Patent GB-636661-A (GB)
Title: Improvements in or relating to d.c. electric motor control circuits
IPC: H02P7/292
Filed: 1944-05-24
Abstract: 636,661. Automatic speed control systems; rectifying. STEVENS, A. H. (Submarine Signal Co.). May 24, 1944, No. 10065. [Classes 38 (ii) and 38 (iv)] [Also in Group XXXVII] In a system for automatically controlling the speed of a D.C. motor, a negative bias, applied to the grid of a gaseous rectifier valve supplying the armature, falls during the positive halfcycle of the alternating voltage applied to the anode, and the rate of change and magnitude of the bias is controlled by the motor speed and torque. In Fig. 1 the armature of a D.C. motor 1 is supplied from A.C. mains 2, 2 through a gaseous triode rectifier valve 3. The grid bias of valve 3 is developed across a resistor 13 by the anode current of a vacuum triode thermionic valve 7, the anode-cathode voltage of which is supplied partly from the voltage across a condenser 11, charged through a diode 8 from a secondary winding 10 on a mains transformer 9, and partly from a potentiometer 14 connected across the motor armature. The anode supply to valve 8 is in phase opposition to the anode supply to valve 3 so that condenser 11 is charged during the early portion of the half-cycle during which valve 3 does not conduct. During the next half-cycle condenser 11 is discharging through a resistor 12 and the anode voltage of valve 7 falls until, at a point determined by the grid voltage of valve 7, the voltage across resistor 13 permits conduction of valve 3 to supply armature current for the remainder of the half-cycle. The grid bias of valve 7 is constituted by the output of a potentiometer 17 supplying, as negative bias, a constant D.C. reference voltage obtained through a full-wave rectifier valve 19 from secondary winding 10<1> on transformer 9, acting in opposition to the output of potentiometer 14 supplying, as positive bias, a voltage proportional to the motor speed. Potentiometer 17 is the primary means for setting the motor speed, any deviation from the speed chosen causing a variation in one sense or the other of the opposition voltage from potentiometer 14 to advance or retard the point of conduction of valve 3 to correct the speed automatically. By appropriate choice of the setting of potentiometer 14 the motor can be given a level or rising speed-torque characteristic. The motor field winding is separately excited through a rectifier 30 from a secondary winding 31 on transformer 9 and the motor may be reversed by means of a field reversing switch 20. As shown, the motor is inoperative, the voltage across a resistor 32 in series with a resistor 33 across the field supply biassing rectifier 19 negatively so that the voltage across potentiometer 17 is zero, valve 7 is conductive and valve 3 non-conductive. To start the motor, a switch 38 is closed and a condenser 35 discharges to zero through a resistor 36 so that the grid bias on rectifier 19 is removed gradually and dangerous surges avoided. The reference voltage for the grid circuit of valve 7 may be obtained from a separate D.C. source and the opposing voltage,. proportional to the motor speed, may be obtained from a tachometer generator. In Fig. 2 (not shown) a full-wave valve rectifier controlled in a similar manner supplies the armature current. Characteristic curves and operating data are given.