SP5103 Wrote:
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> > >Early EMD switchers start in full series, and
> > then change to series-parallel.
> > >"Transition" varies with locomotive design.
> Early
> > EMD switchers start in full series, and then
> > change to series-parallel.
> >
> > EMD had 300v traction motors? If so, never
> heard
> > of it before.
>
> The consideration in EMD's case has nothing to do
> with the voltage rating of the traction motors.
> The apparent issue is the (lack of) capacity of
> the main generator. The typical low horsepower
> rating of switch engines did not justify the
> cost/investment in the largest main generators
> which would rarely even approach their maximum
> ratings. To connect a pair of EMD traction motor
> strings across a main generator requires around
> 1500-2600 amps to work at full capacity (depending
> on model/era of traction motor design). By
> connecting the four motors in series instead of
> series-parallel, a smaller (and cheaper) main
> generator of 800-1200 amps is more than
> sufficient. Since the typical switch engine is
> primarily operated at low speeds, this is not
> really a drawback under most conditions.
>
> The tractive effort formula for diesel-electric
> locomotives is traction hp x 375 x efficiency
> divided by speed, understanding that at low speeds
> the theoretical tractive effort will exceed
> available adhesion. In other words, the
> approximate tractive effort (drawbar pull) halves
> as speed doubles, and the ammeter will reflect
> that.
>
> I run and work on the electrical systems of
> various GE and EMD switchers, and GP units
> built/rebuilt 1950s-1980s; and can attest that
> there is a wide variation in the electrical
> arrangements. I believe all the EMD switchers from
> at least the SW1/NW2 use the full series to
> series-parallel arrangement. The
> SW1000/SW1001/SW1500/MP15DC use a permanent
> series-parallel arrangement with a D32 main
> generator, though there was an option of the
> older/smaller D25 main generator and the older
> transition arrangement on the SW1000/SW1001. Even
> though the MP15AC uses an AR10 which will support
> all four motors in parallel at 4200 amps, the
> motors are in series-parallel with one step of
> field shunting with the AR10 limited to 2500
> amps(per Service Manual). The AR6 would have been
> a good option for the MP15AC/GP15AC/GP15T but
> apparently parts standardization on the AR10 won
> out. IIRC the SW900 manual shows that in switching
> (instead of road), it operates in series-parallel
> instead of series. All these variations were
> apparently done by EMD to improve the operating
> characteristics of switch engines.
>
> The SD45 manual shows it has up to ten steps of
> forward transition: series-parallel with 5 steps
> of field shunting, and parallel with 3 steps of
> field shunting. Field shunting does not typically
> cause the loading to "stutter" compared to motor
> connection changes. Not sure when an SD45 would
> normally make series-parallel to parallel
> transition, but it may actually be fairly high
> (15-25 mph?) due to the number of field shunt
> positions and depending on gearing.
>
> While temperature does have an effect on
> resistance, the primary cemf resistance change in
> traction motors is caused by the magnetic fields
> cutting through each other.
>
> GE, on its trolley, smaller and electric
> locomotive traction motors did have a wide variety
> of insulation valves and voltage ratings, with
> some of the same models having more than one
> voltage rating. (On GE part numbers, the suffix
> can be nothing more than an improved model or an
> entirely different gear ratio, voltage rating,
> interlock arrangement, etc.)