GB473137A - Improvements in control systems for electric motors - Google Patents

Abstract

473,137. Control of D.C. motors. ENGLISH ELECTRIC CO., Ltd., and RAWORTH, A. April 3, 1936, No. 9935. [Class 38 (iii)] [Also in Group XXX] In an electric locomotive or vehicle drawing energy from an external source and controlled by a motor-generator set or booster set adapted to accelerate the traction motors by gradually varying the voltage applied thereto, the set is further adapted, when running by its own momentum, during an interruption of the external supply, to maintain automatically and without interruption a temporary supply of electrical energy to the traction motors. In the system shown, two pairs of traction motors 8, 9 and 8a, 9a are controlled on the buck and boost system by two separate booster machines 12, 12a, driven by motors 3, 3a. The latter motors are connected between the collectors 2 and a common earth return through a starting resistances 5, 5a controlled by contactors 6, 6a and limit relays 40, 40a. The pairs of traction motors with their series field windings 10, 11 and 10a, 11a, are connected in series with the respective booster armatures 12, 12a and in parallel with the circuits of the motors 3, 3a; the common earth return including compounding windings 15, 15a for the latter motors and relay windings 16, 16a. Movement of the master controller to the first notch switches in the motors 3, 3a, which are automatically accelerated under control of the contactors and limit relays ; at the same time the fields 23, 23a of the boosters 12, 12a are fully excited, through switches 25, 26, 32 and 32a, so that a voltage equal and opposite to that of the supply is generated in each of the booster armatures 12, 12a. The traction motor can then be connected to the supply by moving the controller to the next notch. Further movement of the controller opens the switches 25 in sequence to weaken the excitation of the boosters. When the latter reaches zero, and each of the traction motors is across approximately half the line voltage, the booster fields are reversed by closing switches 27 and opening switches 26, and are then strengthened by reclosure of the switches 25, until the voltage generated by the booster, which is now in the same direction as the line voltage, equals the latter. Retardation is brought about by the reverse process. On failure of the supply due e.g., to the locomotive going over a gap in the live rail, each of the motors 3, 3a acts as a generator and circulates current through the closed circuit formed by itself and the associated traction motors and booster armatures, the latter continuing to act in the sense in which they were acting before the failure, i.e. bucking or boosting. Each booster set is designed to have a high moment of inertia, e.g. by coupling it to a fly-wheel. Since the series windings 15, 15a and the relays 16, 16a are not included in the closed circuits, no current flows through them, and the relays trip the contactors 6, 6a to reinsert the starting resistances 5, 5a and also trip normally-closed contactors 29, 29a to insert resistances 28, 28a between the traction motor circuits and the collectors 2. On restoration of the supply, the relays 16, 16a are re-energized, whereupon the contactors cut out the resistances under the control of current limit relays 30, 30a and 40, 40a. The excitation of the booster field windings 23, 23a is obtained from an exciter 21 driven from the supply through a motor 17, the exciter also charging a battery 34. The field regulating resistance 24 is common to the two field windings which are connected in series ; this feature forms the subjectmatter of Specification 473,179. If the power is cut off for an abnormal length of time, the traction motors may slow down to a speed at which they cannot be safely connected, across the lines even with the resistances 28, 28a in circuit, and to guard against this, centrifugal switches driven by the booster sets are adapted to trip a main line contactor 31 when the speed falls below a certain value. The contactor 31 cannot be reclosed until the mater controller has been returned to the off position. In the event of a complete breakdown of the supply, the battery 34 may be used for running the locomotive at a crawling speed, the traction motors being connected directly to the battery by a switch 35, while switches 36, 37 and 37a are operated to disconnect the exciter 21 from the battery and to cut out the booster armatures 12, 12a. In order that one design of traction motor may be used with two classes of load, the field windings 10, 11 and 10a, 11a are provided with tappings connected to change-over switches 38, 39 and 38a, 39a ; this feature forms the subject-matter of Specification 473,180.