I understand that electric energy is produced when the traction motors are turned, which is then sent through resistors making the wheels difficult to turn, but I'd like to know where the electricity continues after the resistors. Back to the traction motors, or to somewhere else to be grounded?
I think the series of resistors slow the output in turn creating a lot of heat which is why there are extra fans on the units with dynamic braking. I don't know where the little left over energy goes...
As the motor slows, less electricity is produced. If the dynamic brakes were being used long enough, the loco would come to a stop. There is a circuit that switches between power to the motors and power to the dynamic brakes. I have worked on industrial drives that use dynamic braking and the operation is essentially the same. At one time, the power from the motor was returned to the system that used overhead wires or third rail to power the locomotives. I have not kept up with that style of braking. If the dynamic brake is not switched out, the motor comes to a stop. The electricity never goes to ground you might say. In fact, electric drives never send electricity to ground in all the systems I am aware of. There is no left over or extra electricity.
http://www.northeast.railfan.net/diesel_faq.html
This is probably more than you will ever want to know about locomotives.
Rich
The resistor grid is a load, just as would be a lamp or your range or electric drill. When the traction motors are thrown into "generator" mode, the power generated goes into that resistor grid, and is consumed, producing heat; hence the fans used for that purpose. Simply put, it goes no where else.
As was said before, as the motors are slowed down by "loading" them, less emf is produced, thereby the locomotive progressively slows down, eventually to a stop.
When the switches are thrown, the system again reverts into motor loads, the generator(s) supply them, and the accelleration process is again activated.
Rich
Let's make the distinction between dynamic braking and regenerative braking. The first is what is used in diesels today--the power is turned into heat in the resistor grids and blown off by the fans. Regenerative braking uses a similar principal--turning the motors into generators. However, in electric locomotives the power was in fact sent back through the overhead wires, providing some saving in operating costs. I'm not sure whether Amtrak still uses regenerative braking or not.
ebtnut,
As I read the reponses in this thread I wondered if anyone was going to point out that energy was being converted to heat, then dissipated through heat sinks/fans. I think that's the truest answer to heart of the original question. You answered it.
Craig
Dear All,
I can't believe that someone hasn't invented a system of storing a (diesel-electric powered) train's dynamic braking energy instead of dissipating it as heat. What a waste.
How about charging a bank of batteries, spinning up a large flywheel, or compressing some air? Any other suggestions? (Would compressing a spring be too dangerous?..)
Sincerely,
Joe Satnik
You answered it.
Craig;
See my abovementioned post...
Rich
Joe,
There is not currently any technology that would be effective to store that energy. Batteries would be the only way and to be usefull in powering the loco, their size and weight would result in more power (and therefore more fuel) being used when they where not in use, so it would most likely be a zero sum game. Waste is a relative term, trains are already 5 times more fuel efficient than trucks, that's a waste.
ebtnut,
It is doubtful if regenerative braking ever saved a penny in generation costs. There is no practical way to raise and lower the generation station output in response to such a quick, short and relatively small change in the total system load of any grid. The biggest advantage to regerative braking was simply that the loco did not need resistor banks and cooling fans. Remember, parts left out cost nothing and cause no service problems.
In the design of any machine, maintenance costs, down time, etc, should always be weighed against small gains in efficiency. This is most likely going to be the case with hybrid cars. I'll just waite for the hydrogen one. It will be simple, fuel cell and electric motor. Again, parts left out....
Sheldon
Rich,
I read your post and I know you are well versed in electricity. I was confused by your post though, and it initially seemed to mostly speak to regenerative braking, i.e.
Quote"When the traction motors are thrown into "generator" mode, the power generated goes into that resistor grid, and is consumed, producing heat; hence the fans used for that purpose. Simply put, it goes no where else."
I admit I inadvertently lumped your response into the "regenerative braking" category at first glance.
It is my understanding that dynamic braking occurs when the electric motors are powered in such a way that the armatures produce torque in the reverse direction that the wheels are turning, thereby offering physical resistance. The banks of resistors absorb the load created and dissipate the energy through heat, as in a rheostat. Several respondents suggested that power generated was going into the resistors. I don't know how accurate that is. The banks of resistors aren't acting as capacitors for power generated, they serve as protection for the load produced when power is applied to the armature windings opposite that of the motor windings. Right?
You wrote:
Quote"When the switches are thrown, the system again reverts into motor loads, the generator(s) supply them, and the accelleration process is again activated."
Correct me if I'm wrong, please, but again it is my understanding that the motor
is loaded during braking, just in such a way that torque on the armature is counter to that of the wheels and the physical resistance is caused by the strength of the motor's magnetic field. And when the switches are thrown, the motor is powered in such a way as to work
with the rotation of the wheels.
Comments?
Craig
Craig;
overloading any mechanical contrivance would have the same results; that being it will bog down and either slow down, or destroy itself. A generator that has no further means of producing the demanded load, will go into an overload status become overloaded, the protective devices (if any) kicking in and keeping it from damage.
The same thing would happen, albiet on a grander scale, during a power blackout; you push a megawatt demand over the capacity of the system on line, and you will shut it down by virtue of OL protection; hence "brownouts" which, by the way, are murder on inductive loads like air conditioning, which might not have brownout capability built into their systems, as well as most non-linear loads out there.
Essentially, most of what has been said in this thread is accurate, either for regenerative or Dynamic braking.
BTW, EBTnut has a good point: For example, the New Haven would, as a practice, do just that, and feed back into the overhead when thier "motors" would brake, thus saving a few sheckles in operation.
Rich
s
Dear Sheldon,
Passenger trains use steam to heat the cars. Could dynamic braking energy supplement the heat to the steam boiler?
How about using the generated electricity to refine aluminum? (Be the first kid on the block with a model aluminum refinery in your model locomotive. Look Ma, no dynamic brakes!)
Assuming perfect conversion of energy (no friction or resistive losses), you would recover the same amount of energy coming down the hill (kenetic energy) as you put in to raising the weight to the top of the hill (potential energy). The weight of the batteries or other storage devices in a loco would just add traction, which, from what I've heard, is not a bad thing.
So, would the friction and resistive losses, material costs and complexity of maintenance still overshadow any gains in efficiency? Hmm.
Sincerely,
Joe Satnik
If anyone still doea not understand, go to www.google.com and search the 'Net fpr locomotive dynamic braking and locomotive regenerative braking. When ever I have a question, I use Yahoo.com, Google.com, Ask.com and most of the time I get the answer. The 'Net is one huge library, although not to well organized. You might have to search through some gar-bage but that is what makes it interesting.
Rich
Joe, Passenger trains have not used steam to heat the cars in the last 20 years or more.
Rich,
I agree that much of what's been said is accurate, particularly where regenerative braking is concerned. There's also quite a bit of potentially-misleading phraseology. The sentences "the power generated goes into that resistor grid, and is consumed" and "overloading any mechanical contrivance would have the same results" both strike me as contrary to my understanding of the principals of dynamic braking, which is as follows:
The motors aren't overloaded during dynamic braking. Braking is achieved because torque is reversed and the armature attempts to run in the opposite direction that the wheels are spinning. The purpose of the resistors is to provide a means of dealing with what happens when the motor's field coils remain energized while the armature current is reversed. The resistors are current-limiting devices with heat sinks. The value of the resistor bank applied across the armature terminals helps to determine the rate of braking. The main difference between a locomotive under power and a locomotive under brake is the current across the armature of the motor. It either works with or against the intended movement of the locomotive.
Comments anyone?
Craig,
The motor does not try to run in reverse, it becomes a generator. The strength of the resistance it faces is roughly equal to the load on that generated output. In this case the resistor banks.
In-depth understanding of what happens requires a complete understanding of how wound field induction motors and generators work. Which is different from how peremant magnet motors (like our models) work.
I suggest you do a web search or go the the library if you are interested in the details. The explainations are best done with drawings that I have no means to post and the text would take pages.
And, yes if the the load is too high a generator will distroy its self just like a motor will burn up if run under too much load for too long. That is why over current and over temperature protection is put on both.
Understand it on this level, as a generator or a motor, a voltage is necessary to excite the windings. The Alternator in you car will not work without the battery in the circuit and without at least some residual charge in the battery. The traction motors in Diesel locomotive work the same way.
Sheldon
Sheldon,
I've just done some brief web searches which produced, among other things, the following information:
" The rolling locomotive wheels turn the motor armatures, and if the motor fields are now excited, the motors will act as generators. For a given direction of travel, current flow through the motor armatures during braking will be opposite to that during motoring. Therefore, the motor exerts torque in a direction that is opposite from the rolling direction."
And also:
"The armature is then short circuited or a current-limiting resistor placed across the armature terminals while the field coils remain energized. In either case, armature current reverses, armature torque reverses, and the motor tries to reverse. The speed in the forward direction rapidly decreases as does the voltage generated in the armature."
Sources:
machinedesign.com
wikipedia.org
I haven't found any publications that contradict my perception of how dynamic braking works. I'll look around a bit more tomorrow.
Craig,
You have it! Understand this, from a magnetic standpoint the motor does "try to reverse" but from an electrical standpoint it " becomes a generator".
The difference is in the "motor" mode current enters the motor to energize all the fields, in the generator mode, once excited, it makes current, it does not consume it. The resistors do the consuming.
Sheldon
Just a quick point of clarification... possibly a little off subject. In engine school, we were taught that the braking action decreases as the speed of the locomotive drops below a certain speed. In fact, I have never had a loco "reverse" on me while in dynamics. As the train slows, you can really feel the braking ... but as you come to a stop, you need to apply more air to hold it. However, if you leave it in dynamics as you continue to roll (perhaps as you approach a slight increase in grade) then the dynamic braking force will again increase as the train begins to increase speed. Sometime a little "this is how it works in practice" info helps in understanding theory.- Mike S.
Craig;
Poor choice of wording on my part. Mea maxima culpa. I think I will go work in my garden now.
Rich
Mike,
You are completely correct and just like any generator, as the speed decreases, the output decreases and the force necessary to turn it decreases, which in this case causes the braking to decrease as the loco slows. At zero speed there is zero resistance so something has to hold the train still since trains are very free rolling with very little friction.
Rich,
It took me a while to get what Craig was driving at, its just a different way to look at what is happening. There is a mechanical/magnetic side and an electrical side to this. You and I tend to look at the electrical side with our electrical background. Craig seems focused on the mechanical aspect.
Having built cars from the ground up, the mechanical part seems obvious to me, but I guess wanting to understand that is just as important as understanding what the electrons are doing.
Sheldon
Sheldon;
I would be the very first person to say my knowledge about dismal locos is limited; however, this stuff is all relative, one way or the other; and having spent the time in the industrial en of this business I have, I have learned that to spend some time looking at it logically is your biggest ally, hence the amount of hot air I blew out.
I will say that if what starts out as a simple congenial debate seems to suddenly turn that corner, I am outta here. I think I will leave this thread to the "engineers".
Well, back to ye olde garden. Hey; what is the best method to sledge large pieces of ledge? I had some backhoe work here the other day planting trees, and ther are some laaaaarge rocks now on the surface. I didn't know there were stones this large anywhere other than planet Neptune.
Rich
Sheldon,
I'm a retired electrician (though I actually just came home from an electrical side job) so I know a bit about the "current" subject. I'm also certified in HVAC and my mechanical background is extensive. I guess, as you said, it's all in one's perspective as to how one approaches this subject.
Rich,
Don't give it a second thought. I can't knock a fellow electrician who also happens to be a fellow gardener.
Craig
Craig,
Rather than saying the motor tries to reverse, a better way to discribe the magnetic action would be to say the fields are energized in reverse order impeding the motor rather than spinning the motor. But again, the only differance between a motor and a generator are few field connections and wheather the shaft is driving something or being driven by something.
No one is challenging your knowledge or experiance either, I was however confused at first by your use of the term "reversing" because that implyed to me that you thought power was beinging applied to the motor in the braking mode, which is not the case.
Or, it could be stated like this: The motor becomes a generator powered only by the coasting momentum of the train, the load on the generator exceeds the available torque and HP of the trains momentum and so the generator speed decreases slowing the train.
Sheldon
Sheldon,
I, too, might reword a few entries in this thread if I were so inclined/enabled. I think simply stating that the fields are reversed, resulting in a reversal of motor torque, would suffice for the casual reader.
I can recall experimenting with a motor and a drill in my misspent youth. I put the shaft of the motor in the drill's chuck and used the motor as a generator, just to see if I could get an arc.
Craig
Getting back to Chris R's original question, it seems that nobody has yet stated that there must be a complete circuit for electricity to flow, and that the resistors are connected across the traction motors at both ends.
And that what happens in dynamic braking is that the kinetic energy of the train is converted firstly into electrical energy in the traction motors and this energy is then converted into heat energy in the resistors and dissipated into the atmosphere, as a result of which the train slows down.
In mechanical braking, the action of the brake shoes on the wheels again produces heat which is again dissipated into the atmosphere, but this time wear (brake shoes and wheels) also occurs.
Terry2foot
Here's a pretty good explanation:
http://en.wikipedia.org/wiki/Diesel_locomotive#Dynamic_braking
As to the "wasting" of electrical energy in the form of heat during dynamic braking, the latest General Electric "green" road unit actually does have batteries to store at least some of that electrical energy to be used in starting and accelerating the train. Don't think any have been sold yet.