On Sun, 26 Oct 2003 15:19:44 -0800 (PST), George William \(Bill\) Newport
wrote
> No,
> but the diesel is still needed to make air for the brake system and
> heat for the crew and to keep the batteries charged up and to
> operate the relays that make everything work, the magnets you speak
> of is the magnetic field of the electrical system which either
> generates power or makes resistance without any of the parts
> actually touching each other, and dynamics cannot stop the train
> like the air brakes can and you still have to use the air brakes,
> you just add dynamics to it as an assist, the Seaboard did not buy
> much dynamics if they had any at all, early Seaboard engineers used
> what some people call Seaboard dynamics, they set the reversers in
> the opposite direction of travel and used a little throttle to send
> reverse current through the traction motors to slow them down, they
> also burned up a few traction motors as the system is not designed
> for that, but leave it up to a bunch of ex steam engineers to find
> something to tear up them diesels that took away their favored steam
> engines G
>
> wollffee <wolfee@onebox.com> wrote:
> OK, thanks, I think I understand. This means that the diesel is not
> used then, right? When the wheels turn, instead of getting pushed by
> the magnets in the electric motors, they get the opposite force and
> the magnets push AGAINST the direction they are turning, right?
>
Let me see if I can simplify this:
If you take a system that applies power to work, but reverse the input, you
reverdse the effect.
Take a windmill, where the moving air turns a shaft. Put a motor on the
haft, and you have a fan producing wind. Take a water wheel, which uses the
weight of falling water to turn a shaft. Apply power to the shaft, and you
have a pump which lifts water.
Take an electric motor which uses electricity to turn, and let the weight of
the moving train turn the "motor". It now generates electricity.
The key to dynamic braking is the bank of resistors, which turns the
generated electricity into heat, exhausted by the DB fan (of whatever
configuration). The same effect could have been achieved by mounting
(remember, this happened during World War II) air-raid seachlights on the
front of the unit and using the generated current to blind approaching
motorists (which would have raised liabulity insurance rates) - far more
seriously, you need to impose a load on a generator to make it "drag", and
the resistor banks in a GP blister or an F hatch "uses" the electric current
to generate discardable heat, and that spent electricity, generated by the
turning traction motors/generators slows the unit(s) . . . see the "Second
Law of Thermodynamics" whence comes the name, "Dynamic Brake".
Marshall Thayer
PS: CB&Q was the third buyer of EMD's FT units - which were the first
diesels to offer DBs . . . the Santa Fe & Q eook them . . . the Southern
(with the notorious Salida grade, passed!
--
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