Patent GB-1222786-A (GB)
Title: Improvements in regulated dynamic braking circuit
IPC: H02P3/12
Filed: 1969-07-17
Abstract: 1,222,786. Control of D.C. motors. GENERAL ELECTRIC CO. 17 July, 1969 [29 July, 19681, No. 35967/69. Heading H2J. Series traction motors 1-4 operate as generators during the braking mode of operation, braking resistors 15, 16, being variable in discrete steps. A phase-controlled impedance 24 comprises a bank of ignitrons, the motors being energized for propulsion from rectifier bridges 28-30 which are connected sequentially to secondary windings of a supply transformer 31. During braking, the converter 23 provides an adjustable direct voltage which aids the series generators in maintaining maximum current at any speed or in maintaining a prescribed current level. The braking effort is increased by increasing the output of the converter in accordance with the current as sensed by reactors 21, 22, until a voltage-measuring reactor 32 operates to decrease the braking resistance by one increment. The reactor 32 detects the output of the converter 23, whilst reactors 19, 20, indicate the voltage on the braking resistance. In response to a given relationship between these two voltages, one of the contactors in each of the banks 17, 18, is closed to reduce the braking resistance. At this point, the output of the converter 23 is reduced to zero. To decrease the braking effort, the output of the converter is reduced to zero and the braking resistance is increased by one increment. The converter-output is then increased to provide the voltage determined by the signal from the reactors 19, 20, after which the output is again reduced. The output of the converter 23 is increased by advancing the firing angle of the ignitrons 24, which is effected by increasing the potential at input terminal 34 of firing unit 23. A unit 50 increases the output of source 59 according to programmed levels of current and in accordance with the instantaneous value of braking current, an indication of which is provided at input terminal 54. The unit 50 may also be used to control wheel slip and to limit the maximum braking effort. A unit 36 receives signals from the detectors 32 and 19, 20, and, when the converter voltage exceeds the voltage on the braking resistance, a pulse is transmitted to unit 45. The latter produces a pulse having a duration equal to the delay in response of a contactor which closes to reduce the resistance. At the end of this pulse, control by means of unit 50 is taken over for a period by a generator 55a which operates to reduce the converter-output to zero. After a predetermined period, control is resumed by the unit 50. When the output of the converter is decreased to zero, detector 32 supplies a signal to unit 40 which transmits a pulse to unit 45. One of the contactors in each of the banks 17, 18, is opened, the unit 50 being by-passed. A command circuit 55 provides a pulse whose magnitude represents the increase of converter voltage to compensate for the increase of braking resistance, this pulse being applied to the unit 59 to advance sharply the firing angle of the converter. Unit 50 resumes control at the end of a predetermined period. The unit 36 comprises a pair of transistors which conduct when the signal at terminal 38 exceeds that at terminal 37. After a delay determined by a capacitive network, a unijunction transistor fires to provide an output pulse at terminal 39, Fig. 4 (not shown). The unit 40 comprises a pair of diodes which are blocked when the signal at terminal 41 is zero and the motors are set for braking. A storage network then responds to fire a unijunction transistor which provides an output pulse on terminal 43, Fig. 5 (not shown). When the unit 45 receives a pulse at terminal 47, a controlled rectifier applies a charge to a pulse generator to deenergize a contactor which establishes a potential at terminal 49 until the contactor has fully opened. As the output of the converter is again reduced to zero, remaining contactors will open, Fig. 6 (not shown). A pulse appears at terminal 61 of unit 59 for a predetermined time after a contactor has closed, a transistor being rendered conductive to reduce the voltage at terminal 60 to zero. When the pulse at terminal 61 disappears, the voltage at terminal 60 is determined by a transistor associated with the terminals 62, 63, Fig. 7 (not shown). The amount of current bled off from the latter transistor is determined by the conduction of a regulating transistor in the unit 50. The degree of conduction of this regulating transistor is a function of the input at terminal 54 associated with a signal line which receives other signals for restricting parameters such as motor current, voltage, and braking effort. A signal at terminal 52 responds to a discrete decrease in the braking resistance to enhance the reduction in converter output, Fig. 8 (not shown). A pulse appearing at terminal 57 transmits a signal indicating the voltage on the braking resistor to terminal 56, whilst an output of predetermined duration is provided at terminal 56a in response to removal of a pulse from terminal 57a, Figs. 9, 9a (not shown). In a modification of the power circuit, a further phase-controlled impedance enables the introduction of a voltage to augment or oppose that of the series generators. When converter 23 provides maximum output, the further impedance is connected into the motor circuit and the braking resistance is reduced by one increment. The phase-controlled impedances then operate to return power to the source. Alternatively the impedances may be controlled to conduct during separate ninety-degree portions of supply current to provide conversion or inversion, Fig. 12 (not shown).