Patent GB-599136-A (GB)
Title: Improvements in or relating to automatic d.c. voltage control systems
IPC: G05F1/00
Filed: 1944-12-22
Abstract: 599,136. Control of D.C. motors. FOSTER TRANSFORMERS & SWITCHGEAR, Ltd., LANCASHIRE DYNAMO & CRYPTO, Ltd., and JENNINGS, R. E. Dec. 22, 1944, Nos. 25798/44 and 32707/45. [Class 38(iii)] [Also in Group XXXV] Voltage; speed. - In an automatic D.C. voltage control system an alternating voltage controlled by the D.C. voltage is utilised for correction purposes, the voltage to be controlled and a constant D.C. voltage being supplied separately to the grid circuits of two thermionic valves the anodes of which are connected through separate load resistances to a source of constant D.C. voltage and to a winding of an A.C. transformer. In the Figure, the voltage across the armature A of a D.C. motor, obtained by fullwave rectification, through discharge-type valve rectifiers V6, V7, of the voltage across a winding W1 of a mains transformer, is maintained, automatically, at a predetermined value. A portion of this voltage is applied through a potential divider R14 to the input circuit of a triode V4 controlling the D.C. grid potential of one triode of a double-triode V5 through a potential divider R4, R5. The grid of the other triode of V5 is maintained at a constant D.C. potential through a potential divider R9 so that at the desired armature voltage the anodes of valve V5, which are connected through separate load resistances R6, R7 to a common source of combined A.C. and D.C., have equal potentials. Deviation of armature voltage from the desired value produces a difference of potential between the anodes of valve V5, the A.C. component of which is applied to the primary of a transformer J so that A.C. voltages dependent in magnitude and phase on the magnitude and sense of the deviation appear across secondary windings S1, S2. These voltages are combined with voltages, in quadrature with the voltage across winding W1, produced by resistance-capacity circuits associated with mains transformer windings W3, W4, and are applied to the grid circuits of rectifiers V6, V7, to correct, automatically, the armature voltage deviation. Armature voltage is preset by a potentiometer R3 adjusting the D.C. bias to triode V4. D.C. for the control valves is provided by a full-wave rectifier valve V1 the output of which is stabilised by neons V2 and V3. A.C. for valve V5 is provided by mains transformer winding W2. R15, C8 is a smoothing and anti-hunt network. In modifications of the above: (a) the volt-drop across a resistance in the armature circuit is introduced into the input circuit of triode V4 to compensate for the internal resistance drop in the armature so that motor speed is maintained substantially constant; (b) the voltage of a tachometer generator (rectified if A.C.) replaces the portion of the armature voltage applied to the input 'circuit of triode V4 so that motor speed is maintained constant; (c) the volt-drop across a resistance in the armature circuit is introduced into the input circuit of an additional triode valve controlling the conductivity of a diode valve so that when, during starting or under overload or stalling conditions, the armature current tends to exceed a predetermined value the diode conducts to reduce a grid potential of valve V5 and limit the armature current to that value. Since this limiting circuit is not effective immediately after switching on the armature supply, a charged condenser is introduced into a grid circuit of valve V5 on switching on to provide bias to limit the current surge. This condenser discharges through a resistance so that the bias is removed after the diode circuit becomes effective; (d) the field voltage is automatically controlled instead of the armature voltage. In this case triode V4 may be omitted and a portion of the field voltage applied directly to one of the grid circuits of valve V5. A short circuit is removed from this grid circuit at the instant of switching on the field supply to ensure full field when starting; .(e) the armature and field voltages are each automatically controlled and a relay system for controlling the starting and reversal of the motor and for protection of the rectifiers is described. The motor is rheostatically braked on switching off. A diode is arranged to conduct and raise the potential of one of the grids of the double-triode controlling the field voltage to limit the armature voltage to a predetermined maximum when increasing the field manually to reduce motor speed. There are references to automatic D.C. coltage control in which the A.C. output from a double-triode is amplified and applied to one winding of a disc or two-phase induction motor the other winding of which is supplied from a constant voltage A.C. source. The motor may operate a switch, rheostat or mechanical means to correct voltage deviations. The first Provisional Specification refers to the automatic control of alternator voltage in which the output is rectified and controls the alternator field.