Air Compressor Horsepower/Wattage/Amperage

On Sun, 24 Jun 2007 07:20:40 -0400, Ron Rosenfeld
wrote:

On Mon, 18 Jun 2007 13:49:59 -0700, "RST Engineering"
wrote:

I thought I had this compressor power thing down to a pretty good science
until they started screwing around with "rated watts" and "peak power" and
all that crap that makes their compressor look really good until you go to
use it.

Back when we were using "real" horsepower I used a figure of 750 (to make it
easy to calculate, I believe 746 is the actual number) watts per horsepower
and an efficiency factor of 85% so that a one horse motor would take 860
watts to do the actual work.

But then you multiplied that times two for "starting" wattage for a couple
of seconds to give 1725 watts under start and then times three for starting
under some volume of air left in the compressor reservoir or about 2600
watts.

That presses my 2200 watt continuous duty (2800 watts peak) fairly close to
the load limit, but certainly gives a margin for error that seems
reasonable.

I happen to have a Sears 1HP compressor (1.5HP Peak). Mine has a nameplate
rating of 10.5A @ 120V. I could not see/locate the nameplate on the motor
itself, so I figured a 52.5A startup surge (5X). My inverter has a 78A
peak capacity (46A continuous) so I figured things would work -- and they
have.

Your 2800W peak generator translates to 23.3A at 120VAC. If you have the
same Sears compressor as I do, I'm not surprised that the generator will
have a problem starting it.

I just measured my little suitcase compressor. It's a dry pump with a
nameplate rating of 115V, 15A, 7.9cfm and 4cfm (presumably at 40 and
90psi). Running off an inverter - tank empty starting current 33.8A.
Running current 11.6A. Normal starting current (after tank drops to
about 80psi, 32.4A (reflects warmed up compressor I guess). Despite
the startup current, this compressor runs fine off my $300 cheapie
3500W rated generator, which in reality is only good for about 2000W
continuous. (inflated rating plus 4500' elevation)

Wayne

On Tue, 26 Jun 2007 22:34:34 -0700, Don Tuite
wrote:

On Wed, 27 Jun 2007 01:25:27 -0400, "Roger (K8RI)"
wrote:

On Wed, 27 Jun 2007 01:05:26 -0400, "Morgans"
wrote:


"Ron Rosenfeld" wrote

A problem with small, inexpensive, gasoline fueled generators is that,
unless you are religious about exercising them, they usually won't start
when you need them.

An exception to that rule is a Honda generator. I'm not a foreign car guy,
or anything like that; quite the opposite. My dad had one that would sit
for a couple of years, and it would start on the first or second pull, with
fresh gas in the tank.

It is amazing how many other uses come up, once you have a generator sitting
around.

Mine is 9500 watts continuous. It doesn't even have the option of a
pull starter. I don't think I could pull it even if it did. I keep
the tank full. When I shut it down I turn the gas off and let it dry
out the carb. It's on wheels, but it'd take a truck to haul it so it
could be called portable. OTOH it'd take three and preferably four men
to put it in the truck. If it were in the shop I could do that with
an engine hoist. I fire it up about every other month and let it run
for about 10 minutes. In the winter I have to keep a small heat lamp
on the battery. It'll run about 10 to 12 hours powering the whole
house sans air conditioner (except for the little spare one stuck in
the bedroom window) on 10 gallons. I had a little 4000 watt Coleman
that used twice as much gas, but it was kinda portable and made a
whole lot more noise.

I purchased it new, *after* Y2K when there were lots of them available
at half price and less. In the last 6 1/2 years I have well over
100 hours on it powering the house. Lots of power outages due to poor
line maintenance and we are about 2 miles from the city limits and
just over a mile from the substation.

As for other uses, with help I some times pull it out of the generator
shed and use it to power my 180 amp MIG welder. It's been a while but
IIRC the engine doesn't even change pitch when welding.

You might want to mention your isolation switch.

Ohhh...yahhh...Power companies get kinda excited about those.
I took a pair a big breakers and built a sliding connector between the
handles based on one I saw on a commercial transfer switch. If you
turn one on the other turns off first, but your can turn one off
without turning the other on. So, I fire up the generator and by the
time I can walk from the shed, around the end of the house, through
the garage and then down stairs the engine is warmed up enough for me
to throw the transfer switch. I left a coupe circuits on the mains
panel so when the power comes back on those lights come on to let me
know I can go switch back to the mains for power. I usually wait
about 5 minutes before switching and another 5 before shutting the
generator down. It'd be nice to have one of the fully automated
systems that runs on natural gas, which would be a lot quieter and
cheaper to run, but the initial cost would be about 10 times what I
have in this system.

Don

On Wed, 27 Jun 2007 13:56:07 GMT, Darrel Toepfer
wrote:

Ron Rosenfeld wrote:

One problem is that frequently, after sitting around for a few years,
there isn't fresh gas in the tank! (I should have been more specific
as to the reasons).
--ron

My 40kw runs on Natural Gas or Liquid Propane and is configured to
automatically switch between them. No worries about fuel geauxing bad
from evaporation or sucking moisture out of the air. Course those fuels
aren't cheap either, but it sure cuts down on the hassle factor of
trying to buy gasoline/diesel when all the retailers don't have power
for any length of time...

Spent over 60 hours on a 5.5kw due to hurricane Lili (6 gallon tank,
used 90 gallons), it eventually burned the muffler off (bought it new in
'85). Needed the 40kw for only 18 hours during/after Rita (didn't even
get a drop of rain from Katrina) and it makes my UPS's very happy too
when needed. Now I can cook and enjoy airconditioned comfort while my
neighbors ice boxes are also plugged in as well...

If I had to buy another portable, NG/Propane would definitely be high on
the list for compatible fuels. Just remember you lose some power output
when using those for fuel...

My backup generator is a 12kW unit; runs on propane and I have a 500 gallon
tank which is usually 60-80% full (also supplies propane for heat, DHW,
cooking, clothes dryer). It automatically runs for 10 minutes every 10
days, and has never failed to start. In the winter, I preheat it for an
hour or two with electricity -- also automatic.

I have a 5.5 kW gasoline unit sitting around. The last time someone asked
to borrow it, it took him four or five hours to get it started, and then I
think he could only use the 115V tap -- not the 230.
--ron