Air Compressor Horsepower/Wattage/Amperage

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.

Jim: Here are the nameplate specs for the compressor that I have.

It's listed as a 4HP

Model 919.152921

125 psi max

120/240 V 1 phase 15/7.5 amps

Calif. Code 462 (L)(2) / Max RPM 3450

Hope this helps a little at least.

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

wrote in message
oups.com...
I built my own
compressor years ago using an old truck compressor, and drove it with
a 1-horse 3450 RPM jet-pump motor.
Dan


How did you supply oil pressure to lube the crankshaft? or did this
compressor have its own closed sump and pump or splash lube system?

TIA

On Jun 20, 12:31 pm, "Private" wrote:
wrote in message

oups.com...

I built my own
compressor years ago using an old truck compressor, and drove it with
a 1-horse 3450 RPM jet-pump motor.
Dan

How did you supply oil pressure to lube the crankshaft? or did this
compressor have its own closed sump and pump or splash lube system?

TIA

It was a base-mounted belt-driven compressor that was supplied
oil from the engine's oil system. I made a steel base plate to close
it off, drilled the side of the case and tapped it for a 1/2" 90° pipe
elbow for a filler/level check plug, and drilled and tapped the con
rod caps for 1/8" pipe street ells that acted as scoops to grab the
oil from the sump and jam it into the bearings. The main bearings were
ball bearings and get plenty of splash to keep them happy. The thing
has run happily for probably 100 hours over the last 20 years and
shows no sign of distress.

Dan

wrote in message
ps.com...
On Jun 20, 12:31 pm, "Private" wrote:
wrote in message

oups.com...

I built my own
compressor years ago using an old truck compressor, and drove it with
a 1-horse 3450 RPM jet-pump motor.
Dan

How did you supply oil pressure to lube the crankshaft? or did this
compressor have its own closed sump and pump or splash lube system?

TIA

It was a base-mounted belt-driven compressor that was supplied
oil from the engine's oil system. I made a steel base plate to close
it off, drilled the side of the case and tapped it for a 1/2" 90° pipe
elbow for a filler/level check plug, and drilled and tapped the con
rod caps for 1/8" pipe street ells that acted as scoops to grab the
oil from the sump and jam it into the bearings. The main bearings were
ball bearings and get plenty of splash to keep them happy. The thing
has run happily for probably 100 hours over the last 20 years and
shows no sign of distress.

Dan

I have had good luck and extensive service from these compressors, but
always by mounting them on engines where I was able to connect them to the
engine's pressure oil and coolant systems. I used an electric clutch pulley
so they did not need to run continuously, IMHO they are very good
compressors. The current high cost of fuel makes it an expensive way to
obtain pressure air in any quantity unless the engine will be operated
anyway.

Happy landings.

On Jun 21, 10:40 am, "Private" wrote:
wrote in message

ps.com...
On Jun 20, 12:31 pm, "Private" wrote:

wrote in message

roups.com...

I built my own
compressor years ago using an old truck compressor, and drove it with
a 1-horse 3450 RPM jet-pump motor.
Dan

How did you supply oil pressure to lube the crankshaft? or did this
compressor have its own closed sump and pump or splash lube system?

TIA

It was a base-mounted belt-driven compressor that was supplied
oil from the engine's oil system. I made a steel base plate to close
it off, drilled the side of the case and tapped it for a 1/2" 90° pipe
elbow for a filler/level check plug, and drilled and tapped the con
rod caps for 1/8" pipe street ells that acted as scoops to grab the
oil from the sump and jam it into the bearings. The main bearings were
ball bearings and get plenty of splash to keep them happy. The thing
has run happily for probably 100 hours over the last 20 years and
shows no sign of distress.

Dan

I have had good luck and extensive service from these compressors, but
always by mounting them on engines where I was able to connect them to the
engine's pressure oil and coolant systems. I used an electric clutch pulley
so they did not need to run continuously, IMHO they are very good
compressors. The current high cost of fuel makes it an expensive way to
obtain pressure air in any quantity unless the engine will be operated
anyway.

Happy landings.

This was an old aircooled unit, circa 1950, that showed up
in some returned cores. Instead of the usual unloader pistons that
lifted the intakes, it has intake ports in the cylinder wall that the
piston uncovered near the bottom of the stroke. The unloaders are
separate small valves in the head similar to small-engine valves,
opened by a lever operated by metal diaphragm that received the air
signal from the governor.
The whole design of these old compressors resembled an
outboard motor powerhead, and in researching the origins of Bendix-
Westinghouse compressors I discovered that the first units were
converted Evinrudes. So much for the similarities.
By the way: of the thousands of compressors we rebuilt, I
would say that at least half of the cores had nothing wrong with them
except for worn unloader piston O-rings, damage caused by a worn-out
$20 governor. New O-rings were a few cents. A shot governor doesn't
apply the unloading pressure suddenly, as it should, and the intakes
bang the lifting pins and work the pistons up and down rapidly and
wear the o-rings out. Leaky o-rings cause a pressure drop in the
governor when it cuts the compressor out, it thinks the tank pressure
has dropped, and it cuts in again so that the compressor is on/off/on/
off rapidly and the driver or mechanic thinks it's shot.
About a fifth of cores were those off Detroit Diesels and had
the phenolic drove coupling that would shear internally off its steel
hub but remain attached to the hub and look perfectly normal. It
couldn't drive the compressor, a compressor that had nothing wrong
with it.
And the rest had been allowed to eat dust through old/missing/
cheap air filters, and dust destroys a compressor faster than it will
an engine. It sticks to the thin film of oil on the cylinder wall
instead of being vaporized and blown out, and the cylinder and rings
eat each other.
Maybe you guys aren't interested in all this, but the point is
that most of our troubles with machinery are our own fault, either
through ignorance or just being plain cheap.


Dan

On Tue, 19 Jun 2007 21:09:51 -0700, "RST Engineering"
wrote:

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

You don't have full information there, but I went to Baldor's site and
picked a 1HP single phase general purpose motor, their L1310 for some
numbers.

Starting, 32A @ 230V (will be roughly double on 115). If less is
available, starting will take longer. No load current is 4.8A. NOTE
that power consumption at idel is not 4.8x230; idle PF will be less
than 0.1, I'd guess.

Following are 230V numbers, double currents for 115

Load Characteristics at 230 V, 60 Hz , 1 HP
% of Rated Load 25 50 75 100 125 150
S.F.
Power Factor 37 52 64 72 79 83 76
Efficiency 43.4 58.2 64 66 65.8 63.7 65.9
Speed (rpm) 1782 1766 1749 1729 1706 1676 1715
Line Amperes 4.9 5.3 5.9 6.7 7.7 9.1 7.3

ASSUMING 100 psi is 90% load, mayne 6.5A on 230. Restart at 80psi, if
compressor is properly designed, will be no worse than 0 psi start,
ie, it starts briefly unloaded. (It won't draw any more current in
any event, but the starting time would be extended)