Air Compressor Horsepower/Wattage/Amperage

[RST Engineering]

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

[[email protected]]

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

[B A R R Y]

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>

[cavelamb himself]

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???

[Peter Dohm]

"B A R R Y" > wrote in message
...
> 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.)

[GeorgeB]

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.

[Montblack]

("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

[George]

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 ?

[B A R R Y[_2_]]

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>

[[email protected]]

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

[George]

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

[kontiki]

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.

[cavelamb himself]

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...

[cavelamb himself]

B A R R Y wrote:

> 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>

Well, there was the 150 horse Franklin.

And the 60 horse VW.

[Don Tuite]

Jim, this is interesting:

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

(Sorry if it's a repost.)

Don

[RST Engineering]

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
>

[[email protected]]

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.

[[email protected]]

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.

[[email protected]]

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

[Private]

> 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

[GeorgeB]

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)

[RST Engineering]

Good Lord, starting current of 64 amps at 115 volts? Most 15 amp breakers
would pop in half a second at 400% overload; how do these little one horse
compressors plug into a 15 amp circuit and start so easily? It is just when
you don't have nearly infinite current available from house mains and you
start fooling with generators that the situation becomes sticky.

Lessee now, lets go to the 100% (one horse) column and we find at 115 volts
a PF of 79, 66% efficiency, and 15.4 line amps. One horse is 750 watts and
at the one horse level this rascal is sucking 1770 watts, or an efficiency
of 42%. But the table shows a 66% efficiency. Hmmm ... cos (79°) = .2 but
that factor of out of phase isn't going to get me into the ballpark, nor is
the sin at .98 going to get me anywhere either.

Hmmmm ...

Jim

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


"GeorgeB" > wrote in message
...
> On Tue, 19 Jun 2007 21:09:51 -0700, "RST Engineering"


>
> 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)

[[email protected]]

RST Engineering wrote:

> Lessee now, lets go to the 100% (one horse) column and we find at 115 volts
> a PF of 79, 66% efficiency, and 15.4 line amps. One horse is 750 watts and
> at the one horse level this rascal is sucking 1770 watts, or an efficiency
> of 42%. But the table shows a 66% efficiency. Hmmm ... cos (79°) = ..2 but
> that factor of out of phase isn't going to get me into the ballpark, nor is
> the sin at .98 going to get me anywhere either.
>

It's them dadgummed vodoo phased lectrons with them giant p states
that are
casuing you the problems Jim. Just remember that all a tech cares
about is that
missing volt, the guy with the BS. is chasing the extra milliamp, the
MS guy is
putzing with the missing microvolt and the PhD is still trying to
figure out where that
extra electron came from....



Craig C.

[[email protected]]

On Jun 20, 3:17 pm, "RST Engineering" > wrote:
> Good Lord, starting current of 64 amps at 115 volts? Most 15 amp breakers
> would pop in half a second at 400% overload; how do these little one horse
> compressors plug into a 15 amp circuit and start so easily?

Those little one-horse compressors that run on a 15-amp
circuit probably aren't really one horsepower as we know it. More
marketing hype. They might be basing the 1-hp thing on starting
current or some similar sleight-of-hand.

Dan

[[email protected]]

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

[cavelamb himself]

Ya know, it's funny - ( he said, changing subjects abruptly).

I have a Hobart 135 wire welder set up for 120VAC.
At 120 VAC the placard calls for 20 amps.

It had a dedicated 30 amp breaker for it in my shop.
Sweet machine.

But here in the garage of this house there is only one plug.
And it's on a 15 amp breaker.

Trying to tack some 1/4" steel brackets together was about the most
frustrating welding experience of all times.

Just about the time the puddle going nice - the breaker tripped.
The tacks were very "tacky" - down right sloppy looking in fact.

I real quick gave up and went down to the Tractor Supply and got a
heavy duty (30 amp) twenty foot extension cord and plugged into the
dryer circuit.

The Hobart was back to it's sweet personality again.

At least the welder rating is right...

But my 6 HP shop vac can't blow that breaker even if it wanted to.

[Blueskies]

Maybe try to find some info re: air conditioners for RVs that run off generators...

FWIW, do you have a compressor you can instrument?

I'm not a motor guy, but the inrush will always be high until the back emf stabilizes things. I would imagine the
limited source from the generator would only slow down the start, and if the motor turns at all it should get up to
synchronous speed. How much it would overheat due to the low voltage brown out is another issue...


"RST Engineering" > wrote in message ...
> Good Lord, starting current of 64 amps at 115 volts? Most 15 amp breakers would pop in half a second at 400%
> overload; how do these little one horse compressors plug into a 15 amp circuit and start so easily? It is just when
> you don't have nearly infinite current available from house mains and you start fooling with generators that the
> situation becomes sticky.
>
> Lessee now, lets go to the 100% (one horse) column and we find at 115 volts a PF of 79, 66% efficiency, and 15.4 line
> amps. One horse is 750 watts and at the one horse level this rascal is sucking 1770 watts, or an efficiency of 42%.
> But the table shows a 66% efficiency. Hmmm ... cos (79°) = .2 but that factor of out of phase isn't going to get me
> into the ballpark, nor is the sin at .98 going to get me anywhere either.
>
> Hmmmm ...
>
> Jim
>
> --
> "Dream as if you'll live forever. Live as if you'll die today."
> --James Dean
>
>
> "GeorgeB" > wrote in message ...
>> On Tue, 19 Jun 2007 21:09:51 -0700, "RST Engineering"
>
>
>>
>> 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)
>
>

[GeorgeB]

On Wed, 20 Jun 2007 14:17:58 -0700, "RST Engineering"
> wrote:

>Good Lord, starting current of 64 amps at 115 volts? Most 15 amp breakers
>would pop in half a second at 400% overload; how do these little one horse
>compressors plug into a 15 amp circuit and start so easily? It is just when
>you don't have nearly infinite current available from house mains and you
>start fooling with generators that the situation becomes sticky.

If you go to Baldor's website and look at this motor (and the 3600RPM
is different), you'll note that this 1HP will acutally supply 2HP and
not stall ...

>Lessee now, lets go to the 100% (one horse) column and we find at 115 volts
>a PF of 79, 66% efficiency, and 15.4 line amps. One horse is 750 watts and
>at the one horse level this rascal is sucking 1770 watts, or an efficiency
>of 42%. But the table shows a 66% efficiency.

that's 1770 VA, not watts ... watts will be that times PF (already the
cos of phase angle applied) gives 1400 watts ... which my pea brain
shows as a hair under 54% ... not the 66% they show.

I work with 3 phase motors, and have found the calculations to be
better ... wonder where the extra power is?

>Hmmm ... cos (79°) = .2 but
>that factor of out of phase isn't going to get me into the ballpark, nor is
>the sin at .98 going to get me anywhere either.

0.79 is cos(38 degrees)

>Hmmmm ...
>
>Jim

I agree that the data doesn't fit itself ... but it was a copy/paste,
not hand entered, so I didn't check carefully.

If you got a store bought 1HP compressor, I'd bet ti would have a more
accurately defined 0.5HP unit ... see following ... at 100% the
numbers check out ... 3.9A*230V*0.69*0.607=375W ...

Product Nameplate Data :
Rated Output .5 HP Hertz 60 NEMA Nom. Eff. 57
Volts 115/230 Phase 1 Power Factor 71
Full Load Amps 7.4/3.7 NEMA Design Code N Service
Factor 1.25
Speed 3450 LR KVA Code K Rating - Duty 40C AMB-CONT

(Typical performance - Not guaranteed values)
General Characterstics at 230 V, 60 Hz, 0.5 HP
Full Load Torque 0.75 LB-FT Starting Current 18.5
Amps
Start Configuration DOL No-Load Current 2.95 Amps
Break Down Torque 2 LB-FT Line-line Resistance @ 25° C
4.72 Ohms
Pull-Up Torque 1.6 LB-FT Temperature Rise, C @ FL (in deg) 78
Locked-Roter Torque 3 LB-FT Temp. Rise @ S.F. Load (in
deg) 92
Load Characteristics at 230 V, 60 Hz , 0.5 HP
% of Rated Load 25 50 75 100 125 150
S.F.
Power Factor 39 49 61 69 74 78 74
Efficiency 38.3 49.7 57.4 60.7 60.9 59.8 60.9
Speed (rpm) 3530 3490 3450 3420 3380 3340 3380
Line Amperes 3 3.2 3.5 3.9 4.4 4.95 4.4

[GeorgeB]

On Wed, 20 Jun 2007 14:17:58 -0700, "RST Engineering"
> wrote:

>Lessee now, lets go to the 100% (one horse) column and we find at 115 volts
>a PF of 79, 66% efficiency, and 15.4 line amps.

I think you slipped a column ...

100% 13.4A (115V), 0.66 eff, 0.72 PF ... 732 watts

> One horse is 750 watts and
>at the one horse level this rascal is sucking 1770 watts, or an efficiency
>of 42%. But the table shows a 66% efficiency. Hmmm ... cos (79°) = .2 but
>that factor of out of phase isn't going to get me into the ballpark, nor is
>the sin at .98 going to get me anywhere either.
>
>Hmmmm ...
>
>Jim

[Private]

> 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.

[[email protected]]

On Jun 21, 10:40 am, "Private" > wrote:
> > 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.

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

[[email protected]]

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

[Maxwell]

> wrote in message
ps.com...
>
> 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.
>

I don't know what it would cost to buy and fit a truck compressor, but have
you seen this?

http://www.wttool.com/product-exec/product_id/4086/nm/10_HP_Compressor_Pump

I built a 2 hp from a Mopar A/C compressor that I used for several years.
But I think if I had to do it all over, I might just buy a pump from China.

When my 5 hp Saylor Beal failed a few months ago, I bought a complete 10 hp
that I believe uses this same compressor. It makes a lot of air even at 175
psi. Almost 35 CFM I think.

[[email protected]]

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

[Morgans[_2_]]

"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

[Ron Rosenfeld]

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

[[email protected]]

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.

[Ron Rosenfeld]

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

[Morgans[_2_]]

"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

[Roger (K8RI)]

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.

[Don Tuite]

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

[Ron Rosenfeld]

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

[Darrel Toepfer]

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...

[[email protected]]

On Sun, 24 Jun 2007 07:20:40 -0400, Ron Rosenfeld
> wrote:

>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.

>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.

I just measured my little suitcase compressor. It's a dry pump with a
nameplate rating of 115V, 15A, 7.9cfm and 4cfm (presumably at 40 and
90psi). Running off an inverter - tank empty starting current 33.8A.
Running current 11.6A. Normal starting current (after tank drops to
about 80psi, 32.4A (reflects warmed up compressor I guess). Despite
the startup current, this compressor runs fine off my $300 cheapie
3500W rated generator, which in reality is only good for about 2000W
continuous. (inflated rating plus 4500' elevation)

Wayne

[Roger (K8RI)]

On Tue, 26 Jun 2007 22:34:34 -0700, Don Tuite
> wrote:

>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.

Ohhh...yahhh...Power companies get kinda excited about those.
I took a pair a big breakers and built a sliding connector between the
handles based on one I saw on a commercial transfer switch. If you
turn one on the other turns off first, but your can turn one off
without turning the other on. So, I fire up the generator and by the
time I can walk from the shed, around the end of the house, through
the garage and then down stairs the engine is warmed up enough for me
to throw the transfer switch. I left a coupe circuits on the mains
panel so when the power comes back on those lights come on to let me
know I can go switch back to the mains for power. I usually wait
about 5 minutes before switching and another 5 before shutting the
generator down. It'd be nice to have one of the fully automated
systems that runs on natural gas, which would be a lot quieter and
cheaper to run, but the initial cost would be about 10 times what I
have in this system.
>
>Don

[Ron Rosenfeld]

On Wed, 27 Jun 2007 13:56:07 GMT, Darrel Toepfer >
wrote:

>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...

My backup generator is a 12kW unit; runs on propane and I have a 500 gallon
tank which is usually 60-80% full (also supplies propane for heat, DHW,
cooking, clothes dryer). It automatically runs for 10 minutes every 10
days, and has never failed to start. In the winter, I preheat it for an
hour or two with electricity -- also automatic.

I have a 5.5 kW gasoline unit sitting around. The last time someone asked
to borrow it, it took him four or five hours to get it started, and then I
think he could only use the 115V tap -- not the 230.
--ron

[GeorgeB]

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.

I'm not Ron, but IN GENERAL, it will be 4 to 8 times running current
.... with no load, the peak won't last long at all, just until the
motor gets to perhaps to 90% of speed.

I looked on the Baldor website, and that is high current for a 3/4 HP
motor from them. A 3600 rpm is 4.75A, an 1800 is 5.3A.