Shop Talkers,
Regarding the recent discussion on the ratings of air compressors, I
think many manufacturers (perhaps some more than others) are being
rather optimistic.
I have a basic single-stage Campbell Hausfeld "6 HP", cast iron (oil
type) compressor with a 60 gallon tank. It is rated at 9.1 cfm @90 psi
and 11.5 @40.
It has a Doerr 230 volt, 15 amp motor, rated for continuous use with a
service factor of 1.0 (you can multiply the hp*service factor to
determine the actual continuous HP potential safely delivered at 104
degrees or less ambient temperature). To compute electrical HP, you
typically multiply the wattage by .00134. For this particular motor, the
HP would be:
15 amp*230 volts*.00134 or 4.62 HP
Now I do know there have been advances in motor efficiencies, but I do
think some manufacturers are taking considerable liberty in their
ratings. If you compare the more conservative ratings used by industrial
grade manufactures, say Quincy, they have two "5 HP" compressors, one
rated at 22 amps, and another rated at 28 amps. The 22 amp motor
computes out to 6.78 HP and the 28 amp to 8.63 HP, this would give the
28 amp motor a service factor rating of 1.73, clearly built for
industrial applications.
I guess what I am trying to say here is that if you see a 15 amp motor
rated at "6.5 HP" and a 28 amp motor rated at "5 HP", it doesn't take a
rocket scientist to figure out that there are very different
"approaches" to ratings here.
IMHO, the Sears ratings seem high even compared to Campbell Hausfeld.
Regarding the oiless compressors, they tend to be more compact, lighter
and considerably noisier than the oil type. They originally were used
for portable applications (nail gun compressors, etc.) where weight and
size are major considerations, but now are making their way into
stationary models, my guess is because they are considerably less
expensive to build.
The Cambell Hausfeld literature describes the oiless type as being
appropriate for lighter-duty use and the oil type compressors as being
for heavier duty applications. Since they make both types, and price the
oiless lower, there may be some merit in this comparison. For what it
is worth, all of the industrial grade, dual stage compressors use cast
iron, oil type pumps.
I don't know if any of this helps, but I would suggest that you look at
the amperage ratings of the motor and not the "Big 6.5" painted on the
tank.
doug shook
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