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Why is DC power in a locomotive cab 74 volts?

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Posted by Erik_Mag on Wednesday, July 14, 2021 11:15 PM

Just for grins, I took a look at the DIGI-Key website and found two power supplies that would convert 74VDC to 13.7VDC (normal battery voltage with engine running. One is selling for $163 and can supply 22A at 13.7VDC (Delta B62SR13722A), the other selling for $235 supplies 36.5A at 13.7VDC (Delta B70P13736AC). Both would need a heat sink which would add a bit to the cost.

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Posted by BaltACD on Wednesday, July 14, 2021 8:57 PM

oltmannd
I always thought it was to creat maximum inconvenience to all of us trying to do anything at all in the cab with normal, commercial or consumer equipment!

Plug in a 110 VAC chart recorder?  Nope.  Charge a laptop?  Nope.   

We used to rent a Honda generator to power a chart recorder, etc, when doing testing on locomotives.  Only later did some decent sized invertors come along.

In the days when CB radios were the tools that crews used to had end to end communications on the territories I worked.  I would see 'cable' running from the battery boxes to the locomotive cabs - I suspect the locomotive battery was being tapped to provide 12v DC to power the CB radio in the cab.  I don't recall seeing what arrangements Conductors used to power their CB radios on the caboose.

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Posted by oltmannd on Wednesday, July 14, 2021 8:45 PM

I always thought it was to creat maximum inconvenience to all of us trying to do anything at all in the cab with normal, commercial or consumer equipment!

Plug in a 110 VAC chart recorder?  Nope.  Charge a laptop?  Nope.   

We used to rent a Honda generator to power a chart recorder, etc, when doing testing on locomotives.  Only later did some decent sized invertors come along.

-Don (Random stuff, mostly about trains - what else? http://blerfblog.blogspot.com/

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Posted by rdamon on Tuesday, July 13, 2021 1:04 PM

You also get the benefit of smaller transformers and capacitors to make DC from 400Hz

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Posted by blue streak 1 on Tuesday, July 13, 2021 12:44 PM

Abut the 400 Hz current.  The main advantage for aircract is that motor size is much smaller there for weight is less.  The motors can run much faster.  An example is the Lockheed L-188 Electra.  It had an airconditioner freon motor pump 25 HP  24,000 RPM ,that was about 18" in diameter and 24" tall.   Unfortunately transmisssion over long sdistances has proven 400 Hz not reliable over longer distances.  Will not work at all on aluminum wire..

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Posted by Erik_Mag on Thursday, July 8, 2021 9:38 PM

Assuming a per cell float potential of 2.3V (corresponds to 13.8V for a 6 cell lead acid battery), 32 cells will produce 73.6V. FWIW, a partially discarged lead acid cell will put out 2.1V, from where we get the 6.3V (3 cell) and 12.6V (6 cell) filament/heater voltages for vacuum tubes.

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Posted by Overmod on Thursday, July 8, 2021 7:49 PM

Simmons-Boardman, coincidentally in 1954, says the charging current for the 'double car-light-plant' size battery represents 32 cells x 2.32V per cell, which is where the "74 volt" number came from.

I believe some of the Copper River Baldwins had nominal 112V (56-cell) batteries (with charging current a bit north of 128V, cf. 2.32V charging voltage per cell) so there is likely a bit more to the story behind the choice of that number of cells -- perhaps a multiple of 8 that exceeded 110V OTS?  Apparently 56 cells was an option chosen enough that Exide and Gould provided figures for them in their contemporary literature...

Just to make it worse, both the Japanese and the British heavily used 48-cell batteries -- which involve nearly 112V at that 2.32V 'optimal' charge... the British call this '110V' charging, rounding down a volt and a fraction.

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Posted by SD70Dude on Thursday, July 8, 2021 6:34 PM

timz
SD70Dude
Weren't some early ALCOs 110VDC

A 1954 manual for 244-engine ALCos says 74 volts.

By early I meant really early, like this one.  My memory must have been off a bit, the video says 112V.  

https://m.youtube.com/watch?v=JHi9mKhZqfg

I wonder when ALCO switched?

Also, some smaller industrial units like GE 45-tonners did use 24V, and I believe our 80-tonner is 32V as it has four batteries.  

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Posted by timz on Thursday, July 8, 2021 5:36 PM

SD70Dude
Weren't some early ALCOs 110VDC?

A 1954 manual for 244-engine ALCos says 74 volts.

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Posted by rdamon on Thursday, July 8, 2021 2:52 PM

75V triggers a different union trade?

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Posted by Overmod on Thursday, July 8, 2021 1:59 PM

SD60MAC9500
Since we're talking about cab and batteries here's Progress Rail's newest addition to the battery field.

This is an interesting detail. Siemens was supposed to have a lithium-ion battery arrangement for the 'testicles' on the Charger locomotives around the time they were first provided to railroads... but the early examples were delivered with conventional battery chemistry. It would be interesting to see and compare when Siemens did get the higher-energy-density battery system into production, and how they packaged it.

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Posted by SD60MAC9500 on Thursday, July 8, 2021 1:36 PM
 

Since we're talking about cab and batteries here's Progress Rail newest addition to the battery field.

 
 
Rahhhhhhhhh!!!!
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Posted by Paul Milenkovic on Thursday, July 8, 2021 1:26 PM

Overmod

 

 
CSSHEGEWISCH
Wasn't aluminum cabling the reason the U50C's had such short service lives?

 

ONE reason.

 

The U50C and the C855 might have been used for a Monty Python skit... "Among our reasons are..." Smile

 

 

I believe I read this in a railfan magazine or newsletter, but the poor reliability of the ALCo C855 had one UP official quoted as describing those locomotives as "having laid an egg" and they were retired from service because the UP didn't have much patience for an unreliable locomotive disrupting operations.

That ALCo left the US market for mainline road-service locomotives also had to do with their partner GE coming out with their own locomotives.  But were the Century series problematic or was this specific to this locomotive model?

If GM "killed the electric car", what am I doing standing next to an EV-1, a half a block from the WSOR tracks?
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Posted by Overmod on Thursday, July 8, 2021 12:45 PM

CSSHEGEWISCH
Wasn't aluminum cabling the reason the U50C's had such short service lives?

ONE reason.

The U50C and the C855 might have been used for a Monty Python skit... "Among our reasons are..." Smile

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Posted by CSSHEGEWISCH on Thursday, July 8, 2021 10:03 AM

Wasn't aluminum cabling the reason the U50C's had such short service lives?

The daily commute is part of everyday life but I get two rides a day out of it. Paul
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Posted by Overmod on Thursday, July 8, 2021 6:09 AM

Erik_Mag
A 12 or 24 volt starting system would have required batteries with very large individual cells

Unlikely these would be any larger than the ones in existing locomotive battery elements; I think you'd get the necessary amperage via lots of parallel connections... with more heavy cable.  It might have been amusing to see where this would have gone during Alco's misbegotten but mercifully brief experimentation with aluminum cabling in the '60s...

Of course you'll note that heavy-separator 6-cell (12V) elements, like locomotive-grade truck batteries, could easily be strung to 72 nominal volts, with the float-charging voltage correspondingly higher.  My guess is that safety and internal-resistance considerations are behind what we saw evolve...

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Posted by Overmod on Thursday, July 8, 2021 5:55 AM

SD70Dude
Weren't some early ALCOs 110VDC?

You'd probably have better knowledge about that.

I do know that many passenger cars ran on 110V to optimize their air conditioning, I believe in part to make better use of OTS components.  I would not be surprised to see a similar argument advanced for 'new' diesel-electric locomotives... not for HVAC, of course, in those more rugged times Wink 

As a potentially amusing aside, there were apparently quite a few discussions starting around the end of WWII regarding the adaptation of aircraft-style power, which is 115V but 400Hz.  As you might imagine there were enormous stocks of war-surplus parts making this very cheap as well as efficient.  Periodically this  would get dusted off, usually after a few beers or sherries or whatever, and be jazzed around a bit before better common sense prevailed.  I suspect Forner Car Maintainer may recall similar 'optimization' discussions over the years as generations of bright young engineers came into the industry...

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Posted by SD70Dude on Wednesday, July 7, 2021 11:14 PM

Weren't some early ALCOs 110VDC?

Greetings from Alberta

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Posted by Overmod on Wednesday, July 7, 2021 10:50 PM

A somewhat less simple answer, though, is why 74VDC and not, say, the 120VDC used extensively in the Navy, or some other series/parallel multiple of 6 or 12 nominal volts (which come from cell chemistry in 3 or 6 cells respectively).  The reason I was taught is that it's around the 'float' voltage for 64V nominal batteries and is about the highest voltage that doesn't begin to pose a lethal shock hazard.

Note that 'traditional' power from steam-locomotive dynamos was in the 32V range, and that could easily be applied to two strings in parallel (which could then be connected in series to deliver the higher voltage...)  I don't know if that has direct relevance to the choice of 74V over 72.

In the event, the locomotive batteries I'm familiar with were composed of units of four, not three or six, cells, and the actual electrochemical voltage, as in car batteries, will be the appropriate multiple of cell voltage per string (traditionally nominal 64V, "8x8") This has led to some fun from time to time on RyPN when someone wants to build a charger or arrange a jump starter...

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Posted by Erik_Mag on Wednesday, July 7, 2021 10:30 PM

A perhaps too simple answer is the amount of power to turn over a locomotive engine versus a truck or car engine. A 16 cyl 567 engine has a displacement of >9,000 cu in, versus 600 to 900 for a truck engine versus 100 to 400 for a car engine.Getting sufficient cranking power would have involved a LOT of amps at 12 or 24 volts and going from 24 to 74 would substantially reduce the amount of copper in the cable for the starter motor.

Another possible reason: The traction generator equipped locomotives could use the generator as a starter motor, but would need more than 24 volts to get sufficient cranking speed.

Yet another consideration: A 12 or 24 volt starting system would have required batteries with very large individual cells, 74 volts allowed for a larger number of smaller cells.

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Why is DC power in a locomotive cab 74 volts?
Posted by aegrotatio on Wednesday, July 7, 2021 9:26 PM

Simple question: Why is DC power in a locomotive cab 74 volts?  Why not 12 or 24 volts?

 

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