How is the resistance of a diesel engine's dynamic brake calculated?

It used to be in pounds of dynamic braking effort.  250,000 pounds of braking effort was considered to be the maximum allowed before lateral buff forces within the train and damage to track structures could begin to become excessive. Today's AC locomotives measure it differently.

Tom:  I'm sorry if i misled you.  The resistance I'm asking about is in ohms, not pounds.  Its related to the design of the dynamic brake 'grid'; i.e., the heat exchanger.

Do you mean that you want to know the total rated ohmage of the grids? I'd like to know the rated ohmage of a single grid unit. HOT WATER, HELP!!!

Just my opinion but, I don't think he knows what he wants. Anyway, the two standard options, depending on customer requirements, were: 0.66 or 0.86 individual grid resistance, for Dash 2 series units..

The grids have varying resistance, and this depends on the characteristics of the motors and their number, and the speed range where the grid effort is required, and the ventilation system to cool the grids, as well as the permitted temperature rise during operation. For a number of years, the grids have been the "extended range" variety and there are locomotives now available with grids that can supply dynamic braking effort  close to zero mph. If you look at a DB vs speed curve, the curve is a "sawtooth", and this reflects the reconnections that take place as speed changes. There are a number of "additional things to consider" if the DB system is used on an AC traction locomotive vs one with DC traction motors.

This is not an easy question to answer since the DB system is almost as complicated as the propulsion system, but E still equals IR!

Thanks, Hudson, for an interesting post.  But B 4 I can say more, I must ask:  What does DB stand for?  Surely not my 1st guess:  Drawbar.

No, "DB" stands for Dynamic Brake.

Thanks Hot!

The units of measurement for the retarding power of a DB system is "pounds" (i.e. lb.) In the metric system it is kg. We used to joke that if tractive effort was "positive" pounds, then DB must be "negative pounds"! Obviously, dynamic braking effort is still a positive number!

Thx, Hot Water:  Are your 2 numbers [.66 & .86] in units of ohms?

Hudson:  What's the value [in ohms] of the total grid resistance when in maximum DB, in which something like 1000 amps are passing thru?

OK, here are some 'rough' calcs, using E = IxR , P = IxIxR = ExE/R.  For R = 0.7 ohms & I = 1000 amps, I get E = 700 volts [seems a bit high], and P = 700 kw =~ 1000 Hp, which seems a bit low.....UNLESS it is 'per traction motor'.  Next?

Phil,

I do not know the answer to your question. To the best of my knowledge, GE does not publish the resistance of individual grid assemblies. The curves that I used were speed (in mph or kph) on the x axis and DB effort in lb or kg on the y axis. In our applications, I know that most railroads do not bring loads downhill at a greater speed than they run uphill. And of course the severity of the grade and number and degree of curvature also is considered. Once we knew the application, that is, the grade percentage and the limiting speed on grades both uphill and downhill, it was easy to refer to the DB curve so that the greatest braking effort was available below the downhill speed. We also applied some braking margin so that, with any INCREASE in speed, GREATER braking effort was always available, so if the train started to "run away", for whatever reason, the DB effort available was greater. There are a lot of additional factors involved here, things like minimum wheel to rail adhesion, and others, so "don't try this at home" is certainly good advice!

I don't know where you got that information, but, you are very much mistaken.

Let me clarify what I meant. It was customary for us to refer to employee timetables for the division of the RR that was the limiting factor with regard to tonnage ratings due to adverse compensated grades. On CSX, for example, it was Cranberry grade which was at 2.87% compensated. I don't know whether this is still true, but I do know for this area on the original B&O there were different downhill speed limits depending on the type/class of train. We always determined what the maximum amount of braking effort that was required. For example, a loaded coal train might have a downhill speed limit of 15 mph, while a trailer or van train might have a maximum speed limit of 25 mph on a grade this severe. We determined not only what the maximum retarding effort in pounds would be for each type of train, but also the speed at which this braking effort was required. The size of the DB package was derived from a review of the curves, looking at this basic information. As an example, if 240,000 lb of max DB effort was required at 15 mph, that works out to a DB package of 4800 hp per unit, assuming two units. If a van train required 240,000 lb at 25 mph, the DB required is 8000 HP per unit, assuming two units. The point I was trying to make is that any DB package applied assumed that an increase in the size of the DB package would be required such that, if the 15 mph train "got away" on the hill, we would be on the "upward ramp" of the DB curve which would indicate that additional DB would be available to slow the train. (In the DC days, a DB curve was a series of "sawtooth" shapes depending on the speed of the locomotive, and therefore the grid connections.

NS may be different than most since I know that on NS, coal loads used to go "downhill" and empties uphill. I also know that NS was (and is?) regarded as a "tractive effort" type RR and would run under MCS as a relatively standard practice. Most RR's however, on ruling grades WHERE THERE IS CURVATURE, don't go downhill any faster than they go uphill. Cranberry has 10 degree curves, I don't believe there is curvature on the NS mainline nearly that severe.

The Cranberry grade, just like the Saluda grade (and I am sure there are a few others Cajon being one), being very steep is why the trains go down so slow. And they do it with the air brake applied (there is a limit as to how many units with operating DB can be used). The grades being so steep that the air brakes will overheat and lose their effectiveness if the train speed gets too high. 

Most grades aren't that steep and trains go down much faster than they go up.

Not on the old SP, UP, NP, and GN. Regardless of whether going up, or coming down, the track speed are pretty much limited by curvature.

Wyhog,

Have you ever been over the UP Blue Mountain grade in Oregon, or the GN Cascade Tunnel line in Washington, or the SP Cascade Summit line in Oregon, or the SP Donner Pass Line in California?

We always assumed that "some" air would be available...but in an emergency situation we always assumed that "additional DB can't be all bad" and the applications were based on this. I agree that most railroads limit the  amount of DB as it can cause severe train handling problems if concentrated, on the head end on downhill and in curvature, for example.

There are a lot of individual differences among railroads in the restrictions on use of DB at "high" speed on level track. I remember Conrail guys telling me that on the old NYC, which is flat, between Buffalo and Collinwood, they used the DB for braking and did not have to touch the air until they needed to stop at the yard limit.

There are a number of individual RR differences in the (and if) the DB performed in an emergency. Some RR's, when you went into emergency, the DB cut out and you could not recover it. On other RR's, if you went into emergency the DB would continue to function. On still other RRs, if you went into emergency, the DB would cut out but you could recover it. I forget now which was which but I often wondered, with run throughs, why everybody didn't just do the same thing. I know that the FRA has some rules that builders and the RRs have to follow as well, but do not remember any details.

Finally, I agree that the examples that you all provided show the differences in operating practices based on the RR profile. a 50 mph downhill run on a "moderate" grade with no curves would enable higher speeds than if you are at 8-10 mphdownhill and in severe curvature.

Thanks to all for the comments.

There is always a lot of politics involved in that sort of a decision.  On the Santa Fe, we tossed around the idea of allowing the dynamic brake to remain operable while the brakes were in Emergency, but the decision to stay with the existing policy of having Emergency cancel dynamic braking was based on the (at that time) great variation in skill between our best and our worst Engineers.  It took a tool away from our best ones (who were easily capable of properly using the engine brakes instead, when needed) and protected the public and the equipment from the possible result of a weak Engineer mishandling dynamic braking in Emergency.  I personally think that BN went in the right direction, certainly for them, but BN did not run the same number trains down steep, curvy, grades on the sidehills of a narrow mountain pass, over a hostile foreign railroad which owned the track, in a state where there were thousands of environmental activists within a short drive of the state Capitol. 

There will be no clarification over just exactly where I am referring to.  Everyone should be able to figure that one out.

How basic mechanical principles can change over time.  Forty years ago on the Southern Rwy when an undesired emergency brake application occurred while pulling a train we "bailed-off" the Independent Brake and left the throttle in #1 position in an attempt to prevent the rear of the train from running into the head end.  When in Dynamic Brake and the undesired emergency occurred we applied the locomotive brake to keep the slack compressed on the entire train.  At this point in time no rule could require an engineman to fully apply the independent brake.  All units were equipped with cast-iron clasp brakes whose coefficient of friction varied with the speed of the locomotive.  An engineman then had the skill to graduate the independent brake consistent with speed to prevent flat spots on the wheels as the train speed decreased. As an RFE we always instructed " a rotating wheel has more retardation value than a sliding wheel". When depositions were taken after incidents occurred, we always managed to have this statement included. 

Today with hi-friction material brake shoes and relair valves to control Brake Cylinder pressure to a multitude of different brake rigging designs in a mixed consist I guess one can stand by the decision to fully apply the independent brake and let nature take its course.  One thing todays generation will never experience is how cast iron shoes would anchor a locomotive in a hurry at speeds below 20 MPH;  and when flat switching no having to back up after getting a truck length over the switch points. 

URRENGR:  Please elaborate on your last statement, "....and when flat switching no having to back up after getting a truck length over the switch points. "

Phil,

The composition brake shoes used today do not hold as well as the old cast iron shoes. What he means is that when flat switching, especially when a quick burst of speed is needed to "Kick" a car, it is not as easy to stop the train before the next car's truck gets by the switch points necessitating a wasteful backup move to send the next cut to the proper track.

Muchas gracias, Grande Jim!