Patent GB-804361-A (GB)
Title: Improvements in or relating to the control of dynamo-electric machines
IPC: H02P7/34
Filed: 1956-02-28
Abstract: 804,361. Exciting generators; automatic speed control systems. SQUARE D CO. Feb. 28, 1956 [Feb. 28, 1955], No. 6163/56. Classes 35 and 38 (4). [Also in Group XXXVII] A D.C. generator 20 which supplies a motor 30 has a main field winding 49 for controlling the speed of the motor and a supplementary field winding 22 arranged to add to the main field when the generator voltage exceeds a reference voltage developed across rectifier 25, and to oppose it when the generator voltage is less than the reference voltage. The winding 49 is supplied through the output windings 53, 54 of a magnetic amplifier 7 having a control winding 40 energized by a signal voltage dependent upon the motor speed, this signal voltage being the resultant of components derived respectively from potentiometer 36 subjected to the generator output, and potentiometer 37 which is energized through rectifier 23 from mains transformer 46. The magnetic amplifier has a supplementary control winding 5 which corrects the excitation of the generator for IR-drop and variations of A.C. line-voltage, this winding being biased to cut off the amplifier by means of a voltage derived from transformer 47 and rectifier 51. The IR-drop compensation is dependent upon two components which are respectively proportional to current only, and to both current and generator voltage. The signal is obtained by the voltage-drop across generator field windings 19, 21, motor series field winding 17, and the windings of current-limit relays 13, 15. When the motor current increases, the voltage-drop between lines 1 and 34 also increases and causes winding 5 to increase the excitation of the generator 20. The motor is started by actuating push-button 60 to energize contactor 56, the setting of potentiometer 37 establishing the speed at which the motor is operated. If the acceleration current is excessive, relay 15 opens contacts 44 to de-energize winding 40 and allow the magnetic amplifier to be biased to cut-off. The relay 13 operates similarly on deceleration to close contacts 41 and open contacts 43 whereby winding 40 is connected directly to the highspeed terminal of rectifier 23, so reducing the decelerating current. The potentiometer 37 controls the IR-drop compensation voltage as well as the speed of the motor so that increased compensation is afforded at high speeds. This is necessary because both the magnetic amplifier and the generator have non-linear characteristics requiring additional driving voltage as they reach the higher voltage end of their operation. Compensation for line voltage variation is afforded by the non-linear characteristics of rectifier cells 55, the number of which is so chosen that small changes in the A.C. supply voltage, reflected through transformer 47 and rectifier 51, produce relatively larger changes in the bias current of the magnetic amplifier. The purpose of the field winding 22 is to obtain generator voltages less than that otherwise provided by means of the winding 49 alone, the winding 22 subtracting from the main generator field to afford the desired minimum speed of the motor. Conversely, the winding 22 adds to the main field when the generator voltage is above that derived from the rectifier 25, so permitting reduction in the size of the amplifier and preventing hunting. Specification 804,362 is referred to.