Patent GB-1168246-A (GB)
Title: Improvements in or relating to Railway Vehicle Propulsion Systems
Inventor: ARMSTRONG DONALD STUART
IPC: B60L50/11
Filed: 1966-10-28
Abstract: 1,168,246. Control of D.C. motors. BRITISH RAILWAYS BOARD. 24 Oct., 1967 [28 Oct., 1966], No. 48499/66. Heading H2J. [Also in Division G3] In a diesel-electric railway traction system, the excitation of the generator and of the motors is controlled by a combination of vehicle speed error and the difference between actual motor current and a limiting value set on a separate controller. In addition, the fuel supply is controlled by the difference between the engine speed and a set value, the set value being determined by the electrical power developed. The term " excitation " used in the figure includes means whereby the generator excitation is first increased to maximum with full motor field, followed by field weakening. The arrangement is such that, from rest, acceleration occurs at the maximum current set by the current con - troller, the generator excitation increasing with speed, so that the motors run at constant torque; when full excitation is reached the current is still held constant so the motors operate at constant power. When the desired speed is reached the speed loop assumes control and the current decreases. The engine speed system first controls the fuel supply, but if this reaches a maximum for the set speed obtaining the current control signal or the excitation direct, is modified via a function generator F.G. so that engine speed is maintained by weakening generator excitation, the motors then continue to operate at constant (but reduced) power. Any abrupt change in the vehicle speed error is detected by a differentiating circuit " Transient " which adjusts the fuel supply immediately, while the signal to the excitation control is delayed. The transient signal may alternatively be superimposed on the engine speed error signal. A sufficient speed error in the opposite direction causes rheostatic braking by means not shown, during which generator excitation is zero and the current limit is maintained by adjustment of motor fields. Wheel slip is prevented partly by the action of the above transient response, but in addition the ratio of torques on each axle is adjusted, either manually or automatically when wheel slip occurs. For this purpose the motor armatures are preferably connected in series and their fields are individually thyristor controlled, the thyristors being controlled by motor driven potentiometers (Fig. 3, not shown) with manual override. The potentiometer motors are connected across frequency sensitive devices such as inductors which are in series with diode bridges across individual tachogenerators. The tachometer circuits are all connected in parallel across a single resistor (Fig. 4, not shown); the voltage across the resistor constitutes the vehicle speed signal, and with no wheel slip the voltage across each inductor is zero because the tacho. voltages balance. If wheel slip occurs the resulting imbalance causes resetting of the appropriate field control potentiometer, but it is never reset by this means, because the inductor voltage is unidirectional. Thus the effect of wheel slip is to automatically balance the load between the various axles. Alternatively the motor armatures may be connected in parallel and their currents regulated by saturable reactors on the A.C. side of rectifiers, the generator being A.C. in this case. Reversal is effected by reversing armature connections only.