WO1999009566A1

WO1999009566A1 - Magnetic flux concentration device

Info

Publication number: WO1999009566A1
Application number: PCT/GB1998/002492
Authority: WO
Inventors: Roger Francis Golder; Christopher Davies
Original assignee: Cambridge Consultants Limited
Priority date: 1997-08-20
Filing date: 1998-08-19
Publication date: 1999-02-25
Also published as: GB9717668D0

Abstract

A magnetic flux concentrating device made of a single piece of magnetically permeable material. The device comprises a continuous outer wall (2) surrounding an aperture (3) extending through the device. A dividing wall (4) is magnetically coupled with the outer wall and extends across the aperture (3) between opposed portions of the outer wall to divide the aperture into a pair of subsidiary apertures (8, 9), the dividing wall defining a gap (5). Magnetic flux generated by an electric current carried in use on a conductor passing through the subsidiary apertures (8, 9) is concentrated in the gap (5).

Description

MAGNETIC FLUX CONCENTRATION DEVICE

The invention is related to a magnetic flux concentration device, for example for use with a magnetic flux detector such as a Hall effect device.

Magnetic flux concentration devices are used in a variety of applications to concentrate magnetic flux for purposes of detection and the like. A typical example is an electricity meter which includes a magneto/electric element which responds to an external magnetic field to generate corresponding electrical output signals. If the magnetic field is generated by a line carrying a current to be monitored, then the output from the magneto/electric element will be proportional to the product of a bias current applied to the magneto/electric element which can be derived from line voltage and the applied field, derived from line current, and thus is proportional to power.

Not only is it important to concentrate magnetic flux, but it is also important to shield the magneto/electric element from external magnetic fields, provide a flux level which is substantially independent of other effects such as minor variations in the relative position of the conductor, magnetic field sensor and flux concentrating element, and imperfections in the flux path, such as variation in the size of the air gap in the flux path.

It has been proposed in the past to house the magneto/electric element within a "box" type flux guide with a pair of plates held within and insulated from the box to define a gap, a Hall sensor or the like being located within the central gap. This arrangement is complex to construct, achieves insufficient flux concentration, and is not an efficient screen.

In accordance with a first aspect of the present invention, a magnetic flux concentrating device made of a single piece of magnetically permeable conductive material comprises a continuous outer wall surrounding an aperture extending through the device; and a dividing wall magnetically coupled with the outer wall and extending across the aperture between opposed portions of the outer wall to divide the aperture into a pair of subsidiary apertures, the dividing wall defining a gap, whereby magnetic flux generated by an electric current carried in use on a conductor passing through the subsidiary apertures is concentrated in the gap.

This device significantly improves upon the known devices by increasing the amount of flux concentration within the gap and more efficiently screening the gap from external magnetic fields. This comes about because of the single piece nature of the device and the resulting continuous magnetic coupling of the outer and dividing walls together with their geometric arrangement. Further improvement in screening can be obtained by maximising, within acceptable constraints, the ratio of the cross- sectional area of the dividing wall in the Z projection (that is the direction extending across the gap and which approximates to the flux intercepted from a field in this dimension) to the magnetic reluctance of the adjacent, side portions of the outer wall .

Typically, the device is sintered from a suitable material or constructed from a stack of laminates. The one piece construction is considerably cheaper than a multipart construction and overcomes several problems of multipart constructions. For example, the separate parts of a multipart construction must have their mating faces ground as flat as possible to obtain good magnetic continuity and low reluctance while the glue used to join the parts needs to have a very strong adhesive effect and a carefully controlled composition. If the glue composition changes, for example by the inclusion of particulates and the like, this will vary the quality of the join between devices which is undesirable. The outer wall will typically have a rectangular form but other shapes including a circular wall or a polygonal wall are possible. In general, a single dividing wall will be provided which would typically bisect the aperture into two substantially congruent subsidiary apertures. However, in other applications, more than one dividing wall may be provided with corresponding gaps being defined. Conveniently, the gap is defined between the end of the dividing wall and the outer wall. This B-core reduces manufacturing complexity. Also, sensitivity variation after assembly with electrical conductors is eliminated. As has already been mentioned, the magnetic flux concentrating device is particularly suited for use with a magnetic flux monitor which includes a magnetic flux detector within the gap and an electrical conductor looped through the subsidiary apertures . The detector can be any known magneto/electric detector such as a Hall effect device.

If the monitor is to be used in an electricity meter, the output signal from the detector will be fed to a processing device for integrating the output over time.

Since the device is constructed in the form of a single piece, it is necessary to thread the current conductor through the subsidiary apertures. This could be done by threading the conductor through one subsidiary aperture and then looping it around and back through the other. However, in the preferred approach, a prelooped conductor is passed through the gap and then moved transversely away from the gap so as to extend about the dividing wall. Subsequently, a magnetic flux detector can be located in the gap. This provides a very simple way in which to construct a magnetic flux monitor using the new flux concentration device and allows the gap to be minimised in height. Typically, for example, the height of the gap will be less than the combined thickness of the magnetic flux detector and electrical conductor.

This leads to a second aspect of the invention which provides a method of constructing a magnetic flux monitor, the method comprising providing a magnetic flux concentrating device made of a magnetically permeable material the device comprising a continuous outer wall surrounding an aperture extending through the device, and a dividing wall magnetically coupled with the outer wall and extending across the aperture between opposed portions of the outer wall to divide the aperture into a pair of subsidiary apertures, the dividing wall defining a gap; passing a looped electrical conductor through the gap and then displacing the electrical conductor relative to the dividing wall out of alignment with the gap; and thereafter locating a magnetic flux detector in the gap.

This method may be used with magnetic flux concentrating devices made from a number of parts which have been preassembled together but is particularly suitable for use with devices according to the first aspect of the invention.

Some examples of magnetic flux concentrating devices and an electricity meter according to the invention will now be described with reference to the accompanying drawings, in which: - Figure la is a perspective view of a first example of a concentrating device;

Figure lb is a perspective view of a second example of a concentrating device;

Figure 2a is a section taken along the line X-X in Figure 2b;

Figure 2b is a view in the Z direction of Figure 2c showing part of the first example of a concentrating device and other components of an electricity meter;

Figure 2c is a section through the B-core of Figure 2b and assembled components; and

Figure 3 is a schematic, block circuit diagram showing the meter connected to an AC supply.

The magnetic flux concentrating device shown in Figure la is in the form of a B-shaped member or B-core 1 formed as a single piece pressed and sintered ferrite. The core has a continuous outer wall 2 having a rectangular shape and defining an aperture 3 extending through it. A dividing wall 4 formed integrally with the outer wall 2 projects downwardly within the aperture 3 towards an opposite part of the outer wall, stopping short of that part so as to define a gap 5. The dividing wall 4 divides the aperture 3 into two congruent subsidiary apertures 8,9. The device in one example has a width of 15 -20mm, a depth D of about 5mm and a height H of about 8mm. Typically, the projection length "h" of the dividing wall 4 is about 3.3mm and the height of the gap 5 "g" is about 1.0mm. Figure lb illustrates a second example of a magnetic flux concentrator which is similar to that shown in Figure la except that the dividing wall 4 is formed in two parts 4A,4B projecting from opposite surfaces of the outer wall 2. Since the core 1 is preformed as a single piece unit, it cannot be constructed around a conductor. When constructing the meter, therefore, a U-shaped conductor 6 is provided, typically made of copper, the conductor having a reduced thickness section 7 (Figures 2a and 2b) which can fit within the gap 5. The conductor is then pushed through the aperture 3 with the reduced thickness section 7 within the gap 5 until the bight of the U extends beyond the wall 4. As can be seen in Figure 2c, the conductor 6 can then be dropped down around the wall 4 so as to be fully accommodated within the subsidiary apertures 8,9 defined between the wall 4 and outer wall 2. A printed circuit board 10 carrying a Hall effect device 11 is then inserted into the aperture 3 with the Hall effect device 11 aligned with and received in the gap 5. The components can then be secured in place in a conventional manner, for example using a suitable adhesive or the like. It will be noted that the height of the gap "g" is significantly less than the combined thickness of the device 11 and the conductor reduced thickness section 7. When a current passes along the conductor 6, a magnetic field will be generated around each section of the conductor, the components of those magnetic fields within the wall 4 being in the same sense so that the wall 4 concentrates magnetic flux into the Hall effect device 11. The height of the gap "g" can be minimised when the threading technique described above is used to assemble the core with the conductor and Hall effect device thus enabling magnetic flux concentration within the gap 5 to be maximised.

The device 11 is shielded from external magnetic fields by the outer wall 2. Figure 3 illustrates a typical circuit for an electricity meter making use of the monitor arrangement shown in Figure 2. A pair of supply lines 20, 21 carry an AC current from a supply 22 to a load 23. The Hall effect device 11 is supplied with a bias current via lines 24, 25 connected to the lines 20, 21 via a resistor 26. The line 21 is coupled with the conductor 6. Output signals from the Hall effect device 11 are fed to a differential amplifier 27, typically mounted on the PCB 10, the output from the differential amplifier being fed to a processor 28 also mounted on the PCB 10.

The processor 28 integrates the output signal from the differential amplifier 27. Since the current in the Hall effect device 11 is based on the supply voltage and the magnetic field to which the device is exposed is related to the supply current, the Hall voltage sensed by the differential amplifier 27 is directly related to the power supplied to the load 23. This can therefore simply be integrated to provide indication of power usage.

It will be appreciated that no winding of high current conductors is necessary and it has been found that once assembled, the sensor is tolerant of minor changes in the relative position of the core, conductor and sensor/PCB assembly. To ensure that the gap 5 is the dominant reluctance and that the ferrite characteristics do not affect the transfer function, μ should exceed 4000 over the whole working range.

Claims

1. A magnetic flux concentrating device made of a single piece of magnetically permeable material, the device comprising a continuous outer wall surrounding an aperture extending through the device; and a dividing wall magnetically coupled with the outer wall and extending across the aperture between opposed portions of the outer wall to divide the aperture into a pair of subsidiary apertures, the dividing wall defining a gap, whereby magnetic flux generated by an electric current carried in use on a conductor passing through the subsidiary apertures is concentrated in the gap.

2. A device according to claim 1, wherein the outer wall is substantially rectangular.

3. A device according to claim 1 or claim 2, wherein a single dividing wall is provided.

4. A device according to claim 3, wherein the dividing wall divides the aperture into a pair of substantially congruent subsidiary apertures.

5. A device according to any of the preceding claims, wherein the gap is defined between an end of the dividing wall and a facing surface of the outer wall.

6. A device according to any of the preceding claims, wherein the magnetically permeable material is ferrite.

7. A device according to any of the preceding claims, within the gap has a height of about 1mm.

8. A magnetic flux monitor comprising a magnetic flux concentrating device according to any of the preceding claims; a magnetic flux detector located within the gap; and an electrical conductor looped through the subsidiary apertures .

9. A monitor according to claim 8, wherein the gap thickness is less than the combined thickness of the magnetic flux detector and the electrical conductor.

10. A monitor according to claim 8 or claim 9, wherein the magnetic flux detector comprises a magneto/electric element .

11. A monitor according to claim 10, wherein the detector comprises a Hall effect device.

12. An electricity meter comprising a magnetic flux monitor according to any of claims 8 to 11, the electrical conductor being connected in use to or forming part of an electricity supply line to be monitored; the magneto/electrical element being supplied with a bias current from the electricity supply lines in use; and a processing device for integrating the output of the detector with time.

13. A method of constructing a magnetic flux monitor, the method comprising providing a magnetic flux concentrating device made of a magnetically permeable material the device comprising a continuous outer wall surrounding an aperture extending through the device, and a dividing wall magnetically coupled with the outer wall and extending across the aperture between opposed portions of the outer wall to divide the aperture into a pair of subsidiary apertures, the dividing wall defining a gap; passing a looped electrical conductor through the gap and then displacing the electrical conductor relative to the dividing wall out of alignment with the gap; and thereafter locating a magnetic flux detector in the gap.

14. A method according to claim 13, wherein the magnetic flux concentrating device is constructed according to any of claims 1 to 7.

15. A method according to claim 13 or claim 14, wherein the gap thickness is less than the combined thickness of the magnetic flux detector and the electrical conductor.