Group,
I have read several responses to Wollffee's questions about diesel
starting systems and dynamic brakes systems. All of the responses are
either partially correct or incomplete. I will address primarily EMD
locomotives since that is what I know. However, with a few exceptions,
the GE units operate in a similar manner. Because changes have occurred
in the various system designs over the years, locomotives built in
different time periods do not all operate exactly the same way. EMD
locomotives can be separated into four distinct design periods, World
War II and before, post World War II to the mid 1960s, mid '60s to the
early 1990s and those built later.
Starting shortly after World War II, locomotives were built with three
distinct power generation systems on board. These include the main
generator which provides the primary propulsion, a small (usually 10
kw) generator which produces 64 DC used for control circuit, battery
charging, lights and some generator excitation and a an AC alternator
"piggybacked" on the main generator shaft, which produces AC power for
accessory equipment such as cooling fans and some blowers and much
later, air compressor drive motors. Theses locomotive all were started
using DC power from the batteries that were connected to a dedicated
winding in the main generator that converted it to a very powerful
motor. This cranked the engine. It never failed unless the batteries
were dead.
Starting in late 1965 with the delivery of the first 40 series
locomotives (645 series engines) all EMD high horsepower locomotives
were equipped with AC alternators in place of the DC main generators.
(The alternator output was fed into a built in rectifier bank (external
on GE locomotives) which converted the electrical power to DC for the
traction motors. With these locomotives, it was not possible to use an
internal starting winding. Thus, the diesel engines were equipped with
two modified highway truck starter motors to crank the engines. These
drive the crankshaft through a large gear mounted on the flywheel plate
between the engine and alternator. While successful, this arrangement
does not have the durability of the previous starting system.
The next change in the early 1990s was the introduction of the AC
traction motor. However, the starting system unchanged until the
introduction of the four cycle 6000 h.p. engines. These locomotives are
equipped with air operated start motors and include an extra air
reservoir to support them. They are very reliable, but without an air
source, there is only about two start attempts possible with a fully
charged air reservoir. Fortunately, most railroads have air sources
available almost anywhere. Some ALCO locomotives also used air start
motors as do most diesel on ships.
Dynamic brake systems date back to early electric locomotives and even
some street cars. As noted previously, the traction motors become
generators driven by the coasting motion of the locomotive and train.
The power output of the electric locomotives was fed back into the
power line to create the "load" which retards the traction motor.
Technically, the power output was available for another locomotive on
another train to use if it was in the power mode. As noted earlier, on
a diesel locomotive, the power output from the traction
motors/generators is fed into a resistor grid, usually located on top
of the locomotive where it is dissipated as heat. To operate in dynamic
brake, the diesel engine and generator or alternator, indeed, need to
be operating. Traction motors do not have permanent magnets in the
fields. Instead, they have electro magnets. These receive their power
source from the main generator. Without the electro magnetic fields,
there would be no resistance to slow the rotating armature/wheel
assembly.
Most later model diesel locomotives control the amount of dynamic
braking by varying the grid resistance. However, some RRs , such as SP,
used "field loop" DB control where the fields of all the traction
motors of all the locomotives were connected together in one giant
circuit. The locomotives had an extra three wire jumper cable between
each unit to accomplish this circuit. To control the dynamic brake, the
engineer varied the amount of power to the traction motor fields. Use
of this system declined after the 1950's.
On last note. A comment was made that dynamic brake could not stop a
train. This is correct with standard DB, however, some RRs have
locomotives equipped with extended range DB which automatically
increases the level of DB resistance at low speeds (below 12 - 15 mph)
so that additional braking is available that can all but stop a train.
An even better improvement occurred with the advent of the AC traction
motor. On these locomotives, the motors can literally be brought to a
stall with no damage since there is no commutator and brush
arrangement. In fact, on some grades, the AC units can hold a coal
train at a standstill with the air brakes released using only the stall
force of the motors. It is truly amazing to see the tractive effort
meter (there is no amp meter on AC units) sitting at well over 100,000
lbs. of tractive effort with no damage to the motors and no forward
motion. I've seen it.
Bill Barber
On Monday, October 27, 2003, at 09:19 AM, BRHSlist@yahoogroups.com
wrote:
> Date: Sun, 26 Oct 2003 15:06:57 -0800 (PST)
> From: "George William \(Bill\) Newport" <gwnewport2@yahoo.com>
> Subject: Re: Don't stop now.
>
> The engineers controls isolate the electrical power of the generator
> if I remember correctly,
> I have never had the pleasure of running one big enough or modern
> enough to have dynamic braking,
> the traction motors being turned by the wheels that they are connected
> to fed power back into the system and it is in turn fed into resistor
> banks, not the phaser banks like on the Enterprise on Startrek, the
> more electrical resistance the engineer adds by advanceing the control
> the harder it is to turn the traction motors which slows the train
> down, some, brakes also are used along with dynamic, you do not knock
> off the brakes, you leave some brake on and do not reduce braking when
> you use dynamic, dynamic will not stop a train but just slow it down,
> the dynamics effectiveness decreases as the train slows, and you would
> not normally use dynamics on the level or uphill, just in the downhill
> travel, the generator is directly connected to the flywheel of the
> diesel so it always turns the same way as long as the diesel is
> running whether or not the generator is what they call on line, when
> the generator is working and making power to turn the traction motors
> it is called loading up, so
> you have two complete and different circuits in a locomotive with
> dynamics, the resistors build up a lot of heat absorbing electrical
> power and are large and located in the top of the locomotive body
> where they can be cooled without adding any heat to the operation of
> the diesel, there is a separate system charging generator which keeps
> the batteries up to voltage and the electrical controls are opeated by
> the secondary sixty-four volts to handle the main generators output,
> the number that comes to mind is six hundred volts of main generator
> output, this way the large main generator output is not running
> through the controls that the engineer has to handle to operate the
> train but the smaller sixty-four which is a lot compared to a
> automobile, but safer that way, if that is unclear or I have
> misrepresented the facts because of short term memory loss from head
> injuries from military parachuting there should be someone out there
> who will say, hey, stupid, you got that wrong
> G
>
> wollffee <wolfee@onebox.com> wrote:
> OK, next question. How do dynamic brakes work? Does the diesel have
> to be turning the generator in order for them to work? Does it turn
> the generator backwards? Please keep in mind that I am the kind of
> guy that has to call an electrician to change light bulbs so don't
> tell me all kinds of complicated stuff, ok?
------------------------ Yahoo! Groups Sponsor ---------------------~-->
Rent DVDs Online - Over 14,500 titles.
No Late Fees & Free Shipping.
Try Netflix for FREE!
http://us.click.yahoo.com/vhSowB/XP.FAA/3jkFAA/8ZCslB/TM
---------------------------------------------------------------------~->
Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/
|