How to keep a constant speed

[e-paw]

Well I'll see if I can explain it Charlie. Unlike our toy trains that use permanent magnets to produce a magnetic field in witch our DC motors turn in. Locomotives, as do most large industrial DC motors, use a electro magnet called a field winding in there traction motors. Transition is the act of switching the field winding and armature winding ( the part of the motor that turns) in different combinations of series & parallel wiring to get either more torque or allow for more speed.I was able to find this drawing that helps to show what I am talking about.

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and this one too.
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Different manufacturers had different steps to transition in what gets wired how, but the all are after the same results. In straight series wiring you get more torque as you work your way up to full parallel you get more speed.