GB664681A - Improvements relating to electric motor control systems - Google Patents

Abstract

664,681. Control of D.C. motors. BRITISH THOMSON-HOUSTON CO., Ltd., FORSTER, E. W., and FARMER, W. J. R. Sept. 14, 1948. No. 24115/48. Addition to 613,917. Class 38 (iii). [Also in Group XXXV] The system described in parent Specification is modified by having a common capacitor 55 connected in the return conductor to the cathodes in the control circuits of the main rectifying valves 3, 4, so that the alternating potentials applied to the valves is similarly influenced by a component of direct voltage appearing across the capacitor. The armature of motor 1 is supplied from transformer 5 through valves 3, 4, which may be thyratrons. A potential of fixed phase relation to that of the anode voltages of the valves is supplied to such control electrode through transformer 16 and resistor-reactance network 52 which is connected to tappings in the secondary winding of the transformer and to the grids of the valves. The capacitor 55 is connected between the centre-tap of the transformer secondary winding and the cathodes of the main valves, a direct voltage control component being developed across the capacitor by control valves 23, 24, which are respectively connected in parallel with parts of the control circuits of the valves 3, 4, those parts each including the capacitor. The valves 23, 24 are connected in series so that, when the impedance of each of the parallel circuits in which they are connected is equal, no resultant direct potential appears on the capacitor 55. If the impedances become unequal, a direct potential appears across the capacitor by rectification of the secondary voltage of the transformer 16, which potential varies the conductivity of the main valves. Any alteration of the direct potential equally affects both main valves, so providing stable operation. The control valve 23 has its impedance varied in response to a control potential developed between its cathode and grid whereby to determine the value of direct potential across capacitor 55 and thus to control the conductivity of the main valves. The control potential has a motor-speed setting component derived from potentiometer 25, a potential component dependent on the motor armature current applied across potentiometer 37, and a component dependent upon the motor armature terminal voltage from potentiometer 34. The second and third components are simultaneously adjusted in value with the first so that the motor speed remains substantially constant at any setting from no-load to fullload. The control voltage derived from the potentiometer 37 is modified by one obtained from a potentiometer 57 so as to be proportionately higher for a given armature current at high speeds. A two-phase supply source may be used.