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Hotel Load of train: Why separate generator & not locomotive

  1. Mar 1, 2015 #1

    rollingstein

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    While on a long distance train in India I noticed that there were two huge cars devoted to power generation, one at each end of the train (approx. an 18 car consist). A pic. below. Apparently diesel powered generators. Each car had two exhaust pipes on top (one at each end) so presumably these are four generators per train.

    I'm wondering why they wouldn't use the locomotive power itself for this load. What's the typical horsepower of a big railway loco versus what's the size of the lighting / HVAC loads. Is stepping down the voltage a problem? What's typical railway practice regarding this?

    It's like Hoover dam running a thermal power turbine for their offices instead of a tapping from a step down transformer. Just curious.

    https://c2.staticflickr.com/4/3055/5827726839_66d9e74c49_b.jpg
     
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  3. Mar 1, 2015 #2

    Drakkith

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    I'd guess that the generators are more efficient than the engine at producing electrical power, or that there is some limitation on the engine's ability to produce electrical power.
    Based on some quick reading, it looks like attaching generator cars may help reduce the load on the engine since it no longer has to provide power for the rest of the train, which frees up horsepower for starting, getting up hills, and makes the engine more efficient, though all that is just a guess.

    Some links:

    http://en.wikipedia.org/wiki/Head-end_power
    http://trn.trains.com/railroads/abcs-of-railroading/2006/05/head-end-power
     
  4. Mar 1, 2015 #3

    rollingstein

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    Interesting. I'd have thought adding a little bit of hotel load to an already ginormous engine makes more sense than having separate small generation.

    The additional initial cost, the weight, the higher effeciency of one large engine than multiple small generators et cetra.
     
  5. Mar 1, 2015 #4

    rollingstein

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    I find that hard to believe because it's essentially like saying that a 4000 hP diesel-to-electric generator is less efficient that a (say) 200 hP diesel-to-electric generator.
     
  6. Mar 1, 2015 #5

    Drakkith

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    Per the second link above:

    Amtrak's locomotives can be set to produce traction power only, head-end power only, or both - the speed of the prime mover varies with each setting. "An electronic circuit [reduces] excitation of the main generator for propulsion to the extent that head-end power is needed to maintain that rpm," explained John Wood, who was manager at Amtrak's 14th Street shop in Chicago. He recalls a bitterly cold January day when the 3-trains-in-1 California Zephyr, often 16 cars long, drew 700 kw when it was first plugged in before departure. (The 3000 h.p. diesel in an F40 - the long-distance locomotive then in use - turned at 893 rpm in the traction/HEP mode.) "We almost didn't release the train," he recalls, "but gradually the heaters began shutting off and the load dropped to around 550 kw."

    It can be. You can set up an AC generator (perhaps a DC too?) so that it runs at a very efficient RPM for a given load. It doesn't have to worry about anything else. But the train engine also has to pull the train, which means that it can't simply be set at the most efficient RPM for a given electrical load since that may mean that it's running too fast or too slow to be efficient at pulling the train.

    You can see from the above quote that the electrical power required for a train can sap a significant amount of power from the engine. Also in the same link that quote came from is a part where the writer talks about the train crew having to cut all power to the cars just to be able to get up a steep incline.
     
  7. Mar 1, 2015 #6

    Baluncore

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    Firstly, if the locomotive is generating DC for DC traction motors, then control of field windings can be used to optimise power transfer. The extraction of HVAC from such a variable system would be quite involved.

    Secondly, there are few trains that require significant HVAC at a stable voltage and regular frequency. It is more economic to have a few generator cars that can stay with the special train than to modify every locomotive that might pull the train.
     
  8. Mar 1, 2015 #7

    rollingstein

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    Agreed, but isn't that one reason why no modern diesel locomotive is directly linked to the wheels by a transmission? What you already have,( even in a pure freight loco with no HVAC loads) is a massive electric generation plant on wheels that is driving electric motors connected to wheelsets.

    Ergo, you are already running a generator at its Best Efficiency point (or close to) no matter how fast or slow the train is moving. If so, you have a very efficient high power generator Maybe I understand it wrong.
     
  9. Mar 1, 2015 #8

    rollingstein

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    Agreed that the HVAC power is significant. From the figures you cited perhaps as much as 20% of the total loco power.

    But the fact is you must provide this power from somewhere. The question is why from elsewhere rather than just go for a beefier loco. Not sure if that makes sense.

    Some reasons I can think of (a) We are just stuck with legacy locos that are underpowered (b) Having separate power cars adds backup in case the loco fails the cars don't have to stay unheated (c) If there are long periods where the train must stop then it might be more efficient to run a small 200 hP generator than a large 4000 hP loco.
     
  10. Mar 1, 2015 #9

    rollingstein

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    You mean if the loco does a mixed freight + passenger duty? All PAX trains must need a large HVAC load I suppose though the exact amount might vary.

    Is it typical to use the same loco for both freight & PAX services?
     
  11. Mar 1, 2015 #10

    jim hardy

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    Locomotives nowadays generate AC power but at varying frequency. It's converted electronically to whatever the traction motors need.

    Think about the practical side -
    It'd sure be simpler to have a smaller fixed frequency generator somewhere in the line of passenger cars for house load.
    That way any old locomotive can be used to move the passenger train, even a steam one with only a DC lighting dynamo.

    this old locomotive is being restored and will likely see passenger service for nostalgia tours. If you're ever around Cheyenne Wyoming visit the Union Pacific roundhouse...

    e68a68119990aede6aa8f2606f2062db_L.jpg
    http://www.railwayage.com/index.php...urn-to-steam-for-big-boy-4014.html?channel=35
     
    Last edited by a moderator: May 7, 2017
  12. Mar 1, 2015 #11

    Drakkith

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    Huh. I didn't realize most diesel locomotives were diesel-electric hybrids.

    A beefier locomotive is more expensive to build/purchase and more costly to run. It seems to me that a separate generator car is much more flexible and cost efficient than using the engine itself.

    It's running at the best efficiency for providing the type of power needed by the traction motors, not by the rest of the train. For example, in a modern diesel-electric locomotive, as the speed of the train increases, the engine-generator moves from initially producing low-voltage, high-current power at low speeds to high-voltage, low-current power at high speeds. This change in voltage and current means that you need a method of transforming the power produced by the engine-generator into power usable by the cars since their current and voltage use does not change in this manner. Transforming the power inevitably incurs losses.
     
  13. Mar 1, 2015 #12

    Baluncore

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    The original DC power control was based on series or parallel connection of motors, known as “group up” or “group down”. When combined with control of the field windings, a very effective electrical equivalent to a continuously variable gearbox was implemented.
    The technology of ship propulsion and train traction is always needing to change. But railway systems do not replace expensive plant and equipment, such as million dollar locomotives, until the service life is reached, maybe 30 years. Purchases are political decisions, made by conservative managers, on the advice of accountants.

    The alternator sets are built into the baggage cars. For a long train, every joule available for traction is valuable, unfortunately long trains require the most traction and the most HVAC. If the length of trains was always the same a more integrated solution might be possible. The present system is flexible and it works.
     
  14. Mar 1, 2015 #13

    jim hardy

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    DC locomotives indeed start with the traction motors connected in series because at standstill they make no counter-emf so appear basically a dead short to the engine driven generator. At speed of a slow walk big relays re-arrange them from series to parallel connection , you hear the engine unload for a split second as the contactors clatter.
    The predominant instrument on the dashboard was a big kiloamp-meter.
    I remember locomotives with no speedometer. It was impressive if quaint to watch the engineer and fireman pull out their pocket watches and count telegraph poles, announcing speed together on i think twentieth pole....


    Big copper commutators and brushes are expensive to fabricate and they require frequent maintenance. When power electronics became equal to the task it was a no-brainer.
     
  15. Mar 1, 2015 #14

    rollingstein

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    I could be wrong. But I think most, relatively large, recent diesel locos are indeed hybrids. Maybe someone else can tell us.

    In fact, even if not for efficiency reasons, the mere task of fabricating a mechanical transmission that was up to the task of transmitting those huge torques would be pretty challenging. Perhaps enough to kill the idea. (As an aside, are there large, say 5000 hP+, mechanical geared transmissions in other applications?)

    I think the only feasible options that remain are diesel-electric & diesel-hydraulic (but that too doesn't seem very popular)
     
    Last edited: Mar 1, 2015
  16. Mar 1, 2015 #15

    jim hardy

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    Diesel electrics indeed are predominant.


    I took the engine room tour on Alaska Ferry's "M V Columbia". It has 6,000 hp diesels mechanically geared to its two propeller shafts.
    It was built circa 1974.
    columbia3.jpg
     
  17. Mar 1, 2015 #16

    rollingstein

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    Fascinating. How big were those darn gear wheels anyways. Took any pics of its innards? Must have been a sight.

    Wonder why they made that design decision. Any idea? I though in marine applications too diesel electric was fairly common.

    On second thought, do propellers need as much gearing as a locomotive? Is the varying resistance offered by water with propeller rpm more forgiving in terms of torque-speed characteristics? Are there many speed-gears or just a reversing gear?
     
  18. Mar 2, 2015 #17

    Baluncore

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    Hydraulics give the most compact and maximum power to weight ratio. Pumps and motors can be variable displacement, so the matching of diesel engine to track speed would be good. But the fundamental problem is that hydraulic devices are really only about 85% efficient, (Apparently viscosity has something to do with it, thick fluid gets hotter, thin fluid gets through the seals and requires precision clearances). 15% of the total power is an awful lot of heat to remove from the hydraulic fluid, but it might be possible. Unfortunately hydraulics gets a little more difficult than that. Both the pump and the motor are 85% efficient. The product of the combination gives 72% efficiency, which requires the removal of 28% of the total generated energy as heat. I think that explains why hydraulic transmissions are rarely employed. Using that 28% to heat the train in winter could work, but in summer it would be a really expensive problem.
     
  19. Mar 2, 2015 #18

    jim hardy

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    Didnt get pictures, sad to say. Light wasn't the best and it's a crowded equipment room where one would need professional wide angle lenses.
    The reduction gear is in a closed box about size of a medium moving van.
    It is a sight. As you know you feel more than see big machinery in operation.

    http://gallery.pasty.com/images/image.gif?id=1791088179625 http://gallery.pasty.com/images/image.gif?id=1791088179625 I did find this youtube time-lapse of a recent engine replacement on Columbia.
    www.youtube.com/watch?v=y7BAQ4NPKB8


    I think it was the time. Columbia was built early 1970's when big electric drives were in infancy.
    As you see, 6000 hp is a very modest electric drive nowadays.
    http://www.gepowerconversion.com/product-solutions/medium-voltage-drives/mv7000


    Columbia was just forward-reverse gear. I think the engines ran at 650 RPM propellers around a third of that- but that's just memory so i dont trust it very much.

    Yes a propeller is easy to turn at low RPM whereas to move a train you're overcoming immense inertia.
    So a marine engine starts against low torque(after overcoming relatively small inertia of the rotating parts) ,
    but a locomotive is pulling against a near immovable object.
    Once inertia is overcome the locomotive's task is a lot easier just friction and hills.
    I guess my old railroad days made me somewhat obsessive about inertia and momentum...... if you've ever watched from a locomotive cab as people in a car try to beat the train at a crossing, well, the feeling of abject helplessness is profound .

    see "Propeller Law" http://atljsoft.com/html_help/Propeller Law.htm
     
    Last edited by a moderator: Apr 18, 2017
  20. Mar 8, 2016 #19
    Here is a low tech actual use of the theoretical question in India. When travel time needs to be saved in an elite train, a secnd locomotive is being used to push the train removing the power cars.
    https://in.news.yahoo.com/cut-travel-time-rajdhanis-shatabdis-010200712.html [Broken]
    The second engine besides handling the hotel load, mainly saves time while accelerating or decelerating. This can happen frequently due to technical speed restrictions and short stoppages. Besides two passenger carriages can be attached to earn more revenue also.
     
    Last edited by a moderator: May 7, 2017
  21. Mar 8, 2016 #20

    rbelli1

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    The only locomotive I have ever worked on was full hybrid. It had diesel, 600VDC, or HV catenary power input. Everything ran off of the centenary power bus. Hotel power was inverter based. One little software bug and everything went dark. The hotel inverter was very particular about load changes.

    I was working on the air conditioners on the passenger cars so unfortunately never got to tour the main engine room.

    BoB
     
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