Patent GB-686799-A (GB)
Title: Improvements in or relating to electrical control systems for electrical power converters
IPC: G05F1/00
Filed: 1950-08-17
Abstract: 686,799. Control of D.C. motors. BROWN, W. J. Aug. 17, 1950 [Aug: 17, 1949; Aug. 17, 1949; Aug. 17, 1949], No. 20474/50. Class 38 (iii) [Also in Group XXXV] The output of a converter (valves 1, 2) to a load 4 is controlled by varying the ignition time of the converter by means of a phase shift circuit including a capacitor 10 and a reactor 11 variably saturable by the difference between a reference flux and that produced by a D.C. current (in windings 13) derived from the converter output, e.g. load current from shunt 19. The phase shift circuits may be of the kind described in Specifications 656,778 and 656,817. In the Ward-Leonard control system shown in Fig. 5 the speed of a motor 26, fed from a generator 25 energised through a discharge type rectifier 29 across which is connected relief valve 33, is regulated by varying the phase of the supply to the grid of the rectifier from a network comprising resistances 5, 6 capacitor 10 and saturable reactor 34. To keep the motor speed constant, the difference between the voltage from a tachometer generator 28 driven by the motor 26 and voltage between taps 42, 45 of a potentiometer 38, is applied to a winding 36, the current through which varies the value of reactor 34 and the phase of the grid volts of rectifier 29. Rectifier 46 ensures that this current does not reverse and the motor stop when the speed setting control 45 is suddenly moved to reduce the potentiometer volts below the tachometer volts. Alternatively rectifier 46 may be connected in parallel with winding 36 Fig. 7 (not shown). An antihunt winding 37 is fed through a condenser 48 from the motor volts. When the forward armature current increases until the sum of the voltage drop across series resistance 40 and winding 36 is equal to the voltage drop between terminal 42 and tap 52 of potentiometer 38, current flows through rectifier 51 in opposition to the normal current and reduces the rectified current to the generator field 24. Under regenerative conditions rectifier 53 acts instead of 51 as a current limiting device. Fig. 9 shows a system in which the speed of motor 26 is kept constant by feedback from the armature volts, the value of inductance 82 being controlled to vary the phase of the volts of grids 73, 74 controlling the anodes 67, 68 of rectifier 66 which supply the motor armature. Motor field 27 is energised by auxiliary anodes 69, 70. Separate reactor control windings carry respectively the reference current from potentiometer 38 (84), feedback current from resistance 58 across the motor armature (85), antihunt current through condenser 94 (86), current limiting current through rectifier 96 (87), and compounding current (88) derived from a resistance 40 carrying the main armature current. Reversal prevention rectifier 98 with filter circuit 101, 102 is connected effectively across those ends of windings 85, 85 not connected together. The current limit control is improved by rectifier 104 which limits the voltage difference between terminals 105, and 107. The current limiting and compounding control currents may be derived through current transformers from the input A.C. through rectifiers, in which case the latter current may be compared with a supply through a rectifier and voltage transformer from the A.C. supply, Fig. 17 (not shown). In other modifications the reference potentiometer 38 may be fed from the A.C. input through a separate rectifier and if necessary stabilised by connecting across it a glow discharge tube, Figs. 14, 15, 16 (not shown). Instead of using constant control voltages from a potentiometer, permanent magnet 145 may form part of the saturable reactor in which fluxes are compared, Fig. 11 and Fig. 10 (not shown). This magnet is rotatable to vary the effective value of reference flux and thereby the setting of motor speed. Instability on rapid rotation of the magnet is prevented by movement of a polarized arm 158 when the main flux reverses in direction which movement breaks at 168, 169 the connections of reactor windings 148 to the phase shift circuit, thereby allowing the grid volts to reduce to a minimum the output of rectifiers 1, 2 which supply the field of a generator 25 connected to a motor 26. In a different kind of reactor the pivoted arm may be energised by the field current. Fig. 10 (not shown). Specification 656,816, [Group XXXV], also is referred to.