Air Compressor Horsepower/Wattage/Amperage

On Jun 21, 2:30 pm, wrote:
Dan : Which models of truck compressors would work good for a
homemade,
engine driven unit? I need to have the capability of developing about
25-30 cfm at
70+ psig for s shop project. Tankage is a 650 gallon propane tank and
I don't mind
ganging two compressors together and having split cut-in/cut-out
pressures.

Craig

None of the truck compressors will keep up with that. I've
been out of the business for 15 years now, but I don't think there's
anything bigger than the 24 CFM V-4 TF1000 made by Bendix or the 24
CFM Cummins twin. And it would cost so much that you could buy a
couple of big industrial compressors for the same money. The core
charges alone on these things were on the order of $750 15 years ago.
One of our shop compressors was an old deVilbiss originally
installed in a lighthouse to drive the foghorn. Around 50 CFM at 70
psi. I reset the thing to run at 150 psi to keep the bead blasters
going, and it had been running for five or six years like that when I
left. It wasn't supposed to be able to handle that load, since the con
rods and bearings were so slender, but it did. It had been bought at
auction by guys in the head office who didn't know any better and just
wanted to get something cheap. I had hoped to blow it up to teach them
to buy the right stuff in the first place, but the old thing wouldn't
cooperate.

Dan

"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 01:25:27 -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.

Don

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

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...