GB1233822A - An electric current monitoring device - Google Patents

Abstract

1,233,822. 822. Current and magnetic field measurement. COMPAGNIE GENERALE D'ELECTRICITE. Feb.26, 1969 [Feb.27, 1968; Oct.3, 1968; Dec.3, 1968], No.10248/69. Headings GIA, G1N and G1U. In an electric current transforming arrangement a magnetostrictive rod is placed in the field of the current to be transformed or measured and has a mirror (grating or diffuser) mounted thereon whereby light from a source falling on a detector is modulated by changes in the current. The line conductor may be looped into one or more turns and the rod may be polarized by a coil or a permanent magnet, the former of which may be energized by A. C. for the apparatus to serve as a D.C. transformer. Two identical rods one of which is screened from the magnetic field may be used for temperature compensation and the rods which may be cylindrical may be interconnected by a good heat conductor. In a null arrangement Fig. light from a source 2 which may be a sodium vapour lamp or a laser is split by a mirror 3 and supplied to a pair of interferometers 4, 5 the former having a mirror carried by a rod responsive to the current to be transformed and the latter having a similar rod responsive to a field proportional to the error current developed by a comparison in unit 6 of the two interferometer outputs. The current affecting the rod in interferometer 4 may be indicated by ammeter 8. The use of Michelson, Twyman Green, Fabry- Perot and Jamin interferometers is referred to and in a null system described with reference to Fig.4 (not shown) the output light from one current responsive interferometer arrangement is applied to a succeeding differential pair having a common nulling coil around a magnetostrictive rod. Embodiments in which the rod is a tube surrounding the conductor and is supported at one of its ends or at its centre are also described, the tube carrying at its free end or ends a concave mirror and deforming torsionally with changes of current. Further arrangements in which the movable mirror is rotated relative to the fixed mirror or in which the two mirrors are rotated and the light source is a particular wavelength or group of wavelengths have light guides and a vibrating slot aperture whereby pulses corresponding to the number of fringes may be counted. In a further null method Fig.10 in which mirror 142 is mounted on the magnetostrictive rod, the light from mirror 142 is split by a separator 143 and the compensating wedge 147 is adjusted to maintain equality of pulses directed alternately by a mixer 150 on to a photo-cell 152. A further embodiment uses variations in the reflection of diffused light and two still further embodiments are directed to the automatic control of the output power of a laser by a permanent magnet biased rod carrying a mirror in the beam path in which the current derived by a photo-cell from the beam is passed through the rod itself.