In respect of the EMD field-loop system of dynamic brake control, my understanding is that originally it was a high-current system. That is the field-loop trainwires and jumper cables carried the actual main generator battery field current for all units (up to four) connected in series. And that this battery field current was controlled by the master controller DB rheostat (consequently a large, heavy-current device) in the leading unit.
But somewhere along the way, the field-loop control system might have been changed to become a (relatively) low-current system, which effectively piloted some form of heavy-current control system in each locomotive.
GEJ-3815, the U25B educational manual, provides schematics for the DB system including the optional field-loop control. When field-loop control is switched in on a leading locomotive, the field loop trainwires are fed from the XB trainwire, which is in turn controlled by the low-current resistances switched (in 16 steps) by throttle handle action. Here the current level would be that required by the exciter battery fields, several in parallel, which would be much lower than that required by main generator battery fields. Not shown is how field-loop DB control is executed on U25B trailing units, but possibly the exciter battery fields are simply connected in series in the loop.
The book “D-Day on the Western Pacific” by Virgil Staff provides some additional clues. In talking of the EMD GP-20 it was said: “The field loop brake concept had been used by Electro-Motive from the early days of dynamic braking in 1,350-hp units. In this system, each unit was equipped with a separate three-pronged field loop receptacle connected by a jumper cable between each unit to hook the main generator battery fields in series. In dynamic braking, current from the batteries of the lead unit passed through the battery fields of each trailing unit, and then back through the other wire in the cable to the batteries of the lead unit.
“Control of excitation was done by manipulating the amount of current through the field circuit, and this in turn controlled the dynamic braking effort. The design was a proven one, and the only major difference on the GP-20s from that on older power was the presence of a load regulator commutator rheostat to control the amount of battery field current rather than a huge separate dynamic brake control rheostat in the control stand such as on the F-7s, GP-7s, and GP-9s. This was an improvement over the previous models since the builder eliminated the cost of a duplicate commutator rheostat and used a micro-positioner relay to control the position of the load regulator, and therefore the amount of dynamic braking current.”
Then Staff goes on to say that the WP GP-35s were equipped with potential wire DB control, and in addition: “To make the GP-35s compatible with older power, Electro-Motive included a field loop circuit which necessitated a motor-driven rheostat, and some extra transductors, capacitors, and rectifiers to produce a second dynamic braking system.”
And furthermore: “...when the GP-35s came to the property, the field loop system with which they were equipped was far from reliable, and there were problems with the motor-driven servomotor positioning the big faceplate commutator. So with the dual-system, whenever a GP-20 found itself in the consist with GP-35s, a dynamic braking problem developed for which the GP-20 tended to receive the blame, but for which the GP-35 was the culprit since it was forced to use its own inadequate field loop braking system instead of the potential line brake control circuit.”
I don’t think that there is enough information there to develop a complete picture as to how the dual-DB control systems worked, but they did seem to involve some complexities. Possibly the motor-driven commutator rheostat mentioned was intended to set field loop current from the lead unit and was slaved to DB potential wire voltage.
Returning to the GE U25B, when in trail this also had two modes for potential wire DB control. If it was trailing another U25B, then exciter battery field current was provided direct from the XB (DB excitation) trainwire. In turn XB trainwire voltage was controlled from the leading unit throttle handle in 16-steps, just as in motoring. On the other hand, if the U25B was trailing a non-U25B locomotive with potential wire DB control, then the exciter battery field was controlled by the load regulator, which in turn was controlled by a micro-positioner working from the XB trainwire. Switching between two modes was “automatic” from the GE-unique MR-trainwire, which was energized when a U25B was in the lead. Not really covered in GEJ-3815, but it would appear that during motoring, when a U25B was trailing a non-U25B, the exciter battery field was controlled by a combination of a local resistor matrix switched by relays in 8 steps from the AV, BV, CV and DV throttle control trainwires, and also by the load regulator rheostat.
It might be noted that notwithstanding what EMD did in its home market, quite early on it adopted potential wire DB control for its export models. The first locomotives so equipped were probably the Victorian Railways (Australia) B class of 1952. These were the Clyde-GM ML-2 model, essentially a lower-profile, 6-motor version of the F7. Certainly the New Zealand Railways EMD/GMD G12 fleet of 1955 had potential wire DB control.