Electricity: measuring and testing – Measuring – testing – or sensing electricity – per se – Using radiant energy
Patent
1998-06-02
2000-11-28
Williams, Hezron
Electricity: measuring and testing
Measuring, testing, or sensing electricity, per se
Using radiant energy
324117R, 3242441, 250225, G01R 3100
Patent
active
061540226
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to a method and an arrangement for measuring an alternating magnetic field.
BACKGROUND OF THE INVENTION
An alternating magnetic field is a magnetic field which has in its frequency spectrum only frequency components differing from 0, and is thus, in particular, temporally variable.
Conventional optical measuring arrangements and measuring methods measure a magnetic field using the magnetooptic Faraday effect. The Faraday effect is the rotation of the plane of polarization of linearly polarized light as a function of a magnetic field. The rotational angle is proportional to the path integral over the magnetic field along the path covered by the light, with the Verdet's constant as the constant of proportionality. The Verdet's constant is generally a function of material, temperature and wavelength. A Faraday sensor device comprising an optically transparent material such as, for example, glass is arranged in the magnetic field for the purpose of measuring the latter. The magnetic field causes a rotation of the plane of polarization of linearly polarized light, transmitted by the Faraday sensor device, by a rotational angle which can be evaluated for a measuring signal. Such magnetooptic measuring methods and measuring arrangements are may to be used to measure electric currents. The Faraday sensor arrangement is arranged for this purpose in the vicinity of an electric conductor, and detects the magnetic field generated by a current in the electric conductor. In general, the Faraday sensor device surrounds the electric conductor, so that the measuring light runs round the electric conductor in a closed path. The absolute value of the rotational angle is in the current to be measured. The Faraday sensor device can be constructed as a solid glass ring around the electric conductor, or else can surround the electric conductor in the form of a measuring coil comprising a light-conducting fiber (fiber coil) with at least one turn.
The advantages of magnetooptic measuring arrangements and measuring methods by comparison with conventional inductive current transformers are the electrical isolation and the insensitivity with respect to electromagnetic interference. However, temperature influences, and in particular influences of mechanical bending and vibration in the sensor device and the optical transmission links, in particular optical fibers for transmitting the measuring light, present problems.
International Patent Application No. WO 95/10045 describes a conventional device where two lineraly polarized light signals are transmitted in opposite directions through a Faraday sensor device. The Faraday sensor device surrounds an electric conductor, and has a circular birefringence which is negligible by comparison with the Faraday effect. After traversing the sensor device, each of the two light signals is decomposed by a polarizing beam splitter into two mutually perpendicular linearly polarized component light signals. For each light signal, an intensity-normalized signal is formed, which corresponds to the quotient of a difference and the sum of the two associated component light signals. A signal processor derives from the two intensity-normalized signals a measuring signal for an electric current in the electric conductor, the signal being virtually independent both of the temperature and of vibrations in the sensor device.
In three other conventional devices, two light signals traverse an optical series circuit comprising a first optical fiber, a first polarizer, a Faraday sensor device, a second polarizer and a second optical fiber, doing so in mutually opposite directions of circulation. After traversing the optical series circuits, the two light signals are transformed into an electric intensity signal in each case by appropriate photoelectric transducers.
In one conventional measuring system described in U.S. Pat. No. 4,916,387, a solid glass ring which surrounds the electric conductor is provided as Faraday sensor device. The axes of polarization
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Bosselmann Thomas
Hain Stefan
Willsch Michael
Jolly Anthony
Siemens AG
Williams Hezron
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