GB418933A - Improvements relating to electric transformers - Google Patents

Abstract

418,933. Current transformers. BUCHANAN, J. H., Carnethy, South Drive, Timperley, Cheshire, and ASSOCIATED ELECTRICAL INDUSTRIES, Ltd., Crown House, Aldwych, London. Aug. 23, 1933, No. 23461. [Class 38 (ii).] A dual ratio current transformer comprises two units, one of which has its secondary connected across the burden when operating on either ratio whilst the secondary of the other unit is adapted to be connected across the burden in parallel with the first secondary when operating on the lower ratio and to be short-circuited or reversely connected to the burden on the higher ratio, means being provided for adjusting the ratio and phase angle errors (e.g. as described in Specifications 286,431, [Class 38 (ii)], and 399,611) on one or both units whereby the combination is caused to function with substantially equal errors on either ratio. The first unit comprises a core 3 linked by a bar primary 1 and having its secondary 5 permanently connected to the burden at terminals 7, 8. The second unit comprises two main cores 11, 12 linked by the same primary and carrying series-connected secondaries 14, 15 adapted to be paralleled with the secondary 5 by interconnection of terminals 6, 7 for the lower ratio and to be short-circuited by interconnection of terminals 6, 8 for the higher ratio. A winding 16 connected in parallel with the secondaries 14, 15 is provided on the core 12 for adjustment of ratio as described in Specification 399,611. Phase angle adjustment is effected by providing on an auxiliary core 13 a secondary winding 17 connected in circuit with windings 18 embracing the cores 11, 12 and with an impedance 19 as described in Specification 286,431. In an alternative arrangement, Fig. 3 (not shown), phase angle adjustment is also effected on the first unit by providing an additional secondary winding on the auxiliary core 13 connected in circuit with a winding embracing the core 3. In this arrangement the secondary winding 5 is arranged in two parallel-connected parts of slightly different turn numbers thus modifying the ratio error variation and affording fractional adjustment of the turns ratio. Variation of ratio error with current variation in the other unit is reduced by the provision of a further secondary on the auxiliary core 13 connected in series with the main secondaries 14, 15 and their parallel-connected winding 16.