GB668636A - Improvements relating to electric motor control systems - Google Patents

Abstract

668,636. Control of D.C. motors. ENGLISH ELECTRIC CO., Ltd. March 23, 1950. [April 4, 1949] No. 9076/49. Class 38 (iii). A rheostatic braking system for a series type direct-current motor in which, during braking, the motor is connected in series with its field winding so as to act as a generator supplying a braking resistor, comprises in combination an exciter connected in parallel with the field winding and having its own field winding excited in accordance with the difference between a reference quantity and one varying with the speed of the motor whereby the motor excitation is automatically varied in a sense to maintain the generated voltage substantially constant, and means, responsive to the armature current or voltage of the exciter, arranged to reduce the amount of resistance in the braking circuit each time the current or voltage exceeds a predetermined value whereby the braking current is caused to increase in steps, at least over a part of the braking range, as the speed of the motor decreases. As shown two traction motors having armatures M1, M2, and series field windings M1a, M2a, are connected with a braking resistor BR sections of which can be short-circuited in sequence. An amplifying exciter E, which may be of the type described in Specification 579,570, -[Group XXXV], is connected in series with the coil P1 of a polarised relay P across the motor field windings. The exciter bucks or boosts the motor excitation according to whether the braking-current is greater or less than that required in the motor field windings to produce the required voltage. To this end the exciter is provided with a reference field winding E1, which is supplied through variable resistor VR from substantially constant source V, and with a control field winding E2 which is connected across armature M2. Any deviation of the generated voltage from a set value causes unbalance of the excitation of windings E1, E2, so that the exciter alters the motor excitation in a sense to correct the deviation and maintain the generated voltage and braking-current substantially constant, independently of the vehicle speed. The relay P is polarised by coil P2 and closes its contacts when the exciter current reaches a maximum value in the boosting direction. The relay P is arranged alternately to energize relay X or Y whereby contactors R1-R4 are closed in sequence to remove sections of the resistor BR in a stepped manner as the vehicle speed dcreases. During the periods between adjustments of the resistor BR the braking-current thus remains substantially unchanged, the exciter current varying from its maximum braking to its maximum boosting value. When the braking resistance is reduced the braking-current changes abruptly to a higher constant value. Upon the braking- current reaching a pre-determined value the connections of the machine E may be changed so that the exciter is energized in accordance with the difference between a reference quantity and one dependent upon the output voltage of the exciter, the motor excitation being maintained substantially constant. The means responsive to exciter current or voltage may control a motor-driven rheostat so that, each time the current or voltage exceeds a predetermined value, the rheostat moves to reduce the braking-resistance. The rheostat may comprise a rotating brush-arm bearing on a commutator having segments connected to the braking-resistance. In another arrangement the braking-resistance is controlled by a motordriven camshaft controller which is advanced in steps. Means may be provided to increase the reference quantity applied to the field winding E1 at the same time as resistor BR is reduced.