Patent GB-953914-A (GB)
Title: Improvements relating to apparatus for supplying a d.c. load from an a.c. source
Inventor: CALVERLEY HENRY BOLTON
IPC: H02M7/06
Filed: 1959-09-18
Abstract: 953,914. Transformers. ENGLISH ELECTRIC CO. Ltd. July 20, 1960 [Sept. 18, 1959], No. 31861/59. Heading H1T. [Also in Division H2] Apparatus for supplying a D.C. load from an A.C. source comprises a transformer and rectifier means for connection between the load and the source, the transformer having a tapped secondary winding with tap changing switches connected between the tapped secondary winding and the rectifier means in an even number of groups in an arrangement in which adjacent ones of the top charging switches belong to different groups, and means for operating the tap changing switches sequentially to connect the rectifier means and the load in a sequence in which no switch is opened until the next switch is closed, said rectifier means including rectifier devices connected between each group of switches for preventing the transformer winding from being short circuited when any two switches of different groups are closed at the same time. In one arrangement (Fig. 1) for supplying a variable voltage to traction motors M1, M2, a supply transformer 10, has a tapped secondary with tapping contactors T1-T8, divided into two groups, one connected through the saturable reactor arrangement A which is capable of absorbing the full inter-tap voltage, and the other through the similar saturable reactor arrangement B, and rectifiers R1-R6, to the traction motors M1 and M2. The control windings A3, A4, B3, B4, of the saturable reactors A and B, which have biasing windings A5, B5, are fed by tapped potentiometer 28, supplied by a tertiary winding 26, operation of the potentiometer tapping switches TR1-TR7, and switches TA1, TA2 and TB1, TB2 being linked with the transformer tapping contactors T1-T8 so that on connecting a transformer tapping to the load, the voltage step then applied to the load is reduced by the full impedance of the windings of the appropriate saturable reactor, this impedance being progressively reduced before the next transformer tapping is connected by operating the potentiometer switches to increase the saturating current to its maximum. Motors M1, M2 are fed through a bridge circuit having two branches, containing rectifiers R7, R8 and R9, R10, connected to the low voltage end of the transformer secondary, and two opposite branches, each being connected to both groups of transformer tappings, one through rectifier R5 and a load winding on each of the saturable reactors A and B, through the associated rectifiers of the group R1-R4 and the other, through rectifier R6 and the remaining two load windings of A and B and the other rectifiers of the group R1-R4. The group of rectifiers R1-R4 comprise the rectifier devices protecting the intertapped portion of the transformer winding from short circuit. A voltage limiter such as a silicon carbide resistor or a capacitor may be connected between the reactor sides of R1 and R2 and similarly for R3 and R4. The potentiometer may be replaced by a tapped auto-transformer or a continuously variable potentiometer. The whole potentiometer circuit may be replaced by two transducers of the Ramey-flux reset type (Fig. 3), the saturation of one of the saturable reactors A and B being effected concurrently with the desaturation of the other, the saturation currents for saturable reactors A and B being derived from transductors C and D of the Ramey-flux reset type with their control rheostats C1, D1, mechanically linked with the main cam-shaft controller operating the contacts T1-T8. In a further embodiment (Fig. 6) the saturable reactors A and B and associated rectifiers may be replaced by controlled silicon rectifiers, having their associated variable phase gate control units regulated by the potentiometer circuit of Fig. 1, or, both saturable reactors and potentiometer circuits may be omitted (Fig. 4, not shown). In the (Fig. 5) arrangement, a centre tapped secondary 15, is used, with the mid point between the motors M1, M2, connected to the centre of the transformer secondary, and tapping contactors for both halves of the windings, supplying the motors, through rectifiers as shown. During tap charging the corresponding contactors for the two halves of the winding may be operated simultaneously or alternately.