Dynamic brakes

[Anonymous]

All right..could somebody explain me what are dynamic brakes and how'll they perform versus the regular brakes? Are electric engines capable of having dynamic brakes too?

Thank you very much in advance,

Rodolfo

[Anonymous]

Simply put,Dynamic braking is a system using a locomotives electric traction motors as generators,taking power from the rotation of the axle and slowing the train electromagnetically. It's a brake system which does not work off of kinetic friction like road vehicle brakes. This allows less brake wear and better train handling,it supplements the train's conventional air brake system. The current generated by the dynamic braking of a diesel electric locomotive is sent to large roof mounted electric grids,which act like giant electric space heaters and dissipate the energy generated by the brakes. Modern electric locomotives frequently use a more efficient system called regenerative braking whereby the electrical current generated by the brakes is fed back into the overhead trolley wire. One interesting future application for diesel electrics is the possibility of using large,superconducting flywheels(or maybe,more simply,a battery system) to store the brake energy for later traction use.
That's a simplified answer,I'm sure there are others out there who can explain in greater detail.

[Anonymous]

Dynamic Braking
A train brake system where the traction motors are used to provide a braking force by reconnecting them in such a way that they become generators. Al Krug, referring to diesel-electric locomotive braking in a reply to a question in a newsgroup, put it this way.
Dynamic brakes are fundamentally no different from locomotive air brakes. Both systems convert the energy of the rolling train into heat and then throw away that heat. If you apply the loco air brakes, the brake shoes are pushed against the wheel treads and the resulting friction produces heat. The energy required to produce this heat power makes the loco hard to keep moving. The heat power is thrown away into the air by radiating from the hot brake shoes and hot wheel treads into the surrounding atmosphere.

A loco with air brakes applied is hard to keep moving but it will keep going, particularly if it has energy to move it in the form of a train pushing it from behind. The energy (kinetic energy, it's called) comes from the rolling train that is pushing it. The trouble with using engine brakes alone is that eventually (rather quickly actually) the shoes and wheels get very hot. Hot enough to destroy them. This is because heat is produced faster than it can be dissipated by radiating it into the air. So dynamic brakes are used to move the heat dissipation away from the brake shoes and wheel treads to the dynamic brake grids instead. Like an electric bathroom heater, the dynamic brake grids are designed to handle this amount of heat power (as long as the grid cooling blowers are operating).

Train air brakes work in the same manner as loco air brakes. They convert the rolling energy of the train into heat and throw it away. But when using train brakes, the heat generated is dispersed through out the entire train. It is spread over (say) 800 wheels instead of just the few wheels of the loco. Because of this, the train's wheels do not normally get overheated. They will get warm or even hot but not normally so hot as to cause damage. On prolonged downgrades, however, the braking energy required is sufficient to overwhelm the heat dissipating ability of even all the train's wheels and overheating occurs. This is the main reason for using dynamic brakes, to move the heat dissipation away from the wheels to the dynamic brake grids.

Remember that it takes a 3,000 HP diesel engine just to turn the generator on a 3,000 HP loco. Commercial generating power plants require 100s of thousands of HP to turn the generators that supply your household power. Generators are hard to turn when they are producing power. This is because you never get anything for free. If you take power out of a generator you must put at least equal power into it. (Actually more than equal since nothing is ever 100% efficient either).

In locomotive dynamic brakes, the traction motors are acting as generators. That means the traction motors are hard to turn. The loco's wheels are what are turning the traction motors. They are geared to the traction motors. This means the loco's wheels are hard to turn. They resist turning because they are geared to the traction motors which are hard to turn when generating power, as they are doing when in dynamic braking. Because the loco's wheels are hard to turn when in dynamic braking the loco is hard to move or in other words it resists movement just as if the airbrakes were applied making the wheels hard to turn. The energy required to pu***his "hard to move" loco comes from the rolling train. This removes energy from the rolling train slowing it.

Note that dynamic brakes are used by electric multiple unit trains as well. In these designs, careful blending with air braking is required to maintain a smooth braking profile. Electronic control is used to determine that the brake effort demanded by the brake controller is matched by the brake effort achieved by the train. Preference is given to the dynamic brake to save wear on brake blocks (shoes) or pads and air braking is added if necessary to achieve the braking rate required.

Dynamic braking can be used on electric railways to convert the energy of the train back into usable power by diverting the braking current into the current rail or overhead line. This is known as regenerative braking. It is used in the same way as rheostatic braking but the energy can be used by other trains requiring power. The power developed by a braking train may not be accepted by the line if no other trains are drawing power so trains equipped with regenerative braking will usually have resistor grids as well to absorb the excess energy. The balance between regenerated current and rheostatic current is also controlled electronically

[Anonymous]

dude...a bit long winded arent we...

[Soo2610]

May be long winded, but it was still a pretty nice explanation of dynamic braking.

[Anonymous]

Why some railroads acquire locomotive with the dynamic brake and others without this device? When there is convenience to use the dynamic braking?
Thank's in advance.

[Anonymous]

Firstly,all modern North American road locomotives come with Dynamic Brake systems as standard equipment. In fact one of the major advantages of A.C traction units is the low speed performance of their dynamic brakes.
Back in the "old days" (1980's and before) some railroads who operated in territory with mild grades would specify air brakes only on part or all of their locomotive fleet. This not only lowered the initial purchase price on the engines,but saved money on maintenance as the Dynamic Brake system adds to the complexity of a units electrical/electronic equipment. The Missouri Pacific comes to mind as a major user of non Dynamic brake units,but some of the midwestern roads favored these as well.

[Anonymous]

Jonathan you are state a lot exhausting.
Thank you.

[Anonymous]

I can't describe dynamic braking systems much more than the descriptions before me. However, I can tell you that most class 1 railroads "prefer" (require) the use of maximum dynamic braking before resorting to the air brakes. Also, Most older conventional cab units have a seperate handle to operate the dynamic brakes, however, modern units have the throttle and the dynamic braking on te same handle. When left in the middle, the train is in idle, when pulled towards you, it increases the throttle, and when pushed away, dynamic braking is setup and used. This is called a Combined Power Handle.

-Brian

[Soo2610]

I'm not the expert and don't profess to be, but I suspect it has to do primarily with the type of terrain the railroad operates in. Lines that run in relatively flat terrain with little or or no grades don't really need dynamic braking. The roads with heavy grades definitely need it to save their brakes and help prevent runaways. You usually don't see many locomotives with dynamic brakes on the midwestern lines like the WC.

[Anonymous]

WOW !!!

Thank you all.

Best regards,

Rodolfo
"Catenary to the coasts"

[Anonymous]

all i use to slow down..and sometimes even to stop is dynamic brake... useing the air has to many potental problems... you run the risk of getting a kicker if you use the air...as well as stuck brakes...moreso if the weather is cold outside... the last thing i want is to have to have my conductor out at 3 am in 10 degree cold walking a train becouse we got a defect detector for a stuck brake... dynamic = less chance of problems... air = problems... that is what i tell all the engineer trainees i get... use the air only when you have to... to slow down use dynamo

[wabash1]

i use air often. i dont get stuck brakes. if you draw enough off they will release and as far as a kicker goes there are tricks to prevent that. also if its been snowing out you will have a build up of ice on shoes i like getting air early to get rid of the ice so if i need them i have them.

[Anonymous]

i know about feed valving it to get rid fo a kicker...but you cant do that on the new computer controlled locomotives... its not a matter of how much you draw off...it all comes down to the fact that in the cold..it takes longer for the air to pump up... if you draw down 10 pounds or more...and you have to do a quick release... you have a bigger chance of getting stuck brakes... ill say it one more time... air= problems.... dynamic=less problems... why use the air if you can get the same results from dynamo and with less risk of train problems...it goes without saying for the snow.... in that instance..you have to do that..... but any other time..why use air if you have enought dynamic..... i guess alot of what it comes down to is who trained you... if you where on with an engineer that strechbrakes...then you learn how to strechbrake... most of the engineers that trained me...use dynamic...

[wabash1]

what ever...... i dont use the feed valve to by pass the kicker problem. its done with the automatic. i dont get sticking brakes even in below 0 weather. the risk of using dynamic vs air is no differant. ive heard that junk for years and i still have not seen a train wreck from using air. ive seen cars derailed from dynamic but not air. and where i run if you dont use air your going to get into some lady house when you get to the bottom of the grade. and she dont like being woke up to a dash 9 sitting in her living room.

[Anonymous]

woow woow woow.... you never said anything about desending a grade.... that is a totaly differnt deal... i will agree with you on that one... no way but to use the air... im talking about strechbrakeing on nongrade territory...for controlling speed...not for desending a grade... how long have you been an engineer anyways

[wabash1]

does it matter. the question was ask about how dynamic brakes work. and i think the first person had it nailed down then comes all the other stuff. i will use air to stop or ill slow down with dynamic what ever i feel like at the time. and if i have a shooter i use a trick shown me by a roadforman . works everytime. and its not the feed valve. they say you caant teach a old dog new tricks . but everyday i learn something new and hope i do til i retire or die.