Air Compressor Horsepower/Wattage/Amperage

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.

Before I tell elebendy bazillion Kitplanes readers that the Harbor Freight
Subaru 2200 watt generator will drive the Sears 1 horse compressor (and even
worse, buy the Sears compressor only to not have it work), will somebody
please do a reality check on me for horsepower/watts for this lashup.

(Note ... convert watts to amperes by dividing watts by 120 volts).
860w=7a 1725w=14a 2600w=21a

Sears and Harbor Freight are damned near clueless about this sort of stuff.
If anybody has a source I can reference for running/starting/starting under
load for air compressors it would be well received.

I'd LIKE to buy a 3/4 horse compressor but they go from the kiddie's 1/4
horse toy straight to one horse with darned little in between.

Jim

--
"Dream as if you'll live forever. Live as if you'll die today."
--James Dean

On Jun 18, 2:49 pm, "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.

Before I tell elebendy bazillion Kitplanes readers that the Harbor Freight
Subaru 2200 watt generator will drive the Sears 1 horse compressor (and even
worse, buy the Sears compressor only to not have it work), will somebody
please do a reality check on me for horsepower/watts for this lashup.

(Note ... convert watts to amperes by dividing watts by 120 volts).
860w=7a 1725w=14a 2600w=21a

Sears and Harbor Freight are damned near clueless about this sort of stuff.
If anybody has a source I can reference for running/starting/starting under
load for air compressors it would be well received.

I'd LIKE to buy a 3/4 horse compressor but they go from the kiddie's 1/4
horse toy straight to one horse with darned little in between.

Jim

--
"Dream as if you'll live forever. Live as if you'll die today."
--James Dean

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.

Charletons. BH

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.)

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

and then times three for starting
under some volume of air left in the compressor reservoir or ...

That SHOULD not be an issue. A properly designed compressor will
start "unloaded" so that only the motor inrush, assumed at 4-8 times
running for an industrial motor, less for the capacitor start single
phase units, is an issue.

Before I tell elebendy bazillion Kitplanes readers that the Harbor Freight
Subaru 2200 watt generator will drive the Sears 1 horse compressor (and even
worse, buy the Sears compressor only to not have it work), will somebody
please do a reality check on me for horsepower/watts for this lashup.

(Note ... convert watts to amperes by dividing watts by 120 volts).
860w=7a 1725w=14a 2600w=21a

Sears and Harbor Freight are damned near clueless about this sort of stuff.
If anybody has a source I can reference for running/starting/starting under
load for air compressors it would be well received.

I'd LIKE to buy a 3/4 horse compressor but they go from the kiddie's 1/4
horse toy straight to one horse with darned little in between.

Based on the units with bigger motors, the HP is a marketing game, not
real. Industry assumes ... ASSUMES ... about 4 cfm (to 100 psi) per
horsepower. Small units will be less efficient ...maybe 3 cfm. Large
(50 hp+) units will be a little better, perhaps 4.4 or so.

It is often missed that capacity is INLET air.

From what I've read and seen, trying to do calculations, most CFM
ratings are 30% to 100% unrealistic. Most horsepower ratings are 50%
to 300% unrealistic.

Look at it this way ... 2 HP is 1500 watts ... throw in single phase
efficientcy of maybe 65%, you are at 2200 watts in for 2HP out ...
about 20 amps on 120V. It would be my OPINION that the 2200 watt
generator would start and run a 1HP motor with little trouble.

There is a guy (a lawyer, darnit) who has done some research in this;
I understand that he was involved in a complaint against the wild
numbers used in advertising ... take alook at
http://users.goldengate.net/~kbrady/motors.pdf for some interesting
info.

("GeorgeB" wrote)
There is a guy (a lawyer, darnit) who has done some research in this;
I understand that he was involved in a complaint against the wild
numbers used in advertising ... take alook at
http://users.goldengate.net/~kbrady/motors.pdf for some interesting
info.


Very interesting. Good info. However, I must take exception with this
observation:

"Since most people find mathematics to be about as fascinating as watching a
guy making a weiner dog out of balloons, I'll keep the calculations simple."

I would like to go on record as being one person who loves watching balloons
get made into wiener dogs.


Paul-Mont

On Jun 19, 12:58 am, "Montblack" Y4_NOT!...
wrote:
("GeorgeB" wrote)

There is a guy (a lawyer, darnit) who has done some research in this;
I understand that he was involved in a complaint against the wild
numbers used in advertising ... take alook at
http://users.goldengate.net/~kbrady/motors.pdffor some interesting
info

My old memory has it that motors are rated by the heat that they can
dissipate, rather than the torque/ revs that they produce, or
electrical energy they consume.
An example would be: if we have a constant load that will raise the
frame temperature to an acceptable level, then that torque/rpm is the
rated hp. The motor will produce much more torque, but it will
overheat at that hp output. You see the effect of this rating on the
frame style, with open frame having a much higher hp rating than a
sealed explosion proof motor, with similar dimensions and windings.
The really wild ratings are the "protection ratings", where they
advertise the hp load required to trip the overheat circuit breaker,
by definition that is more hp than the motor can safely produce. How
is that for useless information ?

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

On Jun 18, 7:25 pm, GeorgeB wrote:

Based on the units with bigger motors, the HP is a marketing game, not
real. Industry assumes ... ASSUMES ... about 4 cfm (to 100 psi) per
horsepower. Small units will be less efficient ...maybe 3 cfm. Large
(50 hp+) units will be a little better, perhaps 4.4 or so.

It is often missed that capacity is INLET air.

This issue has been beaten to death on the rec.crafts.metalworking
discussion group. Everyone knows that Sears compressors are wildly
overrated. I was in the transportation air brake industry for years,
and me and my guys rebuilt about 12,000 compressors in that time and
tested every one of them on a dyno. We found that, as you have said,
that one HP will pump around 4 CFM. We had the cutout at 120 psi, but
of course, as also mentioned, there's nowhere near 4 CFM being
delivered at 120 psi. The 4 CFM is free air, at atmospheric pressure.
A really good compressor has as little volume as absolutely
possible when the piston is at TDC. This is to drive out as much of
the compressed air as possible; any air left in the cylinder at TDC
will expand as the piston travels downward again and so the intake
valves won't open until the cylinder pressure drops below atmospheric
pressure. A cheap compressor might have so much unswept volume that,
at the higher pressures, the intakes don't open until the piston is
halfway down. Not efficient at all. Unswept volume includes that
between the piston and head, whatever cavities the intake and
discharge valves may have, and so forth.
So the CFM rating is a zero discharge pressure, and it will
drop, depending on the efficiency and overall design of the
compressor, to considerably less as the tank pressure rises. You can't
take cylinder area and multiply it by stroke and RPM to get a reliable
CFM figure, but I think that's what the retailers do. You will be
disappointed if you have a 4 CFM spray gun and expect the 4 CFM
compressor to keep up with it. The spray gun requires 4 CFM at around
40 or 60 psi, the compressor is rated at zero.

Dan