Patent GB-656817-A (GB)
Title: Improvements in or relating to electrical control systems
IPC: H03H11/20
Filed: 1948-12-07
Abstract: 656,817. Rectifying. BROWN W. J. Dec. 7, 1948, Nos. 31698, 31699, 31700, [Convention dates, Aug. 28, 1947], and 31701, [Convention date, Oct. 15, 1947]. [Classes 38 (ii) and 38 (iv)] [Also in Groups XXXVII and XL (b)] A load such as an electric motor is supplied from an A.C. source through rectifying valves, the power supplied being controlled by varying the time of ignition of the valves by means including a phase shifting network comprising a mutually variable inductance and capacity in series energized from a reference voltage, the dephased output being taken from a terminal between the inductance and capacity, the potential of which follows an arcuate locus in the vector diagram, and from another terminal whose potential lies within the space on the vector diagram bounded by the arc and the reference voltage. In Fig. 1, the armature of D.C. motor 1 is fed through rectifiers 5, 6 from A.C. supply 3. The valve grids are fed through a self-biassing resistance-condenser combination 8 and grid loading resistancecondenser combination 9 from a transformer 7 the primary of which is energized from terminals O, P of a phase-shifting network 10 of which the equivalent circuit and vector diagrams are shown in Figs. 2, 3 respectively. The variable inductance 15 is actually constituted by a valve 16 provided with a quadrature feed-back circuit 17. Anode and screen voltage for valve 16 and part of its grid bias which determines the value of its impedance come from a potential divider 24. A glow discharge valve 26 maintains constant direct reference voltage and constant screen volts. For keeping the motor speed constant a tap on a potential divider 27 across the armature is connected to a contact 29 movable on a speed control potentiometer 28 connected through adjustable acceleration control resistance 33 and condenser 33A, to potential divider 24. Change in motor speed and therefore in the armature volts alters the grid bias which comprises the sum of voltages across parts of potentiometers 27, 24, 28, and therefore the value of impedance 15 and the phase of the output voltage from O, P to the grids of rectifiers 5, 6, the output of which varies to bring the motor speed back to its set value. This may be predetermined by adjustment of potentiometer 28. To overcome the adverse effect of blocking condenser 35, the quadrature feed-back circuit may be suitably altered (Fig. 4, not shown). The phase shifting network 10 may be of the kind described with reference to Fig. 4 (not shown), or to the vector diagrams of Figs. 2, 5, 20, 22, 26 and 28 of Specification 656,816. These networks all have the characteristic that the initial phase angle of the output volts corresponds to that of the source volts so that upon failure of the reactance valve 16 or its D.C. source 25 the rectifier output will be reduced to zero. Also described are two circuits in which a 3-phase A.C. source supplies a D.C. motor through valve rectifiers with grids supplied from variable phase shifting networks (Figs. 5, 8, not shown). The Figures show the connections for one phase only in each case. Figs. 6, 9 (not shown), are simplified equivalents of the respective networks actually employed, although any of the phaseshifting networks described in Specification 656,816, might be used. The field and not the armature of the motor might be energized by the rectifiers or a load of an entirely different character might be supplied.