I understand DC dynamic braking. A DC motor can be a generator when rotated and the force acts as a brake but from what I have read not an AC motor. So what do they do on an AC diesel engine to create the electric drag?? Any real experts who can explain ??
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I'm not a "real" expert, so I'll let someone else give you the answer. Lots of complicated electrical stuff place in these locomotives, and it's not just a "prime mover--> alternator --> A-C motor" traction system.
But you might consider that the alternator/rectifier/excitation/phase&voltage controls/inverter/traction motor combination in an A-C locomotive is a very complicated series of components. Trying to visualize a traction motor as simply a generator when running against outside mechanical influence, is simplistic. You need to understand that A-C traction motors are controlled by varying their voltage and phase angles, and that during any time that their mechanical speed exceeds their electrical speed, energy can be "sent back" (in simple terms) through the supply circuitry. The larger the difference between the actual RPM and the excitation frequency RPM, the more power it will generate. This power is changed back to D.C., and then dissipated through one of the usual methods: sent back to the generating source via the third rail or catenary, in the case of electrical railways; or sent to resistor grids (or in rare occasions, to a battery) on board the locomotive.
AC prime movers are now making use of 3 phase variable frequency drives. Dynamic braking is accomplished by dissipating the (heat) energy via a bank of resistors.
For more information there is good info here: VFD Dynamic Braking
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