Patent GB-656778-A (GB)
Title: Improvements in or relating to electrical control systems
IPC: H02P7/26
Filed: 1947-09-12
Abstract: 656,778. Rectifying; changing phase displacement; automatic speed control systems. BROWN, W. J. Sept. 12, 1947, No. 25008. [Classes 38 (ii) and 38 (iv)] [Also in Group XXXVII] In an electrical control system of the kind in which an electrical load is dependent on the output of a converter supplied from an A.C. source and comprising one or more space-discharge devices provided with control electrodes, the power supplied to the load being controlled by varying the ignition time of the converter, voltages derived from at least one resistance in series with the load and from a shunt resistance in parallel with the load and with at least a portion of the series resistance, are combined with one or more controlling voltage or voltages to produce at least one resultant voltage which is applied to a control device for varying the ignition time of the converter. The armature of a motor 1 is supplied from single-phase mains through two grid-controlled rectifying valves 5 and 6, the grids of which are supplied through a transformer 7 with alternating voltages which are varied in phase relatively to the anode voltages of the valves. The primary winding of the transformer 7 is connected to terminals 16 and 17 of a phaseshifting network 10 which is supplied from the mains through a transformer 11. The network comprises a condenser 14 connected in series with a thermionic valve 15, preferably of the pentode type. A quadrature feed-back circuit for the valve 15 is provided by the connection of circuit elements 18 and 19 and the valve functions as a variable reactance so that the control grid-potential thereof determines the phase-displacement of the voltage applied across terminals 16-17 relative to the supply-voltage, and therefore the output of the rectifying valves 5 and 6. The grid cathode circuit of the valve 15 comprises adjustable portions of a speedcontrol potentiometer 30, a. resistor 26 which is connected across the motor armature, and a resistor 27 which shunts a resistor 28 in the motor armature circuit. When the motor is lightly loaded, the voltage-drop in the resistor 27 is small and the grid-bias of the valve 15 is substantially determined by the difference of voltages derived from the elements 30 and 26. The corresponding phase-displacement of the grid voltages of the valves 5 and 6 is such that the motor operates at a speed in accordance with the position of contact 31 on potentiometer 30. An increase of load produces a higher voltage-drop across resistor 27 and the valve 15 is afforded an additional component of gridbias voltage and, if the tapping on the resistor 27 is in an appropriate position, the motor speed is restored to its original valve. The adjustment of the resistor 27 determines the degree of compounding which is obtainable. The motor-current is limited, for example during acceleration, by the connection of a potentiometer 38 and a resistor 29 in the grid circuit of the valve 15. Grid bias voltages of opposite polarity are derived from these resistors, but they do not normally become effective to control the valve owing to the connection of a rectifier 60 in circuit. If, however, the armature current rises to a value such that the voltage derived from the resistor 29 exceeds that from the potentiometer 38 by a predetermined amount, the valve 15 will become negatively biassed since the rectifier 40 is now conductive and the load-current is limited to a value dependent upon the position of contact 39 on the potentiometer 38. The circuit constants are preferably so chosen that the current-limiting arrangement has a definite over-riding control when it becomes operative. A reversing switch (not shown) for the motor has auxiliary contacts for applying, by means of condensers negative bias to the valves 5, 6 and 15 before the main contacts open. After the main contacts have closed, the auxiliary contacts operate to allow the condensers to discharge slowly, so affording smooth reversal. In the modification of Fig. 2, the converter energizes a field winding 42 of a Ward-Leonard generator 41 which supplies the motor 1. The potentiometer 30 is replaced by three independent potentiometers 43, 44, 45 controlled by a selector switch 49. Condenser 55 and resistor 32 are arranged to counteract the ripple across resistor 29. An auxiliary contact 56 of a main contactor affords an initial negative bias for the valve 15 which is gradually removed by the charging of a condenser 57 when the contact 56 opens, so rendering the current-limiting feature effective when accelerating from stand-still. The regenerative motor current is limited by the provision of potentiometer 38, rectifier 60, and resistor 29 in the grid circuit of the valve 15. When a reversed voltage of predetermined value is developed across the resistor 29 during regeneration, the rectifier 60 becomes conductive to increase the output of the generator 41. The phase-shifting network is modified in Fig. 2 in that the circuit elements 14, 15 are connected across a resistor 65 which is in series with a condenser across the mains. Resistors 67, 68 are also connected to the mains, their functionpoint being the terminal 16 of the phase-shifting circuit. The magnitude of the voltage derived from terminals 16, 17 remains substantially constant while its phase may be shifted through more than 180 degrees. The converter may be adapted to supply the field winding of a separately-excited motor, or the armature or field windings of a generator-driving motor in a Ward-Leonard set. Specifications 656,816 and 656,817 are referred to.