Transition - MagnumMacKivler/RLCPT2 GitHub Wiki
What is Transition?
Transition is a simple, but obscure part of locomotive operation that is incredibly important to a locomotive's performance. Transition allows a locomotive to make the most use of its horsepower over a wide range of speeds.
Locomotive electrical equipment, namely the main generator and traction motors, is subject to certain limits. For example, they can only take (or put out) a certain amount of current before they fry. And they can only take (or put out) a certain voltage before they fry. And they can only spin at a certain speed before... you guessed it... they fry.
The problem is unless you have generators and traction motors that are very big, you have to pander to one limit or another. It's like gear ratios in an automobile: You can have a low gear ratio, which makes it easy to start and pull things with a top speed of 10 mph, or you can have a high gear ratio, which allows you to go 55 mph if you can get someone else to push you from a dead stop. Automobiles solve the problem by having multiple gear ratios that you can shift between. For locomotives, it's exactly the same... except instead of gear ratios, you change how the motors and generator are connected.
There are two kinds of transition: Motor Transition and Generator Transition. Both accomplish the same thing, but in two different ways.
Motor Transition
Motor transition is the changing of the traction motor connections to allow the locomotive to operate over a wider range of speeds. You'll often hear words thrown about like "Series-Parallel" and "Parallel-Shunt" if you read into locomotive manuals, but it can be a bit tricky to understand.
Let's examine a 4-motor locomotive setup. We'll assume the generator can put out 3000 Amps peak, and that the motors can take 1500 Amps peak (The GP38 has this exact setup). 4-motor locomotives can have 6 different transition states depending on the locomotive's purpose and function. Those 6 states are:
- Series
- Series Shunt
- Series-Parallel
- Series-Parallel Shunt
- Parallel
- Parallel Shunt
To begin, let's examine a locomotive wired up in the Series configuration.
In the Series configuration, all four traction motors are wired in series with each other. For motors (or any electrical device) in series, the current through each one is exactly the same, and the voltages across them add up (Kirchoff's Laws). At a complete stall, the traction motors behave like short circuits, and the voltage drop across each one is very small. And because the current is the same for each motor, the generator only has to put out 1500 Amps to max out every single motor. This makes the Series configuration great for starting really heavy trains. As a result, Series is a good starting configuration for switchers, which typically have smaller generators.
There's a problem, however, when the train begins to speed up. As the traction motors begin to spin, they generate a Back-EMF (Voltage) that opposes the flow of current. As long as the generator can output enough voltage to match the traction motors, the train can continue accelerating. But since the voltages of the traction motors add up due to them being in series, the generator has to put out 4x as much voltage to keep up with them! As a result, the generator very quickly becomes unable to accelerate the train past 10 or 12 mph.
So how do you get more acceleration once you max out the generator? You shift into Series-Parallel!
Now instead of having all four motors in series, you divide them into two parallel groups with two motors in series. Kirchoff's Voltage Law tells us that the two parallel motor groups have the same voltage (and that it's equal to the generator voltage). That means the generator only has to put out two motors' voltages instead of four, and the train can now accelerate to double the speed it could while in Series!
The price of this extra speed, however, is current. In order to max out all four motors, the generator now has to output twice the amount of current. For most 4-motor road locomotives, this isn't a problem because the generator is big enough to do that.
As the train continues to accelerate, it'll run into the same problem it had before: the generator will eventually bump up on its voltage limit and the train won't be able to accelerate any further. That's when you shift into Parallel!
Now, all four traction motors are in parallel. The generator usually isn't able to supply enough current to all four motors to max them out, so the engineer has to settle with what he gets. But the generator now only has to supply enough voltage for one traction motor, and thus the locomotive can accelerate even further!
These three steps will cover most of a locomotive's speed range, but they are pretty big jumps. Going from Series to Series Parallel doubles your voltage capacity and halves your current capacity, which can make for some pretty big jolts when the transition takes place!
To smooth things out, locomotives can take advantage of Field Shunting to bridge the gaps. Field shunting means connecting a resistor in parallel with the traction motor's field coils. This means less current is available to go through the field coils, even though the armature current is the same. The physical impact of field shunting is that the amount of torque you get per Ampere of armature current is reduced. In return, the back-EMF that is generated by the motor spinning is also reduced, making it easier on the generator. As a result, the motor requires more current for the same amount of torque, but the locomotive can go faster. Sound familiar? Field shunting does the same thing as changing the series/parallel arrangement of the motors, and by playing with the resistance value, you can get any number of current and voltage ratios, smoothing out the locomotive's tractive effort curve nicely. The downside to field shunting is that some of the energy that would be used for traction is just dissipated as heat from the resistor.
Field shunting can be used with the Parallel configuration to allow even higher speeds than what Parallel can do alone! And of course, the field coils can be shunted in the Series configuration to help bridge the gap between Series and Series-Parallel.
Generator Transition
To Do