• EMD Field Loop Dynamic Brake Control

  • Discussion of Electro-Motive locomotive products and technology, past and present. Official web site can be found here: http://www.emdiesels.com/.
Discussion of Electro-Motive locomotive products and technology, past and present. Official web site can be found here: http://www.emdiesels.com/.

Moderator: GOLDEN-ARM

  by Engineer Spike
EMD used the field loop system for controlling the dynamic brakes. I have heard that it limited the number of units in a consist. Is that true?

Did they give the option of using the 27 pin train line? The reason for the question is for companies which mixed brands. Was Alco offering a system which could MU the db with EMD?

Some rewired the old units to have the db run through the 27 pin. On CPR's they did this. The GP9s have the slightly taller mu stand, like those with the double 27, and field loop receptacles. They put a new socket where the field loop was, to give power for a snowplow. This is now even redundant because the plows have a 27 pin, which is used to supply power.

How difficult was the rewiring job to run the db control through the 27 pin jumper?
  by Bright Star
EMD Field Loop was limited to four units in a consist. There are set-up switches that need to be manipulated. The train line was used for d/b setup, with regulation through the FL jumper.

Alco (and GE) units with Potential Control DB could control/be controlled by EMD units with Potential Control DB. Some EMD's (GP-30,35?) had the Potential Control System overlaid over the FL control-so the EMD could work with Alco and GE and older EMD's like the F-7,GP-9 etc.

Potential Control uses two mu trainline wires-one for set-up and one for control of d/b excitation. Some Alcos may have been able to pass the FL signal, but I doubt they had the ability to convert it to a potential control signal. That would have required several additional contactors, relays and some cabling modifications.

By the time of the GP-40, Potential Control was the standard on EMD.

  by Typewriters
Fairbanks-Morse units in the mid 1950's became available with "Universal Dynamic Brake Control." This was briefly mentioned in the book 'Train Master," published by Diesel Era in 1997. Bob Stacy, former F-M employee, described the fact that F-M engineers invented this equipment, which could work with field loop dynamic brake units or with potential control units.

Information in technical literature about this has proven scarce but findable. Fairbanks-Morse Bulletin 1502H (in my collection) which is the operating manual for B&O H16-44 units 6705-6709, published 11/57 includes information about this option, which in the wiring diagram table listing "Electrical Specialties" is called out as "Universal Dynamic Brake." The description of this optional equipment (standard equipment for this locomotive was no dynamic brake at all; dynamic brake was an extra cost option, and the Universal dynamic brake a further cost option) is in Sec. 110A, and indicates that upon railroad specification, a field loop circuit can be added that "is designed to energize simultaneously with the voltage control of the GE braking system in the braking range of the controller. From big "B" to little "B" on the controller, field loop amps vary from zero to fourteen (14) amps plus or minus 10%. The field loop circuit controls only the excitation of trailing units loop-equipped."

If the consist (up to four units) included a mix of units such as F-M and EMD units with the EMD units only having field loop control, then the lead unit's unit selector switch would be placed to match the number of units, all other units' switches placed to "1" and the field loop jumpers hooked up. If all units (any number) used potential control, then the unit selector switch was to be placed in "1" on the lead unit and field loop jumpers left down.

There is however one very important note that follows the operational description:

"Units with field loop control, with which it is desired to multiple GE-equipped units as described in this manual, must have certain minor control wiring changes made. Fairbanks-Morse Locomotive Modernization 212 describes the changes which apply to field loop control units of F-M manufacture."

This last mentioned F-M modernization reference probably applies most widely to the F-M C-Line units, all of which that had dynamic brakes used a field loop control system deliberately matched to EMD's. (Of course, all C-Liners used the "common standard" Westinghouse eight notch electric throttle, which is to say the XM-781 controller.)

No information in my collection supports any as-built ALCO-GE units with 244 engine as being able to operate with field loop equipped units. However, the RS-11 / RSD-12 operating manual does in fact mention loop control, and although it does not specifically call out or describe a universal control, it's clear that such was being used. Look at the following portion of the Dynamic Braking Operation instructions (this is manual TP-412, February 1956.)

"Dynamic Brake Unit Selector Switch.

1. When operating all ALCO units in multiple"
a. Place unit selector switch on all units in No. 1 position.
b. Do not install field loop dynamic braking jumpers between units.

The instructions go on to detail what you'd do if you had a mixed lashup... but it's instructive to note that if you didn't have universal control, then you wouldn't have the ability to place all unit selector switches in No. 1 and get control on units that had field loop jumpers. (Remember, it says "all ALCO units.")

We can then guess pretty safely that although F-M might have originally invented the universal control, ALCO had one too - sooner rather than later.

These units all use a 27 point jumper and the separate field loop jumper as standard equipment. (There were exceptions, of course, such as Union Pacific who had everything built with ALCO-GE pattern 21 point and 12 point MU jumpers, and field loop jumpers as well.)

I'll keep looking around and post more information as I find it. It's an interesting topic.

-Will Davis
  by Typewriters
{Further on the topic of EMD Field Loop Dynamic Brake Control, and compatibility with other makes of locomotive.}


Sharp eyed historians might have noticed that the very first prototype U25B units had, centered on their end platforms, a 27 point MU jumper and a field loop jumper. General Electric knew who was in the lead and fitted field loop compatibility from the start. Let's look briefly at some operating manuals I have to see what the operational compatibility really was.

GEJ-3807 GE 751-752 5/60

This is the manual for the U25B prototypes. These units had an MU Braking Selector Switch on the Engine Control Panel. Directions:

"This switch is mounted on the Engine Control Panel. It has two positions - GE Braking and Loop Braking and should be set to correspond to type of dynamic braking in the consist. If all locomotives in the consist have GE excitation and dynamic brake equipment the MU Braking Selector Switch must be set in "GE Braking" position. If this locomotive is operated in a mixed consist with one, two, or three locomotives having dynamic brake equipment requiring loop braking circuit control, the MU Braking Selector Switch must be set in "Loop Braking" position.

GEJ-3810 U25B 12/60

This is the manual for high short hood U25B early production units. There is a slight difference in the operational instruction.

"This switch is mounted on the engine control panel. It has two positions: "GE Braking" and "Loop Braking" and should be set to correspond to type of dynamic braking on each unit in the locomotive consist. If all units in the locomotive consist have GE excitation and dynamic brake equipment, the MU Braking Selector Switch must be set in "GE Braking" position. If this locomotive is operated in a mixed locomotive consist with one, two or three locomotive units having dynamic brake equipment requiring loop braking circuit control, the MU Braking Selector Switch must be set in "Loop Braking" position on any GE unit that is directly coupled to an EMD unit."

GEJ-3816 U25B 3/62

This is the manual for what we'd think of as normal production U25B units. The description of all-GE excitation is the same, but the second part changes yet again.

"If a U25B locomotive is leading in a mixed locomotive consist with one, two or three locomotive units having dynamic brake equipment requiring loop braking circuit control, the selector switch must be set in "Loop Braking" position on the lead unit only."

GEJ-3816 as modified by pink insert GEJ-3818A dated 9/62 contains the following important mechanical and operational change:

"Dynamic Braking - MU Selector Switch. A 5 position Dynamic Braking MU Selector Switch is located beneath the Engine Control Panel in the Operator's cab. This switch must be positioned according to the type dynamic brake units (sic) in the consist. Operate the switch as follows:

1. Position the switch at the top (U25B trail) when all units in the consist are not equipped with loop braking or when a U25B is a trailing unit in the consist.

2. Position the switch at one of the 4 numbered positions corresponding to the number of trailing units equipped with loop braking when a U25B is leading. Do not count the U25B as a field loop unit.


GEJ-3816A U25B 12/62

Production U25B units. Description of operation of MU switch modified - manual now contains essentially the data added by the 3818A pink insert. The description of the operation of the switch is just as found in the earlier manual. I'll reproduce the whole thing for clarity and to show the third added paragraph that reflects the new switch design addition.

"MU Braking Selector Switch -- This switch, when equipped, is mounted on the engine control panel. It has two positions: "GE Braking" and "Loop Braking" and should be set to correspond to the type of dynamic braking on each unit in the locomotive consist. If all units in the locomotive consist have GE excitation and dynamic brake equipment, the MU Braking Selector Switch must be set in "GE Braking" position. If a U25B locomotive is leading in a mixed locomotive consist with one, two or three locomotive units having dynamic brake equipment requiring loop braking circuit control, the selector switch must be set in "Loop Braking" position on the lead unit only.

The Selector Switch should be set before leaving the terminal. Positioning must not be changed even if the engine is isolated enroute. If locomotive units in the consist are changed enroute, then the position of this switch may have to be changed.

Some locomotives may be equipped with a multi-position dynamic braking MU Selector Switch. This switch is used to furnish full braking effort at all locomotive speeds when operating U25B locomotives and loop type locomotives in a consist with a U25B leading. Position the switch according to the number of Loop Braking units in the consist and as specified by railroad rules.

GEJ-3816B Model U25 1/64

This manual is the first to cover four and six axle units. In all previous manuals, the descriptions I've given (except for the pink Revision insert) were located in the section actually describing operation of the locomotive in dynamic brake mode. There was a corresponding mention of the switch in the rundown of all control panel switches in each of these manuals. In GEJ-3816B the description is OMITTED from the operating instructions except for a brief mention. In the rundown of operating controls and switches we find the following, clearly in line with the equipment change described by GEJ-3818A:

"MU Braking Selector Switch: The MU dynamic braking selector switch (when installed) is furnished in several styles depending on the request of a railroad. Position the selector switch according to the directions on the nameplate and according to railroad rules. The selector switch must be positioned before leaving the terminal and must not be changed even if the engine is isolated enroute."

GEJ-3834 Model U28 1/66

Four and six axle U28, early body style. Directions as GEJ-3816B.

GEJ-3837 Model U28 5/66

Four and six axle U28, new body style. Directions as GEJ-3816B.

GEJ-3844 Diesel Electric Locomotive. 11/66

Four and six axle U30. Directions as GEJ-3816B.

GEJ-3856 Diesel Electric Locomotive. 2/68

Covers U23, U30, U33. Two lever control stand. Directions as GEJ-3816B.

So in looking at all of these manuals, even just briefly, we see that field loop compatibility was built in from the start and appears to at least have been available through the February 1968 issuance of the first "all inclusive" U-series manual... even if no one may have ordered such equipment in years. THAT would be a much bigger research project -- looking at all the various U-boats of all the various roads to see if dynamic brake field loop jumpers were there.

The instructions for the early units are confusing when compared with each other once through. I'll take a look at my very large GE technical manuals and see if I can't figure out just how the dynamic brake controls were wired, so that I can figure out what the functions actually were. It might be that there was also an unmentioned "Unit selector switch" just like you'd find on an EMD F-unit with dynamic brakes.. or for that matter an F-M C-Liner with dynamic brakes... but I'm not certain. Usually all the control switches are mentioned in the operator's manual, so this is just the slightest bit curious.

-Will Davis
  by Typewriters
EMD units..

Looking at manuals here, we found the following information regarding field loop dynamic brake compatibility and EMD units in the late 50's and early 60's...

GP-20 2nd Ed. May 1961 -- Field loop dynamic brake control only.

GP-30 3rd Ed. April 1963 -- Potential control standard, field loop dynamic brake optional. No dual compatible system.

GP-35 1st Ed. October 1963 -- Potential control OR field loop dynamic brake control in two options -- lead only in field loop, or lead/trail in field loop. If the latter option was fitted, the units had an Operation Selector switch inside the electrical cabinet whose positions were "Field Loop" and "Potential Wire." There are directions on use of this switch, and on when to reposition it. The manual states that "GP-30 and GP-35 units which are not equipped with an Operation Selector Switch cannot trail in field loop."

GP-35 2nd Ed. March 1964 -- Exactly as 1st Ed.

SD-35 / SDP-35 1st Ed. October 1964 -- Exactly as GP-35 previously described.

SD-35 / SDP-35 2nd Ed. April 1965 -- Exactly as GP-35 previously described.

SD-45 3rd Ed. July 1967 -- Same control system as previously described. Controls now located in short hood, with Unit Selector Switch and "Field Loop / Potential" toggle switch on the same panel.

SD-38 1st Ed. February 1970 (Static Excitation). Same as SD-45.

This seems to nail down the change from standard dynamic brake being field loop to being potential control with the GP-30. Compatibility with field loop units was offered for years, just as we found out with the previous look at GE manuals.

A look at the excellent photo book "Norfolk and Western - First Generation Diesels" (Withers/Bowers 1990) however shows a large number of N&W GP-9 units (and GP-18 units for that matter) which don't have field loop jumper receptacles, but do have dynamic brakes. I also see units which had field loop jumpers originally in photos have them blanked off and no longer used as early as 1962. So my guess is that N&W rapidly converted its units to potential control, and began substituting the short MU stand without room on top for the field loop jumper receptacle. N&W was known to be modifying its locomotives, so seeing this change very early isn't a big surprise.

-Will Davis
  by Typewriters
ALCO Century series locomotives...

ALCO Products publication TP-447C - Operating Manual, Century Series Road Locomotives / D.C. Transmission, Rev. August 1967.

Page 46 - Dynamic Brake Unit Selector Switch (if used)

1. When operating all ALCO units in multiple:
a. Place unit selector switch on all units in No. 1 position.
b. Do not install field loop dynamic braking jumpers between units.
2. When operating ALCO units in multiple with units of other manufacture:
a. Place unit selector switch on all trailing units in No. 1 position.
b. Place unit selector switch on lead unit to correspond with number of units in consist.
c. Install field loop dynamic braking jumpers between all units.

We can see thus that ALCO also offered field loop compatibility on the Century series locomotives through at least the third quarter of 1967, just looking at one manual.


Why am I doing all this?

The answer is simple. Many things have been written in railfan oriented publications and on railfan forums about multiple unit compatibility between what we might now think of as "early" diesel locomotives in the US. Much of that, I'm sorry to say, has either been mistaken or at least confused.

Now that the subject of field loop dynamic brake control came up, it was time to get out all the original manufacturer's materials and really look at what was offered when. This material does no one any good if it just sits.. and it does little good if it isn't read and interpreted in this sort of way to answer questions. And preserve the actual history of the design and manufacture of these units.

Now, none of this is to say that railroads were buying EMD units or GE units or ALCO Centuries in the mid-late 1960's with field loop dynamic brake control. It is to say that compatibility was offered for a very long time after EMD began to offer potential control on its units. It is also to say that some units were one way or the other, and that some units could operate with either type of control.

This might even help really die-hard model railroaders out there who want to absolutely ensure that their layout consists would have actually been compatible in all aspects. Who knows.

I think I'll have one more post to make in this series for completeness, and that should be it.

-Will Davis
  by Pneudyne
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.

  by Pneudyne
As noted in my previous post, in contradistinction to its domestic market practice, for its export models, EMD adopted potential-wire dynamic brake control from the start in 1952.

Possibly the reason for this is found in US patent US2745050, available on-line at Google patents. This was filed 1952 January 02 and published 1956 May 08. So the development work was likely done during 1951.

In synopsis, EMD wanted an improved dynamic braking control system for light axle load (40 000 lb) locomotives so as to limit maximum braking effort to that which matched the available adhesion. Whilst simply limiting maximum armature current with the existing (field loop) control system would have achieved the objective of a lowered peak braking effort, it would also have reduced the braking effort available at higher speeds to below that desirable. So EMD adopted decompounding of the main generator in DB mode. The shunt field was used for decompounding, being connected across part of the DB load resistance (grid) in a sense that bucked the battery field.

A consequence of this decompounding was that much higher battery field currents were required for a given braking effort than with the field loop system, 55 amps being mentioned in the patent. This was necessarily supplied from the local main generator and not via jumper cables from the leading unit. Thus EMD adopted potential-wire control, with “translation” to battery field variations being done via the load regulator rheostat using a positioner relay (sensitive to differences as small as 1 volt) and the overriding solenoid.

The 40 000 lb axle loading mentioned in the patent corresponds approximately with that of the B-B variant of the export B12 model. The Victorian Railways B class (Clyde-GM ML-2), the first to use potential wire dynamic braking, had an axle loading of around 43 000 lb.

So it may be seen that EMD was essentially forced to abandon field-loop DB control for its export locomotives, and once it was determined that remote control of local excitation was needed, potential-wire was likely seen as a logical choice. In that sense, MU compatibility with GE-equipped locomotives was evidently not a primary driving factor, but it might have come into play once the deliberations on remote excitation control started, although perhaps weakly so. In 1951, GE-equipped locomotives with potential-wire DB control were in service in Guatemala and the Congo, and orders would have been in hand for Argentina, Australia (NSWGR), Brasil and Columbia. Except perhaps for NSWGR, none of the railroads involved appeared to be particularly concerned about inter-make MU compatibility at that time.

The essence of the EMD patent appears to have been not so much potential-wire control in and of itself, but the method of decompounding, and the use of a load regulator positioner relay. Decompounding of the main generator during DB operation was not new; GE used the voltage across the commutating pole winding as a decompounding input to a transductor in its amplidyne system, and English Electric (EE) used the main generator starting winding for that purpose. Neither was use of the main generator battery field load regulator rheostat for DB excitation control new, as EE had previously used the same, albeit with direct up/down (impulse) rather than potential-wire control. And EMD itself had used direct up/down control of the load regulator rheostat on its early non-MU hump control. (The MU version, which may have debuted with the -9 series locomotives, used a load regulator positioner relay.)

One may also deduce that notwithstanding the special WONA (world-outside-of-North America) requirements, in 1951 EMD had not conceded that potential-wire DB control was superior to its own field-loop control for the North American market which it by then dominated. That would explain why it was a decade or so before it reconsidered the DB control case, by which time a different set of circumstances obtained.

Still, in respect of EMD domestic locomotive practice, there was an interesting intermediate step that I think arrived with the SD-24 and other 567D-engined models. Rather than controlling field loop current directly by a “high-current” rheostat in the control stand, it was controlled by the load regulator rheostat in the leading unit. In turn the load regulator was controlled by a positioner relay, whose operation was controlled by a low current, 500-ohm potentiometer in the control stand. Thus there was in effect an internal potential-wire control system between the control stand and the load regulator rheostat.

A corollary is that contrary to what I said in my previous posting, the field loop itself never changed from a high-current (15 amps) to low-current system. The change was simply internal.

  by Pneudyne
I have also had a closer look at the GE U25B case, where apparently paradoxically, both the field loop (high-current) and potential-wire (low-current) control systems are essentially one and the same. So the potential control must have been able to supply up to 15 amps as required by the field-loop. There was not the customary potential-wire potentiometer, but instead a resistor ladder that was switched in various ways to control the voltage applied to the exciter battery field of the leading unit, and via the DBX trainwire, the exciter battery fields of any trailing GE units in the consist. The resistors in the ladder summed to around 52 ohms, which I think is about the same order of magnitude as that of the typical governor-integral load regulator used for controlling exciter battery fields. I can only guess at what a typical exciter battery field maximum current might be, but let’s say 5 amps. With four units in a consist, the resistor ladder would have to supply up to 20 amps. In that case, the 15 amps maximum required for the field loop would be well within its capabilities. Anyway, it seems that in the GE case, the DBX trainwire could at times be carrying quite a bit of current, at least as compared with its original Alco-GE role as a voltage reference. That would explain why the U25B was equipped with load regulator positioner relay control for DB when it was trailing a non-GE lead unit in a consist. The potential-wire control system on say an older Alco unit would not have had the current capacity to directly supply the GE exciter battery field.

One is then left to wonder how Alco had earlier implemented dual-mode DB control in the RS11, etc. I’d guess that it worked with GE on this; unlike Fairbanks-Morse, I don’t think it was so inclined to undertake its own electrical development work.

  by Pneudyne
From the foregoing series of posts, one might endeavour to construct a chronology of dynamic brake control systems in American practice from the beginning through to the time when potential-wire control became the standard.

Here is a rough first attempt, with gaps and no doubt errors.


EMD electric holding brake on the original FT.


EMD field loop system, with high-current rheostat in control system. Initially used on later FTs.


GE potential-wire system, in conjunction with the Amplidyne control system. Initially used on Alco-GE road units and F-M Erie-built units.


Baldwin pneumatic DB control and associated CE-100 control stand. Used on Baldwin road units. Probably also used on Baldwin export models, and definitely used on Cockerill (Belgium) licence-built units into the early 1960s.


Westinghouse XM-781 compatible control stand and system, optional on Baldwin locomotives and standard on Lima-Hamilton locomotives. The XM-781 options included DB with either field-loop control (EMD-compatible) or potential-wire control (Alco-GE compatible), although the actual availability (and use) of the latter needs to be confirmed. Did Lima-Hamilton ever offer a DB option?


F-M adopts the WH XM-781 controller with field loop DB control as standard for its C-Line series.


EMD adopts potential-wire DB control as standard for its export models. It uses a load regulator positioner relay.


Apparent first use by GE of its potential-wire DB control in a non-Amplidyne equipped locomotive, namely the WP&Y shovel-nose design. Details unknown, but possibly there was direct control of the exciter battery field from the DB control potentiometer.

1955 or 1956:

F-M introduces its universal DB control, compatible with both field-loop and potential-wire systems. From Sweetland/Diesel Era (“Trainmaster”), this was done initially for the SOU, whose F-M Trainmasters had GE Amplidyne control. It would appear that with the changeover from WH to GE electrical equipment, F-M had also changed from field-loop to potential-wire DB control as standard. One may reasonably infer that the universal DB control was an option on late GE-equipped (both Amplidyne and static excitation versions) Trainmasters and other FM units, but that it might not have applied to any WH-equipped units.


Alco offers optional field-loop DB control – in addition to its standard potential-wire control - on its RS-11 – and presumably other contemporary models.


GE’s new export Universal series of locomotives uses potential-wire DB control, apparently with direct control of the exciter battery field from the DB control potentiometer.


EMD modifies its field-loop control system, replacing the high-current rheostat in the control stand with a low-current potentiometer and then using a load regulator positioner relay. This is an internal change and does not affect the external field-loop circuit per se. This appears to have arrived with the SD-24.


GE’s new U25B has optional field-loop DB control in addition to its standard potential-wire control. Both types of control are fed from the same 16-step excitation resistor ladder as used for power control. For operation in trail with “traditional” potential-wire control units leading, GE also included a load regulator positioner relay.


EMD provides a choice of either field-loop or potential-wire control on its GP-30 model. The revised internal control system introduced with the SD-24 would appear to have made the potential-wire option quite easy to implement.


EMD offers universal DB control on its GP-35. According to Staff (“D-Day on the Western Pacific”), the GP-35 was basically a potential-wire locomotive with a field-loop system overlaid, using a servomotor-driven commutator rheostat.

  by Allen Hazen
Pneudyne: Thanks for this series of posts about d.b. systems and their history. You ask, in the most recent, whether Lima-Hamilton actually offered d.b.: yes, and some of their CC transfer units for PRR had it.
  by Pneudyne
Thanks, Allen. I suppose that the next question is whether those Lima-Hamilton units had field-loop or potential-wire control. Field-loop seems more likely.

One may imagine that EMD originally developed its field-loop system as being a relatively simple way of ensuring synchronized main generator excitation of several units during DB operation. The loop enforced the same current through all battery fields, and did not depend upon for example the correct calibration and operation of “slave” mechanical devices in trailing units. At the time, four units was likely the maximum locomotive consist envisaged, and the available battery voltage was enough for four main generator battery fields in series (at maximum DB), but perhaps not more than that.

On the other hand, once Alco-GE had adopted its amplidyne excitation system, then voltage control was the logical choice for DB, with the potential-wire providing the DC input to the appropriate transductors. This essentially static approach would have provided adequate synchronization without dependence upon mechanical devices.

Of course, slave devices did come to be used later on, particularly in the form of the load regulator positioner relay, which placed the load regulator rheostat as the follower device in a bridge circuit. I have heard anecdotally, at least in respect of the New Zealand EMD fleet, that the positioner relay required very careful adjustment to keep it properly calibrated.

  by Allen Hazen
Re: Limas
I don't know where to look for more information on the Lima "transfer" units. Kalmbach Publishing Co has photos of both the non-db and db variants: the same photo of the db variant (a nice side view: the db boxes are on the roofline of the hoods, maybe a couple of feet from the cab, with a large square vent on the side and what looks like the profile of a fan housing on top) is used in a couple of their books. I don't even know how many db-equipped units there were. PRR divided their fleet of 22 into two classes, LS25 (= Lima, Switcher, 2500 hp)and LS25m: I think their classification system used "m" to mark a switcher with multiple-unit connections. The db-equipped Limas (referred to in various places as "road switchers") are, I think, LS25m, but I don't know whether all LS25m had db.
I'll post a request for information on the Baldwin/Lima railroad.net forum.
  by Pneudyne
Thanks again, Allen.

How Westinghouse implemented field-loop dynamic braking may be deduced from the schematic in the Fairbanks-Morse Train Master operating manual that may be found at the excellent Fallen Flags site; http://www.rr-fallenflags.org/manual/FM-TMom.pdf" onclick="window.open(this.href);return false;.

EMD of course used something of a “brute force” approach to excitation control, eschewing a separate exciter for a high-current load control rheostat that determined main generator battery field current, which was probably above 50 or 60 amps at full field during motoring, and apparently 15 amps maximum during DB. On the other hand WH used a separate split-pole differential exciter to supply the main generator battery field, with a low-current load control potentiometer, in this case of the integral kind in a Woodward PG governor, controlling the exciter 4-pole battery field. (Earlier iterations may have used an external load-control rheostat with the Woodward UG8 governor – I don’t know for sure.) The maximum exciter 4-pole battery field current during motoring was probably of the order of 5 to 6 amps, with less than that required during DB. So there was something of a mismatch between the EMD field-loop and the Westinghouse exciter. From the above-mentioned schematic, WH evidently shunted the exciter battery field with a 1.2 ohm resistor during DB, and most of the field loop current would have gone via the resistor. Let’s say, at a guess, that with 15 amps in the field loop, about 13 amps went via the shunt resistor, and about 2 amps via the exciter battery field. That equates to just under 16 volts across the shunt resistor (and the battery field), which seems not unrealistic considering that up to four units could be in the field-loop.

The schematic also shows the humping control, which was quite simple, being a manually operated potentiometer switched in to directly control the exciter battery field. Quite possibly this is what was fitted to the PRR Lima transfer units. I’d say that it was of the non-MU variety. F-M later patented an MU humping control of the bridge-circuit, follower rheostat type. (US2908852 filed 1956 September 10). For its locomotives equipped with its own form of pneumatic throttle control, F-M had applied its high-power/low-power switching, with the low-power setting probably serving as a hump control. (That was covered by US patent 2434413 filed 1945 October 04.)

Returning to the WH case, it would certainly be understandable had field-loop been the default standard for the DB option with the XM-781 controller, simply because EMD locomotives were the dominant type. And it looks as if F-M offered only the field-loop DB on WH-equipped Train Masters. That could have been its own choice or it could have been because that was all that WH offered. On the other hand, it is not unreasonable that WH would also have offered potential-wire control, for those customers who wanted to mix-and-match their WH-equipped units with Alco-GE units. If so, then the WH approach to potential-wire would be of specific interest as it would likely have been the first case where this form of control was applied outside of the Amplidyne application.

  by Pneudyne
Looking again at the EMD DB changeover in the early 1960s, it would appear that it [EMD] adopted a governor-integral low-current load control rheostat with the GP35, whose static excitation system is described in a dedicated manual available at the Fallen Flags site; http://www.rr-fallenflags.org/manual/gp35-static.pdf" onclick="window.open(this.href);return false;. Essentially the load control rheostat provided one of the reference voltages for the static control system during motoring. Similarly the DB control potentiometer provided a reference voltage during DB. Absent a conventional high-current load control rheostat, field-loop DB had to be done by an “add-on” system including a motor-driven high-current rheostat, as described by Staff in “D-Day on the Western Pacific”.

The preceding GP30 had a somewhat similar excitation control system, except that as far as I know, it still used the EMD “traditional” conventional external load control rheostat, which in turn could be used for field-loop DB in the same way as had been established for the SD24, etc.

So it would appear that with the increasing sophistication of excitation control schemes, field-loop DB became something of a misfit, requiring an “add-on” approach, whereas potential-wire control happened to be a good match for integrating with the voltage-operated devices used in static systems. In that sense it appears that EMD’s changeover from field-loop to potential-wire was motivated primarily by its own needs, and not because potential-wire had become more popular with the growth of the GE installed fleet. Compatibility with other makes does not seem to have been a major driver, given that both Alco and GE locomotives were available with universal DB control. Of course, EMD was unlikely to invent a new system to replace its field-loop once the latter ran out of utility, and it made sense to use the alternative established system if it was a reasonable fit, which it was. Not only that, EMD had a decade’s worth of experience with potential-wire in the export market.

Provisionally at least then, we have deduced a plausible “why” for the EMD change, in addition to some insights into the “how”.