For many years (beginning with the EMD FT model) dynamic brakes were a simple matter:
...the direct current traction motor was electrically changed (field windings charged???) so that it became a generator instead of a motor.
...the DC current generated was sent into a heavy duty resistor grid in the roof of the locomotive
...this grid was cooled by a fan which blew air over it.
Remember the hand crank light bulb we all played with in school science class? The harder you cranked, the brighter the light, but also the stronger the resistance to turning the crank. Same principle for dynamic brakes, but on a much larger scale.

So, how does this work on current AC traction diesels? I can think of two possibilities:
1) the traction motors contain a DC winding for the sole purpose of providing dynamic braking
2) can the traction motors "generate" AC current which then goes into the grid to provide the resistance.

Would appreciate hearing from somebody who knows.

---------------
Bruce Butler