Patent GB-812698-A (GB)
Title: Improvements in and relating to dynamic braking control circuits for electric traction motors
IPC: B60L7/04
Filed: 1956-02-24
Abstract: 812,698. Control of several D.C. motors; controllers. GENERAL ELECTRIC CO. Feb. 24, 1956 [Feb. 25, 1955], No. 5833/56. Class 38 (3). Relates to a dynamic braking system for series-connected traction motors 11, 12 having reversible field windings 13, 14, and a load resistor 15 regulated by a cam controller 19 driven by pilot motor 17. The traction motors are supplied from pantograph 21, over-current protection being provided by relays 22, 23. The vehicle is started by energizing train wires 106, GS and 108 so that relays 25, 30 are closed to connect the traction motors to the pantograph. At the same time, the pilot motor is energized from a local battery to remove a portion of the load resistor by closing contacts 1-8 in a predetermined number of steps. When the pilot motor has been driven to the full acceleration position, contacts 40, 41, are closed whereby relay 30 is maintained energized from the battery upon the train wire 106 being de - energized in preparation for dynamic braking. Thus, de-energization of the train wires 108, 106 and GS releases relay 48 and the traction motors are connected in a loop circuit with resistor 15 through contacts 34 and overload coil 23, the amount of resistance in circuit being initially sufficient to prevent appreciable deceleration of the train. The de-energization of train wire 106 releases relay 25 to disconnect the traction motors from the power source. Dynamic braking may be introduced by applying a signal to train wire 105 and energizing relay 45 so that the pilot motor is started to remove portions of the load resistor, and the coil of recalibration relay 55 is connected across the field windings 13, 14. Energization of relay 45 opens contacts 54 to connect resistor 56 in series with coil 58 of calibration relay 59 which has contacts 64 for regulating the advance of the pilot motor. As the braking-current increases, relay 55 operates to open contacts 60 whereby resistor 66 is also connected in series with coil 58 which is connected across a portion of the load resistor to sense the current therethrough. A series coil 62 of the relay 59 senses the current in the loop circuit directly and, when the current increases, the coils 58, 62, both act to open the contacts 64, the calibration relay responding at a low current until the contacts 60 are opened. A differential coil 70 is energized when relay 55 is actuated to re-calibrate the relay 59 so that it operates at a higher value of braking current. A further means for re-calibrating the relay 59 may consist in having the relay 55 resiliently coupled to increase the bias on the relay 59 by means of spring 69. At the time the train line 105 is energized, the pressure in airline 74 increases to move piston 75 against spring 76 so increasing the resistance of rheostat 77 to reduce the effect of differential coil 71 of relay 59. The movement of the piston also closes switch 46 to ensure energization of relay 45 independently of the line 105. Thus, the traction motor current is controlled during the initial application of dynamic braking by halting the pilot motor at a predetermined value of current which is much less than the maximum value, thereby allowing the field excitation of the motors to build up. The calibration relay is then re-calibrated so that it will re-open at the rated dynamic braking current. This allows the pilot motor to run at full speed during the initial stages of braking without danger of exceeding the safe value of braking current. Specification 692,099 is referred to.