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'[EE] volts Vs amps Vs generator efficiency'
2010\07\12@141605 by Robin D. Bussell

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Hi Folks,  

        When considering a generators powered by internal combustion
engines (probably diesel) are there any large efficiency differences in
terms of fuel consumed per KWh output if the output voltage is mains
voltage (240VAC in my case) or down in the 12VDC range?  The power
output required is in the region of 2KW and the power is going to be
used close to the generator so no great transmission losses to consider.

At first glance I suppose it ought to make little difference (assuming
the load doesn't mind what voltage it runs off ... that's another set of
problems not in scope here, for now assume simple resistive heating) but
I know nothing of the subtleties of generator design, does anyone here
know or have a good reference I can check?



Cheers,

      Robin.



2010\07\12@143806 by Isaac Marino Bavaresco

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Em 12/7/2010 15:16, Robin D. Bussell escreveu:
> Hi Folks,  
>
>          When considering a generators powered by internal combustion
> engines (probably diesel) are there any large efficiency differences in
> terms of fuel consumed per KWh output if the output voltage is mains
> voltage (240VAC in my case) or down in the 12VDC range?  The power
> output required is in the region of 2KW and the power is going to be
> used close to the generator so no great transmission losses to consider.
>
> At first glance I suppose it ought to make little difference (assuming
> the load doesn't mind what voltage it runs off ... that's another set of
> problems not in scope here, for now assume simple resistive heating) but
> I know nothing of the subtleties of generator design, does anyone here
> know or have a good reference I can check?


With 12VDC you will have currents in the range of 170A. The generator
will be very expensive, with thick wires and large diodes.
And if your generator is basically a 3-phase alternator, expect some ripple.
Lots of heating in the diodes also.

I don't know if a dynamo can handle this type of current. Dynamos are
very old stuff anyway. Probably the brushes will wear very quickly.


Isaac
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2010\07\12@144511 by Olin Lathrop

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Robin D. Bussell wrote:
>          When considering a generators powered by internal combustion
> engines (probably diesel) are there any large efficiency differences
> in terms of fuel consumed per KWh output if the output voltage is
> mains voltage (240VAC in my case) or down in the 12VDC range?

Generators can be wound over a fairly wide range of voltage versus current
for a given power rating.  However, you're probably pushing the limit with
2KW and 12V since that requires 170 amps.  Now we're getting into copper
conductors with some serious diameter, which will force other tradeoffs in
the design that will likely compromise efficiency.  Look around at
commercial 2KW generators and see what voltage and current combinations they
tend to come in.  I'm guessing 2KW at 12V is going to be rare, probably with
good reasons.


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2010\07\12@162710 by Steve Smith

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Just a simple matter of rectification as I assume you mean 12v DC.

Therefore 2000 / 12 v = 166A X 0.8v(x2) = or 266w loss in the rectification
diodes.

This compares directly to nothing if you use 240v at 8.3 A AC... you will
save 13% of the lost energy that it takes to rectify the low voltage

Enjoy

Steve

{Original Message removed}

2010\07\12@170023 by Isaac Marino Bavaresco

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Em 12/7/2010 17:26, Steve Smith escreveu:
> Just a simple matter of rectification as I assume you mean 12v DC.
>
> Therefore 2000 / 12 v = 166A X 0.8v(x2) = or 266w loss in the rectification
> diodes.
>
> This compares directly to nothing if you use 240v at 8.3 A AC... you will
> save 13% of the lost energy that it takes to rectify the low voltage
>
> Enjoy
>
> Steve


If his alternator is 3-phase he will need 6 diodes and the current will
pass through 2 diodes in series, so double the loss.

He could use synchronous rectification to reduce the loss, but it would
require a lot of very big MOSFETS.

And don't forget the ripple (around 13% for a 3-phase system). I think
that filtering has little use at these current levels.


Isaac

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2010\07\12@175653 by Oli Glaser

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--------------------------------------------------
From: "Robin D. Bussell" <spam_OUTRobinBTakeThisOuTspamexcelerate.info>
Sent: Monday, July 12, 2010 7:16 PM
To: "Microcontroller discussion list - Public." <.....piclistKILLspamspam@spam@mit.edu>
Subject: [EE] volts Vs amps Vs generator efficiency

{Quote hidden}

As mentioned, I don't think finding a 2kw 12V generator will be too easy so
I suppose it makes your choice a bit easier - all the ones I've seen are
something like 110V/230V. Apart from these very expensive Marine generators
(>£7500!):

http://www.onboardenergy.com/acatalog/info_AGT4000-12.html

However, some "normal" ones I've seen do come with a 12V supply too but at
a lower current/wattage (like 8.3A). Here are a couple of examples (second
one is petrol, but looks like quite a nice little generator - only 28.5kg!):

cgi.ebay.co.uk/BDE-Silent-Red-Diesel-Generator-5KVA-230v-110v-12V-_W0QQitemZ230493914706QQcmdZViewItem?rvr_id=&rvr_id=&cguid=163327b91270a0aad5321e52fdc039e5
(you will have to stick this link back together again)

http://www.genpoweruk.co.uk/product.php?productid=16186&cat=0&page=1



2010\07\21@063907 by Robin D. Bussell

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Thanks for the info, Folks. It has helped convince the guy specifying 2KW that he really needs to look at the power budget again. Turns out he was adding in a rather large "safety margin" based on dead fish waving and "you can't have too much genny capacity":)

Cheers,
   Robin.


{Original Message removed}

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