GB645075A - Improvements in or relating to electric servo arrangements - Google Patents

Abstract

645,075. Automatic current control systems. GENERAL ELECTRIC CO., Ltd., and DALE, S. H. Feb. 10, 1948, No. 3891. [Class 38 (iv)] In a closed-loop servo system hunting is substantially eliminated by a negative feed-back path, for a grid-controlled thermionic valve in the loop, containing a series inductancecapacity network resonant at a frequency less than that at which the system would hunt in the absence of the feed-back path. A D.C. motor 1 is supplied from A.C. mains 2 through a transformer 9 and gas- or vapour-filled valves 4, 5 forming a full-wave rectifier 3. A D.C. transformer 30 (not shown in detail), which includes saturable reactors, metal rectifiers and a resistance, and is associated with A.C. mains 2, produces a D.C. voltage, dependent on motor current, which is opposed to a reference D.C. voltage, obtained through a potentiometer 36 from a battery 37, and applied to the gridcathode circuit of a triode amplifier valve 28 direct-coupled to a second triode amplifier valve 29 energizing saturating windings 27 of a reactor 19 thereby controlling the phase of voltages applied to the grids of valves 4 and 5 from A.C. mains 2, so that motor current is maintained constant at a value determined by the setting of potentiometer 36. To eliminate hunting a negative feed-back circuit between anode and grid of valve 28 includes a capacitor 47 and a reactor 48, series resonant at a frequency lower than that at which hunting would otherwise occur, together with a resistor 49 having a value lower than the reactance of the reactor at the hunting frequency. The following modifications are referred to : (a) supply to the motor may be obtained from a polyphase source; (b) in place of the D.C. transformer, current transformers may be included in the anode circuits of valves 4 and 5 and their output rectified; (c) valve 28 or 29 may be omitted, feed-back being applied to the remaining valve; (d) the Provisional Specification refers to a shunt as an alternative to the D.C. transformer.