Patent GB-423618-A (GB)
Title: Improvements in and relating to electric transformers and choking coils
IPC: H01F27/38
Filed: 1933-10-25
Abstract: 423,618. Transformers; changing wave form. SIEMENS - SCHUCKERTWERKE AKT.-GES., Siemensstadt,- Berlin. Oct. 25, 1933, No. 29576. Convention date, Oct. 25, 1932. [Class 38 (ii)] [See also Group XXXVI] In arrangements for obtaining a high magnetization output from induction apparatus two or more magnetically saturated three phase choking coils or transformers are connected in series the windings of the choking coils or transformers being arranged in star or zig-zag, and a magnetic short-circuit path for the harmonics of the flux divisible by three being provided. Fig. 1 shows two saturated three phase choking coils 1 and 2 supplied from the mains 3 and so arranged in series in star-zig-zag on a common core structure that the fundamental waves of the fluxes in the aligned limbs of the structure are 30‹ out of phase, and no fifth or seventh harmonics arise in the resulting current or potential. The current taken by the coils 1 and 2 is then of sine form, as is the potential between the lines connecting the coils to the mains 3 while a magnetic short-circuit path, for the harmonics of the flux divisible by three, is provided by the unwound limbs. The flux waves of the coils thus contain a pronounced third harmonic and are substantially rectangular. A high magnetization output of the induction apparatus is thus obtained and the phase potentials of the separate choking coils or transformers contain a marked peak in each half wave, the voltage peaks in windings on any two aligned limbs differing in phase by 30‹. Transformers of this character may therefore be used for controlling the grids of rectifying apparatus &c. In Fig. 3, two sets of series connected choking coils are shown arranged in star-zig-zag, on a common core structure, and so connected to the mains that the fundamental flux waves produced in the two sets are of opposite sign. The harmonics of the flux divisible by three produced in one set are then of opposite sign to those produced in the other set, and additional unwound limbs are therefore not provided. In a modification, series connected polarizing windings supplied with direct current through a variable resistance, are arranged on each limb of the core structure. These windings are arranged to produce a constant flux of the same amount and in the same direction in each of the wound limbs, thereby preventing the formation of even harmonics, and additional unwound limbs are provided to form a return path for this constant flux. In a further modification the two sets of coils are so connected to the mains that their fundamental fluxes have the same sign, the additional unwound limbs providing a return path for the harmonics of the flux divisible by three, while the series connected windings supplied with direct current are arranged on each limb so that the constant fluxes in adjacent limbs are in opposite directions and no even harmonics are produced. An arrangement comprising four choking coils having their windings connected in series and arranged each on a separate set of limbs of a common core structure is described and is stated to permit compensation of all the harmonics up to the 23rd.