GB622573A - Improvements in and relating to control systems for electric motors - Google Patents

Abstract

622,573. Control of D.C. motors. BRITISH THOMSON-HOUSTON CO., Ltd. March 28, 1947, No. 8491. Convention date, March 28, 1946. [Class 38(iii)] A reversible D.C. motor 1 is supplied from an A.C. source and is provided with split series field windings 1b and 1c which are respectively adapted for forward and reverse motor operation. These field windings are connected respectively in series with half-wave rectifying valves 2 and 3 which may be of the gas-filled type. The rectifying valves are grid-controlled by means of individual condensers 6 and 11 respectively. The charge on the condenser 6 is varied by means of control valves 7 and 8, which are connected reversely in parallel. The valve 8 is of the triode type and has a signal voltage applied to the grid thereof. The valve 7 is adapted for diode operation. The condenser 11 is similarly controlled by means of a triode valve 14 and a diode valve 13, the grid of the valve 14 being connected to a voltage divider 16. A signal voltage is developed across resistors 18 and 19 which, for example, are connected in series with a photo-electric cell 20. The signal voltage is amplified by a valve 25 so that, with an increase of the signal voltage above a predetermined value, the conductivity of the valve 25 is increased, that of the valve 8 is decreased with respect to the valve 7, and the resultant voltage across the condenser 6 becomes such that the valve 2 is rendered conductive for forward motor operation. The effect of a common resistor 10 in the cathode circuits of the valves 8 and 14 is to increase the conductivity of the latter when the current through the valve 8 is reduced. The resultant voltage on the condenser 11 is such that the valve 3 remains at cut-off. Increase or decrease of the signal voltage, within a certain limit, leads to increase or decrease of the motor speed in the forward direction, but, if the signal voltage falls below a pre-determined value, the conductivity of the valve 8 is so increased that the valve 2 is cut off, the conductivity of the valve 14 is decreased, and the valve 3 fires for motor operation in the reverse direction. Further decrease of the signal voltage leads to increased motor speed in the reverse direction.