Rheostat Arrays

I have completed some work on the undertrain equipment for the 81-717 train, specifically the rheostat arrays. They are an array of resistances which are connected together in a specific way – as the train accelerates or brakes, the extra electric power that cannot be utilized by train is dissipated on these resistors as waste heat.

The rheostat array in Subtransit is all wired up correctly on each type of train – every single resistor, wire and conducting link are in their correct places and correspond to the physically simulated model. Of course, I don’t think anybody would notice if it was wrong, but these are the kinds of details that I want to see in the game.



In the 81-717 trains, the rheostat array is made up of nine groups (boxes): eight KF-47 power resistors and one KF-50 group with extra resistances used for engine field reduction/shunting. The eight KF-47 groups make up two resistance strings which are connected in series to two engine groups to provide extra resistance when needed.

All of the resistors in the array are connected by flat conductive strips of metal and are optimized to connect only neighbors. The specific rheostat shown on the picture above belongs to the 81-717.5 type of trains. Every type of train has a different rheostat setup – this actually provides a very reliable way of distinguishing them (as different variants may look the same otherwise).

The 81-717.5M type of trains carries a slightly different rheostat. Most notable difference is that the sloped stripe between boxes 6 and 7 is removed. The rheostat setup is unique for each type of train and permits distinguishing different models of trains from each other even when they look very similar. Here is the connection schematic for the rheostat array pictured above:

And for clarity, here is the same schematic unrolled into a more readable form. The array represents two main rheostats and two extra braking resistances as well as some extra resistances used for voltage measurements and other purposes: