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Basic principles of Diesel-Electric Traction.

The diesel-electric locomotive is essentially an assembly of three well-known machines, as shown in Fig 46.

A diesel engine crankshaft rotates a generator armature. The current produced causes a traction motor to rotate and drive the wheels through a reducing gear set. Such a simple arrangement would not be satisfactory. Means must be provided to start the engine. It should also be possible to run the engine without running the vehicle. Reversing gear, a cooling system for oil and water is required and the generator will require regulation. Forced ventilation will be needed to keep the drive motor(s) and generator(s) compact, and arrangements for vacuum or pressure braking systems provided, as well as provision of auxiliary electric power supplies.

Fig 47 shows the diagrammatic arrangement of generator and motor windings.




The current for the electromagnetic field of the generator is supplied by a separate constant voltage supply, giving the basic diesel traction arrangement of a separately excited generator electrically connected to a series wound dc motor. This is shown schematically in Fig 48.




Fig 49 shows a main generator’s field windings supplied by an engine driven auxiliary generator via a load regulator, the latter’s field windings being supplied by a battery which it keeps charged by means of a charging regulator [not shown]. The main generator output feeds the traction motor. The load regulator is a variable resistance, which in practice is automatic, being under the control of the engine’s governor mechanism.

In Fig 50 is shown the addition of a ‘field diverting resistance’ in parallel with the motor’s field, which will weaken the motor field strength when introduced. This enables the motor’s traction characteristics to be varied.




Fig 51 shows arrangements to enable motor reversal, as field current flow reversal causes the motor armature to reverse rotate.

Fig 52 shows arrangements for starting the diesel engine. Battery current flowing through the main generator starting field and its armature causes the generator to function as a series wound motor. Battery capacity and the starting field-windings are arranged to provide sufficient torque to turn the engine over at sufficient rpm for starting. To prevent current flowing to the traction motors, their circuits are disconnected during the starting operation.

Fig 53 shows the battery charging arrangements.



Fig 54 shows the services connected to the respective main and auxiliary generators.