> On Wed, 30 Nov 1994, after I wrote:
> John Lupien replied:
> > That can't be right. It must mean "230v only, 15 amps @ 230 volts".
> > 15 amps at 115 volts is 2.3 hp, can't do any better, sorry...
> And I now retort:
> Well, what they said exactly was 120 volts, 15 amps.
OK, so it's 2.4 HP.
> Maybe they *mean* 230 volts, but they said 120 in two
> different catalogs. I know it doesn't add up; I said it doesn't add up.
> I'm not a dummy, John.
Nor would I think you are. I think there is a dummy involved somewhere,
but it seems to me that it's somewhere upstream of the catalog mailers.
> But if, as you suspect, the 120 volts should read 230 volts, then you have
> to explain why a 4 hp compressor that draws 15 amps @ 230 volts produces
> about the same air output as a 2 hp compressor that draws 15 amps @ 115
> volts.
Well the actual energy that can be extracted from 90psi at 6.4cfm is
about 1.5 HP, but there's a fair amount of inefficiency in a single-stage
compressor, so that's not right out of the ballpark.
> I tried to call Harbor Freight to check, but got tired of being on hold.
> It's 1-800-423-2567 if you want to try.
Thanks, I've tried that, and it's known to be hopeless.
> It does not matter; my point was that hp ratings should be taken with a
> grain of salt as predictors of output.
You are quite right about that. I have the 2.5HP 60 gallon CH compressor,
and I can say that it doesn't quite meet my needs. I have plans to build in
a much bigger tank, like 120 gallons extra, when I do the air plumbing for
the garage. That way I can do a good bit more work between waiting for the
pressure to come back. Naturally, it will take 3 times as long for the
pressure to build again, but that's not too bad, I can take a coffee break
or something. Currently I'm getting about 5 minutes of work, 2.5 minutes of
waiting. 15 minutes of work would be nice...
> It's irrelevant, but I suspect
> horsepower ratings should be taken with a grain of salt as expressions of
> horsepower, too.
Yeah, I think that it may be more of a "motor starting" power impulse
rating or something.
--
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John R. Lupien
lupienj@wal.hp.com
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