GB327837A - Improvements in or relating to electric current transformers and measuring instruments - Google Patents

Abstract

327,837. Shotter, G. F., and Elliott Bros. (London), Ltd. March 13, 1929. C u rrent and pressure meters. - A current transformer for use with a rneasuring instrument has an adjustable gap in the iron circuit, so that different ranges of current may be measured, the iron core being in separable parts, for example hinged together, so that the core may be placed around a main conductor. The core is built up of laminae clamped in an aluminium or other non- magnetic casing of U-section and is in two parts 1, 2 hinged together by a pin 3 passing through parts 4, 5 of non-magnetic material extending from the casing, so that the pin is offset from the core. By this means a gap 7 in the magnetic circuit in addition to the main gap 6, is formed when the parts 1, 2 are opened out, the projection 14 being bifurcated and the projection 15 moving into it. A magnetic shunt is thus formed which increases in value as the parts are opened. The projections 14, 15 may be parts of the laminations or separate members. The secondary winding is in two parts 18, 19 mounted on the divided and hinged member 8. A spring 13 is provided for normally maintaining the parts 1, 2 closed, and handles 11, 12 permit them to be opened. The gap between the members 1, 2 in the closed position is measurably adjusted by means of a screw 27, on the part 2 abutting against a hard steel stop 26, Fig. 5, on the part 1. The screw 27 has a cylindrical part 29 in which are depressions 30 filled with materials of different colours. Each of these, when brought opposite a hole 31, indicates that the gap 6 is of the correct size to measure a particular range of current. Sheaths of insulating material 34 prevent contact between the main conductor and the core. In a modification, the magnetic shunt, the handles 11, 12, and the spring 13 are omitted and the hinge is not offset, the parts 1, 2 being held in the closed position by a hinged stirrup embracing both parts and having a U- shaped spring to press them together. A wound primary may be connected into the conductor carrying the current to be measured. A single high-resistance instrument may be used for measuring current or voltage, switch-controlled resistances being inserted in the latter case. The transformer may be employed in connection with rectifiers to operate direct-current permanent magnet instruments. The measuring instrument may have a reference point for the indicator, instead of a graduated scale, in which case the gap in the transformer core is adjusted to cause the pointer to coincide with this mark, the current being indicated by the size of the gap. According to one of the Provisional Specifications, the core is held in the closed position by thumb screw on a pin swivelled on one part and passing through a gap in the other, also the screw 27 has a pointer and a scalp to indicate the size of the gap.