A Question on Dynamic Braking

[Chris R]

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?

[Guilford Guy]

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...

[richG]

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

[r.cprmier]

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

[ebtnut]

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.

[Craig]

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

[Joe Satnik]

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

[r.cprmier]

You answered it.

Craig;
See my abovementioned post...

Rich

[Atlantic Central]

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

[Craig]

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.

"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:

"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

[r.cprmier]

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

[Joe Satnik]

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 

[richG]

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

[RAM]

Joe, Passenger trains have not used steam to heat the cars in the last 20 years or more.

[Craig]

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?