Western Railroad Discussion > Dynamic Braking

I was asked recently what dynamic braking was, and how to tell if a locomotive built in the 1960s or 1970s was equipped with extended range dynamic braking. To answer the question, I went looking in all my railfan and industry source material, but nothing really did a good job answering the question. So I wrote my own answer, which I have added to my web site:

http://utahrails.net/up/up-loco-features.php#extendedrangedynamicbraking

Sad to say, my locomotive knowledge hits a glass wall at UP's SD70s in 2000, so maybe someone can fill in those details.

Don Strack
http://utahrails.net/

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Dynamic braking is an electrical retarding device that utilizes the main generator and traction motor to retard the speed of a train. Dynamic braking converts the kinetic energy of the moving train to electrical energy, passing the resulting electricity to large resistance grids which then heat up. The heat is dissipated by a cooling fan (or fans) that receives its power directly from a connection on the resistance grids. Locomotives with dynamic braking were equipped with one large resistance grid for each traction motor. Each resistance grid is limited to 700 amps, with some high capacity dynamics being rated for 920 to 945 amps.

On a train controlled with standard dynamic braking, the engineer applied the independent brake and shut down the dynamic brakes as the amperage dropped between 250 and 200 amps, which usually occurred at 10 to 13 mph. Failure to make this change from standard dynamic brakes to air brakes could result in the head-end of the train running out ahead of the train as the dynamic brakes faded. The ensuing slack could cause a broken knuckle and a train separation.

The effectiveness of standard dynamic braking begins to decrease at approximately 25 mph. At speeds below 25 mph the effectiveness of standard dynamic braking declines rapidly, but can be used at speeds as low as 13 mph.

Extended range dynamic braking allowed greater speed control, which in-turn would allow better train handling. The use of extended range dynamic braking allowed less use of the automatic air brake, thus reducing wear on car wheels and brake shoes. With extended range dynamic braking, the low-speed range is extended to allow dynamic braking to be fully effective at speeds between 25 mph and 6 mph.

Extended range dynamic braking reduces grid resistance in steps as track speed decreases below 25 mph, allowing maximum amps to be maintained until the locomotive slows to approximately 6 mph.

Using a combination of shorting contactors, and center taps on the resistance grids, extended range dynamic braking increases the resistance of individual grids in steps. All of the grids are used at speeds above 25 mph; 3/4 of the grids are used at 18 mph; 1/2 of the grids are used at 12 mph; and 1/4 of the grid surface is used at 6 mph.

At each reducing step, and as individual grids and portions of grids are shorted out by the contactors and center taps, amperage can return to the maximum 700 amps at each of the three step-down speeds: 18 mph, 12 mph and 6 mph.

The combination of shorting contactors and center taps gave locomotives with four axles a total of eight dynamic braking sections, and six axle locomotives had twelve sections. The lowest speed used the fewest number of sections, allowing maximum braking effort and amperage from the traction motors to be dissipated. To reduce damage caused by the grids overheating, and as an option, locomotives as early as the GP30s and GP35s could be equipped with extended range dynamic braking with larger, high-capacity grids.

Dynamic braking on the locomotives and air brakes on the train may be used together to stop or slow down train. This is called blended braking. Locomotive independent brakes usually are not applied along with dynamic braking at speeds above 6 mph, as this can cause the locomotive wheels to slide resulting in flat spots.

Many observers have wondered about external features of extended range dynamic braking. On EMD locomotives built in the 1960s and 1970s, research indicates that the presense of a small access door with two T-handle latches, located on the dynamic braking hatch is an accurate indicator of extended range dynamic braking.

The GP30s were the first on UP with extended range dynamic braking. This includes both the earlier 800-series in 1962, and the later 700-series in 1963. On both these classes, the access door was on the fireman's side. (The original GP30 demonstrator locomotive, which later became UP 875, had an access door on both sides.)

Union Pacific's GP35s and DD35s were delivered in 1964, and also had extended range dynamic braking. The DD35s had the small door to access their extended range dynamic braking shorting contactors on one side. The front access door was on the right side and the rear door was on the left side.

UP received their DD35As in 1965, and these units had the same layout as the DD35 for their access doors: front access door was on the right (engineer) side and the rear door was on the left (fireman) side.

UP's 1400-series SDP35s, also delivered in 1965, were the first SD six-axle units on Union Pacific with extended range dynamic braking. There were six individual dynamic braking grids, one for each traction motor. The units' extended range dynamic braking shorting contactors were split, three behind a door on the right side, and three behind a door on the left side.

The SD40s delivered in 1966, and SD45s delivered in 1968 had the same extended range dynamic braking as the SDP35s. The DDA40X Centennial locomotives delivered in 1969 and 1970 were a radical departure in locomotive control circuitry but were also equipped with extended range dynamic braking. As UP continued to receive new locomotives (more SD40s in 1971 and hundreds of SD40-2s in 1972-1980), all were equipped with extended range dynamic braking.



Edited 1 time(s). Last edit at 03/04/17 05:44 by donstrack.

Very good write up. What about blended dynamics brakes used on some F40's. Thanks.



Edited 1 time(s). Last edit at 02/26/17 16:39 by usmc1401.

When coming to a stop with extended range EMD's, you could hear the fans rev up as the braking steps up, kinda like the down shifting. Not so much with GE's, the fan speed never sounded like it changed much, but the engine stayed reved up. Pretty effective down to about 5 mph or so. Basic DB started to fizzle out somewhere below 25.

Posted from Android

usmc1401 Wrote:
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> Very good write up. What about blended dynamis
> brakes used on some F40's. Thanks.

What about it?  That was developed for Amtrak ,and other, passenger units only.

Extended range dynamic braking dates to the PCC streetcars of the latter 1930s.

How does blended braking work on Amtrak engines?

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stash Wrote:
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> How does blended braking work on Amtrak engines?

Very well.

Blended braking on Amtrak units are controlled by the computer, or in the case of F40's by a module. When the engineer makes a brake pipe reduction to set the brakes, the computer or module makes a determination as to how much braking effort is being requested. The unit goes into full dynamics and and magnet valves are energized to control the air brakes to supplement the dynamics. The dynamics are the primary braking effort and air brakes secondary. Hopes this helps! 

Excellent. Just what I wanted to know.

Posted from Android

>> How does blended braking work on Amtrak engines?

> Very well.

Most of the time.

> The dynamics are the primary braking effort and air brakes secondary.

To pick a small nit: with DC (traction motor) drive locomotives, the primary braking system is the air brakes because the dynamics can't hold the train sationary. During blended braking, the dynamics provide most of the braking force (and the air brakes on the engine aren't release 100%), and they begin to fade at about 13-11 mph. No Amtrak locomotive has extended range dynamics.