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Re:Re: (erielack) Re: EL Mail List Digest V3 #2336

The reason for transition is that as traction motors gain speed, their armature and field windings begin to act as generators and produce drag -- back electromotive force, or EMF.  To counter the EMF, transition changes the relationships of the motors to each other, and the relationships of the individual motor fields to their armatures.  The steps are Full Series (all fields receiving full excitation and motors wired in series); Series Shunt (motor field current is shunted around the fields to lower excitation; Full Parallel (fields on full current again, but the motors wired in parallel, usually in pairs); and Parallel Shunt (reduced excitation to the motors, which are connectd in parallel.

This is why an accelerating diesel locomotive will sound like it is "shifting gears" -- because, electrically speaking, it is.  Most switchers didn't have transition; they were meant to drag.  Road locomotives had it, to enable them to go faster.  In early road locomotives, it was manual, at the discretion of and executed by the engineer.  Frequent misuse led designers to make it automatic after the FT.

Randy Brown
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> PS, What is transition? 


"Transition" - at a high level - is when a diesel-electric's electrical system is changed from a parallel to series (and backwards) system. I can't remember what the initial state is, but the traction motor can be wired in parallel (output from generator is fanned out to each traction motor) or in series (output of generator goes to one traction motor, which then outputs the next traction motor, and so on)

     - Paul


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