The most significant change in dynamic braking was that, after the FT, excitation of the motor fields came from supplemental windings in the main generator. The FT used battery power for this excitation, which limited the range and power of the dynamic brakes; the engine would stay at "idle". The later design meant not only that the engine would run during dynamic braking, but also that the speed of the engine would vary the excitation and incresase the range of application. It would be noisier, too, accounting for the difference in sound. Randy Brown - -------------------------------------------------------------- Bob, Sam, et al, . . .About the dynamic brakes, three things come to mind in order of increasing likelihood which would affect the sound: First the way the dynamic brake vents were located on the FTs could present a rather different frequency distribution heard from outside the units. Second, the FTs had mechanically driven cooling fans rather than electrically driven fans of the other F units. And probably the most important and likely factor is that the original FT dynamic brakes had a very narrow operational range. All through EMD production the dynamic brakes were improved with a wider usable range. The lowest usable, controllable speed improved with each model release. As the electrical characteristics of traction motors and other components improved, the lowest safe speed of the dynamic brakes improved. This certainly would impact what was heard, different harmonics, etc. Preston or Jimmy might be the people to ask for a more conclusive response. Regards, Will The Erie Lackawanna Mailing List Sponsored by the ELH&TS http://www.elhts.org ------------------------------
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