GB616325A - Improvements in or relating to the measurement of frequency variations in alternating current circuits - Google Patents

Abstract

616,325. Frequency measurements; electromagnetic ammeters. ELLIOTT BROS. (LONDON), Ltd., and HAWKES, H. D. Aug. 13. 1946, No. 24039. [Class 37] Variations from a nominal frequency are measured by means of two tuned circuits, having resonant frequencies above and below the nominal frequency, the 'currents in these circuits being rectified and their ratios measured. A step-down transformer T has a primary winding P and two similar secondary windings S1, S2, each of which is included in a separate tuned circuit comprising a condenser C1, C2, inductance L1, L2, and full-wave metal oxide rectifier R1, R2. One circuit is tuned to resonate at a frequency above the nominal frequency and the other at a frequency below the nominal, the currents in the tuned circuits being substantially equal so long as the main A.C. is at the nominal frequency. The outputs from. the rectifiers R<1>, R2 are supplied to the moving coils 3, 4, mounted on the common spindle of two permanent-magnet instruments. The pole pieces are so shaped that the flux density in the air gaps is substantially the same in both at the centre of the length of the gaps and increases throughout the length of the gap for one instrument, while decreasing for the other instrument. The ratio of the currents in the tuned circuits is thereby measured and the scale may be graduated to indicate frequency. The instrument may alternatively be of the crossed-coil type, with a single permanent magnet, or of the " circular " type.