GB708290A - Improvements relating to electric circuits - Google Patents

Abstract

708,290. Control of D.C. motors. BRITISH THOMSON-HOUSTON CO., Ltd. June 26, 1952 [July 27, 1951], No. 17874/51. Class 38(3) [Also in Group XXXV] A reversible direct-current is supplied to a load 5, which may be the field or armature winding of a servo motor, from two power supplies 1, 2, which are connected respectively to grid-controlled discharge valves 3, 4, in such a manner that the difference of the valve-currents passes through the load. A signal voltage is applied to the grid of the valve 3 from a source S, the voltage-drop developed across a resistor 6 causing an increase of current through the valve 3 to lead to an equal decrease of that passed by the valve 4. In order to avoid degenerative feed-back the valves 3, 4, may be provided with screen grids which are each connected to the anode of the opposite valve, Fig. 2 (not shown). If the power supplies 1, 2, have appreciable regulation, the screen grid potential of either valve is increased when the valve passes more current: Resistors, each carrying the anode-cathode current of one of the valves, provide a constant bias potential for the valve 3, an increase of the current in one resistor being balanced by an equal decrease in the other, Fig. 3 (not shown). In order to reduce the power loss in the resistor 6 it may be connected to an auxiliary D.C. amplifier valve so arranged that an increase of current in the resistor biases the auxiliary valve positively and reduces the potential of the grid of the valve 4 which is connected through resistance to the anode of the auxiliary valve, Fig. 4 (not shown). The supply sources 1, 2, may be either A.C. or D.C. in character. Smoothing is not necessary if the sources are of rectified A.C.