Patent GB-593449-A (GB)
Title: Improved methods of and means for regulating the d.c. output of grid-controlled a.c.rectifiers for motor control or other purposes
IPC: H02M7/15
Filed: 1945-05-30
Abstract: 593,449. Control of D.C. motors. ROWLANDS, T. W., GRIFFITHS, E. W., and THOMAS, W. W. May 30, 1945, No. 13596. [Class 38(iii)] [Also in Group XXXV] Armature control. - In a circuit in which the D.C. output of a rectifier is regulated by grid bias control, two control voltages which are functions of the load current are applied to a selector stage in opposite senses, one control voltage overriding the other when the load current exceeds a predetermined value. In a control circuit for a D.C. motor 3 having a shunt field winding 1 fed from a constant voltage D.C. supply, the voltage applied to the armature from the three phase rectifier 4 is controlled to rise with load to keep the speed constant, which speed can be varied by a manual setting of the potentiometer 15, until the load current reaches a predetermined maximum when it remains constant although the motor speed may drop to zero. The voltage, proportional to the D.C. rectifier load, across resistance 7 is applied between the cathode and grid of a valve 11. By applying the voltage from a slider 15 of a high resistance potentiometer 12 connected between the negative brush of motor 3 and a tap 13 on the anode load resistance 14 of the valve 11, to the grid of a valve 16, one anode 17 of the two anode diode 18 which is connected to the anode of valve 16, receives a voltage which is equal to a part of the sum of the armature voltage and a voltage proportional to the armature current, and which varies inversely with the armature current. A second anode 19 of the valve 18 receives a voltage direct from the anode of valve 11 which is directly proportional to the armature current. Valve 18 acts as a selector switch and passes the higher voltage of anodes 17, 18 through valve 23 to the grid of a valve 25 which forms a resistance in one arm of a bridge of which the other three resistances are 26, 27 and field winding 1. The output terminals of the bridge, which is fed from the D.C. source 2, 2, are connected to the star point 29 of a third winding on the transformer 30 which supplies the rectifier 4, and the rectifier cathode 5. Thus the grid voltage is varied by changes in the bridge resistance formed by valve 25, to alter the rectifier D.C. voltage in such sense that it rises with increase in load current. When this exceeds a predetermined value such as the maximum rating of the rectifier, anode 19 has higher potential than anode 17 and controls the current so that it remains constant, a condition which prevails during starting, acceleration and overload conditions. This current limiting feature may if desired be used in an arc lamp supply or other constant current system. While the potential of cathode of valve 18 can rise suddenly, with speedy charging of condenser 21, it can fall but slowly as the condenser discharges through resistance 20, thus reducing risk of damage to the rectifier with sudden overloads. In a simplified modification, the voltage from the anode resistance tap 13 of the first valve 11 is connected through the anode resistance of a second valve and a variable D.C. voltage to the grid of valve 25, there being no compounding with the armature volts. The speed is selected by varying the D.C. voltage. The overriding current control is obtained by connecting the anode of valve 11 through a biasing battery to the grid of the second valve which acts as a non-linear shunt across the first control circuit. It is also stated that the selector stage may comprise a diode having one control voltage connected between a common lead and the anode and the other between the common lead and the cathode.