Dynamic Braking Models for DC and AC Traction Motors?

[PeterB]

I don't want to get into the complexities/details of how exactly dynamic braking occurs within DC and AC traction motors.

It is my understanding that all locomotives available for use with MSTS and Open Rails use a basic dynamic braking model based on the (old) DC traction motor dynamic braking characteristic regardless of whether the locomotive model uses DC or AC traction motors.

I understand that modern locomotives with AC traction motors and associated electronics and microprocessors can just about maintain constant dynamic braking effort for particular dynamic brake lever setting (almost) over the entire speed range of the locomotive by adjusting the AC pulses to the AC traction motors. In fact the train can be just about be brought to a complete stop using just the AC dynamic braking.

And I understand that DC dynamic braking does not have a constant braking effort for particular dynamic brake lever settings as the dynamic braking effort varies with speed; at high locomotive speeds the dynamic braking effort is not very high, but as the locomotive speed slows down, the dynamic braking effort increase (a little), then at low speeds the dynamic braking effort falls off rapidly.

On observation of locomotive performance under MSTS and Open Rails, it appears as though that ALL locomotives use the DC dynamic braking model regardless of AC or DC traction(?)

Strictly, I believe this to be incorrect, in that for (modern) AC traction locomotives the dynamic braking model should be different to that of DC traction dynamic braking model. So there should be two dynamic braking models available depending on whether the traction is DC or AC.

Does Open Rails have two dynamic braking models, one for DC locomotives and the other for AC locomotives?

[pschlik]

You can make dynamic brakes behave however you like with ORTSDynamicBrakeForceCurves. Now that feature is Open Rails only but the original MSTS dynamic braking parameters have absolutely no bearing in reality and can't reliably replicate any type of traction system, so everyone should be using ORTSDynamicBrakeForceCurves.

[R. Steele]

Does the ORTS parameter --- ORTSTractionMotorType ( AC ) ( used in engine section of Eng file ) ---- have any bearing on dynamic brakes?

[pschlik]

No, there's no magic sauce that figures out dynamic braking for you. Whether the locomotive acts as if it has DC or AC dynamic braking depends entirely on the definition of dynamic brake curves.

[R. Steele]

Thanks Phillip, currently I'm using Joe Realmuto's Dynamic Brake Calc. Spreadsheet available in a forum post at Elvas ---- appears to be sufficient for my needs at my current understanding of dynamic brake curves. Small steps.

[PeterB]

Thank you for the comments.

So how do you utilize the statement "ORTSDynamicBrakeForceCurves" exactly? I haven't (as yet) found any documentation on how to utilize this statement, and what parameters are required and in what format?

Thank you.

[pschlik]

This isn't exactly explained fully in the manual, probably because the implementation is near identical to ORTSMaxTractiveForceCurves, which is explained fully, but the general setup is as follows:

ORTSDynamicBrakeForceCurves (
     0.0 (
          0.0mph    0lbf
          xNmph     0lbf
     )
     z (
          0.0mph   y1lbf
          x1mph    y2lbf
          ...
          xNmph    yNlbf​
     )
     ...
     1.0 (
          0.0mph   y1lbf
          x1mph    y2lbf
          ...
          xNmph    yNlbf​
     )
)

Where z is the dynamic brake setting as a decimal (0.5 means 50% dynamic brake, the table should at least have entries for 0.0 dynamic brake and 1.0 (100%) dynamic brake, but more entries can be placed in between for more detailed control).

x is some speed, usually you want to start the data at x = 0 and go up to the max speed of the locomotive gradually (you can have as many or as few speed values as you like) and each x value has a y value for the force the dynamic brakes should apply at x speed (note that for the first entry where z = 0, you probably want just two x values, one for 0 speed and a second for max speed, with both y values set to 0. This means 0% dynamic braking gives 0 force).

The units for x and y can be whatever OR supports, I just run with miles per hour and pound force since that's what my sources use.

Then you just repeat adding x (speed) and y (force) data for every z (dynamic brake setting) until you've exhausted the data available or have enough for the effect in sim to be convincing.

[PeterB]

Thank you very much for your detailed explanation!

I think I now have the gist of what to do. I am currently experimenting with different values for speeds and forces and running test trains to check the dynamic braking effects.