Patent GB-613917-A (GB)
Title: Improvements in and relating to electric motor control systems
IPC: H02P7/298
Filed: 1945-06-05
Abstract: 613,917. Rectifying; Automatic speed-control systems. BRITISH THOMSON HOUSTON CO., Ltd. June 5, 1945, No. 14156. Convention date, June 7, 1944. [Classes 38 (ii) and 38 (iv)] [Also in Group XXXVII] A control system for supplying variable direct - current to a load, such as an electric motor, from an alternating - current source through a pair of main triode valves comprises means for supplying dephased components of the alternating-current to the grids thereof consisting of a resistance-reactance circuit supplied from the source and including a capacitor connected between each of the cathodes and its corresponding grid, means for applying a component of direct voltage to each of the grids comprising a pair of control valves, each connected in a circuit in parallel with a portion of the circuit containing a corresponding one of the capacitors, and means for varying the firing point of the pair of main valves to vary the current in the output circuit thereof comprising means for varying the relative conductivities of the control valves whereby to vary the sign and/or value of the direct voltage. The armature of a directcurrent motor 1 is supplied from a transformer 5 through two gas-filled triode valves 3 and 4, the grids of which are supplied with alternating voltages, which lag the anode voltages substantially by 90 degrees, from a resistancereactance circuit comprising condensers 14 and 15 which are connected across the grid-cathode circuits of the valves 3 and 4 respectively. Direct voltage components are applied to the grids 3c and 4c through a grid-controlled rectifying valve 23 and a diode rectifying valve 24 which are connectable in series with one another and are respectively connected in parallel across a portion of the circuit containing the condensers 14 and 15. The grid of the valve 23 is controlled by a voltage which is derived from the motor armature. and is therefore substantially proportional to the motor speed, applied across a potentiometer 34, resistors 35 and 36, and the active portion of a potentiometer 37, and compared with a reference voltage applied across a potentiometer 25. An increase of conductivity of the valve 23 retards the firing-point of the valves 3 and 4. In order to limit the armature current a triode valve 39 is connected in parallel with the valve 23, and the input circuit of this valve is adapted to respond to a pre-determined maximum armature current, a current transformer 40 which applies a voltage to the grid 39c through a full-wave rectifier valve 41 being connected in the anode circuits of the valves 3 and 4. The cathode of the valve 39 is connected to a source of reference voltage. A valve 48 is provided to prevent undue delay in response of the valve 39 (in the event of an overload after a long period of low-armature current) owing to the time constant of the filter circuit which comprises a resistor 46 and a condenser 47. The coil of a relay 50 is placed in series with the source of reference voltage for the valves 23 and 39. The time of response of this relay affords an additional period for the valves 23 and 39 to cut off the valves 3 and 4 and prevents too rapid motor re-starting or reversal. The signal voltage applied to the valve 23 is compensated for voltage drop in the motor armature by applying a portion of the secondary voltage of the transformer 40 to the potentiometer 37. A dynamic braking resistance 52 is connectable across the motor armature terminals.