----- Original Message -----
From: "Chris Barker" <ftog@xxxxxxxxxxxxxx>
Thanks, Steve, but it doesn't describe the method of synchronising the power,
only centralising the control (which is neat, I grant you).
The following might be more clear:
There's a signal line that connects all the locomotives, called an "M.U.
jumper". M.U. stands for multiple-unit, and it goes back to the days of the
interurban streetcar.
How does it work? Bear with me, I have to talk about trolleys for just a
moment. Before M.U. control, a trolley car would have a controller. It had 4 or
5 "steps", each one connected to a large resistor. Each step had a smaller
resistor, so more power got to the wheels. The full trolley power was switched
right in the control switch, and your hand is on it, yikes!
With M.U. control, you used the same controller, but instead of connecting 600
volts to resistors, you connected 80 volts to relays under the car. The relays
were connected to the resistors. Each time you moved the controller up a step,
the next relay would pick up. It was a "remote control" controller. Safer, but
now watch...
These 80-volt control wires went to 12-pin plugs on each end of the car. When
the cars were coupled together, they were also plugged together with 12-pin
jumpers. Now, when a motorman worked his controller, it activated the correct
relay not just on his car, but also the relays on every other car too! All the
cars powered at once.
You could even have cars which themselves did not have motors, but you could
put a controller there that could control the cars behind which did have
motors. It was genius.
Enter the diesel locomotive. They used exactly the same idea. But instead of
the steps operating relays, they operated a small solenoid on the diesel
engine's governor. The governor (in diesel locomotives) controls the speed of
the engine, which obviously controls power output. But it's the same deal, the
control cab of the locomotive selects idle or 1-8 "steps" of power, and that is
relayed by M.U. cables to every other locomotive which follows those
instructions.
Now, it is a common misconception that multiple engines must be synchronized
exactly. That may be the case on side-by-side machines like airplanes. But with
land vehicles, the ground synchronizes you! Imagine a guy's car breaks down and
he needs to push it. He calls for help, and a bodybuilder and a small child
come over. Now, the body builder is giving 300 pounds of push but the child is
only giving 30. How much push are those two doing? 330 pounds, not 270. The
child does not weaken the bodybuilder, he adds to it.
So yes, if you have a big 6000 horsepower mainline diesel and also a tiny 1000
horsepower switcher, they absolutely can M.U. and they can push the train
together for a total of 7000 horsepower. They don't have to be equal, they just
have to push in the same direction, lol!
There's some fairly tricky wiring in the M.U. cables to make sure the
locomotives push in the same direction. Modern diesel locomotives have I
believe 21-pin M.U. cables, which include several pins for sorting out
headlight controls.
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