Hobo, on 02 January 2017 - 07:09 AM, said:
Isn't that the same thing as " Blended Braking " in MSTS as in a MP40 ?
Not at all : blended braking is mixing dynamic brake with pneumatic brake.
Transitionning, as said above, is needed on electric and diesel-electric locomotives that have a rheostat that is used to control the current in the motors.
You start with the series-parallel wiring (2 blocks of motors in parallel, each block is composed of 2 motors in series or 2 blocks of motors in series, each block is composed of 2 motors in parallel).
Basically, you start with the maximum resistance. The motor starts with the lowest voltage.
When the motor accelerates, current decreases. So you have to decrease the resistance to increase the voltage and to further increase the speed.
Once you reach the minimum resistance, the voltage of the motors is half the line voltage because of the wiring.
You have two choices : decrease the magnetic field of the inductor, or transition to the parallel wiring (all motors are in parallel).
Then you start again with the maximum resistance, and you decrease once again the resistance until you reach the minimum resistance.
Then, you can also decrease the magnetic field of the inductor.
The rule is to never stay too long on an intermediate value of resistance (due to the huge amount of heat produced by the rheostat).
If you are reaching the maximum speed authorized by the signals, you need to decrease the torque, so you have to cut the traction, transition back to series-parallel with maximum voltage, wait for the speed to decrease a bit, then transition back to parallel, reach once again maximum voltage, until you reach once again the maximum speed, etc.
If the needed torque to maintain your speed does not vary too much, you can regulate the torque with the inductor shunts (you decrease the magnetic field => less torque, but not too much because it can also increase the speed).
That type of operation was in nearly all electric and diesel-electric locomotives when the power electronics was not really developed (no thyristors, no transistors, etc.).