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  • Making transition..is that spelled right?

  • General discussion about locomotives, rolling stock, and equipment
General discussion about locomotives, rolling stock, and equipment

Moderator: John_Perkowski

 #124873  by crij
 
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.

 #125059  by txbritt
 
Isn't Parallel-Shunt, the same thing or similar to parallel-reduced field?

I believe thats where they put the field coils of the traction motors across a set of resistor banks.

anyone?

TxBritt

 #125061  by LCJ
 
txbritt wrote:Isn't Parallel-Shunt, the same thing or similar to parallel-reduced field?
I believe this is an accurate statement.

 #127090  by shortlinerailroader
 
I have overlooked this thread until today. I know what transition is, but it bit me in the rear today as we were farting down the main at 25 or 30 in a frickin' GP10 that would not make it. Had to set out 11 cars, but had to wait on a westbound for an hour because we were so darn slow getting there.

Still the best job I've had, tho

 #128411  by EDM5970
 
Some good information here, some not so good. An Alco switcher or RS-1 has Series, Series Parallel and Series Parallel Reduced Field (Yes, RF is the same as shunting. TxBritt hit it right on the head.) The "all the way forward or all the way back" position on the controller is Series Parallel Reduced Field. There is no full parallel on an Alco switcher, and I'm not aware of any common diesel electric that has S, S-P and P connections.

The larger four motored first gen freight units (FT and up on the EMD side, RS-2 and up with the Alcos) generally started with Series Parallel, went to SP-reduced field, Parallel and then Parallel wtih reduced field. One notable exception was the GP-7 and later the CF-7, which had no shunting, just S-P and P. There were some options available on the EMD switchers; field shunting was one, and the basic SW-1 was a series only machine, with transition being an option. (Standard on an S-1, and they had traction motor blowers and a few more ponies!)

As far as contactors are concerned, they didn't do the thinking for you. Contactors are large, usually control air operated switches which connected the traction motors to the generator in the proper sequence.

Some units had manual transition, others had automatic control, based either on the load regulator, generator output or on locomotive speed. The 539 Alcos would automatically transition S to S-P at around 8 MPH, and go to S-P RF at 23 MPH. "V" relay controlled forward transition. Backward transition was made by closing the throttle.

244 and 251 Alcos generally used an axle alternator to determine speed. (Note the use of the word generally, again; there have to be exceptions!)
Top of my head, with 752 motors and 74:18 gearing (65 MPH), an RS-11 or similar would be in S-P to around 17 MPH, S-P RF to around 27, Parallel to around 47, and from 47 to 65 in Parallel reduced field.

EMDs had several systems. (Every Model Different). IIRC, on a GP-7 the load regulator had an arm that contacted a limit switch for forward transition and a thru cable current sensing relay controlled backward transition. (And lets not get into EMD wheelslip and its variations-)

Again, some of this is off the top of my head; all my schematics are upstairs, and its getting late. But hopefully the above will clear up a few things.