but not threading the transducer and other stray magnetic fields.

The inextensible element may comprise a combination of two or more different materials.

The aforementioned inextensible element may, if desired, extend beyond the former at one or both ends and hence may be used for attaching the transducer in position around the conductor being measured.

If the winding is put on tightly, constructing the former as described above, during the winding process, the winding produces a cone-line surface on the former at the end of which the winding is being put on. This helps to guide successive turns into position and contributes therefore to ensuring that the coil is ciose-wound therefore of uniform turns density along its length. Very convenientiy the uniform spacing of the turns is obtained by locating the turns closely adjacent one another.

Provided the former is constricted by the winding and is resilient, it is effectively pre-stressed with a large pressure acting radially outwards which is restrained by the winding. This pressure ensures that, provided the winding itself does not stretch, the cross-sectional area of the former and hence of the coil remains circular and constant even when the transducer assembly is flexed. It also ensures that if, for any reason, the transducer is inadvertently crushed, it will tend to return to its correct shape.

The invention also includes within its scope a current measuring device comprising a transducer as described above with the coil coupled to an integrator feeding an indicator or other output responsive device.

In the following description, reference will be made to the accompanying drawing in which Figure 1 illustrates a transducer for a current measuring device constituting one embodiment of the invention; Figure 2 is a diagram illustrating the winding being applied to one end of the transducer; and Figures 3, 4,5and 6showfourdifferentarrange- ments for applying the transducer of Figure 1 to a current carrying conductor for measuring alternating current flowing through that conductor.

Referring to Figure 1 there is shown a former comprising a sheath 10 of flexible resilient material, such as rubber or a resilient plastics material.

Extending centrally through the sheath is a core 12 of flexible but substantially inelastic material, in other words a core which is substantially inextensible. This core 12 may be made of glass fibre or a substantially inextensible plastics material such as a terylene thread but conveniently is a metal wire or braid. The core 12 serves to prevent any extension of the sheath 10 by tensile loads. The magnitude of such loads would in general be quite small and the core may, in some cases, be formed of a relatively weak metal such as lead which would have the property of retaining the former in its bent shape after it has been put around a current carrying conductor. If the core 12 is not electrically conductive, it may in some cases be desirable to provide also a conductor extending through the former to act as a return electrical conductor from one end of the coil.

A conductive wire 13 to form the coil 14 is helically wound around the former with the successive turns forming a single layer of closely adjacent turns. As shown in Figure 2, the conducting wire 13 forming the coil is put on undertension so as to constrictthe former material. Referring to Figure 2, it will be seen that the diameter of the portion 15 of the sheath 10 around which the coil 14 has been wound is smaller than that of the remaining portion 16 of the sheath.

The tight winding of the wire onto the former causes the sheath to bind onto and grip the central core thereby preventing any slip relative to one another.

Thus, after the winding has been completed, the central core 12 will effectively prevent the length of the winding from changing. The constriction of the resilient material of the sheath produces a cone-like surface in the region 16 where the winding is being put on. This surface helps to guide successive turns into position and contributes also to ensuring that the coil is closely wound and therefore of a uniform turns density along its length. A constriction of the former also prestresses the sheath with a large pressure so ensuring that, provided the winding itself does not stretch, the cross-sectional area of the former remains circular and constant even when the coil is flexed.

In the simplest form, such a transducer may be applied around a conductor 20 carrying an alternating current to be measured, as shown in Figure 3.

The two ends of the coil 14 overlap slightly so that they lie alongside each other; in other words instead of using a toroid as in known devices, the coil is wrapped as a helix around the conductor 20. This enables a sheath lotto be used having a core 12 which extends beyond the ends of the winding on the former thereby facilitating the fixing of the transducer in position. As shown in Figure 4 it is possible to wrap the transducer many times to form a multi-turn helix 22 around the current-carrying conductor. This not only provides increased sensitiv its of the transducer but also tends to average out any imperfections in the winding thereby improving the ability of the transducer to reject stray magnetic fields.

The use of a composite former providing flexibility is of substantial practical advantage. The core 12 prevents stretching and hence permits of bending the transducer without stretching and hence loss of accuracy. Typically a transducer of the kind described above can be bent around a current conductor of a diameter which might be less than twice the diameter of the transducer. The exact route taken by the transducer coil when it encircles the conductor being measured is not critical. In particular it need not fit closely around the conductor or follow a circular path. Successive wraps of the transducer need not be of the same size. The important requirement is that the ends of the winding are held in the same circumferential position on the conductor.If the ends are not in the same circumferential position, the transducer can still be used to measure current but is more liable to interference from stray magnetic fields. Axial displacement from one another of the ends of the winding as shown in Figures 3 and 4 will not cause any loss of accuracy if the current-carrying conductor is straight in the region where the transducer is applied.

In cases where the conductor is not straight or where there is a risk of interference from stray magnetic fields a second layer can be wrapped over the first so thatthe ends ofthewinding are brought as closely as possible to the same position as shown in Figure 6 where two ends 40, 41 of a winding 42 are located close together when the winding is wrapped around a conductor 43.

As an alternative to wrapping the transducer directly onto a conductor, it may, in some cases, be permanently coiled and any conductor in which the current is to be measured is threaded through the toroidal coil. It could for example be wrapped permanently on a piece of tubing and might be potted in a resin or other setting material. Such an arrangement behaves substantially like a conventional toroidal transducer on a rigid former but, by using the present invention, a much simpler method of manufacturing is possible, particularly for small transducers.

In some cases, instead of winding the whole length of the former helically around the current conductor 20, as shown in Figures 3 and 4, it is convenient to bring the two ends of the transducer together as shown in Figure 5, the two ends 23, 24 of the coil extending outwardly parallel to one another.

Such an arrangement, which is referred to as a hairpin mounting, is of particular advantage for many types of temporary mounting of transducers on a conductor for current measuring purposes It will be understood that multiple turns around the current conductor may be used with the ends arranged in this way.

In a current measuring device the two ends of the coil are connected, as shown in Figure 5, to an integrator 35, the output of which is applied to a voltage indicator 36 or other voltage responsive device such as a recorder, alarm system or the like.

Using an electrically conductive core, it is possible, as is shown for example in Figure 5, to make use of this core as a return lead from one end of the transducer so thereby enabling the connections to the integrator to be effected at one end only of the transducer.

Claims (22)

1. A transducer for a current measuring device comprising a flexible former in which there is embedded at least one substantially non-extensible element extending along the whole length of the former, and a conductor helically wound around the former with evenly spaced turns to form a coil.

2. Atransducer as claimed in claim 1 wherein the former is bent round into a loop.

3. A transducer as claimed in claim 2 wherein the two ends of the former are butted together leaving no resultant gap in the winding.

4. A transducer as claimed in claim 2 wherein the two ends of the former are overlapped so that they lie alongside each other.

5. A transducer as claimed in claim 4 wherein the former is wrapped several times around a conductive element carrying the current to be measured.

6. A transducer as claimed in any of the preceding claims wherein the former is cylindrical.

7. Atransducer as claimed in any of the preceding claims wherein the conductor is wrapped on a tube for putting over a current-carrying element carrying the current to be measured.

8. A transducer as claimed in claim 7 wherein the former and conductor are potted in resin or other setting material.

9. Atransducer as claimed in any of the preceding claims wherein the flexible material on which the coil is wound is a resilient material with the winding applied tightly so that the resilient material is compressed by the winding.

10. A transducer as claimed in claim 9 wherein the former is a natural or artificial rubber.

11. Atransducer as claimed in claim 9 wherein the former is a resilient plastics material.

12. Atransducer as claimed in any of the preceding claims wherein said inextensible element comprises a single core axially located in the former.

13. A transducer as claimed in any of the preceding claims wherein said inextensible element comprises glass fibre.

14. A transducer as claimed in any of claims 1 to 12 wherein said inextensibie element comprises a substantially inextensible plastics core.

15. A transducer as claimed in any of claims 1 to 12 wherein said inextensible element is a metal wire or braid.

16. Atransducer as claimed in claim 15 wherein said metal wire or braid is arranged as a return electrical conductor from one end of the coil.

17. A transducer as claimed in any of the preceding claims wherein said inextensible element comprises a combination of two or more different materials.

18. Atransduceras claimed in any of the preceding claims wherein said inextensible element extends beyond the former at one or both ends.

19. A transducer as claimed in any of the preceding claims wherein the turns are closely adjacent.

20. A current measuring device comprising a transducer as claimed in any of the preceding claims with the coil coupled to an integrator feeding an indicator or other output responsive device.

21. Atransducerfor a current measuring device substantially as hereinbefore described with reference to the accompanying drawings.

22. Acurrent measuring device substantially as hereinbefore described with reference to the accompanying drawings.