Patent GB-631892-A (GB)
Title: Improvements in or relating to electric motor control systems
IPC: H02P7/34
Filed: 1947-04-10
Abstract: 631,892. Automatic control systems. GENERAL ELECTRIC CO., Ltd., DAVIES, E. C., and FRIEDLANDER, E. S. April 10, 1947, No. 9608. [Class 38 (iv)] [Also in Group XXXVII] An automatic control system for an electric motor comprises a variable-speed motor wherein, in order to control the deceleration of the motor, a condenser is arranged, during deceleration, to be charged or discharged through an impedance, the value of the impedance and hence the time constant of the condenser and impedance being dependent upon the speed of the motor at the beginning of the deceleration period, the speed of the motor during such a period being dependent upon the voltage across the condenser so that the motor is brought to rest, or thereabouts, after a predetermined movement after the beginning of the deceleration period. A Ward Leonard system comprises a D.C. generator 1 driven by a constantspeed A.C. motor 2, and a variable-speed D.C. motor 3, the fields 5, 6 being separately excited by an exciter 9, the field 5 having a control resistor 10 serially connected in circuit. During acceleration of the motor 3 by variation of the control resistor 10 the change-over switch 44 is positioned such that the speed control condenser 36 is charged to a voltage governed by the speed of the motor 3 by means of a saturable reactor 31 and a rectifier 29. Simultaneously the condenser 51 is charged to a voltage which is the difference between the voltages tapped off the potentiometers 34, 59. When a control is initiated to decelerate the motor 3 the changeover switch is operated, e.g. by a mechanical trip, to its alternate position, thereby causing the speed control condenser 36 to be discharged by the beam tetrode 38 at a rate controlled by the condenser 51. If the deceleration is such that the difference between the output voltage of the rectifier 29 and the voltage across the condenser 36 is appreciable, the grid 28 of the triode 24 is biassed to alter the current flowing in a winding 22 of a saturable reactor 21, thereby varying the voltage applied to the resistor 10 by the rectifier 11 to correct the deceleration of the motor 3 so that it is controlled smoothly according to the discharge rate of the condenser 36. A mechanical brake may be applied when the motor speed is very low to stop the motor after a predetermined movement after the initiation of deceleration. The changeover switch may be remotely controlled. The correcting control may be introduced into an additional field winding in the generator 1, or the motor 3, or both. In another embodiment, Fig. 2, the D.C. motor may drive a tachometer generator, the output of which charges a speed control condenser 62, and also a condenser 110 in conjunction with voltages derived through a potentiometer 100 and resistors 103, 104. When deceleration is initiated a relay 93 is operated to connect the tachometer generator output in opposition to the voltage acquired by the condenser 62, the difference appearing between the grid and anode of valve 66, the output of which is connected to a resistor in the D.C. generator field circuit, thereby to control the motor deceleration. The condenser 62 is discharged by the triode 78, the rate of discharge being controlled by the condenser 110, and by the resistances 105, 106, which control the minimum rate of discharge. The arrangements may be modified so that the condensers 36 or 62 are charged during deceleration.