Patent GB-684438-A (GB)
Title: Improvements in or relating to electronic control systems
IPC: H02P7/34
Filed: 1950-11-08
Abstract: 684,438. Control of D.C. motors. IGRANIC ELECTRIC CO., Ltd. Nov. 8,1950 [Dec. 12, 1949], No. 27300/50. Class 38 (iii). A load is supplied through a discharge device which is connected to an A.C. source, the control electrode of the discharge device is subjected to a dephased potential, and its cathode is connected to the low potential side of a source of D.C. through means affording a potential-drop. A capacitor is charged from the D.C. source through a resistor, a normally closed switch maintains the capacitor in a discharged condition, and means including a normally open switch operates, when the latter switch is closed, to limit the maximum potential to which the capacitor is charged, the two switches being operated simultaneously. Means associated with the D.C. source supply the control electrode of the discharge device with a uni-directional potential varying in magnitude with the capacitor potential, the discharge tube accordingly varying the value of rectified current supplied to the load between predetermined minimum and maximum values. As shown, a direct-current generator 10 supplies a motor 11, the respective field windings 10b, 11b, of these machines being supplied from separate transformer secondary windings 12b, 12c, through gas-filled gridcontrolled valves 13, 14, which are each shunted by diodes 15, 16. The grid control voltages of the valves 13, 14, each comprise a direct-component and a superimposed alternating component, the magnitude and phase of the latter being constant and derived from circuit elements 50b, 51, 46, for valve 13 and elements 50c, 52, 48, for valve 14. Each of these phase-shift circuits provide a component which lags the respective anode voltage by about 90 degrees. The circuit from which the direct component of grid bias for the valve 13 is derived comprises a portion of voltage divider 20 and resistors 40, 39, 41. The potential drop obtained from the voltage divider 20 provides a negative bias which reduces the current passing through the valve 13, and thus the generator excitation, to a low value at the rest condition. Opposing this bias is the potential drop across resistors 40, 39, which are in the cathode circuit of triode 22d, 22e, 22f, the appreciable voltage-drop exists across resistor 41 unless triode 23a, 23b, 23c passes current, whereupon the conductance of valve 13 is limited. A capacitor 26, the voltage across which determines the potential of grid 22f, is charged through resistors 24, 25. At rest the capacitor is shunted through rheostat 38 and resistor 24 so that the generator voltage is low. The circuit from which the direct component of grid bias for the valve 14 is derived comprises a part of potential divider 21 and resistor 43. A positive bias is derived from element 21, and, at rest, the triode 23d, 23e, 23f, is cut off so that no appreciable potential-drop exists across resistor 43 and the valve 14 is substantially fully conductive, the motor field winding being fully energized. Upon closure of push-button 30 relay 31 is energized to connect the motor to the generator, the capacitor 26 being gradually charged to increase the potential of grid 22f and accelerate the motor. If a potential divider 34 is set for low speed operation, when capacitor 26 is so charged that the potential of the function of resistors 24, 25, is comparable with that of the slider 34a, triode 22a, 22b, 22c, becomes conductive so that a voltage is developed across resistor 24 which limits further charging of the capacitor and further rise of generator voltage. Under these circumstances, the motor field excitation remains at full strength. When the slider 34a is adjusted to provide operation of the motor in the field-weakened range, the rise of the potential of anode 23b, and hence grid 13c, is limited by the voltage-drop across resistor 41 and the field strength of the generator is limited to a value deterniined by the setting of slider 42a. Charging of the capacitor 26 continues, and, at a pre-determined potential of the function between resistors 39, 40, determined by the adjustment of slider 44a, triode 23d, 23e, 23f, is rendered conductive to reduce the motor field excitation at a timed rate. The reduction of the motor field excitation is halted when the potential of anode 22b approaches or exceeds that of slider 34a. Operation of stop button 29 deenergizes relay 31 to disconnect the motor from the generator and close a dynamic braking circuit through resistor 35. The capacitor 26 is discharged, the motor field excitation is returned to full strength, and the generator armature voltage reduced to a minimum. The function of rheostat 36 is to limit the maximum positive potential derived from potential divider 34 and thus the motor speed. The potential divider 34 is connected to the common point of resistors 32, 33, so that changes in the greater voltage are partially compensated for by grid control of the valve 13.