Hi Everybody:
I’m new here, and I have a question about locomotives. What is dynamic braking, and why do dynamic brakes slow the train down?
Thanks!
[:)]
…Just a basic answer would be to say the electric traction motors that propel the engine are switched by the engineer to become generators and that current is then sent to giant grids to absorb the current and load the traction motors and causing them to act as brakes slowing the train on the downgrade. Massive fans remove the heat from the grids caused from absorbing the current. [8D]
To take the process a little further, think lower gear in a car going downhill trying to slow down …-or- Jake Brakes on a truck…instead. Instead of making the diesel engine take the abuse and compression, the current generated by the wheels/traction motors is used to generate heat, like in a toaster. The older the dynamic braking system, the more it’s like a toaster with a fan blowing off the excess heat.
Thanks guys. But I’m still missing something. Why does the train slow down? What does the heat have to do with it? Is the inertia of the moving train somehow dissipated through the generation of electricity by the traction motors?
Thanks
…Yes, the last sentence of your statement is correct. The train slows down because when the traction motors become generators…they are saddled with a load of generating electricity and sending it to the grid…which provides the “load”. Heat is the by product. The traction motors are slowed by the “load” of the grid…hence causing a breaking effect on the train. High velocity fans must remove the heat from the grid to allow it to survive.
On downgrades momentum becomes the enemy of the locomotive engineer. The weight of the cars plus very little friction makes for tough going in terms of keeping under a speedlimit. Runaways rarely happen, but when they do, the results are frightening. Classic example is the Southern Pacific Devore, CA accident in 1989 that destroyed Duffy Street (and then ruptured a pipeline in a related blast two months later. )
The free turning axles of the locomotives are connected directly to the traction motors. With dynamic braking applied, those traction motors become electric generators. The current is dissapated by the dynamic brake grid (toasters) thus dissapating some of the inertia of a free rolling mass heading downhill. If you just used the air brake system on the trains, you would be changing out brake shoes faster than Sherman marched to the sea. The whole issue is control, being able to stop and being able to maintain a relatively constant speed.
Think of the grid as a giant toaster over. It was seem that this is a potential source of energy that is now just wasted with the cooling fans cooling down the grid. Why can’t this heat be used?
Woo hoo
Another great passenger Train Question…
Okay, Dynamic Breaks are simply and really simply,. when the gears reverse, in the locomotive, and when Dynamics are engadged (simply by moving the throttle from 8…or 2 to IDLE… or “Point Mort” " the Dynamics engadge, and often we have to apply very little breaks to stop at a station.
Thats didn’t help did it?
…The lady has made a clear and simple statement that pretty well defines what is happening…“The inertia of the moving train somehow is dissipated through the generation of electricity through the traction motors”…[now generators]…We have added somewhat how that is accomplished.
Thanks guys. I appreciate your explanations. I find it amazing that dynamic brakes can slow a train that much.
Welcome to the Trains forum Karen[8D] Glad to see another female on board!
Shame on you guys for not welcoming her after you answered her question[:(]
Now I have a question for you guys:
After all these years, I STILL cannot tell different engines apart!! HOW do you tell them
apart? (I have the same problem with helicopters, and people tell me they can tell by
the sound: helicopters all sound alike to me.) Remember, I’m a self-diagnosed
nimcompoop, so don’t get TOO technical on me.)
Karen [:)]
This is my first visit to your thread. I usually try to welcome new members. Since MC and Q have done a fine job of answering your question, then I will just say,
WELCOME to the forums. [:D]
I know you will enjoy them. [;)]
Sometimes we can’t… I have trouble with them, too.
I’m sure most folks have no problem telling the difference between a cab and a hood, and a switcher. The problems start within those classifications. Key to those problems is the fact that, in many cases, the exterior differences are small.
Perhaps the best way to compare is with road vehicles. If I say pickup truck, you have a picture in your mind, and can easily pick one out on the street, probably including the make. However, if I say F250 or C3500, things get a little confusing. You have to know what to look for in order to tell the difference between that and other models by the same manufacturer. Consider, too, that unlike the 50’s and 60’s where most cars had a distinct look, today you usually have to find the name plate to know what brand you are looking at, much less a specific model.
So - It comes down to a few basic characteristics of various makers (GE, EMD, ALCO, etc), types (BB, CC, cabs, hoods, cowls, etc), and models (SD, U-boat, Century, etc), after that you’re down to the “spotting features.” That might mean that the little window used to check the water level in on the third door on this model, and the fifth door on that. It can get hairy…
Get a copy of the Diesel Spotter’s Guide and play a little “spot the differences” between the pictures.
Ms. Cherokee,
Have you access to Trains Magazine? If so, compare the photos of various SD70s in the October 2003 issue and those of the AC4400CWs in the new March issue. They’re built by the two principal North American locomotive builders: Electro-Motive (GM) and GE, respectively. Differences in the noses should help you distinguish between the two manufacturers. Another thing that’s usually true is that GE units have a flat radiator area toward the rear of the unit, and EMDs have fans up there. The trucks beneath the units are different, too.
I’ve just mentioned these, since these two new “families” of locomotives are accounting for quite a percentage of what you may see in mainline service these days. If you have older units with the narrower noses, the difference in the rear radiator area usually still applies.
But, as others have said, a spotters’ guide is probably your best bet to distinguish between the models once you’ve determined who built it. And, since there aren’t very many SD90s around, GMs and GEs still sound distinctly different!
Oh…and hi, Karen!
When I lecture on dynamic brake systems I find that using the bycicle dynamo analagy helps a lot.
Ever have one of those bycicle dynamos? The kind that mount on the forks and has a little wheel that can rest against the side of the tire when in the operating position. Ever notice that when you engage the dynamo it makes it harder to pedal? This is because it takes energy to create electricity. This energy comes from you pedalling. Dynamic braking systems work essentially the same way. The kinetic energy of the moving train is converted into electrical energy by the traction motors being used as giant dynamos. The electrical energy is dissapated as heat through large resistor banks, like a giant toaster.
I think the point that eludes Karen in all the explanations is what happens in the traction motors themselves. While moving, the magnets (on the axle and aranged around the axle) in the traction motor work to “pull” towards each other to turn the axle and move the train. When they go into dynamic, the poles reverse and now the magnets in the traction motor are “pushing” away from each other. Because a train has so much interia built-up, it overcomes the resistance of the magnetic fields that want to turn the motor in the opposite direction, thereby generating the electric current that is dissapated in the grids. It is the train pushing against the magnetic field that provides resistance to slow the train down.
Hope that helps.
Mookie is printing like mad!
Welcome Karen - it is nice to have another female to “dress” up the place -
So much to learn, so little time.
Mookie
Hi everyone:
Thank you for welcoming me to the forum. It’s great that there are ladies here, too. I didn’t expect to find that.
I’m really impressed with the institutional knowledge everyone has. Wow. I’ve read and reread what everyone said to explain to dynamic braking. I think I finally understand it. It’s really hard trying to figure some of these things out on your own by reading in a text book or magazine. I know a lot of you guys have some practical knowledge – you’ve been out there and done it – and that is something I don’t have.
The part about the bike generator really helped. I was trying to figure why the train slows. And the notion that the generators are turning against the magetic field makes a lot of sense to me. I guess it puts a drag on the drive axles of the locomotive when the generators are working against the field. What I find so amazing is that someone was actually able to figure it out and make it work.
Again, thanks to everyone.
Well, I am bummed - the Mookie still hasn’t figured out the dynamic brakes, so can’t blame it on being female, I guess.
Maybe I should take up brain surgery - surely it would be easier!