GB845534A - Improvements in and relating to ward-leonard equipments - Google Patents

Abstract

845,534. Control of D.C. motors. BRITISH THOMSON-HOUSTON CO. Ltd. Dec. 6, 1957 [Dec. 19, 1956], No. 38749/56. Class 38 (3). In a Ward-Leonard system, the generator G has an exciter 3 provided with a current-limiting field winding 5 which is energized when a current-responsive voltage derived from interpole winding 1 exceeds a reference voltage applied to potentiometer 7. The reference voltage is varied in accordance with the voltage of the generator by means of a magnetic amplifier having a control winding 11 connected to a source of constant voltage, and an opposing winding 12 connected to a voltage divider 13 which is across the generator brushes. A rectifier 14 allows for reversal of the current in the Ward-Leonard circuit. Thus, the value of armature current to which the circuit responds to energize the winding 5 falls progressively with progressive increase of generator voltage. In a modification, three magnetic amplifiers have their output windings connected in delta or star to a three-phase rectifier supplying the potentiometer 7, Fig. 2 (not shown). The magnetic amplifier may be replaced by a D.C. machine in opposition to a rectifier which supplies the potentiometer 7 from an A.C. source. This machine has a field winding connected to the voltage divider 13, a resistor being connected in series with the field winding and providing a voltage in opposition to the voltage obtained from rectifier 14. This arrangement provides that the limiting value of current in the Ward-Leonard circuit will remain constant until the generator voltage reaches a predetermined value, Fig. 3 (not shown). The voltage for controlling the limiting value may be derived from a tachometer generator driven by the motor, Fig. 5 (not shown). The currentlimiting control may be effected by means of the motor excitation instead of the generator excitation, the motor excitation being increased when the armature current is to be reduced.