Air Compressor Horsepower/Wattage/Amperage

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 ?

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

wrote:
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


Dan,

While I agree with almost all of what you have said, and you are right
on the mark, I have seen, and often, compressors that are rated at a set
pressure, ie... indicating a set displacement (in cfm) AT a pressure of
xx psi. Curtis and other old line commercial compressors are often rated
this way, cfm @ psi.

George

"RST Engineering" wrote

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.

Most air compressors have a valve that lets off the air pressure in the line
between the pump and the tank, when they shut off.

You can tell if it has this feature, by seeing a little air line going to
the pressure on/off switch.

You may not need to multiply so big for starting with air in the tank.
--
Jim in NC

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.

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

Jim,

Living off-grid, and generating most of my own power from renewables, I am
somewhat familiar with your issue. Although your approach may make
theoretical sense, in the real world (as you found out) it frequently
doesn't work.

The startup surge for an electric motor is given by a parameter called
"locked rotor amps" (means pretty much what it says). This can either be
measured (with a clamp-on ammeter), or derived from a letter on the
nameplate of the actual motor multiplied by the running amps.

A compressor is one of the more difficult devices to start. Without
specific data, I would figure starting amps to be at least five times the
nameplate amperage rating. Fugedabout trying to convert nameplate HP to
startup surge. It might be less, or not.

So far as your generator is concerned, it is likely that the ratings assume
a power factor of 1. But an induction motor, such as is in your
compressor, will have a power factor considerably less than 1. This
further increases the amount of "real power" the generator must supply.
And may also explain why the nameplate amperage rating seems higher than
what you predict by using HP and an assumed efficiency.

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.

Another issue that comes up with compressors has to do with flat spots on
the rotor as they age. This causes them to draw the locked rotor amp
startup current for a longer period of time when new. This can also cause
an otherwise adequately sized system to "blow" on startup, occasionally.


--ron

On Jun 24, 5:20 am, Ron Rosenfeld wrote:
...
Ron, what peak starting current should I expect on a single-phase,
240V, 3/4h.p., running current=7.8A motor used to lift my hangar door?
I want to rig up a genny to lift the door during a power failure. The
gear train on the door is rigged so that all of the cables are slack
(i.e. only motor inertial) when the motor starts.

On Tue, 26 Jun 2007 09:51:18 -0700, wrote:

On Jun 24, 5:20 am, Ron Rosenfeld wrote:
...
Ron, what peak starting current should I expect on a single-phase,
240V, 3/4h.p., running current=7.8A motor used to lift my hangar door?
I want to rig up a genny to lift the door during a power failure. The
gear train on the door is rigged so that all of the cables are slack
(i.e. only motor inertial) when the motor starts.

mgm17160, I really don't know. I'd be surprised if it were as much as
double the running amps, but there's no good way of telling without
measuring.

With no measurement, I would size the generator to be able to handle a
surge equal to the Locked Rotor Amps as calculated by the nameplate code
rating on the motor. Or just try a genny that can handle a surge of
15A/240V.

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. And, unless you have a mission-critical situation,
it's overkill to put in something that you can set and forget.

I, too, have a door that is not amenable to any kind of mechanical backup.
I happen to have an extra 120V inverter I could hook up -- but the cost of
batteries and a step-up transformer to set up the system is excessive,
especially given that I've only been stymied by a power outage at the
hangar once or twice in the past seven years.


--ron

"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.
--
Jim in NC

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.

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.


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