A Question on Dynamic Braking

[Atlantic Central]

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

[Craig]

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.

[Atlantic Central]

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

[Mike]

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.

[r.cprmier]

    Craig;
Poor choice of wording on my part.  Mea maxima culpa.   I think I will go work in my garden now.

Rich

[Atlantic Central]

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

[r.cprmier]

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

[Craig]

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

[Atlantic Central]

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

[Craig]

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

[terry2foot]

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
   

[morrisf]

Here's a pretty good explanation:

http://en.wikipedia.org/wiki/Diesel_locomotive#Dynamic_braking

[thirdrail]

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.