GB756650A - Improvements in and relating to electric control systems - Google Patents

Abstract

756,650. Automatic control systems. WEST INSTRUMENT CORPORATION. Sept. 16, 1954, No. 26893/54. Class 38 (4). In a closed sequence system for controlling a variable apparatus for correcting the variable is switched on and off intermittently by an electrical relay, the ratio of on to off time of which is controlled by the variable through a saturable reactor. When the temperature of an electric furnace 1, Fig. 1, has a desired value a thermocouple 12, responsive to furnace temperature, energizes an electrical temperature indicator 17 so that a flag 32, carried by the pointer 18 of the indicator, half obscures the light transmitted between a lamp 23 and a lead sulphide photo-cell 28 carried by an arm 20 for setting the desired temperature. Under these conditions photo-cell 28 produces such an energization of a control winding 31 of a saturable reactor 36, 37 in series with the supply from a mains transformer 26 to a selenium rectifier 10, that the output of the rectifier is sufficient to energize a relay 6 and switch on a furnace heater 2. Operation of relay 6, however, energizes a heater 68 and de-energizes a heater 67 so that a temperature-variable resistor 58 is cooled and a temperature-variable resistor 59 is heated to increase the energization of a bias winding 54 opposing winding 31 and thereby de-energize relay 6. The cycle then repeats and heater 2 is intermittently energized with equal " on " and " off periods to maintain the desired furnace temperature. Deviation of furnace temperature from the desired value increases or decreases the light falling on photocell 28 according to the sense of deviation to vary the energization of winding 31, and hence the ratio of " on " to " off " time of heater 2, so that the deviation is corrected. If the temperature falls to such an extent that all the light falls on photo-cell 28, heater 2 is permanently energized while if the temperature rises to such an extent that no light falls on photo-cell 28 heater 2 is permanently de-energized. Feedback dependent on relay current is applied to winding 63 which aids winding 31 to ensure positive operation of relay 6. Application to maintaining a desired pressure in a space by controlling an electric pump or valve supplying fluid to the space is referred to.