GB286431A - Improvements relating to electric transformers - Google Patents

Abstract

286,431. British Thomson-Houston Co)., Ltd., Wellings, J. G., and Mayo, C. G. Feb. 8, 1927. Transformers for use with measuring instruments, tripping devices, relays, &c. are corrected or compensated to any desired extent in respect of phase-angle error, ratio error, or both, by means of auxiliary transformers supplying auxiliary excitation to the main transformers in approximately lagging quadrature. For this purpose the magnetizing current in the auxiliary transformer is made large compared with its output current, preferably by running at a high flux density, by using a core of high reluctance or both. In the arrangement for phase-angle correction shown in Fig. 1, the main tranformer 20 and auxiliary transformer 20<1> have a common bar primary 21 and cores of the same diameter but different cross-section, the auxiliary core being about <1>/4 to <1>/10 of the size of the main core, while a single coil 23<1> on the auxiliary coil is connected through an adjustable impedance 25 to an additional coil 24 of two turns on the main core, the secondary coil of nineteen turns being connected directly to the burden. The arrangement for correcting both phase-angle and ratioerrors shown in Fig. 6 includes also an additional secondary coil 26 of one or more turns connected in series with the secondary coil 23 of the main core; in modifications of this arrangement, the coil 23<1> is connected only to the impedance 25 or is dispensed with. In the arrangement shown in Fig. 14, the main core is divided into two parts 20<a>, 20b, and the divided additional windings 24 are cross-connected to the coil 23<1>; in modifications of this arrangement the auxiliary core may be half the size of the main core, the divisions of the main core may be of unequal size, and the winding 26 may be dispensed with. Specifications 169,093 and 259,198 are referred to. According to the first of the Provisional Specifications, the main and auxiliary transformers are of different current ratio and the cores are of different sizes but the same material, while the ratio of the turns of the secondaries is equal to that of the mean lengths of the magnetic circuits, so that the cores work at equal flux density. The cores may thus be of different diameters, so that one may surround the other. The core of the auxiliary transformer may be a tube or punchings of sheet steel; or of sheet steel in zig-zag formation.