Guide to electric air compressors for home shops

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James Sweet wrote:

If I were ever to buy a stationary compressor, it would be one that you wouldn't be able to run on a 120V circuit. A real 5HP one sounds about right...30A at 240V.
Chris
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5HP on a 120 volt circuit? Somebody is pulling your leg, but looks like you realize it! I bought a Porter Cable 7HP, (sure you betcha), 60 gallon tank compressor a few years ago. Motor amps tell me that it is closer to 3 HP. It does all I want it to do. I can run any air drill, DA sander I have, and it just about keeps up. And what I mean by that is for a home, weekend warrior shop it is just fine. I don't drill or sand continuously for hours on end. By the time the tank starts to run a bit low I am ready for a 30 second break, and the the compressor catches up again. Really I don't even notice any short comings. If you planed on running a sander continuously with no break at all it will not keep up. In my opinion, most any one man home shop can get along nicely with 10 CFM at 90 PSI. Greg
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Greg O wrote:

My main goal was to use it to blow sprinkler system in the fall. What I got does it OK but very slowly. Wish I had a bigger one. Then I hve to run 208 circuit for it. As a compromise I am happy ith what I have. That Hp rating is bogus. They don't tell trun Hp, they invented a term peak Hp to fool general consumers. They do the same with stereo system. When they brag about 200W output power it is barely 50W rms.
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wrote:

I ran into this with Hot Tub motors. The trick they use is to state the horsepower the motor will produce if run using the maximum rated voltage. A motor designed to be run on 220 volts may be "fire rated" up to 300 or more volts. That 3 hp spa motor is really only a 2 hp motor until you feed it 300 volts!
Air compressor motors on consumer level compressors are sometimes marketed the same dishonest way. The HP of the motor on a compressor is not as critical as you might think. What really matters are the specs for pressure (PSI) and volume (CFM)
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snipped-for-privacy@dog.com wrote:

They lie about the CFM (even SCFM) ratings too.
Bob
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wrote:

ergo: my suggestion of the 1.5 or 2 multiplier. The best way to see how a compressor runs what you want to run is to hook it up and run it. But by then it may be too late, and you can't take it all back.
Steve
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Greg O wrote:

They are very creative about horsepower ratings, same with shop vacs and power ratings of cheap audio amplifiers. Ought to be illegal to fudge them so severely but so far it isn't. There's the whole "peak HP" thing they use, worthless number, but it sounds impressive.
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On Sat, 13 Dec 2008 12:49:57 -0600, Greg O cast forth these pearls of wisdom...:

I was pretty much with you Greg, until that last statement. In fact - I might still be with you in light of that last statement, but the DA issue kinda rubs a bit. If a fellow is going to expect to do a fair amount of work with a DA, then 12 is probably a betterr bet for him. It's easy for a DA to overwhelm a compressor and it's even easier to be laid right into a DA for long, continuous times.
--

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Not a bad article, if you don't mind the tone of the Very Authoritative Home-moaner.
But as far as size goes, buying small-ish isn't necessarily a bad thing, as compressors are simply additive. Three small compressors should be exactly equivalent to one big one, with a few advantages:
1. You now have inherent staging, with all of its advantages: reduced electrical surge/"demand"; greater economy at low demand. Would proly need better pressure regulators in each unit, tho, as those in small units are usually not the best.
2. Likely quieter. Doubling the number of sources of the same noise only adds 3 db, vs. perhaps a much greater decibel increase of a larger unit. ie, two trumpets each at the same db is not as loud as one trumpet played more forcefully.
3. Smaller = portable, if nec. 4. May be easier to fit numerous smaller compressors in nooks and crannies.
5. Repair is easier, due to redundancy -- you won't be totally out of air. And Quincy's are nice, but I could proly buy a whole new small HD/c.h. for the price of one Q repair.
6. One can be set up for special apps, like painting, etc.
Proly some other advantages, as well.
HD/campbell hausefeld has a very nice upright 30 gal compressor, dual voltage motor, belt driven cast iron compressor w/oil, sight glass, wheels, VERY quiet. I would buy multiples of these in a minute.
Also, in many cases, larger resevoir volume is all that is needed, not a bigger compressor. I have numerous small makeup tanks for about 70 gals, works great with one small compressor. Would work even for air tools, depending on how much you use them vs. your total gallons.
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I will argue that point! Seems to me that the larger HP cast iron pump compressors are very quiet, certainly when compared to a 1HP oil less unit. My cheapy 7HP, (really about 3HP) Porter Cable is quieter than the 1-1/2 Craftsman it replaced. Much quieter than the oil less models we have at my job. Some of the 5HP cast iron compressors are strangely quiet. Low RPM pump and motor reduce noise a bunch, but at a premium price. Greg
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Air compressors have losses . V belts are a waste , not necessary .
I just bought 6 HarborFright 40400 DIRECT drive ( 8 gallon tank) for
$100 each .
I can series them , output of one could enter the crankcase of the next
stage then directed to the head intake valves .....IF the seals would hold ..
But 1 stage is efficient if PSI is below about 60 ( 4:1 ratio ) ...
At 110 PSI , its a big loss to have only one stage .
Auto a/c is over 10:1 on a hot day and condenser is not gulping 50 MPH
air .
Or i could parallel 4 , staggering the start up by 5 seconds .
This is more efficient than a $600 , 5 H.P. , 12 CFM belt drive ,
I could also put a 6004 2-RS ball bearing on crankshaft ..
I needed these for the H.F. 4 amp PLASMA cutter .
at half the price ( $700 ) of competition , i bought 2 . I also
bought 50 Zirconium/halfnium electrodes ( abt $250 ).
I can reshape the copper cups , easily .
This 40 amp Plasma uses a 4 transistor "H" bridge ... BAD idea ,
but as i said , i got a low price on the cutter ..
Modern circuitry uses only 1 or 2 transistors a single primary winding ,
driving it in one direction , NOT a half bridge , just a simple one sided
forward converter . This way transistors saturate the core , then hit it again
saturate the core etc etc . This is enhernetly safer than the complicated
current mode chips driving alternate NPN's . But i could always take out
the extra transistors , save them for a rainy day , use only 2 for even higher
output . 2 NPN IGBT's inparallel will drive MORE , than the lossy H bridge .
Plasma uses a mcu AT91xxxx . waste of mcu ....
Torch electrode is negative . copper cup with .8 mm hole is POSITVE , but
has a huge choke of 2.2 mil henries . Theres no capacitors
in the circuit , so the pulsating D.C. is limted by the choke and is called
Pilot arc ( aka starting arc ).
The zirconium electrode is pulled away from copper cup as soon
as circuit measures current in the ground cable .
Small cables used here cause plasma uses 90 vdc at
low amps ( 10 amps can cut 1/8" steel )
pull torch away from work , and pilot arc starts again .
NO MOVING PARTs , nothing to wear out ,
except chrome plated tiny electrodes at $6 each .
Lincoln electric inc' said they cut 1/2" plate
and got 450 feet of cut with each electrode .
Plasma cuts fast narrow and encredibly clean.
I bought 3 H.F. Stick/T.I.G. inverter welders ( red box ),
i call it "e-welder" . Buzz boxes can't limit the current , set at 100 amps
they go to 200 amps til you can pull it free !
But e-welder can "fold back current " instantly . And it will go out , if
you pull the electrode away , even to an 1/4" !
Buzz box just increases the voltage !
All my friends are raving about it . im doin diagrams on it .
It will power a Wire Feed . It needs circuit mods to hold D.C Voltage constant .
It does not use mcu's , only a current mode controller and an
IGBT half bridge , . dumb ! MJE13007's cost $0.12 each and are lower
loss when enough are paralled to the same amps as the IGBT's ...
Insulated Gate Bipolar Transistor is much higher loss , much higher cost ,
passing 20 amps , IGBT have about 2.5 to 3.5 volts of LOSS , MJE13007's in parallel can do 0.5 vdc !
paralleld Schottkies can use current sensing to shut them down on
over heat . It has a tiny 60hz transformer to power CONTROL ckt .
Ill replace it w/ 80 to 260 VAC switcher , then add big
Caps and Diodes to MOD it to run on 110vac . Now its 220vac ONLY .
They use a current mode controller chip and 4 surface mnt NPN's to drive
yet another transformer to drive the IGBT gates ! waste of circuitry !
Even if transformer doesnt have a winding to drive the bases , you can add
a tiny toroid in output , as all C.F. flourescents have ,..
ALL cost effective IBMPC power supplies saturate the transformer , to
save a chip . But output has a chip that sends back a signal to control the
MJE13007 , ussualy just to shut it dn prematurely , then allow it to self start
into the next cycle ...
One MJE13007 and a tiny transformer makes 450 watts ! That was my plan til HF
inverter showed up . I bought 20 PC power supplies from MCM and started
figuring how to parallel them to weld with .
Inverters can run 100% duty cycle with low cost MODs .
Plasma cut be MODed to do 40 amps at 100% duty cycle .
e-welder can do 130 amps at 100% .
The Ferrite transformer is the limit .
It stops working at a very low "Curie" temperature .
simply force a little air over it ! e-welder has fan far away ..
I also got 6 HF 13HP gas engines , i will MOD to Atkison cycle ,
since the CAM allready has centrifugal advance ...
I got 2 , 13hp , 5500 WATT gas GenSets .. they use brushes in alternator .
Ill be busy for months modifying this stuff !
BTW WiFi will be your new cell phone . The world will link up WiFi
and a new GUI method of communication . No voice , nor text , you will
have 8 buttons that send ICONs and Thumbnails and European traffic warnings
that will be understood by all humans , no need to translate ...
The B.W. is very wide . You can vu a Hollywood movie on WiFi , and with a dish
you can link at over 80 miles . The world will simply store and forward in
a cooperative way .. Also the new pocket PC is a GP2X game box , w/
80 GB HDD .. some one hooked a WiFi to GP2X .
Much easier to do in Linux ,, than in WXP op systems
snipped-for-privacy@yahoo.com from Guadalajara MX ..
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It may be easier to do in linux. Too bad it turns out unreadable text.
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snipped-for-privacy@yahoo.com wrote:

Hi, Which compressor would last longer? Belt driven or direct driven)lubed or oil less)? de VE6CGX
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wrote:

in general an industrial belt drive compressor will run 24X7 for years, I'd expect an HF compressor in similar duty to be suffering in a month or less. My quincy, bought used, is at least 35 years old.
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Snip

You really dont want all three coming on at the same time or running at the same time either. A breaker could be blown very easily with 2 or more running at the same time not to mention if 2 came on at the same time.

Yeah, that is NOT likely. When considering 3 db, the increase in loudness is not a linear one. Each increase in db level is significantly louder than the previous step increase. Consider that 2 trumpets played at the same db could "sound" 3 times louder than 1 palyed at the same db.
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On Sun, 14 Dec 2008 09:07:39 -0600, Leon wrote:

What I was taught long ago (I have no cite as to its accuracy) is that two instruments are 50% louder than one. Three are 67% louder than one and so on. That no number of instruments ever gets you to twice as loud.
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log 2 = .3 (the definition of a decibel is the 10 x log of acoustical energy, iirc).
The decibel is not "hard" physics, like energy, absolute temp, etc, but rather a description of how we perceive acoustical energy. And is only approximate at that, with freq, loudness itself, spatial considerations, etc.
Play one trumpet at 60 db. Play second trumpet at 60 db. Play both trumpets together, you "hear" 63 db. The acoustical energy has indeed doubled, but not the perceived loudness. Or so I lernt.
The point being, "loudness" is best achieved by modifying the original source, not adding original sources. Which works to a shop's advantage, in this case.
Not saying that a smaller compressor is necessarily quieter than a larger compressor -- I have a Sam's Club 10 gal Alton (an unusual direct drive WITH OIL!) that is so effingly loud, I use it as an alarm for low pressure!!!
The point was, that IF you have a quiet small compressor, you won't greatly add to the noise factor by adding more quiet small compressors.
And, to the other points, indeed, you would not want them coming on all at once, or even operating all at once (unless necessary), as that defeats the purpose of staging. And yeah, circuitry is always an issue, which is why God invented breaker panels.
And, not saying that the three operating together would be more efficient than one big one -- just more efficient if the air load varies widely.
Note that there are probably a few ways to stage smaller compressors. The obvious way is via pressure -- set each pressure switch successively lower pressure, indicating demand. Another way is to use timed relays to sense when one compressor has been operating beyond a preset time, also indicating demand.
I'm a big fan of using volume to compensate for a small-ish compressor head. Can really help if electricity is limited.
Also, a compressor that is a little too large for the electrical service can be helped by changing pulley sizes, which if changed to reduce motor load will also reduce cfm. otoh, you could reverse this strategy to bump up cfm, if nec.
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"Proctologically Violated" wrote:

Only if both are exactly in phase. Anything less, and the increase is under 3 dB. If they are exactly 180 degrees out of phase and at the same level, they cancel each other.
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There ya go! So just get two identical compressors, delay the start of the second one by 1/2 wavelength, and you will have zero db air!!
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