Typewriters wrote: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.
Interesting here is what the above operating instruction implies about unit conditioning in respect of dynamic braking controls.
For trailing Alco units, the unit selector switch was in the #1 position regardless of whether the lead unit was providing potential-wire or field-loop dynamic brake control. Thus such units were evidently able to accept either potential-wire or field-loop incoming DB signals, simply on the basis of whichever happened to arrive. Of course, field-loop signals would arrive only if the field-loop jumpers had been connected.
For a leading Alco unit, one would assume that the #1 position of the selector switch deactivated the field loop. On the other hand, the #2, #3 and #4 positions would have activated the field loop with the appropriate amount of series resistance, and I imagine would have deactivated the potential control – to avoid sending duplicate control signals - although local (to that unit) DB control was still by potential.
Another reasonable inference is that Alco units equipped with this kind of DB control could receive and act upon field-loop signals as well as provide field-loop commands. From item 2b above, one would be counting all units in a consist, not just the non-Alco ones.
Another “mixed-DB case” might have been the members of the UP GTEL fleet that were retrofitted to MU with trailing diesel units. As far as I know UP started the retrofit work in 1958, and in the early days at least, EMD GP-9 diesel units were used in conjunction with the GTELs. If this MU capability included DB control, then it surely would have been of the field-loop type because that is what the GP-9 was fitted with. Again as far as I know, the first batch of 4500 hp GTELs had Amplidyne control; certainly the equipment layout diagram showed that they had Amplidyne exciters. The second batch (“Veranda” type) were shown as having auxiliary alternators, so would seem to have had an early version of GE’s static control. Anyway, most likely both batches had GE’s potential-wire DB control. In the 8500 hp GTELs, DB control was evidently separate to motoring excitation control and was quite simple, thus: “In the dynamic braking control, excitation for the 12 traction motor fields connected in series is furnished by the diesel-generator set. Field no. 1 on this generator is controlled from the engineman's throttle by means of a rheostat and battery power. When the voltage across either the braking resistor or the motor armatures exceeds the motor field voltage, the generator differential field no. 2 is energised through the rectifier. This serves to maintain a relatively constant armature current in the traction motors regardless of motor speed.”
The DB control rheostat would likely have been a higher current device than the potentiometer normally associated with the potential-wire system. And the control might have been characterized as being more like a single-unit field-loop system. I’d guess though that the field-loop for trailing diesel units – if such was fitted – might have been separate from the internal “loop”.
Also interesting from a DB perspective was the SP diesel-hydraulic fleet. The photographic evidence shows that the K-M and Alco DH units were equipped with field-loop jumper receptacles, and that they operated in MU with the SP F7 fleet amongst others, in both leading and trailing positions. That they were equipped with field-loop DB control is a reasonable deduction. In the case of the K-M hood units and the Alcos, the field-loop equipment was likely installed as built. The K-M cab units originally had all-pneumatic motoring and braking controls, and were retrofitted with compatible MU controls by SP. As well as providing field-loop control signals for trailing diesel-electric units, the SP DH fleet must also have been capable of having their hydrodynamic braking controlled from the field-loop system, which would have been an interesting exercise. Unknown is whether the SP DH fleet also had potential-wire DB control. If not, then the Alco DH643 would have been very unusual as an Alco locomotive fitted only with field-loop DB control. Also, by 1964, US domestic locomotives from any builder were either potential-wire only of “universal” with potential-wire and field-loop, so field-loop alone would have been unusual anyway.
Typewriters wrote:It's an interesting topic.
Indeed. And it would appear that much of the detail has not surfaced in the railfan world….