Patent GB-1207753-A (GB)
Title: Electric control system for dc generator/electric motor system with constant current series loop armature coupling
Inventor: COTTRELL GRAHAM ANTHONY
IPC: H02P5/685
Filed: 1968-04-26
Abstract: 1,207,753. 753. Automatic speed and current control ENGLISH ELECTRIC CO. Ltd. July 25, 1969, [April 26, 1968], No.19965/68 HeadingG3R In a circuit arrangement comprising one or more separately excited D. C. generators whose armature windings are in a series loop with the armature windings of one or more separately excited D.C. motors, the loop current is maintained substantially constant by control of the field excitation of the or each generator, the motor(s) is/are controlled by a respective load reference signal which determines the field excitation of the or each motor, and the power supplied by, or regenerated into, the generator(s) is limited to a predetermined value by a control circuit which compares a signal representative of the actual power with a bias signal representative of the predetermined value to derive a further signal which is used as the load reference signal for one or more of the motors during the power limiting period. The described embodiment relates to the motors 13, 14, 15 of a dredger and these are connected in a series loop with two generators 10, 11, whose series-connected field windings 16, 17 are controlled by a feedback loop comprising loop current sensor 21, error amplifier 20 and a further amplifier 19 which varies the mark space ratio of a train of voltage pulses energizing the windings 16, 17. The field winding of each motor is controlled by a feedback loop comprising field current sensor 34, an amplifier 30 similar to the amplifier 19 and an error amplifier 31 having a set point signal 32 derived from a speed error amplifier 33 supplied with an actual speed signal from a tachogenerator 35 and a desired speed signal 36 representative of the load the motor is required to supply. Fig.2 shows the power limiting circuit for the motor 13 and includes a generator field current sensor 38 supplying a real power limit amplifier 39 and a regenerative power limit amplifier 47, which are also supplied with bias signals 40, 48 representative of predetermined values of real and regenerative power and with stabilizing signals from differentiating circuits 58 representative of the rate of change of current in the series loop. When the signal from sensor 38 equals the bias signal in one or other of the amplifiers 39, 47, the amplifier comes out of saturation and produces an output signal which, when it exceeds the signal 32, causes the appropriate one of diodes 41, 43, 49, 51 to conduct so that the signal 32 is overridden by the amplifier signal which then forms a limiting set point signal for the error amplifier 31. The limiting circuit for the motors 14, 15, Fig.3 (not shown) is similar to that for the motor 13 except that the regenerative portion is omitted completely and members 42, 43 are omitted since motors 14,15 are not reversible. By suitable choice of the bias signal values, power limiting can be imposed successively on the motors in a predetermined order as the generator power rises through a predetermined range. In a modification, Fig. 4 (not shown) a common limiting circuit is provided for all three motors and power limiting is intermittent according to the mark/ space ratio of pulses derived from the addition of the output of the real power amplifier 39 and the output of a sawtooth generator. The signals representative of generator power may be derived from the generator armature voltage instead of the field. Limiting may also be effected by discrete periods of complete power limiting, i.e. zero motor excitation.