Making a high efficency 3 Kw DC Generator

Discussion in 'DIY Marinizing' started by kistinie, Mar 8, 2009.

  1. Jimbo1490
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    Jimbo1490 Senior Member

    This has been another constant with these 'electric transmission' threads; The poster comes to the forum with a preconceived belief that electric transmission is universally better than conventional transmissions. When more learned people, many with engineering experience or a least good powers of reason and analysis chime in and show him it just ain't so, the poster becomes frustrated and leaves in a huff :(

    So what should everybody do; lie to him and pat him on the back and say "Attaboy, go for it!"? In the end when he finds out how much it will cost, he is going to be dissuaded, unless hes has more dollars than sense. And in that case some nice marine outfitter will have sold a nice diesel genset to a fool. :D

    Jimbo
     
  2. apex1

    apex1 Guest

    dammn.. we missed that.
     
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  3. mydauphin
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    mydauphin Senior Member

    Stones can fly if you throw them....
    Anyway, I still like discussing ideas with dreamers around here sometimes. I get even crazier ideas to think about.

    Case in point.
    I have an old diesel 400amp welder- wonder if this giant alternator could be used generate ac power via a large Inverter. May or may not be efficient that is the question.
     
  4. Jimbo1490
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    Jimbo1490 Senior Member

    There are plenty of very efficient giant alternators that can be had at scrap prices that will require no more tinkering than your welder. We call them induction motors. Save the welder for welding; working welders are virtually always worth more than scrap.

    Jimbo
     
  5. CDK
    Joined: Aug 2007
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    CDK retired engineer

    That is not correct, Jimbo.
    An induction motor is no alternator. It can be used to generate electricity only when connected to another source, like the power grid. To build a true alternator you have to exchange the rotor.
    And they are not "very efficient". Nothing old or giant can be efficient because at the time nobody cared about it.
     
  6. mydauphin
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    mydauphin Senior Member

    How much horsepower would a 200 amp alternator like on my truck take to run? 20hp?
     
  7. kroberts
    Joined: Mar 2009
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    kroberts Senior Member

    CDK,

    The "old" part I can agree with, to a point. However the "big" part I have to take exception. There are very efficient large alternators and motors out there. Induction motors and other types like PMAC can be equally efficient, but you are right in that an induction motor won't make an alternator all by itself.

    mydauphin,

    Find out what voltage it delivers when it is welding at 200A. It will have a peak voltage when not welding, that doesn't really count. Have somebody with a voltage meter look at it and lay down a big fat bead, and see what comes out. Measure at the generator output, not at the welder output.

    The power being delivered in watts is the voltage times the amperage. Figure around 80% efficient for the generator, so you divide by 0.8 and that's the power to expect to need at the shaft. You should get a bigger number than the one you calculated before taking efficiency into account.

    Oh yeah, that gives you watts. 1000 W = 1 kW. 3 kW = 4 hp, or 1 hp = 750 W.
     
  8. Jimbo1490
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    Jimbo1490 Senior Member

    Connect to the power grid??? all you need is an exciter circuit to get it going! This is the 'tinkering' I was referring to. If he's driving with an engine, he'll have an alternator and battery available to make a short burst of exciter juice; that's not a problem. There are a number of ways to get this done.

    I don't know where the idea came up that I was referring to "old giant" (boat anchor) type obsolete motors. Any NEMA framed motor (older 'giant' motors WILL NOT have a NEMA frame #) will be reasonably efficient as an alternator. If you find one with a rolled steel or cast aluminum case (rather than cast iron), it will be reasonably light as well.

    If you go this route, select a motor with the lowest voltage and highest RPM rating you can find, eg a 230 volt 3500 rpm, to keep from running the motor (alternator) out of it's efficient envelope. The big problem with these conversions is high internal resistance (which you mitigate somewhat by selecting the lowest voltage available) and limited RPM potential (which you mitigate by selecting the nighest RPM rating available)

    You might be surprised at how incredibly cheap you can buy used LATE MODEL three phase motors like this. I recently bought a 15HP motor built in the early 90's for $60. There is (was) a pair of 80's vintage 150HP motors nearby for $100 each. I have seen an entire shipment (24 motors) of NEW 5HP 3phase motors go to the scrap yard as they were 'surplus' and could not find a buyer.

    But even with all the above being true, there are now very good purpose-built inductive alternators available all over the place for very reasonable prices (they're made in China, after all :D) so unless you are needing the absolute rock-bottom, el-cheapo final cost, I don't know why you would do ANY of this, including wreck a good working welder, which you could sell, the proceeds from which sale purchasing the Chinese alternator.

    Jimbo
     
  9. mudman
    Joined: Mar 2007
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    mudman Junior Member

    I don't know much about sailboats, but I'd think that the best way to recharge the battery bank is to harness the power of the wind and use wind turbines. Throw in some solar panels too. Wouldn't you be under the power of the wind anyway?

    I'd think that the genset would of better use to run appliances on such a vessel.

    Heres an idea, lets use a gasoline powered air compressor to run a pnumatic air motor for the drive system. Who wants to try? Anybody? Nobody? How about a generator hooked to a heating element that runs a steam engine? I could go on, but the ideas are silly. People do things a certain way for a reason. That does not mean that we shouldn't try new things, but we need to understand the pros and cons of each idea. There is no magic engine. The belief of the 200 mpg carb is a legend. The engine that runs on water, also fantasy. Flux capacitor......not real.

    A genset defeats this guys purpose. Yeah, you can build a genset, and yeah it will work. Fact is that a gas/diesel drivetrain is more efficient than a genset/inverter/battery bank/electric motor setup.

    Mydauphin
    Amps X Volts = Watts and 750 Watts = 1 hp
    so
    200 Amps X 12 Volts = 2400 Watts
    and 2400 Watts / 750 Watts = 3.2 HP

    I'd get 5 hp cause I think more is better.
     
  10. Jimbo1490
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    Jimbo1490 Senior Member

    If the need is to couple 4500 HP to a propeller, things start to change. If the engine is not reversible, then likely a reversing transmission will be needed. A reduction may also be in order. This will inevitably involve some very big gears turning in thick oil. The losses from a mechanical transmission like this are not insignificant. When the scale is large, then the electric option starts to make sense as you can save on work done churning viscous oil.

    Jimbo
     
  11. apex1

    apex1 Guest

    Jahh... if it was so easy. But it´s not. Even 4500hp are much much more efficient to be installed the "conventional" way, rather than Diesel / El. (talking Yacht business).
    But any way we are far off topic.... our French dreamer thought about charging his battery by using the same propeller that would drive the boat. But he did not like the fact, that his lightweight Cat (2,5 ton), would not sail properly with such a prop down. I estimate that such setup would act like a small outboard in reverse gear on such a light boat, a clever idea.
    Fortunately, at the end that brilliant "engineer" tought me the basic physics of sailing. Now I know that a fast, light Multihull can drag a prop Genny much easier than a heavy displacement yacht. I´m so happy to know that...
    Just read the rest of his crap, you´ll understand.http://www.boatdesign.net/forums/hybrid/diesel-electric-propulsion-sailboats-9310-16.html
    and some other threads

    Regards
    Richard
     
  12. M-Sasha

    M-Sasha Guest

    I went through almost all the posts of the thread starter Kistinie and must say, that ***** flushed you pretty nice with his idiotic ideas. I am shure he went back to his tent in the pampa with a sardonic grin.
     
  13. kistinie
    Joined: Aug 2007
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    kistinie Hybrid corsair

    Lost :) !
    The idiot was sailing in England on a nice race sailing boat, having fun in pubs with friends and doing business about electric motion for boat.
    Cheers !

    And about 3 or 4 Kw DC light geny, what's up ?
     
  14. M-Sasha

    M-Sasha Guest

    I buy that!:D
     

  15. Hägar

    Hägar Previous Member

    Touche...........................

    Hä, hä
    Hägar
     
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