Dynamic breaks, and down hill grades. What speed do they start working?

Question

When a train is decending a grade, and the dynamic breaks are aplyed, does the engineer start aplying dynamics at 100%?or do you move one each notch at a time to reach max dynamics? What speed do they kick in at? Do you also aply the train breaks to avoid over heating of the dynamic breaking systems?

Answer 1

When you apply dynamic you start at a low amperage to bunch the slack up.As soon as the slack is bunched you can increase it to 100%.Normally you start with about 300 amps of it then get more as you need it.They work at any speed.The older units were most effective from about 40mph down to about 5mph.The newer AC units are effective all the way to a stop.You don't need train brakes unless the dynamic isn't strong enough to hold the train.Your only allowed a certain number of axles of dynamic brake.They don't overheat but sometimes you will get one that puts out too much amperage.When that happens we get a warning buzzer and your supposed to reduce the dynamic throttle setting til it stops.If not the dynamic brake grids can suffer a meltdown.It used to happen on the older EMD units sometimes and was a sight to see if it was dark otside.

Answer 2

It is necessary to apply the brakes gradually, there is movement between cars and in the draft gear of cars called slack and if an engineer were to apply the dynamic brakes all at once, there is a very real chance you would pop a few cars out to the side and cause a derailment, at the very least you would be shifting loads and causing freight damage.

There is no danger of overheating a properly functioning dynamic braking system, but Andy is correct, if one is malfunctioning and the grids burn up, it is pretty spectacular. Air brakes are used to supplement dynamic brakes if the train is too heavy for the dyanamics to hold or slow down.

As for the speed and capacity, the above answers have it explained better than I can.

Answer 3

Dynamic controllers do not have "detents" in 8 positions as a power throttle does, but rather are fully "sweepable," for more accurate application of retarding force desired.

Dynamic brake falls into one of two classes:  taper dynamic and flat dynamic.  Taper dynamic provided dynamic according to speed and controller position and were most effective just as Andy describes above, but it has gone the way of the dodo.

Flat dynamic provides retarding effort according only to controller operation, regardless of speed.

In addition, there is standard dynamic and extended range dynamic.  With standard dynamic, the amperage drops off quickly at speeds below 10 mph.  Extended range will provide retarding effort to about 4 mph before dropping off.  As Andy has also said, the newer AC units have phenomenal dynamic right down to a stop.

Most units equipped with dynamic brake also have the DBI (dynamic brake interlock).  When air brakes are used in conjunction with the dynamic the DBI prevents pressure build-up in the locomotive(s) brake cylinders, as a means of ensuring that no sliding of the wheels takes place.  On units so equipped it is not necessary to "bail-off" the independent when using the air, as would be necessary if applying the automatic brake (train brakes) when running in power.  

Another feature nullifies extended range portion of the dynamic brakes when the independent brake (engine brake) triggers the IPS (independent pressure switch) which is usually right around 15 psi.  This feature is incorporated to keep from sliding the wheels and creating flat spots when coming to a stop.

When changing the configuration of the slack in the train, either when in power or dynamic, the change is made gradually to reduce the force levels that run through the train when these changes are made.  300 amps is the magic number to be assured the slack is staying stretched when in power or bunched when in dynamic, however, in undulating territory, this number may need to be higher for potitive slack control.

A very good question.  Thanks for asking.