Patent GB-495370-A (GB)
Title: Improvements in or relating to three-phase transformers and three-phase choking coils
IPC: H01F27/38
Filed: 1937-02-11
Abstract: 495,370. Transformers; inductances. SIEMENS - SCHUCKERTWERKE AKT.- GES. Feb. 11, 1937, Nos. 4112, 4114, 4115, and 4116. Convention dates, Feb. 11, 1936, March 20, 1936, July 20, 1936, and Sept. 5, 1936. [Class 38 (ii)] In a three phase transformer or choking coil having the fluxes in the core yokes differing in phase from the fluxes in the main limbs, the yoke fluxes are arranged to have such magnitude and phase position that harmonics produced in the yoke fluxes balance out the corresponding harmonics in the limb fluxes, means being provided for suppressing the third harmonic in the limb fluxes and auxiliary windings being provided for suppressing the third harmonic in the yoke fluxes. The core yokes are arranged to form a closed magnetic circuit for the three phase fluxes therein and may be dimensioned to be so saturated that the harmonics produced therein are equal in amplitude to the harmonics produced in the limbs, the fluxes in the yokes being arranged to differ in phase by 30‹ from the corresponding fluxes in the limbs. The method of determining the dimensions of the core yokes is set forth in the Specification in connection with a transformer having a five limbed core. In such a transformer the third harmonic in the limbs is suppressed by arranging one of the main windings in delta connection, delta connected auxiliary windings being arranged on the yokes or partially on the yokes and partially on the main limbs. If the transformer or choking coil has a three limbed core the yokes are formed with a longitudinal air gap as shown in Fig. 4, so that a three phase system of fluxes can be developed therein, the dimensions of the yoke sections being determined according to the method referred to above. If the core is composed of three frame cores the large outer core must be so constructed that the air gaps between the yoke sections of the cores are made greater than those between the limb sections, the crosssections and lengths of the yoke sections being suitably chosen. Instead of subdividing the yokes by means of a longitudinal slot this may be effected by inserting conductors 4, Fig. 11, through the yokes and connecting them together at both ends by bridging members to form a damping winding. The yoke sections thus formed may be linked by short circuited delta windings 8, 9, 10, the parts of these windings passing through the yokes being formed by rods 6, 7, insulated from the yokes and the bridging members and preferably separated from adjacent conductors of the damping winding by non-magnetic gaps, e.g. slots. The rods 6, 7 may be replaced by a plurality of rods connected in parallel. In another arrangement the transformer core comprises three frame cores 21, 22, 23, Fig. 16, air gaps being arranged if desired in the inner frames and the cross-section of the outer frame being increased so that the magnetic resistances of the three frames are approximately equal. Short circuited windings 26 comprising rods connected by bridging members are provided between the frames magnetic connection between the frames being produced at certain points by the use of magnetic bridging members 25 or by reduction of the air gap and omission of the conducting bars at these points. The fluxes in the individual frames are arranged to be displaced 180‹/n with respect to each other where n represents multiples of 6. For example, for compensation of the 5th and 7th harmonics the fluxes of the frame parts in each limb have 30‹ phase difference while for compensation of the 11th and 13th harmonics they have 15‹ phase difference. A delta winding 27 is arranged on the frames and serves to suppress the third harmonics therein. In an alternative arrangement the air gaps between the frames are arranged to have a measure of magnetic conductivity over their entire length. The damping winding 26 may be movable or may be provided with a loading resistance so that adjustment of the leakage fluxes can be effected. The transformer or choke may be so designed that the harmonics produced in the yokes overcompensate for the harmonics produced in the limbs. The Specification as open to inspection under Sect. 91 describes arrangements in which the main limbs 1, 2, 3, Fig. 20 (Cancelled), of the transformer core are derived by longitudinal slots the halves of each limb having oppositely wound turns arranged thereon these turns being arranged to form a delta connected winding I, which is supplied from a star connected auxiliary winding II. Alternatively the winding I may be supplied from one of the main transformer windings and may be arranged to surround the yokes instead of the limbs, provided that it passes through the slot in the limbs. A desired phase difference, such as 30‹ or 36‹, is thus produced between the fluxes in each pair of limb halves several other arrangements of the winding I to produce this effect being described. The limb slots may be arranged parallel to the plane of the laminations, the windings being formed as flat copper strip windings or " tube windings ". This subject-matter does not appear in the Specification as accepted.