DC Transition has 3 or 4 positions, in the early locomotives, especially in the switchers, and some of the real early road switchers and passenger road Locomotives (EMD FT, F-2 & 3, ALCo FA-1, PA-1) transition was a manual job, none of those fancy contactors to do the thinking for you. In the ALCo Switchers there was a 7 position reverser ((positions listed from far to near) Forward P-P, Forward S-P, Forward S-S, off, Reverse S-S, Reverse S-P, and Reverse P-P), on EMDs there was a third handle that ratcheted through the positions, not sure how B-L-W and others handled this. The easiest way to think of DC transition is to think of it as a 3 speed transmission.
****Warning**** These numbers are pulled out of my backside, I don't know the true values, but say the generator puts out 600 volts at 100 amps for the discussion below, also assume all discussion examples involves a B-B locomotive (2 power trucks with 2 traction motor in each truck). The voltage at amperage is only in the ideal world, no friction, no feedback, no loss, and is what each motor will see. There is a whole lot more electrical theory that makes it more accurate, but a whole lot more complicated.****
Basically high voltage gives you the speed, and high amperage gives you the torque (rotational force)
Series-Series - Low Speed, High Torque - (1st Gear): Think of the 4 traction motors as knots on a string, which is the path of the electricity. The power comes from the generator, it goes into the first one, then to the second one, and on and on and on..., till it gets back to the generator. The voltage is shared across the motors, but the amperage stays high (150 volts at 100 amps)
Series-Parallel - Medium Speed, less torque - 2nd gear: We will come back to this.
Parallel-Parallel - High Speed, Low Torque - 3rd gear: It is easiest to think of this as a four step (rung) ladder. The left vertical pole is the line from the generator, and the right pole is the line back to the generator. The 4 rungs are the 4 motors (one on each rung) The voltage stays high, but the amperage is shared (600 volts at 25 amps)
Now back to Series-Parallel: The motors in each truck are still in series, but now the 2 trucks are wired in parallel. The power goes from the generator to the first motor in both trucks, then the power in the first motor goes to the second motor, and then the power from both second motors goes back to the generator. Think of a wider 2 rung ladder, the power from the generator is still the left vertical side, and the power going back to the generator is the right side vertical, but now on each rung you have 2 motors running in series. (300 volts at 50 amps)
In the Road locomotives there is a 4th transition, Parallel Shunt, think of it as an Overdrive gear in the transmission. I am unclear on what happens, but IIRC it involves rewiring the generator and using the counter EMF to produce more speed. Maybe one of the other guys can simply explain the parallel shunt operation.
The diesel-electrics and straight electric locomotives are wired this way because an electric motor at 0 rpm technically has 0 horsepower, and the horsepower changes as the voltage and amperage changes. Same thing happens with a steam locomotive, as the throttle valve is opened, the horsepower increases to the maximum, but unlike an electric motor, a steam locomotive can give maximum torque at 0 rpm. This is why when someone opens the throttle too quick on a steamer, her wheels will spin before she moves, even without a load.
Now don't get confused with the talk about counter EMF, this has nothing to do with how and what transition is, but the why it is needed and how dynamic braking works.
Hope this helps,
Rich C.
****Warning**** These numbers are pulled out of my backside, I don't know the true values, but say the generator puts out 600 volts at 100 amps for the discussion below, also assume all discussion examples involves a B-B locomotive (2 power trucks with 2 traction motor in each truck). The voltage at amperage is only in the ideal world, no friction, no feedback, no loss, and is what each motor will see. There is a whole lot more electrical theory that makes it more accurate, but a whole lot more complicated.****
Basically high voltage gives you the speed, and high amperage gives you the torque (rotational force)
Series-Series - Low Speed, High Torque - (1st Gear): Think of the 4 traction motors as knots on a string, which is the path of the electricity. The power comes from the generator, it goes into the first one, then to the second one, and on and on and on..., till it gets back to the generator. The voltage is shared across the motors, but the amperage stays high (150 volts at 100 amps)
Series-Parallel - Medium Speed, less torque - 2nd gear: We will come back to this.
Parallel-Parallel - High Speed, Low Torque - 3rd gear: It is easiest to think of this as a four step (rung) ladder. The left vertical pole is the line from the generator, and the right pole is the line back to the generator. The 4 rungs are the 4 motors (one on each rung) The voltage stays high, but the amperage is shared (600 volts at 25 amps)
Now back to Series-Parallel: The motors in each truck are still in series, but now the 2 trucks are wired in parallel. The power goes from the generator to the first motor in both trucks, then the power in the first motor goes to the second motor, and then the power from both second motors goes back to the generator. Think of a wider 2 rung ladder, the power from the generator is still the left vertical side, and the power going back to the generator is the right side vertical, but now on each rung you have 2 motors running in series. (300 volts at 50 amps)
In the Road locomotives there is a 4th transition, Parallel Shunt, think of it as an Overdrive gear in the transmission. I am unclear on what happens, but IIRC it involves rewiring the generator and using the counter EMF to produce more speed. Maybe one of the other guys can simply explain the parallel shunt operation.
The diesel-electrics and straight electric locomotives are wired this way because an electric motor at 0 rpm technically has 0 horsepower, and the horsepower changes as the voltage and amperage changes. Same thing happens with a steam locomotive, as the throttle valve is opened, the horsepower increases to the maximum, but unlike an electric motor, a steam locomotive can give maximum torque at 0 rpm. This is why when someone opens the throttle too quick on a steamer, her wheels will spin before she moves, even without a load.
Now don't get confused with the talk about counter EMF, this has nothing to do with how and what transition is, but the why it is needed and how dynamic braking works.
Hope this helps,
Rich C.