Patent GB-831274-A (GB)
Title: Improvements in and relating to control systems for electric motors
IPC: H02P7/26
Filed: 1956-04-04
Abstract: 831,274. Control of D.C. motors. WILKINS, F. V. April 4, 1956 [April 6, 1955], No. 10313/56. Class 38 (3). [Also in Group XXXV] The speed of an electric motor 10 is kept at a predetermined value by means of a tachometer generator 22, the voltage of which is compared with that derived from a reference potentiometer 20 having a speed-setting contact 19. The error signal is applied to the input terminal 42 of an amplifier 18 so that the current passed by triode 50 increases when the speed of the motor falls below the desired value. This causes the voltage at junction 56 to be lowered so that the cathode-potential of thyratron 16, through which the motor is supplied, is decreased. An error signal indicating that the motor speed is below the desired value also effects a decrease of the current passed by triode 64 whereby the positive potential at junction 80 is increased, this junction being joined to the grid of the thyratron 16. Thus the conduction period of the thyratron 16 is extended, the power input to the motor being increased to accelerate it to the desired value. When the motor speed is higher than that called for by the setting of the contact 19, the conduction time of the rectifier is cut down to decelerate the motor. Rapid voltage variations at the output leads of the amplifier 18 are prevented by means of a capacitor 88, the series resistor 82 and capacitor 88 acting to filter any A.C. components from the output. This resistor-capacitor combination also provides a time-delay action when rapid changes of the setting of the contact 19 are involved, and when the system is switched on or off by means of switch 90, whereby overloading of the rectifier 16 and motor 10 is prevented. Closure of switch 90 short-circuits the outputleads of the amplifier, so cutting-off the thyratron and stopping the motor. A thermal timedelay unit 96 prevents starting of the motor during the initial warming periods of the cathodes. A grid bias circuit 86 causes the grid potential of the thyratron 16 to lag the anode voltage by more than ninety degrees. The lowresistance armature 112 and series field winding 114 of the motor are shunted by a gas-filled rectifier tube 116 which is connected in inverse relation to the rectifier 16. The energization of the motor may be continued after the thyratron has ceased to conduct at the end of each cycle of the supply-voltage by the discharge of inductive field and armature current through the tube 116. This effect is most noticeable at low speeds and causes the reduction of heating of the motor as well as providing more stable operation. The power supply circuit 26 for the potentiometer 20 and amplifier 18 includes transformer 124, rectifier 127, current-limiting resistor 128, and filter capacitor 130. Modifications of the control circuit comprise a separate capacitor for producing a time delay effect and connected in the grid circuit of the triode 64. This capacitor is adapted to be short-circuited for stopping the motor, Fig. 2 (not shown). Stability of the reference voltage is provided by a pair of parallel-connected regulator tubes, the first of which is connected in series with a resistor across a filter capacitor. The control potentiometer 20 may be connected across a pair of potentiometers which are preset for correct speed calibration.