Patent GB-616494-A (GB)
Title: Improvements in and relating to electric motor control systems
IPC: G05D3/12
Filed: 1946-09-06
Abstract: 616,494. Electric control systems. BRITISH THOMSON-HOUSTON CO., Ltd. Sept. 6, 1946, No. 26891. Convention date, Jan. 23, 1945. [Class 40 (i)] [Also in Group XXXVII] A system for controlling the speed and direction of rotation of a motor according to the magnitude and phase of an alternating signal voltage comprises two power valves, preferably thyratrons, arranged to supply the armature of the motor, in conjunction with an alternating- current source, with current the magnitude and direction of which is governed by the application to the power valves of direct-current potentials derived through filters from asymmetric alternating-current voltages produced by twintriode valves the grids of which are controlled by the signal voltage. The signals may be derived through the relative displacement of a selsyn transmitter and a motor-driven selsyn receiver, the control of the motor serving to effect a follow-up movement of the receiver. Two power valves such as thyratrons 12 and 13 are adapted to be reversely connected in parallel between a motor terminal 1c and a terminal 10 of an A.C. source of which the other terminal 9 is connected to the motor terminal 1d. Direct signal voltages are supplied to the grids of these valves to energize the valves selectively and effect motor rotation in a chosen direction at a speed corresponding to the magnitude of the control signal. To prevent hunting a feedback voltage is supplied to each grid circuit, that for the valve 12 being applied to a gridbias resistor 40 which is connected to the cathode 12b and the armature terminal 1d through a low-pass filter circuit 38. The stabilising voltage for the valve 13 is applied across a grid-bias resistor 43 which is connected to the cathode 13b and the armature terminal 1c through a low-pass filter circuit 41. A voltage, equal in magnitude and opposite in phase to that of the source, is applied by means of a transformer 44 between the grid and cathode of the valve 13 so that the resultant stabilising voltage is equal to that across the motor armature and is indicative of the rate of change of motor speed. As shown, a control signal is applied to the primary winding of a transformer 2 by means of a selsyn transmitter 3 and receiver 4 the rotor of which is mechanically coupled to the motor. The phase of the voltage supplied to the transformer 2 depends upon the direction of rotation of the rotor of the selsyn transmitter, and the magnitude corresponds to the angular displacement of the rotors of the transmitter and receiver. Twin triode valves 16 and 17 control the selective energization of the power valves, the left-hand portions thereof constituting phase discriminators for selective energization of the thyratrons in response to signal voltages of reverse phase. The right-hand portions of these twin triode valves conduct when the anode voltages of the respective left-hand portions are negative. A secondary winding 2b is connected in the gridcathode circuit of the left-hand portion of the valve 16 which is rendered conductive when the control signal is in the appropriate direction. A resistor 19 is connected in the output circuits of both portions of the valve 16 and when unequal currents on opposite half-waves are passed thereby the resultant voltage across the resistor contains both A.C. and D.C. components. The A.C. component is removed in a bridge network 21, the constants of which are arranged to eliminate alternating-current of the appropriate frequency, and the D.C. component is applied to the grid of the valve 12. The valve 17, which is rendered active on reversal of the signal voltage, otherwise operates similarly to the valve 16. A.C. grid bias voltages for the power valves 12 and 13 are supplied through a transformer 34. Time delay in completing or opening the output circuit of the valves 12 and 13 is provided by a valve 45 which controls a relay 46. The heater circuit of the valve 45 comprises a series resistor 47 which allows the power valves to be heated 'before a relay contact 46b is closed.