GB390501A - Improvements in and relating to electric control systems - Google Patents

Abstract

390,501. Control systems for consuming devices ; phase-changing. BRITISH THOMSON-HOUSTON CO., Ltd., Crown House, Aldwych, London.-(Assignees of Fitzgerald, A. S. ; Warwick Road, Wynnewood, Pennsylvania, U.S.A.) Oct. 2, 1931, No. 27451. Convention date, Oct. 2, 1930. [Classes 38 (ii) and 38 (iv).] The current in an A.C. load circuit 11 such as for theatre lighting, motor starting and other purposes is gradually varied between minimum and maximum values, or may be maintained at a desired value, by control of the phase of the voltage applied to the grids of rectifying valves 15, 16 in series with the saturating windings 13 of a reactor in series with the load. The rate of change of current is adjustable. The apparatus comprises a phase-shifting circuit in which the voltage applied to a grid transformer 17 is the resultant of the voltage in the secondary of a transformer 25 and that across an impedance device 23 which is itself connected in series with a condenser 21 across the supply source 10. The impedance 23 has substantially a resistance characteristic and is a transformer, one winding of which is in parallel with valves 26, 27 the conductivity of which may be progressively varied. The voltage of the secondary winding of the transformer 25 is approximately onehalf the supply voltage and is fixed in phase, hence, as the value of the impedance 23 varies from zero to a maximum the voltage applied to the transformer 17 varies in phase from zero to 180 degrees leading with respect to the supply voltage. Thus the average value of the current passing through the rectifiers 15, 16 varies from a maximum to zero and the load circuit current varies from a maximum value to a minimum. The conductivity of the valves 26, 27 is controlled by a switch 34 which is adapted to charge a condenser 32 to alternative polarities through a resistance 33. This resistance may be varied to determine the rate of change of condenser potential and, the condenser being connected between the cathode and grid of each valve, the rate of change of conductivity of these valves. Thus, closing the switch 34 in either of its two positions with the switch 24 on its right-hand contact produces an appropriate progressive change in the current in the load 11. By opening the switch 34 at any time the load current is caused to remain at the value it has at that instant since the charge on the condenser 32 then remains substantially the same. A variable resistance 22 is provided to give an independent control of the load current. Any circuit of high time constant may be substituted for the circuit comprising the resistance 33 and condenser 32. The resistance characteristic of the primary winding of the transformer 23 may be improved by connecting a resonating condenser 37 in parallel with it.