WO2001040811A2

WO2001040811A2 - Screened core current sensor

Info

Publication number: WO2001040811A2
Application number: PCT/EP2000/011763
Authority: WO
Inventors: Kenneth Mcintosh Govan; Alexander Charles Geddes
Original assignee: Honeywell Control Systems Ltd.
Priority date: 1999-11-30
Filing date: 2000-11-25
Publication date: 2001-06-07
Also published as: GB9928246D0; AU2668301A; WO2001040811A3

Abstract

A null balance current sensor (1) enclosed by a shield (2, 3) made from a material with a high magnetic permeability. As a result of this shielding, the magnetic core (11) within the sensor can be made smaller than conventional devices reducing the cost of the sensor and the resistance of the secondary windings (13).

Description

SCREENED CORE CURRENT SENSOR The present invention relates to the field of current sensors .

One popular means for measuring electrical current is a null balance current sensor. This type of sensor comprises a toroidal magnetic core through which there passes a primary conductor that carries the current which is to be measured and which therefore induces a proportional magnetic field in the magnetic core. Around the magnetic core there are also provided secondary windings through which a balance current is passed that produces a magnetic field configured to oppose and cancel out that due to the primary conductor. The balance current is maintained dynamically at the correct value to keep the net magnetic field in the core at zero and a magnetic field sensor provides constant feedback. The output from the current sensor is the balance current which, in the above format, equals the input current divided by the number of secondary windings . The magnetic field sensor is typically a Hall Effect Sensor positioned in a gap in the toroid. The best results are achieved when the edges of the ja are parallel to the measuring element of the Hall Effect Sensor. The resistance of the secondary winding s kept as low as possible to maximise the measuring range of the device. The most costly component of this category of sensor is the magnetic core. As well as its function as a magnetic circuit, the core also shields the Hall Effect Sensor from the influence of external magnetic fields, improving the accuracy of the sensor. This shielding is most effective when the cross-section of the core is as large as possible relative to the Hall Effect Sensor and when the gap is as small and uniform as possible.

It would be desirable to reduce the cost of the sensor by making the core smaller, saving on the expense of the nickel-rich magnetic material. A further benefit of a small core is that the secondary windings would be shorter and therefore have a lower resistance. However, there is a problem which has previously prevented this from being done.

Reducing the size of the core reduces the shielding effect that the core has on the Hall Effect Sensor, diminishing the performance of the device and making it susceptible to stray magnetic fields . These stray magnetic fields create an offset to the measured current. The system accuracy can be improved by subtracting the offset current from the output signal but if the shielding is too low and the offset too large, then the accuracy of the sensor is reduced.

It is an aim of the present invention to find a way of reducing the size of the core without reducing the performance of the device, thereby cutting the cost of the sensor and reducing the resistance of the secondary windings .

The reproducibility of conventional null balance current sensors is limited by the way in which their magnetic cores are manufactured. Typically, these cores are made by the tape winding technique known to those skilled in the art and are then gapped and subjected to heat treatment to give the desired magnetic properties. This technique has certain deficiencies: firstly, it is hard to control the width and uniformity of the gap size, making it difficult to provide reproducible sensors,- secondly, burrs created during the cutting process reduce the magnetic performance of the core . It is therefore a further aim of the present invention to provide a current sensor which can be more reproducibly manufactured, with particular attention to the regularity and reproducible size of the gap in the core . According to the present invention there is provided a current measuring device for measuring a current supplied thereto, the device comprising a magnetic circuit, a primary conductor which carries the current supplied to the device and which passes through the magnetic circuit and so induces a magnetic field in the magnetic circuit, secondary windings around the magnetic circuit which carry a balance current and which therefore induce a magnetic field in the magnetic circuit, a magnetic field measuring means for measuring the net magnetic field in the magnetic circuit and a control means for adjusting the balance current so as to minimise the net magnetic field in the magnetic circuit, wherein the magnetic circuit is contained within a shield against external electromagnetic fields.

Preferably, the magnetic circuit comprises a toroidal magnetic core.

More preferably, the toroidal magnetic core has a gap.

The magnetic field measuring means may be a Hall Effect Sensor positioned in the gap.

Typically, the toroidal magnetic core will be laminated . Preferably, the shield comprises an aperture through which connecting wires to the Hall Effect Sensor can pass .

Preferably, the shield is formed from two separate corresponding parts . More preferably, the shield is the shape of the outside surface of a toroid and the two separate corresponding parts are the separate parts formed when the outside surface of a toroid is cut into two equal parts by a plane orthogonal to the axis of the toroid. Preferably, the shield comprises a material with a high magnetic permeability.

More preferably, the screen comprises a material with a Nickel content greater than 76%.

Most preferably, the screen is made from Mu Metal^®. The invention will now be described with reference to the following drawings in which:

Figure 1 shows an exploded view of a shielded toroid assembly; Figure 2 shows a cross -section through a wrapped toroidal core,-

Figure 3 shows a plan view of a wrapped toroidal core ,- and Figure 4 shows a high scale plan view of a gap provided in a toroidal magnetic core.

The present invention provides a shielded toroid assembly for use with a null balance current sensor. Figure 1 shows an exploded view of a shielded toroid assembly 1 which comprises a wrapped toroidal core 10 and a shield comprising two end caps 2, 3 made from a high permeability soft magnetic material.

Figure 2 shows a cross -section through the wrapped toroidal core 10 and Figure 3 shows a plan view of the wrapped toroidal core 10. The centre of the wrapped toroidal core 10 is a laminate consisting of a plurality of magnetic sheets 11 held in a conventional manner in a plastic core case and cover 12 about which are wrapped the secondary windings 13. The plastic core case comprises a hollow finger 14 which allows access to the Hall Effect Sensor and there is a corresponding aperture 4 in the magnetic shield to allow fitting of the Hall Effect Sensor. Clips 15 hold the plastic core case and cover together. Figure 4 shows a high scale plan view of the gap in the toroid where the Hall Effect Sensor 18 is fitted. The Hall Effect Sensor 18 is fitted between the two end faces 16, 17. The laminations in the toroidal core are ideally constructed from a high permeability soft magnetic material, such as an alloy with a high Nickel content. The preferred material is sold under the brand Mu Metal^® and has a Nickel content around 77%. Individual laminations can be manufactured in a more reproducible fashion than cores made by the tape winding technique and can be accurately fitted together and held in a plastic case. Therefore, this type of core leads to better magnetic performance and smoother, better aligned end faces than with tape wound cores and so a more reproducible current sensor.

The end caps 2, 3 are also constructed from a high permeability soft magnetic material, such as an alloy with a high Nickel content . The preferred material is also the brand Mu Metal^®.

This magnetic shield acts to reduce the influence of external magnetic fields on the Hall Effect Sensor, improving the accuracy for a given size and so allowing the core to be smaller.

The following experiment summarises the results of an experiment into the effectiveness of this construction. A toroidal current sensor according to the present invention was placed a given distance away from a magnetic source. The offset current, i.e. the blank current output when there is no current supplied to the current sensor, was measured with and without the magnetic shield. During these experiments the magnetic field was generated by a 800A dc current flowing through a conductor. The conductor was positioned close to the various faces of the sensor housing. The measurements were taken in the worst case orientation i.e. the external face of the sensor housing closest to the Hall effect/magnetic field sensor.

Distance Offset / μA

/mm shielded unshielded 0 107 1778

5 90 1331

10 80 1126

15 72 958

20 67 809 25 64 697

30 61 622

35 57 536

40 56 478

45 54 432 These results show that providing the magnetic screening leads to a substantial drop in offset current. As a result, the core may be made smaller for a given performance. Due to the smaller size core, the device is cheaper to manufacture and needs a shorter length of wire for the secondary windings, reducing their total resistance and so increasing the measuring range of the device .

Although in the example described the magnetic field measurement is provided by a Hall Effect Sensor it is envisaged that other forms of magnetic field sensor may also be used.

Further modifications and improvements may be incorporated without departing from the scope of the invention herein intended.

Claims

C AIMS

1 . A current measuring device for measuring a current supplied thereto, the device comprising a magnetic circuit, a primary conductor which carries the current supplied to the device and which passes through the magnetic circuit and so induces a magnetic field in the magnetic circuit, secondary windings around the magnetic circuit which carry a balance current and which therefore induce a magnetic field in the magnetic circuit, a magnetic field measuring means for measuring the net magnetic field in the magnetic circuit and a control means for adjusting the balance current so as to minimise the net magnetic field in the magnetic circuit, wherein the magnetic circuit is contained within a shield against external electromagnetic fields .

2. A current measuring device as claimed in claim 1 wherein the magnetic circuit comprises a toroidal magnetic core.

3. A current measuring device as claimed in any preceding claim wherein the toroidal magnetic core has a gap.

4. A current measuring device as claimed in claim 3 wherein the magnetic field measuring means is a Hall Effect Sensor positioned in the gap.

5. A current measuring device as claimed in any preceding claim wherein the toroidal magnetic core is laminated .

6. A current measuring device as claimed in any preceding claim wherein the shield comprises an aperture through which connecting wires to the Hall Effect Sensor can pass .

7. A current measuring device as claimed in any preceding claim wherein the shield is formed from two separate corresponding parts .

8. A current measuring device as claimed in claim 7 wherein the shield is the shape of the outside surface of a toroid and the two separate corresponding parts are the separate parts formed when the outside surface of a toroid is cut into two equal parts by a plane orthogonal to the axis of the toroid.

9. A current measuring device as claimed in any preceding claim wherein the shield comprises a material with a high magnetic permeability.

10. A current measuring device as claimed in claim 9 wherein the screen comprises a material with a Nickel content greater than 75% .

11. A current measuring device as claimed in claim 9 wherein the screen is made from Mu Metal^®.