Rogowski coil
The Rogowski coil is a toroidal air- core coil , which means it does not have a ferromagnetic core. Among other things, it serves as a component of electrical measuring devices for measuring alternating current .
The basic idea for its structure, which uses the voltages induced by alternating currents in concentrically surrounding coils, came from Arthur Prince Chattock in 1887. The name has been known since the publication by Walter Rogowski (1881–1947) as "Rogowski coil" or "Rogowski current transformer" .
General
The Rogowski coil consists of a conductor wire that is wound as evenly as possible around a solid body made of a non-conductive and non- ferromagnetic material (air-core coil). The conductor wire of the coil is wrapped around the entire ring of the coil body so that both connections are next to each other. In the picture opposite, the coil was realized as an open circular arc in order to avoid threading the line to be measured. The second connection of the coil is magnetically neutral to the other end.
In order to measure a current i 1 in a conductor , the Rogowski coil is placed around the current-carrying conductor - for example a cable core or a busbar . The alternating current flowing through the conductor generates a variable magnetic field which induces a voltage u 2 in the Rogowski coil .
The voltage is measured with a high resistance so that the current in the Rogowski coil is almost zero. The following relationship applies under this condition:
The mutual inductance M of the Rogowski coil can be calculated as follows:
where μ 0 is the magnetic field constant , N is the number of turns, A is the cross-sectional area of the Rogowski coil and l m is the mean field line length in the ring.
Since the conductor current i 1 is to be measured, the time integral of the voltage u 2 of the coil must be formed. A quantity proportional to the current i 1 is thus obtained . The time integral is formed in an analogous way with an integrator . In the case of sinusoidal currents, integration can be omitted - the measured voltage can be calibrated in current units, as it only leads by 90 ° phase angle.
Advantages and disadvantages
One advantage of the Rogowski coil over other current measurement methods is the robustness of the structure. On the other hand, the current to be measured can be in a wide range, even up to the level of the highest possible current (short-circuit current), without damaging the transmitter. There are no non-linear influences of an iron core. There is no magnetic influence on the conductor with Rogowski coils; However, it is low even with conventional current transformers with an iron core, since they are operated in the secondary quasi-short circuit.
Rogowski coils can be simply put on and removed without having to open the circuit, i.e. without any assembly work. They are manufactured in many different sizes, so that a wide range of applications is possible, from measurements directly on components on circuit boards to measurements on busbars or machine parts (bearing currents).
Rogowski coils are manufactured with different sensitivities for currents from a few amps up to a few 100 kA . Depending on the series and sensitivity, they are suitable for measurements from below 1 Hertz (Hz) to the double-digit MHz range. Steep current rises, such as those that occur in converters or other power electronic assemblies, can be easily recorded. Rogowski coils are also suitable for measuring harmonics and other higher-frequency interference.
In addition to series with direct voltage output, there are also versions with converters for measuring the effective value . This allows them to be connected directly to the 0–5 V or 4–20 mA inputs of a programmable logic controller , for example.
In contrast to current transformers, high short-circuit currents, such as those that occur in electrical power engineering , do not cause high forces or losses with Rogowski coils. Hysteresis effects and permanent magnetization, as they occur with Hall probes , are eliminated. In contrast to current transformers, Rogowski coils do not pose a risk to operating personnel and are not destroyed if they are not connected.
However, you need auxiliary equipment such as amplifiers and the amplifier's power supply to measure the current. Conventional current transformers, on the other hand, due to the core that bundles the magnetic field (within the scope of their nominal burden ), can provide considerable power that is sufficient to operate measuring mechanisms and overcurrent release devices directly.
Another disadvantage is the positional dependence of the measurement accuracy. For an ideal, d. H. homogeneous coil, the position of the enclosed conductor is irrelevant. In real Rogowski coils, however, there are inhomogeneities (opening for threading, varying winding distances) where the magnetic field induces too much or too little. If the conductor is in the center position, the errors cancel each other out with an ideal ring shape of the coil, but center deviations lead to errors. If the conductor is close to the opening during a measurement, the measured current is lower than the actual value. If the conductor is at a more densely wound point, the measured current is greater than the actual value.
In contrast to some alternative measuring methods, the Rogowski coil cannot measure direct current.
In a study by the University of Twente on intelligent electricity meters ('smart meters'), excessive measurement results were found for devices with a Rogowski coil as a measuring element. In the study, the incorrect measurements are placed in connection with consumers with non-sinusoidal current flow. Such consumers are, for example, many electronic devices with a switched-mode power supply .
literature
- Dieter Kind, Kurt Feser: High-voltage test technology . 5th, revised. u. exp. Edition. Vieweg + Teubner, 1995, ISBN 3-528-43805-3 .
Web links
Individual evidence
- ↑ W. Rogowski, W. Steinhaus: The measurement of the magnetic tension . In: Archives for electrical engineering . tape 1 , no. 4 , April 1912, p. 141-150 , doi : 10.1007 / BF01656479 .
- ↑ Jens Haun: Conductivity measurements on strongly coupled carbon and zinc plasmas . 2001 ( PDF - Dissertation, Faculty of Physics and Astronomy, Ruhr University Bochum. - Contains a derivation of the formula).
- ↑ Electronic energy meters' false readings almost six times higher than actual energy consumption. University of Twente, accessed March 6, 2017 .