Some years back I did look for “hard” information as to the origins of the 74/64-volt standard for North American diesel locomotives. I did not find very much, but this item from the Simmons-Boardman 1954 publication, “Diesel-Electrics..How to Keep ’Em Rolling” (LCC 54-10911) was useful:
“Early diesel-electric locomotives had 16-cell batteries, like railroad car lighting systems. As the diesel engines got bigger, this was not enough voltage to give the heavy cranking currents required. So the battery was doubled, giving the present 32-cell, 64-volt system”.
That book essentially addressed Alco-GE electrical practice of the time, and was written by GE staffers. Given that GE was “in at the beginning”, as it were, then the above commentary reasonably could be taken as being authentic.
Of course, that “answer” simply moves the question back to why was 32 volts chosen for railroad car lighting. That seems to be less clear. 32 volts was also widely used for farm powerplants and industrial safety applications, and also for some road vehicle applications, but these may all have postdated the railroad case. The safety aspect may have been in mind when railroad car lighting was first visualized.
As already outpointed, some care is needed in interpreting nominal voltage numbers, as practice is not consistent, either amongst industries or even within them. Sometimes the battery voltage is quoted, based upon 2 volts per lead-acid cell. At other times the generator charging voltage is quoted, and the basis for this is not always consistent.
To quote again from the above-mentioned book:
“To charge this [64-volt] battery you have to set the voltage regulator for 32 x 2.32 volts, or 74 volts. This may vary some according to the weather. In cold weather, you may have to set it up a bit. Since the locomotive charging system works at all engine speeds, - even idling – you never have to change the setting much. You seldom will go below 72 volts and never above 76 volts.”
So within the railroad industry it had been common practice to refer to “32-volt” systems (nominal battery voltage) for car lighting and small locomotives, and to “74-volt” systems (nominal generator voltage) for larger locomotives. That said, the auxiliary electrical system of the PRR E2b AC electric locomotive was described as being fed by a 37.5-volt DC generator; one assumes that the battery was 32 volts.
Some further information on generator voltages is available from the battery-maker data sheets included in the GE 70-tonner maintenance manual, attached.
Exide stated that the generator voltages should be 74 volts for 32-cell batteries and 130 volts for 56-cell batteries. The corresponding numbers from Gould were 74 volts and 129 volts.
The 56-cell case would, I think, have referred to the “125-volt” auxiliary electrical systems used by Baldwin and Fairbanks Morse on some of their diesel-electric locomotives of the late 1940s. Given that the generator voltage was 129 or 130 volts, and the nominal battery voltage was 56 x 2 = 112 volts, the “125-volt” number may be seen as some kind of weighted average of the two. That is more evidence of inconsistency in the way that locomotive auxiliary electrical systems are specified.
Worldwide, practice has varied somewhat. In the UK, British Rail in 1955 adopted 110 volts as the standard for its line-service diesel locomotives, although other voltages were used for some switchers and EMUs and DMUs. (Car lighting in the UK and other British-influenced countries was usually 24 volts; 12- lead-acid cells.) This 110-volt number, amongst others, had previously been used by all of the major UK electric equipment makers. I suspect that it was adopted in part because 110 volts DC was an established marine and industrial standard, for which switchgear designs, etc., were readily available. It was based upon the use of a 48-cell lead-acid battery, which implies a charging voltage of 2.29 volts per cell. 110 volts was also the norm for British export locomotives from the early 1950s.
The French mostly used what were described as “72-volt” systems on both domestic and export diesel-electric and electric locomotives, and usually with alkaline batteries. Generator voltages and battery cell counts seemed to have varied a little, but one example was 78 volts and 54 (alkaline) cells. In practice this 72-volt standard was probably not materially different to the American 74-volt standard.
Many JNR (Japan) locomotives were described as having “100-volt” auxiliary electrical systems (although some early AC electric locomotives were “52-volts”.) On the face of it, this looks like yet another standard, but in fact it was not. A description for the 1950s DD50 and DF50 diesel-electric locomotives shows that they had 48-cell lead-acid batteries, and that for the DF50 at least, the auxiliary generator voltage was kept in the range 95 to 110 volts. So the “100-volt” number was again a weighted average of some kind. Possibly it was chosen because Japanese domestic electricity supplies are 100 volts. Thus Japanese export locomotives that went to railway systems who specified or accepted 110-volt auxiliary electrical systems did not require anything different to what was used for domestic practice.
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