Patent GB-811571-A (GB)
Title: Improvements in and relating to electric motor control systems
Inventor: WILSON GRAHAM RUDGE
IPC: H02P5/68
Filed: 1954-09-03
Abstract: 811,571. Automatic speed-control systems. ENGLISH ELECTRIC CO. Ltd. Sept. 2 1955 [Sept. 3, 1954], No. 25642/54. Class 38 (4). [Also in Group XXXVII] In a speed control system for a D.C. motor, wherein the motor has a constant excitation and has its armature controlled in accordance with the difference of signals representing a reference quantity and the motor back E.M.F. respectively, the difference signal is modified by a signal which increases non-linearly with increases in the motor load current. A speed regulator 17 for a rolling mill controls the field 16b of a generator 16 setting a mill speed reference on bus-bars 15 for the D.C. motors of the mill stands. One such motor 10, having a constant excitation in its field 10b set by a stand speed regulator 13, has its armature supplied by a generator 11 whose field 11b is energized from the reference bus-bars 15 in conjunction with the " bucking" or " boosting " output of an amplifying exciter 14. The sense and magnitude of this output depends on the excitation in the exciter field 14a, which is connected across the reference bus-bars 15 in series with a stabilizing field 11d for generator 11, a resistor 18 and the motor armature 10a, whereby its excitation is in accordance with the difference between the mill speed reference and the back E.M.F. of motor 10. The latter is derived from the difference between the armature voltage and the D.C. output to resistor 18 from a magnetic amplifier 19 which is responsive to the armature current by virtue of its control winding 19a connected across motor and generator compoles 10c, 11c, whereby to compensate for IR drop in the armature. Changes in motor back E.M.F. due to variations in the air gap flux caused by fluctuations in motor loading are compensated by a control winding 19b of amplifier 19 which is connected across the compoles 10c, lie and is pre-arranged to aid or oppose winding 19a depending on whether the uncompensated system gives an increase or decrease in speed with increase in load. The non-linear characteristics of metal rectifiers 22, 23, connected in series with the winding allow a rapid increase in the energization if the load exceeds a critical value. The parallel connection of the rectifiers allows current flow in either direction depending on the direction of motor rotation. A slider on series resistor 21 is ganged with a slider on regulator 13 to allow variation of the compensation with stand speed. Compounding for producing a desired linear speed-load characteristic is provided by an amplifier control winding 19c, which is energized in opposition to winding 19a and in accordance with a proportion of the load current, the proportion being selected by a manual setting on pqtentiometer 24 or in accordance with the mill speed by means of ganged sliders on potentiometers 17, 25, whereby to achieve a constant percentage compounding over the whole speed range. The back E.M.F. compensation for load changes may be varied by connecting additional rectifier units across the combination of elements 21, 22, 23 and/or across the winding 19b.