GB652052A - Improvements in and relating to temperature control systems - Google Patents

Abstract

652,052. Automatic temperature control systems. BRITISH THOMSON-HOUSTON CO., Ltd. March 16, 1948, No. 7877. Convention date, March 20, 1947. [Class 38 (iv)] A temperature control system for an electric furnace 1 comprises two grid controlled thyratrons 7, 8 supplying the saturating winding of a saturable core reactor 5 connected in series with an electric heater 2, and a bridge circuit including a temperature variable resistance 9 for varying the bias applied to the thyratrons in order to control the temperature of the furnace 1. The grids of the thyratrons are supplied with an alternating voltage from a transformer 18 which lags substantially 120 degrees behind the plate voltage and is of a magnitude equal to the critical starting voltage of the thyratrons. The output of the bridge circuit is also applied to the grids of the thyratrons via transformers 14, 15 and is arranged when the furnace is at the predetermined temperature to be 180 degrees out of phase with the plate voltage and of a magnitude such that the thyratrons pass current over one half of the plate voltage wave. The combined grid voltage of the thyratrons are such that they pass current for a time depending on the phase and magnitude of the output voltage from the bridge circuit, and then on the temperature of the heater. The thyratrons control the current applied to the D.C. winding 6 after rectification at 33 of the saturable reactor 5 so as to vary the imput to the heater in a sense to maintain the furnace at the predetermined temperatures. If desired, one thyratron only may be employed.