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Home > Archive > Alternative Power sources > March 2007 > Generator sizing
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| suniltdpa@gmail.com 2007-02-23, 3:25 am |
| Hi,
I need to find a relaion between load power(non linear load) and the
rating of the load(emergency generator). I saw in different cases they
will consider it as 3 times the load rating. Anybody help me for
deriving a mathematical relation.
Regards,
Sunil
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| Vaughn Simon 2007-02-23, 9:25 am |
|
<suniltdpa@gmail.com> wrote in message
news:1172220645.037720.284900@a75g2000cwd.googlegroups.com...
> Hi,
> I need to find a relaion between load power(non linear load) and the
> rating of the load(emergency generator). I saw in different cases they
> will consider it as 3 times the load rating. Anybody help me for
> deriving a mathematical relation.
Without bothering to look it up, I believe Real Power (in watts) = VA X
Power Factor.
That 3X thing is just advertising. For example: It is normal for
manufacturers of cheap UPS units to assume a .5 Power Factor. That way that can
advertise a 500-watt UPS as a 1000 VA UPS.
Your next question should be "How do I know what my power factor is?"
Vaughn
> Regards,
> Sunil
>
| |
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| On Feb 23, 1:50 am, sunilt...@gmail.com wrote:
> Hi,
> I need to find a relaion between load power(non linear load) and the
> rating of the load(emergencygenerator). I saw in different cases they
> will consider it as 3 times the load rating. Anybody help me for
> deriving a mathematical relation.
> Regards,
> Sunil
My knee jerk would be to ignore power ratings and figure everything
based on current. In that case, I think you would merely need to match
the current output of your generator up with the current requirements
of the load and then add something for start-up current.
There's a great little gadget available that measures power, current
and PF and VA, etc., that only cost $30. Here's an example website:
http://www.thinkgeek.com/gadgets/electronic/7657/
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| Ecnerwal 2007-02-23, 1:25 pm |
| In article <1172220645.037720.284900@a75g2000cwd.googlegroups.com>,
suniltdpa@gmail.com wrote:
> Hi,
> I need to find a relaion between load power(non linear load) and the
> rating of the load(emergency generator). I saw in different cases they
> will consider it as 3 times the load rating. Anybody help me for
> deriving a mathematical relation.
A mathematical relation any better than "3X" would need data better than
"non linear load".
Some motors start hard, and may draw as much as (IIRC) 10X rated load
for a brief period. Some are not so bad. Without trying it, you
generally don't know, as the nameplate rating only tells you what the
running current (and perhaps the locked rotor current, on a better than
average nameplate) is.
Likewise, some generators are more tolerant of a brief startup overload
than others. This also does not show up on the generator nameplate
rating. Thus, the data for both parts of the system is somewhat lacking
in terms of "precise mathematical relations" and you end up falling back
on fudge-factor rule-of-thumb numbers like "3X".
So your best bet is often just to try it and see if it works, since you
can have a system which does not work by the numbers that works, and a
system which works by the numbers but does not work, and in both cases
it's because the numbers are not all that good to start with.
Since diesels scale fuel consumption with load better than gas engines,
get a diesel that's bigger than you need, and let it loaf. Try to pick a
fuel efficient one to begin with - I've seen a factor of 3 difference in
fuel consumed per KWh produced in different diesel generators, shopping
around.
--
Cats, coffee, chocolate...vices to live by
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| nicksanspam@ece.villanova.edu 2007-02-23, 1:25 pm |
| Ecnerwal <LawrenceSMITH@SOuthernVERmont.NyET> wrote:
> suniltdpa@gmail.com wrote:
>
>
>A mathematical relation any better than "3X" would need data better than
>"non linear load".
>
>Some motors start hard, and may draw as much as (IIRC) 10X rated load
>for a brief period. Some are not so bad. Without trying it, you
>generally don't know, as the nameplate rating only tells you what the
>running current (and perhaps the locked rotor current, on a better than
>average nameplate) is.
>
>Likewise, some generators are more tolerant of a brief startup overload
>than others. This also does not show up on the generator nameplate
>rating. Thus, the data for both parts of the system is somewhat lacking
>in terms of "precise mathematical relations" and you end up falling back
>on fudge-factor rule-of-thumb numbers like "3X".
Are we talking about power factor or starting surge power?
Nick
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| Neon John 2007-02-23, 5:25 pm |
| On 23 Feb 2007 00:50:45 -0800, suniltdpa@gmail.com wrote:
>Hi,
>I need to find a relaion between load power(non linear load) and the
>rating of the load(emergency generator). I saw in different cases they
>will consider it as 3 times the load rating. Anybody help me for
>deriving a mathematical relation.
>Regards,
>Sunil
This is a very complex problem with many nonlinearities. I don't know
that I've ever seen a general purpose model. There are so many
dependencies.
A generator's steady-state power rating is only one small part. Surge
rating another part. That is, how much power the generator can supply
in overload and stay in specs. Then there is the de-excitation
overload value. How much overload does it take to de-excite the field
and made the thing stop generating. Almost all small generators
de-excite before the engine is seriously overloaded.
On the load side. How many loads have high inrush? Refrigerators,
air conditioners and other devices that use motors that are at least
partially loaded when started. How many loads have low power factors?
Un-PF corrected switchmode power supplies typically fall in the 40%
range. Both wattless currents and circulating harmonics have a
deletrious effect on the generator, particularly the cheaper
harmonically excited ones.
For large generator installations the loads can be characterized,
corrected if needed and the proper size generator fairly easily
selected from a chart based on heueristics.
The small generator is another matter. The characteristic of the load
can vary widely, depending on the appliances and lighting in use. For
example, a house with exclusively un-power-factor-corrected CF
lighting (most of 'em) will present a load to the generator with high
VAR demands AND high harmonic content. If the home is owned by a geek
with lots of computers with uncorrected power supplies, the same
situation applies. Someone with incandescent lighting, few appliances
and no computer will present a mostly resistive load with low overload
requirements.
The best way I know to go about this is to work with the manufacturer
and benefit from their experience. They've developed heuristics that
work pretty well.
John
---
John De Armond
See my website for my current email address
http://www.neon-john.com
Cleveland, Occupied TN
Don't let your schooling interfere with your education-Mark Twain
| |
| Norman Webb 2007-02-28, 8:25 pm |
| Without bothering to look it up, I believe Real Power (in watts) = VA X
Power Factor. ???????
I thought a power factor of less than 1 gave a real power greater than VA.
Real Power = VA / PF
ie for VA = 200 and PF = 0.5 real power = 200/0.5 = 400
So if you are running a motor you would have to allow for the power factor
correction.
However STARTUP CURRENT is the biggy with motors.
I have a 70W bar fridge and it makes my 500W (750W peak) petrol generator
grunt every time it starts.
Regards
Norm (unashamed top poster )
Vaughn Simon wrote in message ...
>
><suniltdpa@gmail.com> wrote in message
>news:1172220645.037720.284900@a75g2000cwd.googlegroups.com...
>
> Without bothering to look it up, I believe Real Power (in watts) = VA
X
>Power Factor.
>
> That 3X thing is just advertising. For example: It is normal for
>manufacturers of cheap UPS units to assume a .5 Power Factor. That way
that can
>advertise a 500-watt UPS as a 1000 VA UPS.
>
> Your next question should be "How do I know what my power factor is?"
>
>Vaughn
>
>
>
>
>
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"Norman Webb" <tekrec@iinet.net.au> wrote in message
news:45e61d33$0$31083$5a62ac22@per-qv1-newsreader-01.iinet.net.au...
> Without bothering to look it up, I believe Real Power (in watts) = VA X
> Power Factor. ???????
>
> I thought a power factor of less than 1 gave a real power greater than VA.
> Real Power = VA / PF
>
You should have looked it up.
PF = RP / AP
where
PF - power factor (0 - 1)
RP - real power (watts)
AP - apparent power (volt amps)
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| Norman Webb 2007-03-01, 5:25 pm |
|
John wrote in message ...
>
>"Norman Webb" <tekrec@iinet.net.au> wrote in message
>news:45e61d33$0$31083$5a62ac22@per-qv1-newsreader-01.iinet.net.au...
VA.[color=darkred]
>You should have looked it up.
>
>PF = RP / AP
>
>where
>
>PF - power factor (0 - 1)
>RP - real power (watts)
>AP - apparent power (volt amps)
>
>
Thanks. Glad someone is watching out for bullshit.
I confused real power with apparent power. I knew the PF was 1 or less and
why the power companies get pissed if you are running low PF equipment.
Regards
Norm
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