Patent GB-190920867-A (GB)
Title: Improvements in Methods of Obtaining Continuous Currents from Alternating Current Circuits.
Inventor: TAYLOR ALFRED MILLS
IPC: H02M1/22
Filed: 1909-09-13
Granted: 1910-10-13
Abstract: 20,867. Taylor, A. M. Sept. 13. [Cognate Applications, 21,157/09, 21,622/09, 22,791/09, 24,444/09, 1182/10, and 5896/10.] Commutator switching-arrangements; transformers. - For the purpose of producing a rectified current from an alternating-current supply, a transformer having a core highly saturated magnetically has its primary winding connected in series with a choking-coil having an unsaturated magnetic circuit, and the resulting current, which is alternating but with a changed wave-form, is rectified by. two commutators connected to the ends of the secondary winding, auxiliary resistance switches being used in conjunction with these commutators to avoid sparking. Modified methods of changing the wave - form, the object of which is to increase the period available for commutation by flattening-out the curve in the region of the zero points, comprise (1) a transformer having three limbs, primary and secondary coils being wound upon the outer limbs, and the central limb being excited with direct current, and (2) a transformer or choking-coil having a magnetic circuit the flux in which is interfered with in a prearranged manner by a rotating " keeper." An arrangement for converting three-phase alternating current into a continuous current is shown in Fig. 1. The primary windings of three preferably shell-type transformers A, B, C (having their magnetic circuits saturated by reducing the iron section in the central limb) are connected in star connexion to the threephase mains D, one of three choking-coils X, Y, Z (having unsaturated magnetic circuits and provided with gaps) being connected in series with each primary winding. Each secondary winding is connected at each of its ends by two leads with the fixed segments E, O and N, F, respectively, of two commutators J, K. Thus, there are six commutators controlling the end connexions of the three secondary windings, and these are arranged, by connecting the rotating members of the switches as shown, so that the secondaries are always in series connexion R, R. The fixed segments of each commutator are arranged to overlap at their ends for a short are, the rotating member being disposed so as to connect them together twice in each revolution at a period corresponding to the existence of approximately zero electromotive-force in that secondary coil to which the commutator is connected. This period is considerably extended by the deformation of the alternating current wave (to the shape shown in Fig. 9, for example), due to the transformer arrangement described, and admits of the operation of the following devices to prevent the sparking which would otherwise be caused by the transference of the whole of the current in the direct-current circuit R, R to that secondary coil. Each rotating switchmember carries an insulated bridge-piece S1, S2, which establishes connexion, at the period referred to, between the connected commutator segments and a fixed segment T', T<2> connected to one end of a resistance circuit V<1> ... V<4>, the resistance in which is arranged to be quickly but progressively increased during the instant that the secondary-short circuit is being broken, thus forcing the current therein into the secondary coil in a sparkless manner. A modified transformer arrangement, in which the choking-coils X, Y, Z are dispensed with, consists in employing transformers A, B, &c.. Fig. 8, each having three limbs. Each transformer has two primary windings a, b arranged in parallel and connected in star connexion to the mains D, and two independent secondary coils, working in conjunction with the respective primaries, which are arranged on the outer limbs as shown. These secondaries are connected to the commutators J, K, described above, in the manner shown so that they are in circuit alternately during half a wave. The whole of the current in the directcurrent circuit R1 is led around the middle limb c of each transformer and magnetizes it in a constant direction; but the flux passes through the outer limbs alternately according to the direction of the excitation due to the primary coils thereon, and effects a periodic saturation of the magnetic circuit of the secondary coils, which results in the desired deformation of the secondary pressure wave. This arrangement presents the advantage that the extent of the variation of wave-form produced varies according to the secondary load. Instead of this arrangement, ordinary transformers may be used, the primary windings being placed in series with a choking-coil, an air gap in the magnetic circuit of which is bridged at the required moment by a rotating keeper. Another modification is shown in Fig. 11, in which the flux from a primary coil a is caused to saturate the magnetic circuits of each of two secondary coils d, e alternately, at the instant the lowest electromotive-force is being induced therein, by means of rotating " keepers " b, c. An arrangement which is the converse of that shown in Fig 8, is described for converting direct current into alternating current, the only modifications made being in the values of the resistances V<1> ... V<4> and the duration of the short-circuits established by the switches J, K. This last conversion system is described in combination with a system according to that described in connexion with Fig. 1, the two systems together forming a means for transforming continuous currents. Fig. 13 shows a method of direct-current transformation which avoids this complete double conversion. In this case, the converted direct current from the circuit R1, R2, &c. is transformed to the required pressure on the outer limbs of transformers with three limbs and then reconverted. The middle limb is excited by direct current obtained from the primary circuit R1, R2, &c. or the secondary circuit RR1, RR2, &c., or from both; or by a series winding from one circuit and a shunt winding from the other. This gives the waveform required by the commutators. In the foregoing arrangements, three, and if necessary six, phases from at least three transformers are employed, all the secondaries being in series for the purpose of producing a direct current free from pulsations, and, at the same time, destroying spurious inductance in the secondary circuit. The high saturation of the transformer cores at the instant of commutation practically neutralizes the self-induction of the secondary windings, and assists in avoiding sparking. The resistance circuits V1 ... V4 are employed in all the modifications, and are controlled by rotating switches. Three forms are described. As shown in Fig. 3, resistances U, ... U6 are connected between a series of fixed brushes bearing upon a rotating drum. This drum carries an insulated stepped contact segment V. The drum is adapted to short-circuit the whole of the resistances, and, at the moment the short-circuit is removed from the secondary circuit by the commutators J, K, to remove the short-circuit upon these resistances in rapid steps. The switch is shown arranged for operation by a two-pole motor n, s, but a multipole motor may be used, the number of switch devices in association with the drum being increased proportionally. Fig. 4 shows a resistance strip z arranged in a zigzag manner between insulating segments forming part of a cylinder, upon which bear fixed brushes T', T<2> connected to the auxiliary fixed contacts of the switches J, K. The remaining periphery of the cylinder is of conducting material, to the ends of which the resistance strip Z is connected. The brushes or the resistance strip may be set at an angle to the axis of the shaft. In a third form, Fig. 5, a number of notched contact disks V<1> ... V<7> are insulated upon a shaft, each disk being slightly displaced circumferentially in one direction with regard to the others above it. The disks are arranged to pass between the short-circuit pairs of brushes b<1>, arranged in a line parallel to the shaft, and between each of which a section of the resistance U1 ... U7 is connected. When dealing with heavy currents, several resistance circuits may be employed, connected in parallel; to obviate unequal distribution of the currents in these circuits, each is led around an iron core in such a way as to establish a mutual balancing reaction one upon the other. Fig. 6 shows an arrangement for four parallel circuits. The circuits are grouped in pairs, and each circuit is wound in an opposite direction to its fellow around a core c, g, the free ends being connected together. The resultant circuits from each pair are led in opposite directions around another core e. Provisional Specification 20,867/09, describes a further method of converting alternating current into continuous current. Two primary coils and two secondary coils are wound upon the outer limbs of a three-limbed core and are arranged to be worked at high saturation, the fluxes produced in each outer limb by the primary winding being arranged to leave the middle limb unexcited. The primary windings are in parallel, and the secondary windings which are wound in opposite directions are connected in series solidly to the direct-current circuit. The coil which is for the moment producing an electromotive force in the wrong direction is short-circuited by a rotating switch, a current limiting resistance or inductance being included if desired. Between the main segments of the rotating switch, several small contacts are arranged, these having graded resistance or inductance steps connected between them. All the switches and commutators described above are driven synchronously with the alternating current, and may be on one or more shafts. They may be immersed in oil or other suitable spark-quenching medium, or provided with magnetic or air blow-outs to extinguish any sparks. Provisional Specificaticn 21,622/09 states also that the resistances employed to shunt the gaps on the main commutators may be graduated, and t