Dynamic braking in reverse

I;ve just noticed when reversing a GP38 that the dynamic brakes don't work, does anybody know if this applies to engines in real life? I would have thought that the motors would just reverse the current flow direction to the resistors? or perhaps I'm wrong!

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RE: Dynamic braking in reverse, LARKIE, Dec-08-02, 06:40 AM, (1) RE: Dynamic braking in reverse, derekmorton, Dec-08-02, 03:42 PM, (4) RE: Dynamic braking in reverse, LARKIE, Dec-08-02, 04:48 PM, (5) RE: Dynamic braking in reverse, derekmorton, Dec-08-02, 10:01 PM, (9) RE: Dynamic braking in reverse, Richard, Dec-08-02, 12:41 PM, (2) RE: Dynamic braking in reverse, Signalman, Dec-08-02, 12:43 PM, (3) RE: Dynamic braking in reverse, RickCan, Dec-08-02, 05:03 PM, (6) RE: Dynamic braking in reverse, seern, Dec-08-02, 05:19 PM, (7) RE: Dynamic braking in reverse, RickCan, Dec-09-02, 01:18 AM, (10) RE: Dynamic braking in reverse, felix_hasler, Dec-08-02, 05:37 PM, (8)

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Not being a rail engineer,I can only guess.
If your going forwards,and brake the loco dynamically,your load bunches behind you,and you're then slowing the whole train in effect.If you're going in reverse,and you dynamically brake,you're only slowing the loco initially,and your load strings out,in front "behind" you,due to its inertia.
As the masses involved can be pretty high,this could strain your couplers.Thus it's the load's brakes that should be applied first,when slowing in reverse,to keep the load still bunched together with the loco,whilst stopping.Under this condition loco dynamic braking becomes redundant.

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Larkie,Your description of dynamics in reverse is exactly the same as power on going forward, so your comment about air first, I don't think is valid.Derek

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Derek,without splitting hairs too much,when pulling a load properly with a loco,from a standing start forwards,you're only pulling a static load.
What I was thinking more of,was the the far greater potential load imposed on the couplers,by the momemtum of the train's load itself.If only the loco brakes,in reverse,with a load behind it.IE the loco's travelling in reverse pushing a load.
This value is a function of both the mass of the load and the speed of the load.
If analysed fully,as a snapshot,it's as if the loco stops dead,the waggons run out,and then all that kinetic energy is transferred onto the loco coupler,as a deceleration force.This is far greater than a static pull.(assuming the train was bunched before the loco brake was applied).Perhaps I misunderstood the gist of the original question,as I assumed there would be a loco running in reverse pushing a train of waggons.If reversing,pulling a load,on your front coupler,then the dynamics are identical to pulling a load,with your rear coupler,going forwards.

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Larkie,I see where you are going, yes we are playing with hairs.Starting while uphill with a bunched train will have the same effect, and a good engineer would not do it.Derek

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I think this is a bug with MSTS. In a real consist, when the engineer begins using dynamic braking, all units go into dynamic braking mode reguardless of the direction the unit is facing. My guess would be that dynamics should work in reverse, but again this is just a guess. Hopefully an engineer on this forum can verify if dynamics do work when the lead unit is operated in reverse. Rich S.

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Real locomotives have dynamic brakes in both directions, makes no difference whether the cars are in front or back.
I have noticed in Train Simulater that they do not function in reverse. Probably another of those little annoying bugs it has.

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This is a bug in MSTS. Dynamic brakes on a real locomotive will function in either direction. In fact the reverser handle has to be in forward or reverse for the DB handle on the control stand to unlock. DB is disabled automatically if the locomotive has a traction motor cut out feature ( many USA locomotives don't ) and is in TMCO status. TMCO on SD40 can be either 1 & 6, 2 & 4 or 3 & 5. The newer AC4400 and Mac90 #1 or #2 truck is cut out which also disables the DB. The reason for this is all the traction motors of the locomotive have to be in series for the DB to operate.Richard

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LAST EDITED ON Dec-08-02 AT 05:20 PM (EST)

Rick, From what I have read about dynamic braking, the newer AC traction motor engines don't have true dynamics. Whether the engines speeds up or slows down is dependent on how the field excitation is on the stationary vs moving rotor with its magnets. When an AC unit goes into 'dynamic' all that is happening is the field is placed behind the rotor so there is a drag that slows its rotation. Now I make a disclaimer here...I am not an engineer, either train or mechanical. So this comes from reading not hands on. I also realize that it's more complicated then this, but without pictures this is the best I can do.

Sharon Evans
Glen Allen, Virginia
Activities are what makes MSTS run.

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I was trying not to get too complicated Sharon. The newer AC engines don't have the same dynamic engineering as the SD40 or GP38 with grids and DB blowers. If an axle speed or lock sensor is disabled you will also loose dynamic braking for that engine. I should have edited the series remark to apply to DC TM powered engines.Richard

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Hi
On diesel locos in US with DC motors the motors are connected as separatly excited generators working on a constant (extended dynamic braking : switched in 3 steps) braking resistors.Translation : The field current of the traction motor is excited by the main generator and the direction of the current is forced. The armature is more or less short circuited by the braking resistor. The polarity of the field current remains constant while the polarity of the armature current depends on the direction of the locomotive. The braking resistor does not care about polarity so braking effort is generated either way.Self excited reostatic braking : This is the usual form of braking on streetcars, subway trainsets, etc. To build up braking effort it relies on a residual magnetism in the DC motors, therefore you must not change the direction of current in the field but only in the armature. Following this rule braking effort is built up in either direction, too.Some years ago I enjoyed this little physic tutorial on the class 4000 of the MOB (Montreux Oberland Bernois) railroad in Switzerland. These trainsets have a DC regenerative brake with a forced flow of field current through the 4 groups of 2 series connected traction motors, each ammeter has a positive range for traction and a negative range for braking current. But these trainsets also have a connection for self excited reostatic braking. Depending on the last state of the trainset, tractive effort in postive/negative direction, or regeneratif braking with a forced flow excitation current in either direction you get a positive or negative flow of current in the armature and on the ammeter thus confusing the engineer. But braking effort was present ...
Felix

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