Patent GB-397569-A (GB)
Title: Improvements in and relating to electrical measuring instruments
IPC: G01R5/14
Filed: 1932-03-11
Abstract: 397,569. Current, pressure, and power meters; frequency and resistance measurements. DRYSDALE, C. V., Research & Experiment Department, Admiralty, London. March 11, 1932, No. 7299. [Class 37.] In measuring-instruments of the movableiron type, the movable-iron member turns between the poles of a fixed member forming a nearly closed magnetic circuit, both members being made of nickel-iron alloy or like material of high permeability and low hystersis loss, in order that the air gap may be reduced to a minimum, with consequent reduction in the magnetizing . ampÞre turns and losses due to magnetic leakage. In the ammeter or voltmeter shown in Fig. 1, the stationary member 1 is in the form of a ring of nickel-iron stampings with internally-projecting polepieces 2 carrying coils 3. The movable element 4 is made of nickel-iron stampings or of square-section nickel-iron wire and may be of hour-glass shape or otherwise formed to give any desired form of scale. The ends of the movable element may be sealed by thin caps of metal, celluloid or other material so as to make the element of neutral buoyancy in petroleum or other insulating liquid, with which the case of the instrument may be filled. In a modification, the movable element is made of narrow rectangular stampings and the poles of the fixed element are expanded. In another modification, Fig. 6, in which the range of the instrument may be extended to about 300‹ with a single moving element, the fixed element 1 comprises a block of C-shaped nickel-iron stampings carrying the coils 3, and formed with a narrow gap in which the movable element 4 can turn. The movable element is cam-shaped, and may be constructed either of a spiral of nickel-iron strip interleaved with varnished paper, or of nickel-iron particles compressed with binding material and moulded to the desired shape.. In an alternative construction for an instrument with a range of about 300‹, Figs. 8 and 9, the fixed element consists of two laminated rings 1a, 1a, encircling the spindle 5 with the exciting coil 3 mounted between them, and. a number of laminated blocks 1b completing the magnetic circuit, while the moving element 4c, comprises a block of rectangular stampings mounted obliquely on the spindle. In a wattmeter, Fig. 11, the poles of two fixed. elements carry series windings 3, carrying the main current, and shunt windings 3a, arranged to assist the main windings in one element and oppose them in the other. The movable element 4 is in the form of two archimedean spirals in opposite directions and moves in gaps in the fixed elements. In a modification, the fixed elements are wound with single coils on each pole, the coils being connected across the mains through a non-inductive resistance, which may be shunted by a condenser. The secondary windings of current transformers are connected across the coils in such a manner that the main and shunt currents assist each other in one system and oppose each other in the other system. Such a wattmeter is suitable for deflections up to about 150‹, but its range may be extended to 300‹ by employing two axially displaced vanes on the spindle. In an arrangement of this kind, Fig. 17, a single fixed element with two gaps, in which the vanes 4 can turn, is provided. An additional fixed element, as shown in dotted lines in Fig. 17, may be provided for indicating the power in a two or threephase circuit. In applying the invention to ohmmeters. or frequency meters, two fixed magnet systems, operating on one or more camshaped moving elements of different form, are provided,'the deflection depending on the ratio of the currents in the coils of the two fixed magnet systems. If one of the coils is connected in series with a fixed resistance, the arrangement may be used as an ohmmeter, while by connecting the coils in circuits having different variations of impedance with frequency, e.g. a resistance and inductance, in-. ductance and condenser, or two inductance condenser circuits of different resonance frequencies, the instrument may be used to indicate frequency.