Patent GB-634756-A (GB)
Title: Improvements relating to control systems for electric motors
IPC: H02P5/68
Filed: 1947-11-27
Abstract: 634,756. Exciting dynamos ; automatic current control systems. ENGLISH ELECTRIC CO., Ltd., BROWN, H. S., FENEMORE, A. S., and SHAW. N. H. Nov. 27, 1947, No. 31477. [Classes 35 and 38 (iv)] [Also in Group XXXVII] A control system for maintaining at a predetermined ratio the load currents in two groups of direct current motors, either group of which may comprise one or more motors coupled to a common load, wherein the armatures of each group of motors are connected in series relationship with at least one direct current generator and the resulting groups of motors and generators are connected together to form a closed series circuit so that, under load conditions, zero potential, or a low potential relative to the greatest voltage in the series circuit, exists between the two inter-connections comprises a control generator which is comnected across the inter-connections and is excited differentially in accordance with the currents in the series circuit at either side of the inter-connections, whereby on any deviation of the currents from a pre-determined ratio the excitation of the control generator is varied in a sense tending to correct the deviation. Motors, having double armatures 1a, 1b and 2a, 2b respectively, are adapted to drive separate rolls of a slabbing mill. The armatures are connected in series with similar generators 3, 4, 5 and 6 of an Ilgner set. A constant speed control generator 7 is connected as shown to points which are at substantially equal potential when the motors are operating under load conditions. Two sections, 7a and 7b of the field winding of the generator 7 are connected in a bridge circuit which also comprises resistors 9 and 10, the resistance values of which are equal to those of the sections 7a and 7b when these are cold, and which have comparatively low temperature coefficients. An exciter 11 and its field winding 11b are respectively connected across different diagonals of the bridge circuit. The exciter also has field windings 11c and 11d, each of which has opposing sections carrying currents proportional to those in the parts of the main series circuit on either side of the control generator 7. The exciter 11, which may be mechanically coupled to the generator 7 or separately driven, is adapted to operate on the straight portion of its magnetic characteristic, the resistance of its armature circuit being so adjusted that the exciter, which is essentially a self-exciting machine, quickly responds to small changes of the excitation of the field windings 11c and 11d. When the field windings 7a and 7b become warm the armature resistance of the exciter increases, so diminishing its self-excitation and tending to prevent operation in the above-described manner. However, the bridge circuit becomes unbalanced, due to the different temperature coefficients of the field windings and resistors, and the additional excitation of the field winding 11b is arranged to compensate for these temperature-changes. Alternatively, the field winding 11b may afford compensation for the effects of changes of temperature when the machines are cold, the bridge circuit being balanced at the normal working-temperature. Due to energization of the field winding 11b when the currents in the windings 7a and 7b are being increased and the bridge circuit is unbalanced, a quicker rate of change of excitation of these windings is produced. If one motor takes a larger part of the load than the other the exciter field windings 11c and 11d are so energized that the output of the generator 7 is modified to reduce the current of the overloaded motor and increase that of the underloaded one, and so restore balanced conditions. Rheostats 14, 15, 16 and 17 enable adjustment of the ratio between the armature currents of the two motors. Due to the comparative high resistance of rectifiers 12 and 13 with low currents, the machine 11 is rendered ineffective to excite the generator 7 when the motors are operating under no-load conditions. The rectifiers may be replaced by other non-linear resistors. When unexcited the generator 7 affords a low resistance path so that the motors will operate at substantially equal speeds when uncoupled mechanically. A field winding 7c of the generator 7, which is energized from the generator 4 through a reversing potentiometer 18, enables a pre-determined ratio of motorspeeds to be maintained when the motors are mechanically uncoupled. The voltage of the generator 7 is determined by that of the generator 4 and is effective to increase the armature voltage of one motor relatively to that of the other. The winding 7c may be connected across any of the other main generators. The potentiometer 18 may be replaced by a rheostat and reversing switch. Overload control is provided. The double armature motors may be replaced by ones of normal type. If the generator 7 is taken out of circuit it may be replaced by resistance 19 by means of change-over link 8. Specification 595,665 is referred to.