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Re: [TowerTalk] Generator Sizing

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Subject: Re: [TowerTalk] Generator Sizing
From: "K8RI on Tower talk" <>
Date: Sun, 9 Jul 2006 15:18:37 -0400
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> Greetings:
> I am no expert in power generators.  BUT, I do know that there is a BIG
> consideration in sizing generators that could come into play.  Buying 
> extra large
> units could cost you big bucks in maintenance costs.

True, but there are many things to take into consideration when sizing a 
generator. The end result is a compromise. Often it is a large compromise. 
You also have to define extra large unit as what you may see as extra large 
I may see as barely sufficient.

The first thing you need to know is your peak demand. (This not every thing 
on with every motor starting)  Then how much safety factor are you going to 
allow?  Typical is to set the peak demand at 50 and no more than 75% of the 
generator design rating. OR do you quite possibly wish to set the whole 
thing up with the capability meeting the demand of absolutely every thing on 
with every motor going through its start cycle at the same time?

You never use the peak demand as the size of the generator except for non 
critical, intermittent use with no voltage sensitive devices connected. BTW 
that refrigerator motor is probably one of the most voltage sensitive 
devices in the house.

> With something in the order of 15-20 or more KW, you run  serious danger 
> of

15 to 20 KW would not be over size for me and probably not for many if not 
most on this group. My 9500 can not supply the whole house, let alone run at 
65% most of the time.  Also, well designed, large generators tend to have 
better voltage regulation over a wider range than the smaller units.

> damaging the unit if it is not being used at something like 65% of rated

The load varies over a very wide range in most homes. For the engine this is 
good. For voltage regulation it is not.  It's normal to see the load vary 
from 10% to near 90% and we try to avoid coming close to the full generator 
rating.  If I were to try to stay under 75% at peak I'd need at least a 20 
KW installation.

Voltage regulation is at its poorest near each end of the range.  It just 
happens that a constant load of 65% should give the best regulation at the 
nominal design voltage.  Also, if possible, get the low RPM design which 
will be quieter.

Motors such as refrigerators, freezers, furnaces, water pumps, and 
air-conditioners create the most variability.

Where those really hurt is the peak starting current.

> output.  I cannot recall, but I think the rings, valves, etc.  do not fair 
> well

Rings and valves need a varying load (and speed which they won't see in a 
generator installation) to seat well.

If engines have not been pre run it'd be a good idea to fire up the system, 
let it run for a few minutes at no load and then turn on every thing you can 
find and let it run for about a day. (12-24 hours). Keep close track of oil 
usage.  Mine didn't use any in the first 100 hours.  OTOH the system will 
*probably* work just fine for many years if you just hook it up and let it 
do its normal weekly cycle.


For the vast majority of homes you run far more danger of damage (more like 
premature aging) to the engine from sitting without running than you do from 
running it "unloaded." Remember diesel truck engines spend a good portion of 
their lives at idle.


> when running  a low percentage of capacity.  We  installed 25+ KW units in 
> Kuwait
> at customer's insistence and despite strong  warnings from the supplier
> (ONAN?).  Accordingly,

This might be a problem for large generators that run continuously but 
should not be a concern for a unit used intermittently.  It's rare for a 
home back up unit to run more than a few hours at a time although we've had 
several outages that lasted close to two days.

When it comes to fuel consumption you have to be careful about 
generalizations.  The engine in my 9500 watt unit uses about half the fuel 
my little portable 4 KW used.  With the 10 gallon tank full and the 
generator heavily loaded it'll run all night long with a good reserve.

> we installed large load banks to heat up the Kuwait air to  keep units at 
> a
> decent percentage of ratings.

Were those load banks switched in and out?  Typically we do that in industry 
for load regulation and not the health of the engine.


Roger Halstead (K8RI and ARRL 40 year Life Member)
N833R - World's oldest Debonair CD-2

> Regards,  Warren;  W7WY
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