Patent GB-622276-A (GB)
Title: Improvements in and relating to electric motor control systems
IPC: H02P7/26
Filed: 1944-12-11
Abstract: 622,276. Automatic control systems; speed of motors. BRITISH THOMSON-HOUSTON CO., Ltd. Dec. 11, 1944, No. 24788. Convention date, Dec. 11, 1943. [Class 38 (iv)] [Also in Group XXXVII] The armature 10 of an electric motor is connected to an alternating-current supplysource through a triode valve 11, preferably of the gas-filled type, the conductivity of which is controlled by a periodically varying grid voltage having a direct-current component supplied by two reversely-connected valves 17 and 18, the valve 18 being a triode valve, the grid and cathode of which are supplied with voltages corresponding to the difference between a reference voltage and a voltage derived from the counter-electromotive force of the motor armature. As shown, the valve 18 is arranged to be rendered non-conducting when the valve 11 is conducting, the reference voltage is adjustable to vary the speed of the motor, and a condenser 19 is arranged to be charged through the valves 17 and 18 to supply the direct-current component of the grid voltage which controls the conductivity of the valve 11. The motor field winding 10a is separately excited and is connected in parallel with an adjustable potentiometer 23. The valve 18 is controlled in accordance with the difference of voltages derived from the potentiometer and the motor armature. When the valves 17 and 18 are equally conductive, an alternating- voltage, lagging the anode voltage of the valve 11 by approximately 90 degrees, is derived from the condenser 19. If the valve 18 passes more current than the valve 17, a negative direct bias voltage is applied to the grid of the valve 11. Initially, the valve 18 is cut off and the valve 11 is fully conductive. As the motor accelerates, the current passed by the valve 18 increases until an equilibrium condition is reached wherein the motor runs at a steady speed corresponding to the setting of the potentiometer 23. Since cathode 18b and anode 18a are respectively connected to anode 11a and grid 11c, the valve 18 is substantially non-conductive during the conducting-period of the valve 11, and the former valve is responsive to the motor armature voltage when there is no current in the armature circuit. The valve 18 therefore registers a true measure of the motor speed and is unaffected by armature resistance-drop. The current passed by the valve 11 may persist for a period beyond the normal instant of cut-off, due to the inductance of the armature winding. A degree of compounding is thereby obtained. Modifications show different arrangements for obtaining the reference voltage for pre-setting the motorspeed. A potentiometer supplying the reference voltage is arranged across a voltage-regulating discharge tube of the glow type and in parallel with a condenser. This condenser may be placed between the anode of a valve corresponding to the diode 17 and the supply-line, Fig. 2 (not shown), or may be charged through separately-connected valves providing halfwave or full-wave rectification, Figs. 3 and 4 (not shown). In another embodiment, Fig. 5 (not shown), the reference voltage is derived from a battery-supplied potentiometer in the cathode circuit of a valve corresponding to the triode 18, the grid of which is connected through a resistor to the supply-line to which is connected the positive armature terminal. A modification wherein the valve 17 is replaced by a triode which has an increasing negative bias applied thereto as the current transmitted by a valve similar to the triode 18 increases, is illustrated in Fig. 6 (not shown).