Continuing from the previous post, this approach to excitation control appears to have been covered by US patent 3105186:
US3105186 19590826 GE Power Limit & Function Generator p.01.png
It also looks like the forerunner of the system later used on Alco (Type E) and GE diesel-electric locomotives.
As far as I know, dynamic braking control on the GTELs was of the potential wire type, continuously variable and operated by the selector handle.
Most of the GTELs were eventually fitted with equipment that allowed them to control trailing diesel-electric locomotives in MU. This appears to have been a UP initiative, although probably with GE knowledge and possibly involvement. I have not found any details about it, though. Probably some of the existing GTEL master controller trainline outputs could have been used, such as those for forward, reverse, generator field and dynamic brake. But the AV, BV, CV and DV diesel throttle control solenoid trainlines would need to have been added, perhaps by fitting additional cams and contactors within the master controllers. There would need to have been appropriate mapping from 20 GTEL to eight diesel notches. My guess is that diesel notch 8 would have been reached ahead of GTEL notch 20. Firstly, under some conditions (ambient and/or mechanical), full available GTEL power could be reached before notch 20, and it was probably undesirable to also limit trailing diesels to less than notch 8. Secondly, the normal acceleration patterns may have been different, with the diesels getting into notch 8 at lower speed than the GTELs would reach notch 20. Maybe the diesel control reached notch 8 at around notch 15 or 16 of the GTEL control. There was a precedent there with the Milwaukee “Wylie throttle” that allowed the EF-4 class “Little Joe” electrics to control trailing diesels. There the diesel control reached notch 8 when the electric controller was only at notch 24 of 37, reckoned to be at around 15 mile/h at which speed the diesels would be expected not to slip in notch 8. The Wylie throttle coupling system was purely mechanical, and non-linear. Control current for the trailing diesels came from the diesel. I’d guess that in the UP system, there was not a problem with diesel control current coming from the GTEL auxiliary electric system.
Moving from the GTELs to the U50, insofar as the latter was in many ways a double U25B, then one would expect it to have been fitted with the same 16-notch throttle control. But given that the U50 was more-or-less the diesel counterpart to the GTEL4500, it probably wanted a multinotch control anyway. The fact that the same was specified for the Alco C855, and not otherwise used by Alco, was indicative.
The 16-notch control used the GE KC99 master controller, which was an interesting unit. Without any prior knowledge of its workings, one might expect to find that as well as the usual power control trainline outputs, FO (forward), RE (reverse), GF (main generator field), AV (governor A solenoid), BV (B solenoid), CV (C solenoid) and DV (D solenoid), there was an additional trainline that was activated on the half-notch steps.
But from what I can piece together from various sources – not necessarily correctly so – that was not the case. Also, the fact that notches ½, 1 and 1½ all shared the same (minimum) engine speed was a complication that would appear not to have been addressed by a single extra trainline.
Apparently there was an extra trainline, labelled SN, activated in all power notches ½ through 8. What this did was to operate what was known as the MR relay, which “told” any 16-notch units in a consist to operate in 16-notch mode. Absent a closed MR relay, they defaulted to eight-notch mode.
Also, the KC99 contained a large rheostat that was switched in 16 steps and which provided an output to the XB (dynamic brake excitation) trainwire, with the output voltage increasing stepwise from notch ½ through 8. This applied during motoring as well as during dynamic braking. And the XB trainwire was the source for exciter battery field current during motoring as well as during dynamic braking, not only for the leading unit but also for any 16-notch trailing units in the consist. The lead unit supplied excitation current for all 16-notch trailing units. Non-16 notch trailing units in the consist simply ignored the XB trainwire during motoring, and “did their own thing” according to the customary trainline signals.
Overall, that gave three notches (½, 1, 1½) at first (minimum) engine speed, two notches (2, 2½, etc,) at second through seventh engine speeds, and one (8) at eight (maximum) engine speed. One imagines that the notches were arranged to provide relatively closely spaced tractive effort increments along the standstill line, following the GTEL example.
When a 16-notch unit was trailing in a consist led by a regular 8-notch unit, the SN trainline was dead, so the MR relay did not operate, and it defaulted to eight-notch mode. Here motoring excitation was apparently controlled locally by a resistor ladder switched by the AV, BV, CV and DV trainwires. (Whether this ladder was integrated with or separate from the KC99 internal rheostat I don’t know). In dynamic braking, excitation was controlled in a continuously variable way by the load regulator rheostat, in turn operated by a positioner relay controlled by the XB trainwire.
With the three-field system as assumed to have been used for the U50, I think that there would still be just eight hyperbolic constant power curves as determined by the governor and load regulator action. So adjacent notches would share part of the same curve over some of the speed range, although not at lower speeds.
But the Type E excitation system used in the Alco C855 had the ability to construct electrically/electronically close approximations to constant power curves, as shown in the GTEL8500 case. Here then, for normal curves representing the maximum desired power at each engine speed, the load regulator rheostat served mostly as a trimming device, and also as a supervening control in abnormal conditions. But it was also possible to construct constant power curves of less-then-maximum power at any engine speed. If that facility had been used in the Alco C855 case – and whether it was done is unknown - then there could have been 16 constant power curves as well as 16 basic excitation levels. In that case, the XB trainline feed from the KC99 master controller rheostat probably provided the appropriate control voltage input to the excitation system.
The later GE U50C had regular eight-notch throttle control, despite its very high power-to-adhesive weight ratio, higher than any of the GTELs. By this time, improved wheel slip control systems meant that minimizing the notching up tractive effort jump was less of a concern, and there may have been some controlled current ramping at upward notch changes (that I don’t know). As I understand it, the U50C derived eight constant power curves from just three engine speeds, using the capability of the electronic excitation system to construct such curves without help from the load regulator.
Some of the foregoing commentary is based upon dependable source materials, but some, in the absence of such sources, derives from deduction and speculation on my part, so could well be wrong.
Nonetheless, it may be said the subject locomotives do not lack for interesting and unusual features, the running gears and control systems particularly.
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