Air Compressor Horsepower/Wattage/Amperage

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)

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)

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.

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

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

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.

Maybe try to find some info 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)

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

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

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.