Dynamic Brakes - am I overlooking something?

[166Driver]

I noticed a huge amount of wheelslip on the US locos when using dynamic brakes, and also excessive braking force on the German locos.

I think MSTS, SpoorSim and Trainz had it simulated reasonably accurately, the handle has to be moved to a setup position first for the first few seconds before dynamic braking can be selected.

Also haven't seen the dynamic brakes registering on the ammeter either.

[johnmckenzie]

They do seem difficult to use, yes!

[Kariban]

I noticed a huge amount of wheelslip on the US locos when using dynamic brakes, and also excessive braking force on the German locos.

I think MSTS, SpoorSim and Trainz had it simulated reasonably accurately, the handle has to be moved to a setup position first for the first few seconds before dynamic braking can be selected.

Also haven't seen the dynamic brakes registering on the ammeter either.

If you use the F5 display you can see the ammeter going into negative figures when you use the dyna brakes.

[166Driver]

Indeed, I already knew that, but I don't want to have to use the F5 page just to drive the train properly.

[Kariban]

Can someone point me at an ops handbook for a US loco ( if there's a US equivalent of Locodocs, for instance ), I'd like to know the official procedures to use these.

[davveb]

Can someone point me at an ops handbook for a US loco ( if there's a US equivalent of Locodocs, for instance ), I'd like to know the official procedures to use these.

Well I can give you a quote from my 1978 GM SD40-2 operator's manual:

"To operate dynamic brakes, proceed as follows:
1 The reverser handle must be positioned in the direction of the locomotive movement.
2 Return throttle to idle and hold in idle position for 10 seconds before proceeding.

WARNING
The 10 second delay must be accomplished before the handle is moved into SET UP position.
On locomotives delivered after December, 1969, braking delay occurs automatically. Do not misinterpret the delay as failure of the dynamic braking system.
It is possible for a sudden surge of braking effort to occur if the dynamic braking handle is open when the automatic delay times out.

3 Move the braking handle into SET UP position. This establishes the dynamic braking circuits. It will also be noted that a slight amount of braking effort occurs, as evidenced by the load current indicating meter.

4 After the slack is bunched, the dynamic braking handle is moved to control dynamic braking strength. As it is advanced out of SET UP it will be noted that the engine speed automatically increases."

All in all I think the RW approach is pretty close, apart from simulating the engine increase, and a tendancy for the default braking force to be a bit over powerfull.

[Kariban]

Yeah, that is almost spot on then, other than the brake force, although I wonder if the setup position is notched. Is there a dash pic? I'm wondering if there's a seperate dyna brake load meter; the SD70(MAC at least) has a two-way ammeter I noticed earlier.

Wonder what happens if you don't wait the 10s.

[djt01]

I'm wondering if there's a seperate dyna brake load meter; the SD70(MAC at least) has a two-way ammeter I noticed earlier.

If you do a search I posted some pics taken in the cab of a various SD70's while I was at work. I can't remember if I showed the dynamic braking readout though.

Wonder what happens if you don't wait the 10s.

From what I remember the last time I tried that with a set of light engines, nothing, and with a train coupled up behind you I wouldn’t even attempt it because of the run-in you'll get once it does kicks in.

[Kariban]

1: Close throttle
2: wait 10s <- hm. is this to guarantee the engine is idling, and equipment isn't locked in one mode?
3: dyna brake to SETUP, which provides enough brake to bunch the train up
4: apply brake.

Right, that was for the 60s version, and nowadays there's a 10s wait programmed in? so you can't not wait.

[Kromaatikse]

Railworks (and MSTS before it) models the delay as being from the moment the dynamic brake handle is advanced out of the Off position.

But from what davveb posted, it implies that the delay should be from the moment the throttle is returned to the Off position, and that after that timer has expired, the dynamic brakes respond immediately.

The latter makes a lot more sense to me, because it simply allows for various relays and valves to settle in the disengaged position, and the load controller to run back to the minimum position. It is also "sufficiently" similar to the way British equipment (up to and including the PEP-based EMUs) works, as these require the camshaft to run back to the initial position before the rheostatic brake will engage (on the PEPs this is very quick, on the full-sized locos it can take 30 seconds).

[Makeone]

Something of a bit curious nature there, why does the engine rpm rise? I know that Trainz actually mimics this but MSTS we have that hoover that sets in...

As of my understanding, when dynamic brake (or is it the rheostaic brake?) kicks in, it changes the driving motors to act as generators and that current is then ventilated out as thermal energy using resistor banks? Other similar braking method that i understand i have seen in buses (and probably trucks as well?) that's really few electromagnets around main drive shaft that creates electromagnetic force to counter the moving of the shaft. Telma is one such system.

Is it right to generalize that older US engines had actually a magnetic braking and engine rpm's increase to provide electricity to these brakes and later engines (like Dash-9) had resistor banks (ie. rheostatic brakes?). And thinking of, they actually work on same principle...almost anyway

[Kariban]

Naw, I'm pretty sure they all have rheostatic. The engine RPM is curious, unless the system dumps current into the alternator which uses the engine to dump energy. Would seem a bit daft using it to drive cooling gear with all this "free" current around...

Rheostatic braking is just a type of dynamic brakes, does what you describe. Regenerative would dump the power back into the electrical supply - so obviously no use for diesels. There's no point using seperate magnetic brakes with an electrical transmission system, when you can just do the same thing with the traction motors.

[davveb]

Yeah, that is almost spot on then, other than the brake force, although I wonder if the setup position is notched. Is there a dash pic? I'm wondering if there's a seperate dyna brake load meter; the SD70(MAC at least) has a two-way ammeter I noticed earlier.

Wonder what happens if you don't wait the 10s.

Elsewhere in the same manual it indicates there is a mechanical interlock between the throttle and dynamic brake levers. The dynamic brake cannot be moved out of OFF unless the throttle is in IDLE, and visa-versa. There are no other notches on the dynamic brake lever.

Something of a bit curious nature there, why does the engine rpm rise? I know that Trainz actually mimics this but MSTS we have that hoover that sets in...

The engine increase might be to drive the resistor cooling fans that startup when the dynamics are operating. It is also these fans that make the hoovering sounds.

Right, that was for the 60s version, and nowadays there's a 10s wait programmed in? so you can't not wait.

Actually more like the 70s version. Earlier EMD engines did not have a separate dynamic brake lever, but just used the throttle lever for both, operating in the same direction for confusion (ie not like on the Dash 9 and SD70MAC). A separate selector lever was used to select power (possibly including manual transistions) or braking options.

[Kromaatikse]

Some hydraulic transmissions also have a dynamic brake, in the form of a hydraulic retarder. I've been on a small DMU which used this, and even been able to see some of the cab displays while it was happening - the engine RPM does rise when the retarder is in use, possibly to drive the pumps which are used to control the gearbox.

On electric transmissions, another explanation for engine RPM rising is because the field current for the motors still comes from the main generator, with only the armature windings being wired to the resistor grids.