Patent GB-683396-A (GB)
Title: Improvements relating to electric transmission systems for vehicles
Inventor: BONNELL DAVID THOMAS
IPC: B60L7/04
Filed: 1949-10-07
Abstract: 683,396. Control of D.C. motors. ENGLISH ELECTRIC CO., Ltd. Oct. 6, 1950 [Oct. 7, 1949], No.. 25800/49. Class 38 (iii) [Also in Group XXXV] Relates to systems for vehicles comprising a prime mover, a generator driven thereby, at least one traction motor connected to the generator, and means for dynamically braking the vehicle by connecting the traction motor armature across a braking resistor and separately exciting its field winding. The system includes a regulator for varying the excitation of the traction motor during dynamic braking, a pilot motor for operating the regulator, and means dependent on the braking current arranged to control the pilot motor so that, as the speed of the vehicle decreases, the excitation of the traction motor is automatically increased until the maximum excitation position of the regulator is reached whereby, over this range of control, the braking current is maintained within predetermined limits. A main generator 1, driven by a Diesel engine (not shown) and supplying traction motors 2, 3, has a field winding la which is energized through a master switch 5, driver's controller 6, and a pilot motoroperated field regulator 7. When the master switch is engaged with contact 5a contactors 8, 9 100, are energized to close contacts 8a, 9a, 10a, and connect the traction motors in parallel across the generator. On the first three notches of the controller 6 the regulator 7 is inoperative and the regulator arm abuts against a stop 7u which prevents movement of the arm in a clockwise direction. Contactor 21 is energized through contact 7g so that switches 7k-7s are connected to the supply through contact 21a. When the controller is moved beyond the third notch contacts 6h are closed and the controller 7 is set automatically under the control of an engine governor 22. If the speed of the engine rises, for example, the governor operates to energize a relay 23 so that contacts 23a are closed whereby the pilot motor 19 is energized through its field winding 19a. The regulator arm is thus moved to increase the generator excitation until the generator output matches the available engine horse-power whereupon the engine speed is returned to normal and the pilot motor is de-energized. If the regulator arm is in a position to close contacts 7r, for example, and the engine speed falls, the governor 22 operates to energize relay 20 so that the pilot motor is energized through contacts 20a and field winding 19b and the regulator is driven to decrease the field excitation of the main generator. Interlocking of relays 21, 24, ensures that the regulator cannot be operated until contacts 6h are closed. If the controller 6 is moved to open contacts 6h the regulator 7 is driven to its maximumresistance position. If, with the generator operating at maximum excitation, the engine speed tends to rise further, the regulator arm will move to open limit switches 7a, 7b. The contactor 21 is deenergized to insert the full regulator resistance, the relay 23 being maintained energized so that the regulator arm continues to travel in the anti-clockwise direction. Closure of contacts 7d energizes relay 26 to connect resistors 28, 29, in parallel with traction motor field windings 3a, 2a. A second stage of field weakening may be initiated, if the load current does not exceed the value set by relay 35, when the regulator arm closes contacts 7e to energize relay 25 and connect resistors 30, 31, across windings 3a, 2a, When the regulator arm reaches the position shown. contactor 21 is re-energized and the control of the regulator is again effected by means of the governor 22. If the second stage of field weakening does not occur on the first revolution of the regulator, it can only take place if the regulator makes a further revolution upon the engine speed again tending to rise. An interval is provided to allow the generator field excitation to decrease to the low value at which the traction motor field weakening takes place, the pilot motor 19 being operated slowly during this period by means of diverter resistance 32. The traction motor field weakening sequence is prevented from taking place if the contact 6h is open and is then re-closed before the regulator arm reaches the maximum-resistance position. The traction motors are protected against excessive current on weak field by means of relay 27. If the relay 25 is energized the regulator arm is stopped by means of limit switch 7b the opening of which causes de-energization of field winding 19a and the connection of the field winding 19b for momentary braking. When the controller 6 is in the "off" position, movement of switch 5 to engage contact 5b energizes contactors 12-16. Contacts 12a, 13a, connect braking resistors 17, 18, across the armatures of motors 3, 2, whilst contacts 14a, 15a, connect the field windings 2a, 3a, in series across the generator armature. Closure of contact 16a connects the generator field winding 1a to the supply through resistor 41, controller 6, and regulator 7. Controlled dynamic braking may now be initiated by moving switch 11 to engage contact 11a. The relay 23 is energized through contacts 40a of current limit relay 40 so that the regulator 7 is moved to increase the generator excitation. The braking current is thus increased until contacts 40a open. As the speed of the vehicle decreases the braking current falls and the contacts 40a are reclosed at a slightly lower current value, the sequential opening and closing of the contacts 40a being continued until the regulator reaches the maximum excitation position whereupon switch 7b opens to de-energize the pilot motor. The diverter resistance 32 is connected across the armature of the motor 19 to ensure that the regulator arm moves slowly during braking. If at any time during automatic braking operation it is desired to reduce the braking-current, the switch 11 is engaged with contact 11b to energize relay 20. The pilot motor then moves the regulator to reduce the generator excitation until contact 11b is disengaged. The relay 40 may be connected in series with the resistor 17 or in series with either of the fan motors 33, 34. It may be replaced by a voltage relay connected across one or both traction motor armatures or a portion of the braking resistance. A relay responsive to excessive armature voltage may operate a warning signal or initiate automatic air or vacuum brake application. Specification 680,289 is referred to.