http://www.youtube.com/watch?v=O48HvZcKGQU&feature=relatedI know some of you have an interest (or are Locomotive Engineers like myself) in trains, and the above TV doco has made an extraordinary error in describing dynamic braking!!! Anyone see it?

I tried to catch it but didn't. Not beeing an engineer, I've always somewhat equated dynamic braking to something similar to a jake brake although with the presence of the electric motors in the power train, I'm sure it's a bit different . Please elaborate.

Dynamic braking uses the electric motors as generators to turn the train's kinetic energy into electric current.That current is then sent to a set of resistors that turn the electricity into heat which is dissipated into the air.At 6:20 they show arrows going from the electric motors to the dieselgenerator.

EGAD ! How does an error like that get past editing ?I'd hate to think the engineer explained it that way..Briefly.. locomotive dynamic-brakes simply turn the traction motors into generators, and route their output to a massive resistor grid (the grid and its cooling mechanism are obvious exterior features). When you place an electrical load across a turning generator's output, it puts a physical load against the turning action. It cannot bring a locomotive to an actual stop, and is only effective above a certain speed... but above that speed, it's like riding the brakes going down a hill, without the pad wear/tear.Edit: To my understanding, the diesel engine is out of the loop, during dynamic braking..

Seems like a reasonably accurate description to me for a 'quick explanation' of what dynamic braking is. I'm no real life train driver, but as far as I understand dynamic braking, the EMF of the motors in the bogies brakes the axles (a bit like putting an electric motor in reverse) and the heat generated from doing that is dissipated through the big vents on the roof of the loco, which is more or less what the documentary said.I did notice the arrows going to the diesel engine in the animation, but that's not really a big deal, I know they should really be going to a big grille and a fan to indicate it generates a lot of heat, but the words he uses do still adequately describe how dynamic brakes work. After all, it was not a training course, but just a lightweight entertainment programme, so there's no need to turn it into a complex drawn out technical description of the process.Cheers for posting the video though, NS is one of the railroads I model up in my loft in HO scale! Al

OK.. after a bit of research.. the theory is the same, but the diesel engine is in play, in that it decides how much of a generator, the traction motors become (applying electricty to the field).. But the braking action is the placing of a load across the motors-turned-generators, output.

Seems like a reasonably accurate description to me for a 'quick explanation' of what dynamic braking is. I'm no real life train driver, but as far as I understand dynamic braking, the EMF of the motors in the bogies brakes the axles (a bit like putting an electric motor in reverse) and the heat generated from doing that is dissipated through the big vents on the roof of the loco, which is more or less what the documentary said.I did notice the arrows going to the diesel engine in the animation, but that's not really a big deal, I know they should really be going to a big grille and a fan to indicate it generates a lot of heat, but the words he uses do still adequately describe how dynamic brakes work. After all, it was not a training course, but just a lightweight entertainment programme, so there's no need to turn it into a complex drawn out technical description of the process.Cheers for posting the video though, NS is one of the railroads I model up in my loft in HO scale!Al

I was thinking the same thing. If one wants to get pedantic, I guess I could point out a lot of things not explained perfectly.As for Norfolk Southern, I live in "NS country". Nearby here there's an NS yard that employs a good portion of people that live in my town. No less than 4 of my close relatives work for NS (2 engineers, 2 mechanics). When the crew gets hungry and it's time for lunch, you can find the GP-38/40 parked behind my favorite local BBQ. Yes, they park the engine behind the BBQ restaurant, walk up and eat, and then off they go. Convenient that the tracks lead right though town!

The most glaring error:"The resistance in the engine makes the wheels harder to turn"I don't think pointing that out, is pedantic.. it's fundementally wrong.

EGAD ! How does an error like that get past editing ?I'd hate to think the engineer explained it that way..Briefly.. locomotive dynamic-brakes simply turn the traction motors into generators, and route their output to a massive resistor grid (the grid and its cooling mechanism are obvious exterior features). When you place an electrical load across a turning generator's output, it puts a physical load against the turning action. It cannot bring a locomotive to an actual stop, and is only effective above a certain speed... but above that speed, it's like riding the brakes going down a hill, without the pad wear/tear.Edit: To my understanding, the diesel engine is out of the loop, during dynamic braking..

Seems like a reasonably accurate description to me for a 'quick explanation' of what dynamic braking is. I'm no real life train driver, but as far as I understand dynamic braking, the EMF of the motors in the bogies brakes the axles (a bit like putting an electric motor in reverse) and the heat generated from doing that is dissipated through the big vents on the roof of the loco, which is more or less what the documentary said.I did notice the arrows going to the diesel engine in the animation, but that's not really a big deal, I know they should really be going to a big grille and a fan to indicate it generates a lot of heat, but the words he uses do still adequately describe how dynamic brakes work. After all, it was not a training course, but just a lightweight entertainment programme, so there's no need to turn it into a complex drawn out technical description of the process.Cheers for posting the video though, NS is one of the railroads I model up in my loft in HO scale!Al

OK.. after a bit of research.. the theory is the same, but the diesel engine is in play, in that it decides how much of a generator, the traction motors become (applying electricty to the field).. But the braking action is the placing of a load across the motors-turned-generators, output.

Yes correct that dynamic braking uses resistors. The engine is used for cooling the resistors. The engine drives the fans to keep everything cool.But the engine is not in the 'loop' to translate current into heat, which the resistors do. The engine merely is a protective mechanism during dynamic braking to ensure the resistors do not overheat. I have driven locomotives where the grids have blown....... Either due to wear and tear or the Australian heat. I wonder if in Canadian winters they are actually harder to blow?The traction motors become generators in dynamic braking and the current is sent to the resistors in the grid (usually around where the cooling fans are on top). On the SD-40 locomotives the engine goes into Run 5 automatically. Newer locomotives will idle. If heat is high the newer ones will automatically run the engine up (computer controlled).

The most glaring error:"The resistance in the engine makes the wheels harder to turn"I don't think pointing that out, is pedantic.. it's fundementally wrong.

These relativism makes everything seem pedantic. Yes it is fundamentally wrong.The media generally have no idea or clue about anything they 'report' on! No wonder we are led astray.

I guess one could nit pick.I think the general idea is conveyed, 'something' (traction motor, engine etc whatever) is putting resistance upon the wheels causing them to move slower than gravity would normally allow, thereby assisting in slowing down the train.When you start adding descriptions about resistor grids, and heat dissipation etc, you're going to lose most novices anyway, defeating the purpose.Regards.Ernie.

I guess one could nit pick.I think the general idea is conveyed, 'something' (traction motor, engine etc whatever) is putting resistance upon the wheels causing them to move slower than gravity would normally allow, thereby assisting in slowing down the train.When you start adding descriptions about resistor grids, and heat dissipation etc, you're going to lose most novices anyway, defeating the purpose.Regards.Ernie.

Exactly. Some folks are a little obsessessive. :D

Actually Daniel, I am more interested in the fact you are an engineer! Tell me, have you ever ran steam? Or what engines do you drive and where? Use to be into trains as a teenager even working for a big train model company back then... Campbell Scale Models back in it heyday.Clutch

It is relative, I guess.. and this is not a railroading forum full of train enthusiasts.But ponder the reverse.. you all as aircraft enthusiasts who might frequent railroad forums. In a general discussion area of a railorad forum (where they are really calling this out, BTW).. would you just accept a post describing jet reverse-thrust in a completely innacurate way, so long as it conveys that it aids in braking after landing ?Like.. "As the pilot applies reverse thrust.. the engine thrust now powers a mechanism that resists wheel roll".. It is that erroneous.. just sayin'

It is relative, I guess.. and this is not a railroading forum full of train enthusiasts.But ponder the reverse.. you all as aircraft enthusiasts who might frequent railroad forums. In a general discussion area of a railorad forum (where they are really calling this out, BTW).. would you just accept a post describing jet reverse-thrust in a completely innacurate way, so long as it conveys that it aids in braking after landing ?Like.. "As the pilot applies reverse thrust.. the engine thrust now powers a mechanism that resists wheel roll".. It is that erroneous.. just sayin'

 

Brett you are right. The appeasers have stated that I am obsessive to be 'correct' on railroad theory. Now, if a pilot said that I was obsessive for picking him/her up on their engine theory, I bet they are going to have a hard time passing their examinations for commercial licenses.

 

Notwithstanding that, if I was a pilot and said to a flight instructor that it doesn't matter about the speed of the aircraft or the configuration etc, I bet he/she would think I would best be not flying at all until I had a change of attitude. Flying is ALL about attitude, and unfortunately some have the WRONG attitude.