Electric diesel locomotive dynamic braking
A common option on a Electric diesel locomotive is dynamic braking. Dynamic braking takes advantage of the fact that the traction motor armatures are always rotating when the diesel locomotive is in motion and that a motor can be made to act as a generator by separately exciting the field winding.
When dynamic braking is utilized, the traction control circuits are configured as follows:
1. The field winding of each traction motor is connected across the main generator.
2.The armature of each traction motor is connected across a forced-air cooled resistance grid the dynamic braking grid in the roof of the diesel locomotive hood.
3.The prime mover RPM is increased and the main generator field is excited, causing a corresponding excitation of the traction motor fields.
The aggregate effect of the above is to cause each traction motor to generate electric power and dissipate it as heat in the dynamic braking grid.
A fan connected across the grid provides forced-air cooling. Consequently, the fan is powered by the output of the traction motors and will tend to run faster and produce more airflow as more energy is applied to the grid.
Ultimately, the source of the energy dissipated in the dynamic braking grid is the motion of the electric diesel locomotive as imparted to the traction motor armatures.
Therefore, the traction motors impose drag and the locomotive acts as a brake. As speed decreases, the braking effect decays and usually becomes ineffective below approximately 10 mph, depending on the gear ratio between the traction motors and axles.
Dynamic braking is particularly beneficial when operating in mountainous regions, where there is always the danger of a runaway due to overheated friction brakes during descent.
In such cases, dynamic brakes are usually applied in conjunction with the air brakes, the combined effect being referred to as blended braking. The use of blended braking can also assist in keeping the slack in a long train stretched as it crests a grade, helping to prevent a run-in, an abrupt bunching of train slack that can cause a derailment.
Blended braking is also commonly used with commuter trains to reduce wear and tear on the mechanical brakes that is a natural result of the numerous stops such trains typically make during a run.
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