<VV> RE: VirtualVairs digest, Vol 1 #580 - 8 msgs

[srmarti]

> At the risk of starting a long thread about compressors, here goes:
> There's a lawsuit going on now about the way the compressor manufactures
> rate their motors.  Most 'home' compressors are rated at 'peak'
> horsepower.  The Harbor freight unit is rated this way.  I don't know
> about the Lowes unit, I couldn't find it on their website.  Peak
> horsepower is basically 'stall horsepower'.  The motor isn't turning, but
> is developing a lot of torque just before the fuse blows.  Obviously, it
> can't pump any air when it isn't turning.  The amount of air you need
> depends on the rating of your tools, and how continuously you use them.
> A small tank 1 hp compressor will run an air chisel (for about 15
> seconds) and won't be satisfactory for that use. Ditto for an impact
> wrench.  It probably won't be able to remove the lug nuts that you
> tightened by hand last year.  I have a Coleman compressor from Home
> Despot that says 5 HP, but the amps rating on the motor calculates out to
> 2.2 hp.  7.1 CFM @ 40, 5.7 CFM @ 90.  It is adequate for small tools and
> air chisels, but is nearly useless with a 1/2 inch drive impact wrench.
> My electric impact wrench is much better.  Trouble is, the air compressor
> was bought to match the impact wrench rating.  somebody is lying here.
> Either the compressor doesn't put out the volume it is rated at, or the
> impact wrench uses more air than it was rated for.
>
> The real way to compare the output of compressors is to look at the CFM
> ratings.  SCFM stands for standard CFM, where the output is corrected to
> some industry standard.  It is probably a more reliable figure than
> (nonstandard) CFM.
>
> The Central Pneumatic compressor is a direct coupled compressor.  In the
> picture is looks to be a single cylinder unit.  I don't like these and
> prefer a belt drive with at least two cylinders.  I believe the belt
> drive compressors last much longer in use.  Of course, home use is easier
> on tools than industrial use, and you might not use it up for a long
> time, so it's your choice.
>
> Horsepower, if I remember my numbers right, is (volts * amps)/748.  Check
> the label rating to see what the actual horsepower is, so you'll know how
> badly you are being cheated, but go by the CFM ratings.  Occasionally, a
> unit that goes up to 150 psi is really handy with that impact gun, but be
> sure to use a regulator to reduce the pressure, under normal use.
>
> Other things come into question.  If you have to pay freight from Harbor
> Freight, then factor that into the cost of the unit.  It may only come by
> truck to a dock.
>
> My preferences:  I like a belt driven compressor.  They are less noisy
> and tend to last longer.  'Oil-less' compressors are cheap disposables,
> and have to be rebuilt frequently.  Better to use an oil separator/filter
> unit if you are painting.  It's difficult to find a real workhorse
> compressor in 110 volts.  Yes, you can make do, but they are really
> horsepower (and CFM) limited.  My 5hp rated 2.2 actual hp compressor has
> to have a 35 amp circuit simply because of that 5 hp startup torque.
>
> Rules of thumb for air compressors:
> 1) Always buy a larger unit then you think you need.
> 2) A large tank helps by delaying the moment the compressor comes on, but
> also takes longer to fill.
> 3) Don't forget that next year you will buy a sandblast cabinet and need
> a lot of air.
> 4) If the wife says you can have an air compressor, don't tell her that
> there are smaller cheaper ones than the one you want.
>
> John Ryall
>
>

I never understood how they got away with the horsepower claim.  I have a
real 5 hp motor driving my air compressor. That's about as much as practical
for single phase 220vac.  You can sandblast your whole car with it in a
weekend, or run a correctly sized blast cabinet all. day.  It simply isn't
possible to run that big a motor on residential 110vac.

It's about 746 watts/hp or 3730 watts.  That means a current draw of about
34 amps.  That's wrongly assuming the motor is 100% efficient.  Maximum for
110vac is typical 15 or 20 amps.  That why none of the plug in electric
heaters are more than 1500 watts.

The peak hp number can't have any meaning in the real world.  Just how long
could this peak power be produced?  A fraction of a second?

Pity the poor trusting customer that doesn't do the math.
They should the a lawsuit about this one.  IMHO it's fraud.

Steve Marti