Air Compressor Horsepower/Wattage/Amperage

On Mon, 18 Jun 2007 14:53:30 -0700, "
wrote:

Have you ever noticed that in the horsepower place on the motor
dataplate it says something like "SPL"?

The motor mfg doesn't want to be complicit on the out right lying
done by the compressor mfgs, who never heard that 750 watts = 1 hp.

NOTHING beats Shop-Vac HP ratings.

6HP from a 15 amp 120v outlet! Awesome! G

B A R R Y wrote:

On Mon, 18 Jun 2007 14:53:30 -0700, "
wrote:

Have you ever noticed that in the horsepower place on the motor
dataplate it says something like "SPL"?

The motor mfg doesn't want to be complicit on the out right lying
done by the compressor mfgs, who never heard that 750 watts = 1 hp.


NOTHING beats Shop-Vac HP ratings.

6HP from a 15 amp 120v outlet! Awesome! G

Good golly, I hadn't noticed that before!

15 amps at 120 V is 1800 watts

Roughly 746 (100% efficiency) watts per HP

750 x 6 = 4476 watts

Less than 1/2 the claimed power!


I wonder if I could take it back and exchange it for
a REAL 6 hp vac???

"B A R R Y" wrote in message
news
On Mon, 18 Jun 2007 14:53:30 -0700, "
wrote:

Have you ever noticed that in the horsepower place on the motor
dataplate it says something like "SPL"?

The motor mfg doesn't want to be complicit on the out right lying
done by the compressor mfgs, who never heard that 750 watts = 1 hp.

NOTHING beats Shop-Vac HP ratings.

6HP from a 15 amp 120v outlet! Awesome! G

240% efficiency is very impressive--for a single phase motor!

The last time that I wandered through the tool department at Sears, I saw
what appeared to be the same compressor that I used to own. Mine could
manage about 2CFM at 40PSI, and claimed to be 2HP on the aforementioned 120V
circuit--IIRC with a continued draw of about 10A. The new ones were
claiming either 4 or 6 CFM, and I believe that the pressure may have been
higher as well, on the same electrical power. So the problem is not new,
but it is getting much worse.

Peter
(In the future, tools will be rated like sound systems.)

Peter Dohm wrote:

(In the future, tools will be rated like sound systems.)


Let's hope the tradition doesn't extend to airplane power plants. G

B A R R Y wrote:


Let's hope the tradition doesn't extend to airplane power plants. G

Oh don't worry, it will.

kontiki wrote:
B A R R Y wrote:


Let's hope the tradition doesn't extend to airplane power plants. G


Oh don't worry, it will.

Well, there was the 150 horse Franklin...

Jim, this is interesting:

http://www.truetex.com/aircompressors.htm

(Sorry if it's a repost.)

Don

Thanks, Don. As you say, it is "interesting" but it doesn't speak to the
point. So far I haven't had an on-point reply. This link is technically
good as it tries to relate the impossibility of some advertized horsepowers
pumping out SCFM, but SCFM relates to electrical power not.

Forget I said compressor except for the calculation of wattage/amperage
required in a partially full compressor tank. All I need to do is to
rationally or experimentally match a generator with a motor that HAPPENS to
be connected to a compressor.

To hell with inflated horsepower claims, airplane engines, and the like.
Simple question. One horse compressor motor. How much peak
amperage/wattage to start it zero PSI in the tank, how much to keep it
running to max PSI of (say) 100 psi, and how much to REstart it when the
pressure drops to 80 psi.

Jim

--
"Dream as if you'll live forever. Live as if you'll die today."
--James Dean
"Don Tuite" wrote in message
...
Jim, this is interesting:

http://www.truetex.com/aircompressors.htm

(Sorry if it's a repost.)

Don

RST Engineering wrote:


To hell with inflated horsepower claims, airplane engines, and the like.
Simple question. One horse compressor motor. How much peak
amperage/wattage to start it zero PSI in the tank, how much to keep it
running to max PSI of (say) 100 psi, and how much to REstart it when the
pressure drops to 80 psi.



Jim: I'll check at first light and give you actual model numbers for
my old oilless.
I used to run it off a 4500watt/5000 watt surge genset until one of
the brushes in
genset let go and the compressor motor burned up while on a trip. The
compressor
would start under any load condition in the tank on the genset with
one exception,
and that was with over 100 psig in the tank and ambient air temp over
110 degrees
F. and having been used nearly continously for over an hour. At that
point, I
would have to bleed the tank pressure to under 100 psig toi get it to
start.

Craig C.

On Jun 19, 10:09 pm, "RST Engineering"
wrote:

Forget I said compressor except for the calculation of wattage/amperage
required in a partially full compressor tank. All I need to do is to
rationally or experimentally match a generator with a motor that HAPPENS to
be connected to a compressor.

To hell with inflated horsepower claims, airplane engines, and the like.
Simple question. One horse compressor motor. How much peak
amperage/wattage to start it zero PSI in the tank, how much to keep it
running to max PSI of (say) 100 psi, and how much to REstart it when the
pressure drops to 80 psi.

A belt-driven compressor relies on its flywheel to drive
it through a compression stroke. The electric motor produces an amount
of torque that probably won't be enough to run a compressor without a
flywheel, or a compressor that has been stopped and has to start
against pressure. For that reason, most compressors have an unloading
system that exhaust the discharge line to let the compressor spool up
before it has to work against pressure.
I've tried to measure peak amps on a starting motor, and it
isn't easy. The mass it is driving affects it a lot, and even with no
load a squirrel-cage motor draws a bunch, maybe double its running
current, but it's so brief that the circuit breaker puts up with it.
Running off a generator means either a limited current supply that
causes massive voltage drop on start, so that the motor won't come up
to speed. And if you are starting against 80 psi with a non-unloaded
compressor you will need both a BIG motor and a BIG generator.
Sorry I can't give you numbers. Most of us won't be able to. There
are too many variables and I don't have even the basic info. Even the
motor type affects it: capacitor or non-cap start, compressor size and
mass, on and on. What is the motor rated to start? A low load like a
jet pump or table saw, or a heavy load like a big fan? I built my own
compressor years ago using an old truck compressor, and drove it with
a 1-horse 3450 RPM jet-pump motor. This motor has next to no starting
torque, so the compressor is set up to run continuously and the
original unloaders in it are engaged when tank pressure is reached. I
have to manually unload it to start it, and shut it off when I'm done.
Truck compressors have a device that hold the intake valves open when
the tank is full so that it can't compress anymore. The crankshaft
keeps turning.

Dan