Patent GB-629604-A (GB)
Title: Improvements in and relating to electric control systems
IPC: H02P9/00
Filed: 1946-07-29
Abstract: 629,604. Automatic control systems. BRITISH THOMSON-HOUSTON CO., Ltd. July 29, 1946, No. 22553. Convention date, July 9, 1945. [Class 38 (iv)] [Also in Group XXXVII] A control system whereby the excitation of the field winding of a dynamo-electric machine is varied to maintain an operating characteristic of the machine constant comprises a unit for delaying the action of the regulator in response to changes of the characteristic within predetermined limits so that there is no regulator action for temporary changes within these limits. In the Figure, current from transformer 11 is supplied through two grid-controlled gas discharge valves 9 and 10 and resistance 14 to the field winding of a dynamometer 2 which is mechanically connected to a gas engine 1. The dynamometer may act as a motor or a generator. The grid of each of the valves 9 and 10 is supplied with a steady bias voltage and also an alternating voltage which is phase displaced relative to the anode cathode voltage. The bias voltage is varied by automatic control means, to regulate the current through valves 9 and 10 and thus control the field winding current. A potentiometer consisting of resistances 23, 24, 25, 26 and 27 is connected across a voltage supply source provided by winding 15b and rectifier 19. Resistances 29, 30 and 31 are connected across resistances 23 to 26. The cathodes of valves 9 and 10 are connected to point 24a and the grids to a point between resistors 30 and 31. A control valve 32 in series with resistance 14 is connected from a point between resistances 29 and 30 to point 24a. If the conductivity of valve 32 is increased, the voltage drop across resistance 29 then increases, which effectively reduces the grid bias voltage of valves 9 and 10 so reducing the field winding current. The grid of valve 32 is connected through resistance 35 to a tapping on resistance 33, which in series with resistance 34 is connected across the resistance 24. A tachometer generator 38, which is mechanically coupled to dynamometer 2, produces a voltage proportional to speed, across resistances 40 and 39. The potential of the cathode of a regulator valve 37 is derived from the sum of a negative voltage from tachometer 38 through a tapping on resistance 40, and a steady positive voltage which is developed across resistance 41. The anode current of valve 37 passes through resistance 35 and thus controls the conductivity of valve 32, and therefore the field current. The steady voltage across resistance 41 is derived from the anode current of a valve 45 which is supplied from a separate voltage supply. An additional valve 44, the grid of which is connected to the cathode of valve 37, and the cathode to point 25a is provided, and the anode of valve 44 is connected by a condenser 55 to the grid of valve 45. The grid of valve 45 is also connected through resistance 61 to a point of steady potential on potentiometer 57, 58, 59 connected across the separate voltage supply, and two rectifiers 60 are provided to limit the potential variations of the grid to the values existing at the ends of resistance 58. Variations in the voltage from tachometer 38 thus vary the conductivity of valve 44 which effectively applies a voltage to the grid of valve 45 through condenser 55, tending to produce a compensating change in the voltage across resistance 41, so that for small, temporary changes in speed substantially no control voltage is applied to the cathode of valve 37. If the change in speed persists however, the charge on condenser 55 alters and the field current is then varied due to a change in the conductivity of valve 37. Also, if the change of speed, and therefore of tachometer voltage, is sufficient to cause either of the rectifying portions 60 to conduct and thus prevent further alteration of the voltage across resistance 41, the control action through valve 37 then acts normally. Valve' 63 is provided to prevent excessive armature current. If the dynamometer 2 is operating as a generator, an increase in armature current makes the potential of grid 63d more negative which increases the conductivity of valve 32 to reduce the field current and thus reduce the armature current. If the dynamometer is operating as a motor, an increase in armature current causes the grid 63d to become more positive and thus to increase the field current. Adjustment of this effect is effected by connecting the cathode of valve 63 to a tapping on resistance 27. The right-hand triode portion of valve 63 produces an opposing effect to prevent hunting.