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Making sense of a freight locomotive data sheet

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Making sense of a freight locomotive data sheet
Posted by sniffkes on Wednesday, January 26, 2022 1:00 PM

I am looking at a freight locomotive data sheet:

https://www.thedieselshop.us/Data%20RP20BD.HTML

And I am seeing that something is wrong in the calculation of tractive effort and maximal power.

For the maximal power, if I multiply the continuous tractive effort with the indicated velocity, I get:

CTE * mph/375 = 3195 hp

which is much higher than the 2000 hp indicated. This only makes sense if I divide by the number of engines (3) such that I have 1065 and about 50% inefficient engines. 

1. Does this mean that the locomotive can generate potentially 3195 hp? If yes, when the engine is activated to get a train moving from rest, does it potentially reach 3195 up to a certain velocity?

2. The initial tractive effort seems greater than it should as well. In the data sheet we have a starting tractive effort of 88000 lb for 30% adhesion. If I calculate the normal force on the train for this adhesion I am getting

275000lb * 30% =  82500 lb

which means that the initial tractive effort makes the train slide. How do I make sense of that?

 

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Posted by timz on Thursday, January 27, 2022 12:19 PM

You're right -- the data sheet is plainly wrong. No use trying to make sense of it.

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Posted by Overmod on Thursday, January 27, 2022 1:16 PM

I don't really see anything objectively wrong with the datasheet (which claims to be derived from RailPower data and three Kalmbach references, so it could be objectively checked).  Reference to any 'actual horsepower' graph for a diesel-electric shows the same concerns; the dual-mode-lite study for Conrail goes into specific discussion of the relevant issues (with worked-out examples for an SD40-2)

The traction motors are obviously limited by the generator horsepower, which in turn is limited by the prime mover BHP.  This would be trivial to point out except I see the OP thinking differently.  If you look at the rectangular-hyperbolic portion of any diesel-electric horsepower curve you will see the practical implications of this.

At starting, the traction motors can nominally produce high torque, which is of course the thing measured by 'starting TE' in steam locomotives, and accounts for why some ratings of early EMD units in MU were up in the over 200K nominal TE range.  Of course this is an instantaneous rating, probably current-limited, and if applied for more than a short time will cause motor temperature rise beyond safe determined level -- this shows up with typical DC motors including GE 752s  if they are not aggressively cooled, and this has to be figured into the actual locomotive ability to sustain high torque up to some speed, usually in the 8 to 10mph range in my experience, which is called 'minimum continuous speed'

For the sake of completeness there will also be a point where back EMF and armature rotational speed on the 752s hit a practical limit, and the horsepower curve will be effectively truncated from the otherwise-already-falling hyperbola there.

The case of dual-mode 'horsepower' is interesting because an external power source might be able to keep the traction motors supplied at that 'nominal horsepower' and allow the unit to perform as if it were 3000hp plus over a wider range.  The Conrail study pointed out that having the trains built for that higher horsepower would often involve the need for switching the consist or adding more power for unelectrified service (see operations in the 'gap' between electric sections on the PCE) and the costs of that would far outweigh any nominal advantage for 'rating' the dual-mode differently in service except when it will operate only 'under wire'.

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Posted by timz on Thursday, January 27, 2022 3:41 PM

Overmod
I don't really see anything objectively wrong with the datasheet

Must be unobjectively wrong, then. 2000-hp locomotives don't produce 3200 rail horsepower, unless they're adding 2000 horsepower from those series VRLA batteries.

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Posted by Overmod on Thursday, January 27, 2022 4:01 PM

Maybe if I say it differently: those 4 752s have horsepower ratings, as do most designs of traction motor.  If you ran them on external electric power, as from a catenary or third rail, that's what you'd get.

If you supply them from a lower constant-horsepower source (which these gensets are) it should be obvious they won't make 'their' rated horsepower.  Even more obvious that they wouldn't make it if a smaller number of gensets were running or on the line.

The TMs retain their full torque capability regardless of the engine power; they just won't maintain it at higher rotational speed.

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Posted by oltmannd on Thursday, January 27, 2022 4:11 PM

timz

 

 
Overmod
I don't really see anything objectively wrong with the datasheet

 

Must be unobjectively wrong, then. 2000-hp locomotives don't produce 3200 rail horsepower, unless they're adding 2000 horsepower from those series VRLA batteries.

 

 

This makes sense to me.  Otherwise minimum continuous speed for a 2000 HP four axle will be around 11 mph.

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

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