To keep it simple and less wordy: when you set-up the dynamic brakes, the traction motors convert to generators (by reversing the current flow through the armature). Since it takes alot of horsepower to turn a generatorr armature, the locomotive wheels then become a braking force. The power gernerated by the traction motors during dynamic braking is dissapated through resistors.
Actually nowadays the dynamic brake of AC traction engines and the new ES44DCs will stop a train on a dime and keep loading right down to zero.
Many years ago I had the ES44 demonstrators MU'd to the GE research car(dynamometer car with laptops) and on board technicians ,they wanted us to stop next to a lake for photographs in perfect light (couldn't resist) and we stopped with just dynamic and they were amazed that we could and did regularly. Its especially fun behind the old mans house on the GTW in chicago that complains about stopping behind his house as it must cause a small seismic event locally!
Current policy on the unnamed RR that I work for is to use dynamic everywhere in all circumstances which is quite idiotic in many locations (especially uphill) as it uses less fuel than stretch-braking and reduces brake wear as noted elsewhere but it is rough on the train in my opinion and without long gentle terrain to make gradual changes inevitably results in run in bumps which would kill a caboose crew in the old days and is probably not good on refridgerators cars and fine china!
BigJimMy concern was reading the 15 - 45 mph being the "most effective" range part. With a full tonnage train, 15 - 28 mph is the most effective range of conventional DB ( AC units being in a class by themeselves). Over 28/30 mph the DB effort starts to drop off at a proportionate rate to speed increase.
The purpose of my response was to give people with limited technical or operational experience accurate information without overwhelming them. For my explanation the effective range of dynamic brake is the range of speeds where more than half of the maximum retarding force is available to the engineer. For conventional dynamic brakes 15 to 45 mph is that range, for extended range dynamic brakes 3 to 45 mph is that range. 15 to 28 mph is the range if maximum retarding force will be necessary to control train speed as you stated, it is in that range that increasing speed of the train is countered with increasing dynamic brake retarding force. Above approximately 30 mph dynamic brake retarding force reduces as speed increases, however for all but the very heaviest trains on grades with low speed restrictions dynamic brake is still very effective in controlling train speed without heavy air brake applications.
tleary01At track speeds generally above 30 mph, in maximum DB, the armature/grid current is at its regulated maximum and higher speeds will require the motor field current to be reduced which reduces the retarding force accordingly. At track speeds generally below 25 mph the motor field current is at its regulated maximum and if speed continues to drop generated the armature/grid current and the motor’s retarding force reduces accordingly. Between 25 and 30 mph where the field and armature/grid currents are at maximum, the retarding will force be at maximum (for any given DB control handle position). Maximum retarding force for standard dynamic brake is 10,000 lb/ motor, high capacity dynamic brake is 12,500 lb/motor. The most effective retarding range of the dynamic brake is between about 15 to 45 mph, weakened but still good to 65 mph, weakening and not very effective below 15 mph, but DB is a retarding brake not a stopping brake.
DPman,Please excuse me, as I got lost in the flood of your post. I should have read a little closer to what was written above.My concern was reading the 15 - 45 mph being the "most effective" range part. With a full tonnage train, 15 - 28 mph is the most effective range of conventional DB ( AC units being in a class by themeselves). Over 28/30 mph the DB effort starts to drop off at a proportionate rate to speed increase.On the grades of my district, a conventional DB will or just about hold back what tonnage it can pull up the hill. 28/30 mph is riding a very fine line of holding back or having to use the air to maintain speed. If the DB will not hold and the train speed gets over 28/30 mph, then the speed will just keep increasing and eventually the air brake will be needed to control the speed of the train. This translates into a problem if the train is known to have a "snapper" ( read: the train goes into emergency when the air brake is used ). Our rule of thumb is to keep the train down to 25 mph. It may take you a little longer to get down the hill, one with a 25 mph limited curve at the bottom, but at least the conductor isn't out there pounding the ground.With speeds in the 45 - 50 mph range and slight to medium downgrades the DB is at best just keeping the slack bunched up and not much more.
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I believe I've read that C&O had dynamics on the mountain GEEPs, but not on those they ran on the north end, in Michigan, which is relatively flat.
Larry Resident Microferroequinologist (at least at my house) Everyone goes home; Safety begins with you My Opinion. Standard Disclaimers Apply. No Expiration Date Come ride the rails with me! There's one thing about humility - the moment you think you've got it, you've lost it...
What's interesting is some roads, especially in the midwest, ordered EMDs without dynamic brakes. IC, MP and C&NW fielded many EMDs without dynamics. C&NW even had dynamic-less SD45s, and IC's GP/SD40s had no dynamics. IC resisted the trend of dynamics on mainline locos even in the 1990s, and removed dynamic brakes from GM&O SD40s when they were rebuilt into SD40-2Rs. IC also had several former-BN SD40-2s which BN took off lease sometime in the early 90s. As BN locos, these SD40-2s previously had dynamic brakes. IC removed the dynamic brakes on some of these SD40-2s. A pic on page 59 in Paul Schneider's book on BN shows former BN SD40-2 #6751 on the IC with the section of the long hood where the dynamic brakes used to be painted in primer, while the rest of the SD40-2 is still in BN paint. Of course IC's spartan cab SD70s had dynamics, and dynamic brakes was standard on all 60 and 70-series EMDs.
That was very technical DP. Almost too technical, but I did get more insite. Appearently I didn't know the basics, I knew the utter basics, the fact the axle motors are used as generators to create an electrical field that slows down the axle with essentiall electro-magnet force and that juice was fed to a frid and turned into heat. I knew trains had a seperate control to turn the db on and off but I didn't know they had seperate..um..throttle control for lack of better term. I will how ever say that the way it's been described it sounds like something every rr would want in there trains. Use the db as a slowing brake, which saves on mechanical brakes, and then use the air brakes as a slowing/stopping brake at slower speed.
Is there a limit to how much the dynamic brakes can be used? Assuming of course there are no problems with the brake grid or cooling fan(s). Is it a item common to have, not just in hilly/mountainus terrain?
I was still employed by UPRR at the time the SD80 and SD90 were developed. Used two Conrail SD80's at Pueblo to perform the software verification tests and make final corrections. With me were GE-Harris and EMD but no one from Conrail. When the final software was accepted, the same DP/Locomotive software was applied to UPRR SD90 locomotives.
See also Al Krug's sub-page on this, at:
http://www.alkrug.vcn.com/rrfacts/brakes.htm#dynamic
tleary01Check my profile. Experience (40+ years), books, and locomotive manufacturers training and service manuals. In 1985 wrote Dynamic Brake technical manual that is still in use for Union Pacific operation and maintenance training.
Check my profile. Experience (40+ years), books, and locomotive manufacturers training and service manuals. In 1985 wrote Dynamic Brake technical manual that is still in use for Union Pacific operation and maintenance training.
-Don (Random stuff, mostly about trains - what else? http://blerfblog.blogspot.com/)
BNSFwatcherI remember seeing, in my old BN ETT, that a limited number of axles were allowed to be hooked-up for DB. Why was that? More-brakes-the-better, I would have thought. I'll dig out the old ETT and check it out. BN added helper units for their DB capacity over the Rockies, and BNSF still does, if not just for "pushing/pulling" power. Thanks. Bill
I remember seeing, in my old BN ETT, that a limited number of axles were allowed to be hooked-up for DB. Why was that? More-brakes-the-better, I would have thought. I'll dig out the old ETT and check it out. BN added helper units for their DB capacity over the Rockies, and BNSF still does, if not just for "pushing/pulling" power. Thanks.
Bill
Too much braking from the locomotives without enough assistance from the train brakes can cause the train to derail. You will get so much deflection pressure against the couplers and then the rails, that the cars will accordian off to the sides. Alignment control couplers help, but only a little. The use of a "number" of axles is a simple way to calculate the amount of dynamic braking force. That is why modern locomotives are considered to have more axles for calculation purposes that what the locomotive actually has. So modern six-axle DC locomotives are considered to have 8 axles and modern six-axle AC locomotives 9 axles for dynamic braking calculation purposes. Note that these numbers are for locomotives equipped with High-capacity Dynamic Braking.
tleary01 Check my profile. Experience (40+ years), books, and locomotive manufacturers training and service manuals. In 1985 wrote Dynamic Brake technical manual that is still in use for Union Pacific operation and maintenance training.
So you haven't actually been running trains for years on end to know how effective/ineffective the dynamic brakes really are?
tleary01 The most effective retarding range of the dynamic brake is between about 15 to 45 mph, weakened but still good to 65 mph, weakening and not very effective below 15 mph, but DB is a retarding brake not a stopping brake.
The most effective retarding range of the dynamic brake is between about 15 to 45 mph, weakened but still good to 65 mph, weakening and not very effective below 15 mph, but DB is a retarding brake not a stopping brake.
May I ask where you got this information, book or experiance?
MILW-RODR: Don’s response is accurate but terse. You say that you are looking for more in-depth knowledge. I did a quick Google search and found only a Wikipedia hit that has some information about locomotive dynamic brakes but it is only about 50% accurate. I will use Don’s response and expand a bit, but to truly understand the full depth of knowledge would require graphs and diagrams in addition to much more text than this forum can support. Maybe I should edit the Wikipedia page. My answers will be for DC locomotives only.
When the dynamic brake control handle is placed in the SETUP position the motor-brake transfer switch moves to the brake position. The traction motor fields are connected in series across the main generator and the traction motor armatures are connected in pairs to grid resistors; two armature/grid circuits for 4 axle units, three armature /grid circuits for 6 axle units. The braking field contactor (B on EMD) is not closed in setup.
When the dynamic brake control handle is placed in position 1 the braking field contactor is closed and the motor fields receive minimum excitation current from the main generator. If the motor armatures are rotating, generated electrical current will begin to flow from the motor armatures through the resistance grids. Minimum dynamic brake retarding will begin.
On locomotives that have a modular type traction control system it is the DR Module, on micro-processor control systems it is the DR Function. DR looks at the dynamic brake handle position, the motor field current and armature/grid current and determines the amount of main generator excitation. DR also sets the limits of motor field and armature/grid current and limits the main generator excitation to prevent exceeding the limits. The main generator excitation control circuit is modified to produce very low levels of main generator output current since 4 or 6 motor fields are the only load.
As the dynamic brake control handle is moved from position 1 toward position 8, the dynamic brake control signal will increase from about 8 volts (minimum DB) to 68+ volts (maximum DB). DR will in turn increase main generator excitation and motor field current that along with armature rotation speed will produce a grid current relative to the braking handle position/DB control voltage. This is “grid current” type regulation and is the most common way that DB is regulated.
As speed drops, you have to increase the current through the motor field coils to keep the braking resistance up. At some speed, you've hit the max excitation and braking effort tails as speed drops. The purpose of dynamic brake is to produce retarding force at the traction motors. The retarding force is produced by magnetism and is a combination of field current/magnetism and armature current/magnetism. At track speeds generally above 30 mph, in maximum DB, the armature/grid current is at its regulated maximum and higher speeds will require the motor field current to be reduced which reduces the retarding force accordingly. At track speeds generally below 25 mph the motor field current is at its regulated maximum and if speed continues to drop generated the armature/grid current and the motor’s retarding force reduces accordingly. Between 25 and 30 mph where the field and armature/grid currents are at maximum, the retarding will force be at maximum (for any given DB control handle position). Maximum retarding force for standard dynamic brake is 10,000 lb/ motor, high capacity dynamic brake is 12,500 lb/motor. The most effective retarding range of the dynamic brake is between about 15 to 45 mph, weakened but still good to 65 mph, weakening and not very effective below 15 mph, but DB is a retarding brake not a stopping brake.
Extended range braking shunts some of the braking resistors so that you can keep high braking force at low speeds. Current from each armature pair passes through 4 grid “sections” that convert the electrical energy to heat. All of the resistors are necessary at track speeds above 20 mph to dissipate the full amount of energy generated. Below 20 mph armature rotation has slowed to where the resistance of the grids reduces the ability of the armatures to generate current/magnetism. Extended range braking control shunts (bypasses) one grid section at a time as speed is reduced, lowering the grid resistance. Armature/grid current will increase, restoring the retarding force. The DB regulator will adjust main generator excitation and the resulting motor field current to prevent over-shoot of the armature current which can damage the remaining grids and also surge the retarding force making the wheels slide.
As speed decreases below 20 mph, the extended range control will shunt the grid sections at about 18, 12, and 6 mph leaving one grid section in the circuit. The final increase in retardation, though not enough to completely stop the train itself can bring the speed down to where on a nearly level grade a locomotive independent brake application will bring the train to a final stop. The dynamic brake’s effective retarding range has been extended to nearly a stop.
MILW-RODRIs it preffered by a RR to use db's as primary braking over the air brakes?
Dynamics have the advantage of not wearing out brake shoes (important to the bean counters). They also have the advantage of being variable, where on most air brake systems the only way to get less braking effort is to release the brakes. There are some air brake systems where that's not the case, but I believe they are the exception at this time, not the rule.
Most of our engineers prefer dynamics when available and appropriate.
As for a specific railroad policy - I'd imagine, but I don't know.
I know the basic principle of how dynamic braking works, but I'm looking for more in depth knowledge. How exactly does a dynamic brake work? How is the output of braking effort controlled? Besides the visual difference in blisters, what is the difference between a standard dynamic brake set up and an extended range set up? Is it preffered by a RR to use db's as primary braking over the air brakes? Now if I could just remember the last question I had lol.
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