Inductive current sensor

Abstract

An inductive current sensor (1) for contactless measurement of the alternating electric current in an electrical conductor (2), comprising a measuring coil (3) which preferably completely surrounds the electrical conductor (2), wherein the measuring coil (3) comprises a measuring wire (5) which is wound onto a core (4) so that the voltage induced in the measuring wire (5) is a measure of the alternating electric current flowing through the conductor (2), wherein a shunt resistor (6) which is electrically connected in parallel with the measuring coil (3) is provided, wherein the shunt resistor (6) comprises a resistor wire (7) in the form of a bifilar, preferably twisted, double conductor on the outer surface (8) of the measuring coil (3).

Description

[0001] 1 - 65099 / AG / -

[0002] Greenwood-Power GmbH, Industriestrasse B 6-8, 2345 Brunn am Gebirge (AT)

[0003] Inductive current sensor

[0004] The invention relates to an inductive current sensor for non-contact measurement of the alternating electric current in an electrical conductor.

[0005] Inductive current sensors for non-contact measurement of alternating current in an electrical conductor are known in the prior art. Such current sensors or current transformers comprise a measuring coil that preferably completely surrounds the electrical conductor. The measuring coil is generally ring-shaped with a ferromagnetic core onto which a measuring wire, for example made of copper, is wound. The alternating current flowing in the primary conductor induces an alternating magnetic field in the core, which induces an electrical voltage in the measuring wire. The induced voltage is measured in the conventional manner and represents a measure of the magnitude of the alternating current in the electrical conductor.

[0006] A problem with such current sensors, however, is that their measuring range is usually very limited. In particular, conventional inductive current sensors cannot be used simultaneously to measure very low currents, for example below 0.1 A, and very high currents, for example from 10 kA to over 100 kA, such as those that can occur in short circuits, because the voltage induced in the measuring coil would lead to thermal overload of the measuring wire. At low currents, this results in very high measurement inaccuracies.

[0007] Furthermore, a problem with inductive current sensors is that they are susceptible to magnetic interference. 2 - 65099 / AG / -

[0008] Greenwood-Power GmbH, Industriestrasse B 6-8, 2345 Brunn am Gebirge (AT)

[0009] The object of the invention is to solve these and other problems of conventional inductive current sensors and to provide an inductive current sensor that is applicable over a wide measuring range even at very high and low currents and is also as independent as possible against external magnetic interference.

[0010] According to the invention, these and other problems are solved by an inductive current sensor according to claim 1. A current sensor according to the invention has a shunt resistor connected electrically in parallel to the measuring coil, which comprises a resistance wire arranged as a bifilar, preferably twisted, double conductor on the outer surface of the measuring coil.

[0011] A current sensor with a shunt resistor designed and arranged in this way offers several technical advantages over conventional inductive current sensors. Due to its parallel connection to the measuring coil, the shunt resistor can be used to dissipate high currents, for example, in the event of a short circuit. Furthermore, the relatively low resistance of the shunt resistor results in minimal load on the current transformer, allowing for the transmission of much higher primary currents while simultaneously ensuring high accuracy at very low currents. For this purpose, the shunt resistor can have a lower total resistance compared to the measuring coil. The total resistance of the shunt resistor can be, for example, 2, 5, 10, or 50 times lower than the total resistance of the measuring coil. This can be achieved by using a resistance wire with a larger diameter and fewer turns than the measuring wire.

[0012] With a sufficiently large diameter and low number of turns of the resistance wire, its specific resistance can be significantly higher than that of the measuring wire. This allows the use of particularly temperature-stable electrical conductors for the resistance wire, especially manganin, whose resistance is characterized by a particularly low temperature dependence. 3 - 65099 / AG / -

[0013] Greenwood-Power GmbH, Industriestrasse B 6-8, 2345 Brunn am Gebirge (AT)

[0014] This ensures that the shunt resistor arrangement has virtually no impact on the current sensor's measurement accuracy, while significantly extending its measuring range for high currents. In practice, a very large primary current measuring range in the conductor, from 0.1 A to 100 kA, can be achieved within a measurement class of 0.2. For example, a short-circuit current of 100 kA in the primary conductor generates currents exceeding 50 A in the shunt resistor. These currents can be easily and reliably dissipated without affecting the shunt wire, thanks to the shunt resistor's design using a thicker and shorter manganin wire compared to the measuring wire.

[0015] A further advantage arises from the arrangement of the resistance wire as a bifilar and twisted double conductor on the outer surface of the measuring coil. This special arrangement ensures that the current flowing in the resistance wire does not generate a disturbing magnetic field, thus preventing interference with the measurement of the induced current from the primary conductor.

[0016] Furthermore, positioning the current sensor outside the measuring coil provides a certain shielding effect, thus reducing the influence of external magnetic fields on measurement accuracy. This also reduces the current sensor's positional sensitivity, meaning the primary electrical conductor does not necessarily need to be positioned precisely in the center of the measuring coil. The shielding effect is further enhanced by a higher number of windings of the resistance wire and by ensuring the outer surface of the measuring coil is covered as completely as possible.

[0017] According to the invention, the resistance wire can be arranged to encircle the measuring coil at least once, preferably several times, around its outer surface. For example, the resistance wire can be wound once, twice, three times, five times, or more times around the outer surface of the measuring coil. The resistance wire can cover 20%, 40%, 60%, 80%, or 100% of the outer surface of the measuring coil. 4 - 65099 / AG / -

[0018] Greenwood-Power GmbH, Industriestrasse B 6-8, 2345 Brunn am Gebirge (AT)

[0019] According to the invention, the measuring wire can be made of copper or comprise copper. The measuring wire is preferably made of a material with a low specific electrical resistance.

[0020] According to the invention, the resistance wire can be made of or comprise manganin. The resistance wire is preferably made of a material with a higher specific electrical resistance than the measuring wire.

[0021] According to the invention, the measuring coil can be designed as a toroidal core coil. The core of the measuring coil can comprise a ferromagnetic material, for example iron.

[0022] According to the invention, the diameter of the resistance wire can be larger than the diameter of the measuring wire. For example, the diameter of the resistance wire can be larger by a factor of 2, a factor of 10, a factor of 20, or a factor of 50 than the diameter of the measuring wire. Preferably, the resistance wire can have a diameter greater than 1 mm and the measuring wire a diameter of less than 0.5 mm.

[0023] According to the invention, the number of turns of the resistance wire around the measuring coil can be lower than the number of turns of the measuring wire around the core. For example, the number of turns of the resistance wire can be 5, 10, 50, or 100 times lower than the number of turns of the measuring wire. Preferably, the resistance wire can have fewer than 5 turns and the measuring wire more than 20 turns. Correspondingly, the length of the resistance wire can be 20, 50, 100, or a greater factor shorter than the length of the measuring wire.

[0024] According to the invention, the specific resistance of the resistance wire can be higher than the specific resistance of the measuring wire. For example, the specific resistance of the resistance wire can be 10 times, 20 times, 30 times, or 50 times higher than the specific resistance of the measuring wire. 5 - 65099 / AG / -

[0025] Greenwood-Power GmbH, Industriestrasse B 6-8, 2345 Brunn am Gebirge (AT)

[0026] Preferably, the resistance wire can have a specific resistance of about 0.45 Q mm2 / m and the measuring wire a specific resistance of about 0.017 Q mm2 / m.

[0027] Ceramic beads can be threaded onto the resistance wire for thermal insulation. This allows the resistance wire to be thermally decoupled from the measuring wire, so that heating of the resistance wire only leads to a slight heating of the measuring wire, and vice versa.

[0028] Further features of the invention will become apparent from the claims, the description and the drawings.

[0029] The invention is explained below using an exemplary embodiment. The figures shown are:

[0030] Fig. 1a is a schematic three-dimensional representation of an embodiment of a current sensor according to the invention;

[0031] Fig. 1b shows a schematic equivalent circuit diagram of this current sensor.

[0032] Fig. 1a shows a schematic three-dimensional representation of an embodiment of a current sensor 1 according to the invention. The current sensor 1 is designed for non-contact measurement of the alternating current lac in an electrical conductor 2 (primary conductor). For example, the current sensor 1 can be designed for a frequency range of approximately 50 Hz. The electrical conductor 2 runs between the connection points P1 and P2. The current sensor 1 comprises a measuring coil 3 that completely surrounds the electrical conductor 2. The electrical conductor 2 is arranged approximately in the center of the measuring coil 3.

[0033] The measuring coil 3 is designed as a toroidal coil and comprises a thin measuring wire 5 wound on a ferromagnetic core 4 (wound iron core or nickel alloy). In the present embodiment, the measuring wire 5 is made of copper and has a diameter of less than 0.2 mm. More than 200 windings of the measuring wire 5 are arranged substantially uniformly on the core 4. The core 4 has a diameter of approximately 20 cm. 6 - 65099 / AG / -

[0034] Greenwood-Power GmbH, Industriestrasse B 6-8, 2345 Brunn am Gebirge (AT)

[0035] The alternating current lac in conductor 2 induces an electrical voltage in the measuring wire 5, which is a measure of the alternating current flowing through conductor 2. The ends of the measuring wire 5 are brought out via terminals 8a and 8b to allow the connection of a voltmeter (not shown). The magnitude of the alternating current lac in conductor 2 can be calculated from the measured voltage.

[0036] To dissipate high induced currents, a shunt resistor 6 is connected electrically in parallel to the measuring coil 3. The shunt resistor 6 comprises a resistance wire 7, which is arranged as a bifilar, twisted double conductor on the outer surface of the measuring coil 3. In this embodiment, the resistance wire is made of manganin, has a diameter of approximately 1.6 mm, and is wound around the measuring coil 3 with 2 turns.

[0037] In this embodiment, the resistance wire 7 has a specific resistance of approximately 0.45 Ω mm² / m, and the measuring wire 5 has a specific resistance of approximately 0.017 Ω mm² / m. The length and cross-sectional area of ​​the measuring wire 5 in this embodiment are approximately 20 m and approximately 0.04 mm², respectively. 2 .

[0038] In this embodiment, the length and cross-sectional area of ​​the resistance wire 7 are approximately 1 m and approximately 2.5 mm². 2 The total resistance of the shunt resistor 6 is therefore approximately 180 mΩ in this embodiment. The total resistance of the measuring coil 3 is therefore approximately 8 Ω in this embodiment.

[0039] Terminals 9a and 9b of the resistance wire 7 are electrically connected in parallel with terminals 8a and 8b of the measuring wire 5 and are brought out as measuring terminals S1 and S2. During operation, the voltage across measuring terminals S1 and S2 is measured, and the magnitude of the alternating current lac in conductor 2 is calculated from this measurement.

[0040] In an embodiment not shown, numerous ceramic beads are threaded onto the resistance wire 7 for thermal insulation of the resistance wire 7 from the measuring wire 5. 7 - 65099 / AG / -

[0041] Greenwood-Power GmbH, Industriestrasse B 6-8, 2345 Brunn am Gebirge (AT)

[0042] Fig. 1b shows a schematic equivalent circuit diagram of this current sensor 1. The conductor 2 with primary terminals P1, P2 and the alternating current lac is shown. The measuring coil 3 picks up the magnetic field generated by the alternating current lac, producing a measuring voltage Vm, which can be measured at the measuring terminals S1, S2 using a voltmeter (not shown). The shunt resistor 6 with its electrical resistance Rsh is connected in parallel to the measuring coil 3 with its electrical resistance RL, where Rsh < RL for the considered application range, in particular the frequency range.

[0043] The invention is not limited to the described embodiments but encompasses all current sensors within the scope of the following patent claims.

Claims

8 - 65099 / AG / - Greenwood-Power GmbH, Industriestrasse B 6-8, 2345 Brunn am Gebirge (AT) Patent claims 1. Inductive current sensor (1 ) for non-contact measurement of the alternating electric current in an electrical conductor (2), comprising - a measuring coil (3) preferably completely enclosing the electrical conductor (2), wherein - the measuring coil (3) comprises a measuring wire (5) which is wound on a core (4), such that - the voltage induced in the measuring wire (5) is a measure of the alternating electric current flowing through the conductor (2), characterized in that - a shunt resistor (6) is provided which is electrically connected in parallel to the measuring coil (3), wherein - the shunt resistor (6) comprises a resistance wire (7) which is arranged as a bifilar, preferably twisted double conductor on the outer surface of the measuring coil (3).

2. Inductive current sensor (1 ) according to claim 1 , characterized in that the resistance wire (7) surrounds the measuring coil (3) at least once, preferably several times.

3. Inductive current sensor (1 ) according to claim 1 or 2, characterized in that the measuring wire (5) is made of copper or comprises copper.

4. Inductive current sensor (1 ) according to claim 1 or 2, characterized in that the resistance wire (7) is made of manganin or comprises manganin.

5. Inductive current sensor (1 ) according to one of claims 1 to 4, characterized in that the measuring coil (3) is designed as a toroidal core coil.

6. Inductive current sensor (1 ) according to one of claims 1 to 5, characterized in that the core (4) comprises a ferromagnetic material, for example iron. 9 - 65099 / AG / - Greenwood-Power GmbH, Industriestrasse B 6-8, 2345 Brunn am Gebirge (AT) 7. Inductive current sensor (1 ) according to one of claims 1 to 6, characterized in that the diameter of the resistance wire (7) is larger than the diameter of the measuring wire (5), for example by a factor of 2, a factor of 10 or a factor of 20, wherein the resistance wire (7) preferably has a diameter of more than 1 mm and the measuring wire (5) preferably has a diameter of less than 0.5 mm.

8. Inductive current sensor (1) according to one of claims 1 to 7, characterized in that the number of windings of the resistance wire (7) is lower than the number of windings of the measuring wire (5), for example by a factor of 5, a factor of 10, or a factor of 100 lower, wherein the resistance wire (7) preferably has a number of windings of less than 5 and the measuring wire (5) preferably has a number of windings of more than 20.

9. Inductive current sensor (1) according to one of claims 1 to 8, characterized in that the specific resistance of the resistance wire (7) is higher than the specific resistance of the measuring wire (5), for example by a factor of 10, a factor of 20 or a factor of 50 higher, wherein the resistance wire (7) preferably has a specific resistance of about 0.45 Ω mm² 2 / m and the measuring wire (5) preferably has a specific resistance of about 0.017 Q mm 2 / m 10. Inductive current sensor (1 ) according to one of claims 1 to 9, characterized in that ceramic beads are threaded onto the resistance wire (7) for thermal insulation.

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