GB2259780A

GB2259780A - Sensing electric currents

Info

Publication number: GB2259780A
Application number: GB9120170A
Authority: GB
Inventors: Michael Barry Watson
Original assignee: Individual
Current assignee: Individual
Priority date: 1991-09-20
Filing date: 1991-09-20
Publication date: 1993-03-24
Also published as: GB9120170D0; ZA927213B

Abstract

A current, power or frequency sensor for a cable having spaced conductors (22, 24) carrying equal and opposite currents includes a current transformer having a high permeability member (10) defining a pair of jaws (102, 104 Figure 5), the transformer having secondary windings (16, 18) connected to indicating means, such as a current meter (108 Figure 4 not shown). The cable is placed between the jaws in a relatively low permeability part of the flux path of the transformer and magnetic flux is able to flow between the spaced conductors (22, 24) of the cable as in Figure 1. Induced currents in the secondary windings (16, 18) are additively combined (20) and applied to the meter. <IMAGE>

Description

ELECTRICAL CURRENT SENSITIVE TEST APPARATUS This invention relates to electrical current sensitive test apparatus which can be used to derive an indication or a reading of a parameter of an electrical current in a multi-core electrically conducting element. The invention is particularly, though not exclusively, applicable to current indicators and ammeters.

Meters, such as ammeters, and apparatus for indicating the presence of current in an electrically conductive element fall into two main categories. The contact type has leads which are used to establish electrical contact with the element through which measurement or monitoring of electrical current is required. The non-contact type has a probe which is placed in the vicinity of the conducting medium so that the magnetic field produced by the current can be used to derive a current reading.

However, in the case of non-contact type apparatus1 electrical cables, such as single phase supply cables, have insulated live and neutral wires inside a protective outer sheath. As a result, the magnetic fields resulting from the equal and opposite currents in the two wires substantially cancel out. Thus, when a non-contact or 'clamp' ammeter is used it is necessary to isolate an individual conductor somewhere in the cable to be embraced by the probe. Whereas this may be preferable alongside the need to break into the electrical circuit so as to connect a contact type meter, it still requires some level of disturbance of the cable to get at the single wire.

There is described in unpublished International Application No. PCT/GB91/00459 a coupling transformer for a pair of spaced electrical conductors bearing substantially equal and opposite currents, comprising a relatively high permeability member disposed in electromagnetic relationship with the conductor; the member, in use, defining a pair of flux paths and sharing a further portion, extending between the conductors, of relatively low permeability; and a pair of electrically connected secondary elements each also disposed in electromagnetic relationship with a portion of the flux path defined by the high permeability member.

It is stated that the transformer can be used to pick up or induce energy on a two or more core cable, such as a telephone cable, without making electrical contact with the cable provided the core conductors are spaced to permit the passage of flux between them. Thus, the invention provides a very simple tap into a multi-core cable without having to break the insulation or otherwise disturb the electrically conducting lines.

When the transformer is used to pick up energy, e.g.

signals, from the electrical conductors the secondary elements are arranged to provide antiphase outputs related to the current in the conductor. Conveniently, the secondary elements are connected together to provide an addition of the outputs. Commonly, the secondary elements are simply connected together so that the antiphase outputs are additively combined.

It has now been realised that this invention can be used to provide apparatus responsive to the current in a multi core cable which is of the non-contact type but does not require isolation of a single wire.

The cable coupling transformer enables current in twin or more core cables to be monitored anywhere along the length of cable through any non-ferrous sheath material.

According to the present invention there is provided a current responsive monitoring apparatus comprising a cable coupling transformer as described above, having a pair of jaws defining the high permeability member, and indicating means for indicating the voltage proportional to the current induced in the secondary elements which is related to the current in a multi core conductor extending through the jaws.

One of the jaws may define the portion(s) of the high permeability member in electromagnetic relationship with which the secondary elements are disposed. The articulated jaws then define the remainder of the high permeability part of the flux paths through the secondary elements.

One jaw may be swung sideways relative to the other or more conventionally articulated in order to allow a multi core cable to be placed in the relatively low permeability part of the flux path.

Preferably, the output of the secondary elements is rectified (either half or full wave) and possibly smoothed before being applied to indicating means, such as a moving coil current meter or threshold circuit and usual or audible alarm.

On the other hand, the apparatus may be used to measure frequency in which case the indicating means and any conditioning precircuitry are arranged to provide an output indicative of the frequency of the electrical current in the multi core cable.

Alternatively, the indicating means and any conditioning precircuitry can be arranged to indicate a reading of power. This assumes the supply voltage remains constant.

In any case, conditioning precircuitry may also include a scaling potentiometer in order to adjust the signal from the secondary elements to within a range acceptable to the remainder of the apparatus.

The apparatus may also include peak freeze circuitry for indicating peak current level when this circuitry is enabled.

In order to hold a cable in place the jaws may be provided with a resilient material such as foam rubber or other elastomeric substance. The jaws may also be sprung loaded in the closed position to hold the cable in place.

The invention can be put into practice in various ways one of which will now be described by way of example with reference to the accompanying drawings in which Fig.l is a schematic diagram of a cable coupling transformer; Fig.2(a) and (b) are a section and perspective view respectively of another embodiment of a cable coupling tranformer; Fig.3 is a section of a modified form of the transformer is Fig.2(a); Fig.4 is a plan view of a meter according to the invention; Fig.5 is a side view of the meter of Fig.4; and Fig.6 is a circuit diagram suitable for the meter of Fig.4.

It will be known to the skilled person that a current flowing in a conductor induces a magnetic flux in a ring of high permeability material surrounding the conductor. Furthermore, a pair of co-axial conductors carrying equal and opposite currents will lead to cancellation of the flux.

If the conductors are separated, a pair of opposite flux paths are defined by the ring and the lower permeability route passing between the two conductors.

The shared portion between the conductors carries a substantially smaller flux than would be created by a single conductor carrying the same current inducing the flux in the ring. However, the amount of flux is not negligible. It consists of the flux from both paths flowing in the same direction through this common portion between the conductors when the currents in the conductors are opposite.

Referring firstly to Figure 1, a cable coupling transformer is illustrated in which a high permeability, for example, steel, straight sided ring 10 is embraced on opposed limbs 12 and 14 by coils 16 and 18. The coils are wound in opposite senses about the opposed limbs. The lower ends of the coils are electrically connected together by a wire 20. The upper ends carry terminals A and B.

An electrical cable having two enamelled core conductors 22 and 24 passes through the ring. When equal alternating currents in antiphase are passed through the cable cores a magnetic flux is induced in the ring having flux paths of opposite senses (as indicated by the arrows) passing between the cores 22 and 24. This induces a voltage in each of the windings which are additively combined by the electrical connection constituted by the wire 20, such that a voltage proportional to the two equal and opposite currents in the conductors is produced between the terminals A and B.

In an alternative form, the windings 16 and 18 are replaced by lengths of bundles of wires in the opposed corners of a tunnel 10' having a rectangular section to either side of the cores 22 and 24. This form is illustrated in Figures 2a and 2b. The tunnel 10' comprises a channel portion 10a and a lid portion 10b.

In the embodiment shown in the figures the length of the secondary elements is constituted by straight parallel sides 16' and 18' of a coil 28. It is preferable that the connection portions 26 are disposed generally at 900 to the cable conductors in order that they do not receive an induced voltage themselves.

These connecting portions are embedded in a recess in the channel 10a to take them out of the path of the cable.

Preferably, the straight lengths of the coil in the tunnel 10' are embedded in a relatively magnetically inert substance, such as epoxy resin.

As shown in Fig.3 a further winding 29 could be added in the free space between the lid and the side walls.

This could be connected to increase the voltage proportional to the current in the conductor. However, the addition of this extra winding constitutes a further complication in assembling the device and connecting the two cores together.

Referring to Figures 4 and 5, a clamp ammeter comprises a housing 100 on which are mounted a fixed jaw 102 and a moving jaw 104 which is pivotable in the direction of the arrow in Fig.4. Also mounted on the housing 100 is the knob 106 of a manually adjustable scaling potentiometer and a moving coil current meter 108.

As will be seen in Figure 4, a twin core cable extends through a channel defined by the movable jaw 104. Each jaw is made of a magnetisable material of high permeability, such as steel or ferrite for higher frequency applications. The fixed jaw 102 is a flat plate element, whereas the moving jaw 104 defines a base 110 to the channel along which the cable extends and side pillars 112 around which the windings 16 and 18 are wound. In this embodiment the windings each consist of 3000 turns of enamelled copper wire. Of course, the flux paths can be set up without the provision of a closing jaw 102 across the channel defined by the jaw 104.

It will be appreciated by the skilled person that the relative positions of the conductors carrying equal and opposite currents is of importance for an accurate reading. However, very often all that is required is an indication of current, in which case it is found that the position of the multicore conductor within the low permeability path is considerably orientation tolerant.

On the other hand, it is also possible to dictate the orientation of a cable within the low permeability path by arranging for the jaws to clamp the cable and possibly by shaping the engaging surfaces of the jaws to prevent rotation of the cable once the jaws are closed.

The apparatus may also include a threshold circuit arranged to produce an output when the current from the secondary elements passes a particular threshold. In this case, the indicating means may include a visible or audible alarm such that a user may monitor current activity in the wire remotely from the position the test is carried out.

Referring now to Fig.6 an ammeter circuit is connected to the coils 16 and 18. The ends A and B of the coils on the pillars of the movable jaw 104 are connected to the ends of a winding of a scaling potentiometer 120. The wiper terminal of the potentiometer is connected through a capacitor C1 to the non-inverting input of an operational amplifier Al.

The output of the operational amplifier is connected to the input point of a diode bridge rectifier circuit D.

The opposite input point of the bridge rectifier is connected, through a resistor R1 and capacitor C2 in series, to ground potential. One side of a moving coil meter 122 is connected between a voltage supply rail +9V through a protecting diode D1, and to an output point on the bridge rectifier. The other side of the moving coil meter is connected to the opposite output point on the bridge rectifier. Voltage protection for the meter is also provided by a further diode D2 connected across the meter itself.

Feedback to the inverting input of the operational amplifier is provided by a connection of the inverting input thereof to between the diode bridge rectifier circuit and the smoothing resistor/capacitor pair.

The current created as a result of the voltage induced in the secondary elements is rectified and smoothed and then applied to the current meter to derive an indication of the current in the cable between the jaws.

In an alternative embodiment, the high permeability member defines a central stub in the channel. A winding may be wound around this such that the secondary elements are constituted by the flanks of the winding to either side of the stub, which flanks extend along the channel. The cable is then arranged to rest on the stub in the low permeability air gap.

The meter may also be provided with means for switching the parallel connection of the coils to a series connection. In this way the meter can be arranged to work as a conventional non-contact clamp ammeter.

In the case of a central coil wound on a stub in the channel, it is also possible to use this as a reference coil providing a datum about which the outputs of two side coils wound on the outer limbs are measured. An example of the use to which this might be put is in the determination of power factor by comparing the phase and amplitude of the voltage in each coil.

Claims

CLAIMS:

1. Current responsive monitoring apparatus comprising a cable coupling transformer as described, a pair of jaws defining the high permeability member thereof, and indicating means for indicating the voltage proportional to the current induced in the secondary elements of the transformer, which current is related to the current in a multi-core conductor extending through the jaws.

2. Apparatus as claimed in claim 1 in which one of the jaws defines the portion(s) of the high permeability member of the transformer in electromagnetic relationship with which the secondary elements are disposed.

3. Apparatus as claimed in claim 2 in which the jaws define the remainder of the high permeability part of the flux paths through the secondary elements.

4. Apparatus as claimed in claim 1, 2 or 3 in which the jaws are articulated such that one jaw is rotatable sideways across the other of the jaws to allow a multi core conductor to be placed in the relatively low permeability part of the flux path.

5. Apparatus as claimed in any of claims 1 to 4 in which the output of the secondary elements is rectified by rectifier means before application to the indicating means.

6. Apparatus as claimed in any of claims 1 to 5 in which the indicating means comprise an audible alarm or a moving coil current meter.

7. Apparatus as claimed in any of claims 1 to 5 in which the indicating means comprise a meter arranged to provide an output indicative of the frequency of the current in the multi core conductor or power in the said conductor.

8. Apparatus as claimed in any of claims 1 to 7 including a scaling potentiometer connected in the circuit to the indicating means operable to adjust the signal from the secondary elements to within the range of the indicating means.

9. Apparatus as claimed in any of claims 1 to 8 including peak freeze circuitry for indicating a peak current level.

10. Apparatus as claimed in any of claims 1 to 9 including a resilient bush mounted in the jaws to hold in place a multi core conductor passing therethrough.

11. Current responsive monitoring apparatus substantially as described herein with reference to the accompanying drawings.