GB672686A - Improvements relating to speed control systems for direct current motors - Google Patents

Abstract

672,686. Control of D.C. motors. METROPOLITAN-VICKERS ELECTRICAL CO., Ltd. Dec. 5, 1949. [Nov. 29, 1948] No. 30933/48. Class 38 (iii). [Also in Group XXXV] A direct-current motor speed control system of the kind in which increase in speed is effected up to a predetermined value by increase of armature voltage with constant maximum field current and thereafter bv decrease in field current, is provided with means responsive to the back electromotive force of the motor which weaken the field current when the back electromotive force of the motor exceeds a value corresponding to said predetermined value of speed and in proportion to the increase of back electromotive force above said corresponding value. As shown, the armature 1 and interpole winding 2 of a direct-current motor are supplied from an alternating-current source through a device 4 which comprises a rectifier and an amplifier, and operates to control the supplyvoltage automatically so as to adjust the motorspeed and thereby maintain the voltage of a tachometer generator 5 equal to an adjustable reference voltage derived from a potential. divider 6. A field winding 13 of the motor is energized from an alternating-current source through a rectifier 14, the input terminals of which are connected in series with a winding of a saturable reactor 15. The control winding of the reactor is connected in the anode circuit of a tetrode valve 10, the control grid 12 of which is positively biased over a first part of the speed range of the motor so that the control winding of the saturable reactor carries a large current and the field winding 13 is fully excited. The effective voltage applied to the grid 12 is the difference between a reference component derived from a source 11 and a negative component, which varies in proportion to the backelectromotive force of the motor, and which is obtained from a bridge circuit formed by the motor armature, its interpole winding, and resistors 7 and 8. When the back-electromotive force increases above a value predetermined by the selected value of the reference component, the valve 10 becomes negatively biased and the consequent increase of the reactance of the alternating-current winding of the transductor reduces the field excitation of the motor. Quick response to sudden increases in the pre-set speed may be obtained by arranging that the field-weakening control operates before the back-electromotive force reaches a value corresponding to the full armature voltage and field excitation of the motor. The control current for the transductor 15 may be obtained from a second transductor, instead of through the valve 10. Specification 672,687 is referred to.