It is amazing what stupidity one can let slip by the editing and proofreading process late at night. The last sentence in the quote below is obviously incorrect. I blame it on a shift in the stream of consciousness, the hour the original answer was written and a brain flatulent. It's a good thing almost no one reads what I write, so few people were misinformed. Rather than leave misinformation stand, I'll attempt to correct it. >> >>> In dynamic braking, with the possible exception of the very first >>> FTs which used a two position brake, the traction motors are connected to >>> the main generator through the series windings, the commutators are >>> connected to the resistor grids. The series windings supply power to the >>> main generator to run it as a motor. >> In fact, when these units (I'm not sure how they control it in modern locomotives, I strongly suspect the control system is backwards compatible but different, just like when teletype machines were on the way out and we used to use interface devices like opto-isolators or current to voltage converters to connect to the tty current loop) were put into dynamic braking a very interesting and reasonably complex sequence of events happened. The diesel went into idle since you still had to drive all of the auxiliaries connected to the crankshaft/main generator armature drive line. A bunch of contactors, relays and switches reconfigured the wiring of the traction motors and main generator and an initial excitation was applied to the main generator to load the traction motors, now generators just enough to bunch up the slack in the train. At this point the dynamic brake was in effect, being used just like the independent brake was used. Oh, and the main generator output and hence the excitation to the traction motors was controlled by the battery voltage applied via the controller to the battery field winding of the main generator. After the the slack was drawn in, the engineer could increase the dynamic braking effect by advancing the controller which in turn applied more excitation to the main generator with the result of increased excitation to the traction motors turned generators. Increasing the field excitation increases the field strength which causes more current to be produced by the traction motors which of course is dissipated in the dynamic brake resistor grids. The net result is the traction motors require more torque to overcome their rotational resistance which either maintains the current train speed or slows it down if the field strength is greater than the force applied by the weight of the train. The limiting factor was the maximum current rating of the traction motor windings, the cooling efficiency of the traction motor blowers and dynamic brake grid fans and the grid resistor wattage rating. The engine was still running spinning the main generator and auxiliaries like the air compressor, auxiliary generator, and in the case of the FT's the main cooling fans, and whatever else tapped the engine drive line. Otherwise all F units and Geeps had dynamic brakes which functioned in the same manner. Hmm, maybe now it is clear why I was trying to simplify my answer believe it or not, I still get confused when I look at the wiring diagrams. I've always had a great deal of respect for the people who designed electro-mechanical control systems. I find using silicon a whole lot more straightforward myself. Thanks to Randy for catching my misstatement. Will Shultz The Erie Lackawanna Mailing List Sponsored by the ELH&TS http://www.elhts.org ------------------------------ End of EL Mail List Digest V3 #2039 ***********************************
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