Braking

[M.G.Brown]

Train brakes is when all train cars are being used. What is the difference between locomotive brakes and dynamic brakes?

[petitnj]

 The train's brakes operate with the change in pressure of the Brake Pipe -- the continuous pipe that runs thru all of the cars. As the Brake Pipe pressure is reduced a portion of a tank of air in each car is applied to every brake cylinder in the train. That system can apply the brakes proportionally to the Brake Pipe Reduction. Once the Brake Pipe pressure is reduced to about 2/3rds of its highest pressure (when the brakes are all released) the individual brakes are fully applied and there is no more braking available for the train. Any increase in the Brake Pipe pressure will fully release all of the brakes in the train. This causes the tank on each car to recharge to the pressure of the Brake Pipe. So the train brakes are proportional apply -- fully release.

 The locomotive will respond exactly as the train when the train (automatic) brake is used. In general the brakes on the locomotive are more effective than the brakes on the cars. The locomotive will tend to put more braking power on the train with the same train (automatic) brake application. Normally the locomotive's brakes are bailed off (released) when the train brakes are applied to prevent the train from catching up with the locomotive. 

 Now the locomotive's brakes (independent) are proportional apply and release. They are controlled by a separate handle on the brake stand. With the locomotive brake, one can easily control the locomotive and a few cars at slow speeds.

 Unfortunately, the air brake system uses the air in each car when it is applied. If the engineer applies and releases the brakes in many cycles, quickly, the air tank in each car will not recharge between brake applications. That means that every application (Brake Pipe reduction) will have less braking power until eventually the air in each car will be exhausted. 

 One could hold the train back by applying the locomotive (independent) brakes. The locomotive has its own air compressor to continuously supply the brakes. But this would wear the wheels and brake shoes rapidly as they heat up dissipating the energy slowing the train. But wait! We have electric motors on each of the locomotive's axles. This motor has separate stator and rotor wiring and we could convert this motor into a generator and make electricity. Now we have electrical power, what do we do with it? 

Dynamic braking uses the traction motors of the locomotive to generate electrical energy that is routed up to large resistors (much like toaster windings) at the top of the locomotive. These resistors are cooled by fans that push air across the resistors, heating up the air. 

Dynamic braking saves wear and tear on the locomotive's wheels and brake shoes but there are some drawbacks:

1) The motors must be running at speeds above a certain level to generate enough electrical energy to slow the train. Typical minimum speeds are about 15 mph. Newer locomotives can dynamically brake at slower speeds. 

 2) The only braking comes from locomotives so you need lots of locomotives to slow a heavy train. If the train is too large for the number of locomotives, the locomotive wheels will slip when they try to slow the train. 

3) Not all locomotives have dynamic brakes -- it was an "available option" at the dealer and some railroads didn't see the need.

So when a large train goes down hill you have three braking systems at work.

1) The train's brakes-- brake shoes on every car. These can be applied at a steady pressure and are difficult to adjust more or less as that depletes the air in each car's reservior.

2) The locomotives steel on steel brakes -- can be adjusted easily but limited by the weight of the train. 

3) Dynamic brakes on each (installed) locomotive. Again limited by the weight of the locomotives. 

 

Hopefully this helps. 

[Rodney Beck]

Locomotive brakes are air controlled by the independent brake valve, dynamic brakes use inerita when the locomotive is moving by converting the traction motors in to generators by way of a rehoastat by addind amperage restissance to the grids behind the locomotive cab it is then dispalced as heat to the air.

See the website called tales of the krug he is alot sharper on this stuff than I am.

 

Rodney 

 

[chad thomas]

 As Rodney said, check out Al Krug's site, particularly these:

http://www.alkrug.vcn.com/rrfacts/amps_te.htm

http://www.alkrug.vcn.com/rrfacts/brakes.htm