On Fri, 03 Oct 2003 12:28:57 -0000, Chuck wrote:
The compressor currently has a 3/4 hp motor. At first glance I was kind of
depressed thinking "Gee, just about every compressor I see these days has 3
+ hp. Will this thing even work well?" In asking around I was told that
amperage plays a big part in the actual torque of the motor. This is where
I get confused... I thougth torque was the direct product of the motor's
hp. How does the amperage come into play? Can you have a "strong" or
"weak" 3/4 hp motor? What factors actually determine the torque? Or, am I
looking at this equation in the wrong way?

You're looking at things just fine. Most of the consumer grade compressors
you see today have a rating called "peak HP". What that really means is that
they lock the rotor and measure the peak current drawn as the windings
smoke, then calculate a theoretical HP figure from that. It is totally bogus.
HP is *zero* with a locked rotor.

The figure you want is actual continuous load running HP. That's what
your old motor gives you on its nameplate. (You can trust that figure
because the given maximum current draw, which calculates out to an
input power of 2.2 HP, if it were all real and not reactive, confirms they
aren't doing marketing magic with it.)

Where reading the current spec is useful is to cut through the marketing
bull****. Output power must always be less than input power (no machine
is 100% efficient). Input power is voltage times current, in your case
115 * 14.4, which yields 1656 watts, or 1656/746=2.2 HP. Now since
some of that current is reactive, and the 14.4 figure is a maximum instead
of a sustained value, real continuous load HP will be less than 2.2 HP.
Conservatively, probably based on allowable heat rise in the windings,
they're saying it is 0.75 HP, which is very believeable.

Very few (I'm tempted to say none) of the current crop of consumer
grade compressors will quote you a continuous running HP. They'd
call the compressor you've got *at least* a 2 HP unit, and probably
2 or 3 times that much. They lie. But at least now you have a basis
for understanding their lies, and a way to derate their claims to at
least the input power of their compressors. (Figuring actual efficiency
and allowable temperature rise in the motor windings to arrive at
a true continuous running horsepower figure is a bit tougher.)

Another way to compare is to look at the air delivery. A reasonably
well built single stage compressor will deliver about 3 SCFM @ 90PSI
per horsepower. Pitfalls here are that some advertising doesn't use
SCFM, but rather an ill defined CFM or "free air displacement" figure
instead. Also, some will give the air volume at a lower pressure (40 PSI
is common).

The figure given is rarely the continuous delivery rate, ether. They'll
give a duty cycle of less than 100%. That's because they're running
the system harder than temperature rise in the windings (and pump)
would allow it to be run on a continuous basis. So derate by the
duty cycle to get a true comparison.

Note that your motor can produce more than 0.75 HP for short
periods. So you could do like the consumer compressor marketers
do, and claim it to be larger than it is if you allow it to cool every
few minutes rather than running it continuously. This will allow
you to draw higher volumes of air for short periods (a big tank
lets you do that too).

So conservatively, by consumer marketing standards, you have
at least a 2 HP compressor. You should be able to draw 6 SCFM
at 90 PSI from the tank for at least short periods. That's enough
to run many air tools which are only operated in short bursts.

Gary