EP2761309A1

EP2761309A1 - Contactless current sensor

Info

Publication number: EP2761309A1
Application number: EP12756766.7A
Authority: EP
Inventors: Catalin Stoichita; Lionel Fabien Cima
Original assignee: Neelogy SA
Current assignee: Neelogy SA
Priority date: 2011-09-26
Filing date: 2012-08-02
Publication date: 2014-08-06
Also published as: FR2980581B1; WO2013045778A1; WO2013045778A8; FR2980581A1

Abstract

The invention relates to a contactless current sensor, including: - a set of windings (8) consisting of at least one winding (3) wound about the core (2) thereof, said core being made of a material sensitive to the magnetic field, said set being located immediately adjacent to a bar (1) through which the current (I) to be measured passes; and a controller (11), which sends an energizing current (10) and a counter-reaction current (12) towards the set of windings (8), and which receives, as information, the voltage (9) induced by the temporal variation of the magnetic induction (B) in the core of said at least one winding of the set of windings (8), or in the cores of the windings of the set of windings (8) (see figure).

Description

"Non-contact current sensor."

The invention presented here relates to a new non-contact measuring device of an electric current in a conductor through the measurement of the magnetic field generated by said current.

Among the existing devices capable of measuring contactless currents the best known is the transformer. Its main disadvantage is that it only runs on AC power. EP-0499589 discloses a transformer comprising two secondary windings. A current generator supplies one of the two secondary windings. An amplifier circuit is provided for imposing a bias potential at the common connection point of the two secondary windings so as to guard against ripples of the primary circuit during the measurement.

Another known device is the Rogowski coil which has the same disadvantage as the transformer.

Another known device is the Hall effect current sensor using a magnetic circuit to focus the magnetic field of the current to be measured on a Hall cell. Such a current sensor is sensitive to external magnetic fields which can even saturate said magnetic circuit and thus affect the measurement. Another disadvantage is the presence of a hysteresis profile of the magnetic circuit.

Another known device, called "Flux Gate" is based on the saturation of sensitive magnetic materials and suffers from the same disadvantages as Hall effect. US-2004/0201373 discloses a Rogowski type current sensor associated with a fluxgate to cover a wide frequency range of the signal to be measured. This sensor is based on the principle of closed magnetic field circulation measurement, this supposes to make a loop around the driver.

Another device, that presented in the patent US8076931

(also published under the number FR2891917), uses the magnetic non-linearity of soft materials with low magnetic permeability, or super-paramagnetic soft materials, to measure the circulation of the magnetic field around a primary conductor where the current to be measured flows. . By making the complete turn of the primary conductor, its size is large and it exposes itself more to disturbing magnetic fields External. Another disadvantage could be the cost of manufacturing flexible coils. WO2009153485 is also based on sensor technology around the driver. The magnetic core is a flexible elongated element intended to be closed in a loop around the conductor in which the current to be measured flows.

Document EP-0010921 describes a differential current measurement sensor. This sensor comprises at least one measuring element consisting of a core traversed by the conductor whose current must be measured. A winding is performed around the core. This core does not include a superparamagnetic material.

US-2006/0113987 discloses a set of coils arranged around a conductor orthogonally in pairs. This coil set forms a sensor for AC measurement. These coils do not include a core and are preferably flat coils made on PCB (Printed Circuit Board).

The present invention aims to overcome the aforementioned drawbacks by providing a new sensor convenient and easy to use. Another object of the invention is an inexpensive sensor. The invention also aims a compact sensor.

At least one of the aforementioned objectives is achieved with a non-contact current sensor comprising:

a set of coils composed of at least one coil around a hub responsive to the magnetic field, this hub comprising a superparamagnetic material, and

a controller for sending an excitation current and a feedback current to the set of coils and for receiving as information the voltage induced by the temporal variation of the magnetic induction B at least in the hub of said at least one reel of the reels game.

According to the invention, the set of coils is located in close proximity to a bar intended to receive the current to be measured. The set of coils is close enough to be sufficiently sensitive (that is to say obtain a result notable for the skilled person) to the magnetic field created by the bar.

Furthermore, the hub is advantageously rigid cylindrical. With the invention, the hub is a cylindrical rigid element, that is to say straight and open unlike the sensors of the prior art in the form of cable to be arranged all around the driver.

The shape and the structure of the field-circulating sensors according to the prior art are conditioned by the respect of a principle of interpretation of the Ampère theorem which recommends carrying out the measurement by integrating the magnetic field along a curve. closed, this is the principle of magnetic field circulation.

The meaning of the term "magnetic circulation" is that of Ampère's law:

or

^' G is the integral along a closed curve C;

H is the magnetic field F in Ampere per meter;

" ^■ " is the scalar product of two vectors;

di is an infinitesimal element of the curve (a vector with the modulus equal to the length of the infinitesimal element of curve and the direction given by the tangent to the curve);

If, enc is the net current penetrating a surface sitting on the curve C. It is on the basis of this principle that most of the sensors of the prior art were developed, namely a sensor of circular shape around the driver. .

The present invention goes against this prejudice and proposes to use only part of the magnetic field line. To do this, a coil is used around a hub in the form of a cylinder. By cylinder is meant a solid (rigid), such as a cylinder of revolution (circular base), a parallelepiped, a cube, ... The inventors have demonstrated that such a superparamagnetic component placed on a magnetic field line near the a bar carrying the current to be measured makes it possible to effectively measure the current. The advantage is a smaller footprint compared to the devices of the prior art where it was necessary to completely surround the driver. A sensor of the prior art requires:

a length large enough to go around the driver, a sometimes complex mechanism for connecting the two ends so as to achieve a closed loop according to the closed curve prejudices of Ampère's law, a prejudice that the inventors reject, and

- a flexible core (hub) to surround the driver

On the contrary, the device according to the present invention is of smaller size than the sensor of the prior art since it is rectilinear, so does not go around the driver. Its implementation is simple since it is placed near the bar on a field line to be sensitive to the magnetic field.

The invention is particularly, but not only, remarkable in that only part of the near-field line of the conductor is used to make the measurement and not a closed curve of the field line.

According to the invention, the set of coils can contain two coils, in particular independent, identical and wound respectively on their hubs, hubs sensitive to the magnetic field.

Advantageously, the cylindrical shape is a form of one of the following cylinders:

- cylinder of revolution,

- cylinder with rectangular or square base, or

- cylinder with triangular base or prism.

Preferably, the hub is an elongate member which is disposed where the magnetic field lines cross the hub along its length; this magnetic field being created by the bar when fed with current to be measured. It is thus possible to dimension and arrange the hub so that the magnetic field is for example parallel to the generating line of the hub. For reasons of manufacturing cost, this hub is sufficiently small and close to the bar to be disposed in a section where the field line is a straight line.

In order to promote in particular rectilinear field lines, the bar may be flat and have two rectangular side faces, the current flowing along the length of the rectangle. In this case, the hub can be arranged according to the width of the rectangle; the longitudinal distance of the hub being less than the width of the rectangle. As an example limiting, the longitudinal distance of the hub may be less than the width of the rectangle in a ratio greater than Vi.

According to one characteristic of the invention, the sensor comprises holding means for maintaining the set of coils at fixed distances relative to the bar. In this way, the magnetic transformation ratio between the bar and the coil set is fixed and determined by calibration. Then the set of coils and bar can be connected to a conductor by series insertion of the bar with a conductor carrying the current to be measured.

Moreover, this bar can be removably mounted in the holding means.

Of course, the various features, forms and embodiments of the invention may be associated with each other in various combinations to the extent that they are not incompatible or exclusive of each other.

Other advantages and characteristics of the invention will appear on examining the detailed description of a non-limiting embodiment, and the appended drawings, in which:

FIG. 1 is a simplified schematic view of a non-contact sensor according to the invention, this sensor not surrounding the bar whose current is to be measured; and

Figure 2 is a simplified schematic view of a non-contact sensor according to the invention in which coils are illustrated on either side of the bar.

Figure 1 shows the block diagram of the current sensor according to the present invention. A set of coils 8, positioned near a bar 1 traversed by the current to be measured I, receives an excitation current 10 from a controller 11. The proximity between the set of coils and the bar is such that that there is a significant interaction between the magnetic field of the current to be measured I and the magnetic field of the excitation current 10 in the hubs of the coils forming the set of coils 8. For example, this proximity is concretized by a distance between the bar and the set of coils lower than 20mm, even 10mm, even 5mm. The excitation current has a high frequency with respect to the maximum frequency of the current to be measured and its frequency spectrum does not have even harmonics. By interaction between the magnetic field of the current to be measured I and the magnetic field of the excitation current 10 in the hubs of the coils forming the set 8, even harmonics are born carrying the information 9 on the polarity and the magnitude of the primary current I This information is transmitted to the controller 11 which returns a feedback current 12 to the set of coils 8 in such a way as to minimize the even harmonics generated in the set 8 or in other words to cancel the effect of the magnetic field of the current primary. The counter-current current value represents the primary current up to a fixed proportionality factor which is a function of the construction of the coil set 8 and can be delivered directly as a result of the measurement 13 or a post-processing applied as needed.

FIG. 2 represents an exemplary embodiment of the set of coils 8 next to a primary current bar 1 of rectangular section - thickness E and height H - traversed by an electric current I. The ratio between E and H can be understood between Vi and 1/50. In the same way, the ratio between the length of the hub and H can also be between Vi and 1/50. In Figure 2 the bar is drawn in dotted lines to leave visible the other elements of the sensor. On the opposite sides of the primary bar 1 are positioned two identical coils 3 and 5 having hubs 2 and 4 respectively in a material selected according to the constraints on the use of the current sensor. By way of example: a material with a high relative permeability and a low saturation field will be chosen for measurements of small currents without external magnetic disturbance and a material with low permeability but large saturation field will be chosen for the case where the external magnetic disturbances are important. The winding is done on hubs of cylindrical or preferably parallelepiped shape, as shown in Figure 2, thus allowing their easy manufacture.

The hub 2 is excited by an alternating magnetic field created by means of its coil which receives an excitation current, of high frequency, for example 100 kHz for a maximum frequency of 10 kHz of the current to be measured, from a electronic block here called controller. As a waveform, it is possible to use a sinusoidal, triangular, trapezoidal current, square, etc. or any other waveform provided that the semi-alternations of the excitation current are symmetrical, which means that the amplitudes of the even harmonics are zero. Assuming that the primary current I remains zero and given the symmetry of the characteristic B (H) of the material of the hub, the effect of the non-linearity of this same characteristic B (H) is a temporal distortion of the induction B symmetrical with respect to the two semi-alternations, ie a spectral redistribution of the excitation power while respecting the rule of zero-harmonic harmonics. When the primary current I is not zero, the superimposition of the two fields, that of the excitation current and that of the primary current creates an asymmetrical distortion of the magnetic induction B in the hub, therefore the second order harmonics, 4, 6, etc. appear in his spectrum. The temporary variation of the induction B in the coil 3 induces an electromotive voltage at the excitation frequency and its odd harmonics in the absence of the primary current I. When the primary current is present, in the spectrum of the induced voltage, we find even harmonics.

The controller 11, by the interpretation of the phase of the voltage induced in the coil 3 finds the polarity of the magnetic field added by the primary, and therefore the polarity of the primary current. By interpreting the amplitude of the even harmonics (for example the harmonic 2), the controller also finds the magnitude of the primary current but this information remains approximate because it includes the distortions generated by the material of the hub. To make an accurate measurement, the controller sends a feedback current in the coil 3 of such a value that the pair harmonic generation caused by the primary current is canceled. Thus the hub works in a magnetic field condition, other than excitation, almost zero.

From the previous story, it follows that the coil 3 plays a triple role: excitation, harmonic measurement pairs and feedback. For reasons of optimization, it is reasonable to create several windings on the same hub completely superimposed. Thus one could have winding son adapted to the constraints of use of the current sensor and to various technological constraints. A very fine wire winding and many turns for harmonic measurement can be considered, a medium wire winding for excitation and a thick wire winding for feedback. By combining or not combining the three functions can therefore use a two or three windings on the hub 2. In Figure 1, for simplification of the drawing, only a winding is suggested for the coil3.

The term "even harmonics" does not mean the obligation to use several even harmonics at a time. In the case of using a sinusoidal excitation, it suffices to use harmonic 2 only, for example.

The construction and operation of the spool 5 on its hub 4 is identical to the spool 3 on the hub 2. The particularity that differentiates the two is the fact that they are on the opposite sides of the bar 1. Let us observe that the magnetic field around the primary bar 1 created by a current I enters in opposite directions the hubs 2 and 4 while a magnetic field of a distant source, being more or less a parallel field, penetrates in the same direction said hubs. This advantage is exploited: by choosing the connection direction of the two coils, the addition of the original primary current signals and the extinction of the signals of external magnetic field origin are arranged. On the same subject of immunity to external magnetic fields, it is necessary to emphasize the interest to realize a structure as compact as possible, so as to have fields of disturbance on the two coils, the most possible as possible. To this end, the use of a bar 1 for the primary current whose thickness E and very small compared to its height H is an advantage because it allows to bring together the coils 3 and 5.

The use of two coils 3 and 5 is not obligatory, it remains just an option to evaluate according to the needs of immunity to the external magnetic field, manufacturing price, template, etc.

The current sensor may contain two other coils 6 and 7 with similar geometries to the coils 3 and 5 but without hubs sensitive to the magnetic field. Given the proximity of the coils 3, 5, 6 and 7 to the primary bar, when this is traversed by an alternating current, an electromotive voltage is induced in each of said coils. The number of turns for the coils 6 and 7 is adjusted to generate the same voltage induced by a primary current as the coils 3 and 5.

The coils are connected together so that the voltages induced by a variable primary current in the coils without hubs sensitive to the magnetic field cancel by subtraction the voltages induced by said current in the field-sensitive hub coils. magnetic. The interest for the rejection of the signals generated by the baseband primary current depends on the frequency spectrum of said primary current, the permissible dynamics at the measurement input of the controller and, in general, the effects that the presence of the band signals base could have on the accuracy of the null detection on even harmonics by the controller.

The magnetic sensor is intrinsically insensitive to external magnetic fields, since the material used for transduction has a very high saturation field. However, in the presence of a very intense external field, such as the presence of a return bar, or another electrical conductor of another electrical pole or a permanent magnet, it may be that it disturbs the measurement. It is then beneficial to use a magnetic material to provide a shield for channeling the outside field and diverting it slightly from the transducer. This material may be made of a soft or hard magnetic material. The shape and thickness of this shield is optimized according to the disturbing field level.

The form of the shield may be a simple plate interposed between the source of disturbance and the transducer. It can be a cage with or without gap.

In general, the non-contact current sensor according to the invention comprises at least:

a set of coils consisting of at least one coil around its hub made of material sensitive to the magnetic field, a set located immediately close to a bar traversed by the current to be measured, and

a controller which sends an excitation current and a feedback current to the set of coils and which receives as information the voltage induced by the temporal variation of the magnetic induction B in the hub of said at least one reel of the game of coils or in the hubs of the coils of the set of coils.

The set of coils can contain two coils, identical and wound respectively on their hubs, hubs sensitive to the magnetic field. The set of coils may also contain at least one coil without a magnetic field sensitive hub which generates the same voltage induced by the primary current as the magnetic field-sensitive hub coil. Moreover, the material of the hub or hubs can be super paramagnetic.

Of course, the invention is not limited to the examples that have just been described and many adjustments can be made to these examples without departing from the scope of the invention.

Claims

Non-contact current sensor comprising:

a set of coils (8) composed of at least one coil (3) around a hub (2) sensitive to the magnetic field, this hub comprising a superparamagnetic material, and

a controller (11) for sending an excitation current (10) and a feedback current (12) to the set of coils (8) and for receiving as information the voltage (9) induced by the time variation of the magnetic induction B at least in the hub of said at least one coil of the set of coils (8),

characterized in that the set of coils is located immediately adjacent a bar (1) for receiving the current to be measured (I); and in that the hub (2) is cylindrically rigid.

2. Non-contact current sensor according to claim 1, characterized in that the set of coils (8) contains two coils (3, 5), identical and wound respectively on their hubs (2, 4), hubs sensitive to the magnetic field. .

Non-contact current sensor according to Claim 1 or 2, characterized in that the set of coils (8) contains at least one coil (7) without a magnetic field-sensitive hub which generates the same voltage induced by the primary current ( I) as the magnet-sensitive hub coil (3).

Non-contact current sensor according to one of the preceding claims, characterized in that the set of coils (8) contains two coils (6, 7) without a magnetic field-sensitive hub which generates the same voltage induced by the current. primary (I) as the coils with magnetic field sensitive hub (3, 5).

Non-contact current sensor according to one of the preceding claims, characterized in that the controller (11) supplies an excitation current to the coil set (8) with a spectrum without even harmonics.

Non-contact current sensor according to Claim 5, characterized in that the controller (11) analyzes at least one of the even harmonics of the voltage spectrum (9) induced in the magnetic field-sensitive hub coils to determine the magnitude. a feedback current (12) to be returned to the set of coils (8) to minimize the sum between the magnetic field of the primary bar (1) and the magnetic field of the feedback current (10).

7. Non-contact current sensor according to claim 6, characterized in that the feedback current (10) represents the measured value (13) of the current (I) except for a multiplicative constant dependent on the construction of the sensor.

8. Non-contact current sensor according to any one of the preceding claims, characterized in that the cylindrical shape is a form of one of the following cylinders:

- cylinder of revolution,

- cylinder with rectangular or square base, or

- cylinder with triangular base or prism.

A non-contact current sensor according to any one of the preceding claims, characterized in that the hub is an elongate member which is disposed at the location where the magnetic field lines pass through the hub along its length; this magnetic field being created by the bar when fed with current to be measured.

10. Non-contact current sensor according to any one of the preceding claims, characterized in that the bar is flat and has two rectangular-shaped side faces, the current flowing along the length of the rectangle, the hub being arranged according to the width of the rectangle; the longitudinal distance of the hub being less than the width of the rectangle.

11. Non-contact current sensor according to claim 10, characterized in that the longitudinal distance of the hub is smaller than the width of the rectangle in a ratio greater than Vi.

12. Non-contact current sensor according to any one of the preceding claims, characterized in that it comprises holding means for maintaining the set of coils at fixed distances relative to the bar.

13. Non-contact current sensor according to claim 12, characterized in that the bar is removably mounted in the holding means.