I have noticed that the units with dynamic brakes seemed to do some sort of transition. It seemed to happen at about the speed where normal range DB fades out (10-12 mph). How does this work? Is extra resistance added somewhere, or does the circuit between the motors, and the grids change, as in parallel-series?
As far as I know, extended range dynamic braking with DC traction motors is obtained by reducing the braking resistance in several steps starting at the speed at which regular dynamic braking is starting to reduce, and progressing as speed further reduces.
I do not have any EMD curves on hand, but the attached set provide a general illustration. At each resistance reduction, there is a significant spike in traction motor current, which might be felt on the locomotive as being much the same as a power transition spike.
Cheers,