GB2154806A - Clip-on current measuring device - Google Patents

Abstract

A clip-on current measuring device for generating an output current signal representative of an alternating current passing through an electrical conductor onto which the device is clipped. The device comprises a multi-part core 9, 10 of magnetisable material which can be clipped around the conductor so as to define an aperture through which the conductor passes. A multi-part substantially short circuited secondary winding is support on the core, each of the parts of the secondary winding being a metal tube which supports a respective portion of a tertiary winding 7. The tertiary winding has a high impedance so that its output is in the form of a current signal. <IMAGE>

Description

SPECIFICATION Clip-on current measuring device The present invention relates to a clip-on device for generating a measuring output signal representative of an alternating current passing through a conductor onto which the device is clipped.

Electrical supplies are fed to a user's premises through a wattmeter or other meter device which provides a measure of the amount of energy consumed by the user. This requires a measurement of both the voltage of the supply and the current consumed. in conventional electronic wattmeters the current is measured using a transformer in the form of an annular magnetisable core through which the current carrying conductor is threaded. The current carrying conductor forms the transformer primary winding and a secondary winding is wound on the core. The secondary winding has a low impedance and a high capacity low ohmic value resistor is connected across the terminations of the winding. The resistor typically has a resistance of ten ohms so that the secondary winding is effectively short circuited.The voltage across the resistor provides a measurement output signal which is supplied to suitable metering instrumentation.

There are two major drawbacks with the known device. Firstly, it is difficult to ensure that the resistor has the desired resistance to a high degree of accuracy, and as the output signal is a function of this resistance the device can be inaccurate. Typically errors of the order of not greater than one percent are deemed "acceptable" by electricity supply authorities. Secondly, if the secondary winding is not shorted out by the resistor high and potentially lethal voltages are generated across the secondary winding. The electricity supply authorities, who own most meters, enforce very strict procedures to protect personnel from injury in the event of the secondary winding not being correctly shorted out but nevertheless accidents do still occur.

Swiss Patent No. 537085, which corresponds to British Patent No. 1 404 719, describes a current measuring transformer having an annular core supporting a short circuited secondary winding and a tertiary winding. The secondary winding is in the form of a conductive tube through which the core extends. A voltage is generated across the tertiary winding that is proportional in magnitude to a current flowing in a conductor that extends through the core. The core is permanently secured onto the conductor which is in the form of a bus-bar. As the secondary winding is permanently short circuited lethal voltages cannot be generated across it or the tertiary winding.

The transformer known from the above Swiss patent derives a voltage from the tertiary winding. In such an arrangement the temperature coefficient of the voltage will be of the order of 1 per cent per three degrees Celcius. Thus it is necessary to carefully compensate for temperature if accurate current measurements are to be obtained. Furthermore, the secondary and tertiary windings of the described arrangements are disposed at separate positions on the core. As a result the voltage derived from the tertiary winding is a function of the position of the current carrying conductor relative to the aperture in the core through which it is threaded.Therefore the relationship between the derived voltage and the current to be measured cannot be accurately predetermined unless the size and cross-sectional shape of the conductor and its position relative to the core can be precisely predetermined. This is a major problem, and one that cannot be solved if the transformer is to be used in a portable "clip-on" wattmeter which must be capable of being temporarily fixed on a wide range of different current carrying conductors.

A variety of other current transformers are described in U.S. Patent No. 3 584 299, U.S. Patent No. 1 955 317 and "IEEE Transactions on Parts, Hybrids and Packaging, vol.

PHP-11, no. 3, September 1 975 in an article by T.W. Moody, pages 221-225. Each of these documents refers to the provision of a shorted secondary winding, and the latter document refers to the use of a bifilar wire to form the secondary and tertiary windings to obtain constant coupling between the current carrying conductor and the secondary winding regardless of the orientation of the conductor.

There is however no description of a way to achieve this desirable feature in a core which can be split open as is necessary in a clip-on current measuring transformer.

It is an object of the present invention to provide a clip-on current measuring device which obviates or mitigates the above problems.

According to the present invention, there is provided a clip-on current measuring device for generating an output signal representative of an alternating current passing through an electrical conductor onto which the device is clipped, comprising a core of magnetisable material formed from a plurality of parts which may be secured together around a conductor so as to define an aperture through which the conductor passes, a substantially short circuited secondary winding wound on the core, and a tertiary winding wound on the core, characterised in that the tertiary winding provides the said output signal in the form of a current signal, the secondary winding comprises a plurality of elements each forming a single turn short circuited winding, the elements being supported on the parts of the core so as to be regularly spaced apart there around when the parts are secured together around the conductor, and the tertiary winding comprises a plurality of interconnected sections, each tertiary winding section being supported on a respective one secondary winding element.

Preferably the secondary winding is in the form of a plurality of tubes of circular or other cross-section through which the core parts are threaded. The tubes are of a conductive nonmagnetisable material such as aluminium or copper.

An embodiment of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic illustration of a conventional device for generating a measurement output voltage representative of the amplitude of an AC current; Figure 2 is a schematic illustration of a known device having a shorted secondary winding and a tertiary winding providing an output measurement voltage; and Figures 3 and 4 illustrate one configuration for a device according to the present invention, Fig. 3 being a section through the device in its operating position around a current carrying conductor and Fig. 4 being a simple end view of the device.

Referring to Fig. 1, a conventional device is illustrated which comprises an annular core 1 of high permeability magnetic material through which a conductor 2 is threaded. In practice the core 1 is often in two parts to enable it to be clamped around a conductor.

The conductor 2 forms a primary winding and a secondary winding 3 is wound onto the core. The winding 3 has an impedance of a few ohms and is effectively short circuited by a high capacity low impedance resistor 4.

When an alternating current passes through the conductors 2 the resultant magnetic flux which is established links with the secondary winding 3. This changing magnetic flux induces an electromotive force in each turn of the secondary winding, driving current through the resistor 4. If the resistor 4 is omitted high voltages are generated across the secondary winding 3 so that great care must be taken when installing such devices.

Assuming the resistor 4 is present however, a measurement output signal appears across terminals 5, which output is a voltage signal proportional to the resistance of the resistor 4 and the induced current. Clearly the accuracy of the output at terminals 5 is a function of the accuracy to which the resistance of the resistor 4 is known and the accuracy to which the induced current is a representation of the primary current. In practice the terminals 5 are connected to instrumentation which provides a measure of the current through conductor 2 on the assumption that the resistance of the resistor 4 is the nominal resistance and the induced current is an accurate representation of the primary current.It is known that resistors are used which may have resistances differing slightly from the nominal and that the magnetic materials linking the primary current and the induced current are non-linear. Such factors can be very significant financially particularly for users of large quantities of electrical energy such as industrial plant operators.

Referring now to Fig. 2, a known transformer construction having secondary and tertiary windings is illustrated. The core 1, conductor 2 and secondary winding 3 are the same as in Fig. 1, but the winding 3 is permanently short circuited by a direct electrical connection schematically represented by line 6. A tertiary winding 7 is also wound on the core 1 however and provides an output at terminal 8. This output is used in the known transformer to provide a voltage signal input to suitable measurement instrumentation.

The arrangement of Fig. 2 has four advantages over that of Fig. 1. Firstly the permanent short circuit of the secondary winding 3 prevents the generation of high voltages by either of windings 3 or 7. Secondly the accuracy of the measurement is not dependent upon the tolerance of a low ohmic value resistance. Thirdly, the magnetic materials linking the primary current and the induced current are operating at a much lower flux level and hence in a more linear manner.

Fourthly, the accuracy of the device is not significantly dependent upon the position of the conductor 2 relative to the core 1. The procedures of installers of the device can thus be relaxed and the accuracy of the device can be increased significantly.

Thus the device of Fig. 2 has several advantages over that of Fig. 1, but cannot be used in a clip-on meter as this core 1 is not split.

Figs. 3 and 4 illustrate schematically the features of one embodiment of the present invention. The core has a square cross-section and is in the form of two L-shaped parts 9 and 10 which are clamped together around the conductor 2. The tips of the core sections are shown spaced apart but in practice would be clamped together. The shorted secondary winding is in the form of four square-section tubular elements 11 of for example aluminium or any other electrically conductive non-magnetisable material, the tubes 11 being slipped over the limbs of the L-shaped core sections 9 and 10. Each of the tubes 11 supports a respective section of the tertiary winding 7 (Fig. 4). The regular distribution of the four elements of the secondary winding and of the four parts of the tertiary winding makes the device insensitive to variations in the position of the conductor 2 relative to the assembled core and yet the core can nevertheless be split to enable its use in a clip-on meter. Thus a compact, safe and accurate clip-on device can easily be assembled.

It will be appreciated that the core does not have to be in two L-shaped sections. Any suitable multi-part core structure and any suitable multi-element form for the secondary and tertiary windings may be selected. Furthermore, it is not necessary for the core and the tubes to have a square cross-section. They may for example have a circular or any other shaped section and the cross-section of the core need not match that of the tubes provided that the dimensions are such that the tubes can fit over the core.

In Fig. 4, the tertiary winding is shown as four suitably connected windings each of which is supported on a respective tube. Such an arrangement maximises protection against stray fields.

The secondary and tertiary windings preferably have substantially the same temperature coefficient of resistance. Alternatively if the secondary and tertiary windings are of different materials, e.g. an aluminium secondary and a copper tertiary, a suitable temperature compensating element may be provided to compensate for the difference between the coefficients.

It will be appreciated that the purpose of the secondary winding is to control the magnetic flux in the core and, in particular, to permit substantial reduction of that flux as compared to prior art constructions. Accordingly, it is possible to achieve this object by incomplete, but nevertheless "substantial" short-circuiting of the secondary winding, i.e.

the secondary winding has a very low ohmic value, typically less than 0.00001 ohms.

The impedance of the tertiary winding should be high, e.g. 2000 ohms, so that the output signal is a current rather than a voltage signal. In effect, the tertiary winding should be substantially short circuited by the instrumentation connected to the output 8. Such an arrangement results in the temperature coefficient being less than 0.1 per cent per degree Celcius, a significant improvement on the prior art devices which as described above have temperature coefficients of the order of 1.0 per cent per three dgrees Celcius. Even further temperature stability can however be obtained by providing a low ohmic value zero temperature coefficient resistor in series with the tertiary winding, the resistor having a resistance typically less than 1 5 per cent of the total tertiary winding resistance.

Claims (3)

1. A clip-on current measuring device for generating an output signal representative of an alternating current passing through an electrical conductor onto which the device is clipped, comprising a core of magnetisable material formed from a plurality of parts which may be secured together around a conductor so as to define an aperture through which the conductor passes, a substantially short circuited secondary winding wound on the core, and a tertiary winding wound on the core, characterised in that the tertiary winding provides the said output signal in the form of a current signal, the secondary winding comprises a plurality of elements each forming a single turn short circuited winding, the elements being supported on the parts of the core so as to be regularly spaced apart therearound when the parts are secured together around the conductor, and the tertiary winding comprises a plurality of interconnected sections, each tertiary winding section being supported on a respective one secondary winding element.

2. A device according to claim 1, wherein the secondary winding is in the form of a plurality of tubes through which the core parts are threaded.

3. A device according to claim 2, wherein the or each tube is of square cross section.