Bob;
Watch out for those "7.5A Motor" (or whatever) ratings that you find in ads
for vacuum cleaners, power tools, etc. The current that a motor draws
(electrical power INput) is only loosely related to its mechanical power
OUTput.
The key is to have a motor with high efficiency; a really cheap motor will
look great in an ad that touts its "amps" while a high quality motor will
have a higher "HP" rating with far fewer "amps". It isn't simply a matter of
"watts".
Wiring that motor to 240V will reduce its amperage because you are putting
the AC voltage across a different winding than you use for 120V. The power
will be about the same but the motor might have a slightly higher efficiency
on 240V.
Look for what you need from the compressor-- CFM @ 90psi is my criterion--
then compare the AC motor ratings. Get one with a BIG tank.
Home Depot has some nice compressors from time to time.
Regards, Neil Tucson, AZ
-----Original Message-----
From: Bob Jepson [mailto:Bobbyhotrods@attbi.com]
Sent: Saturday, March 01, 2003 3:14 PM
To: Land Speed
Subject: Electric motors and compressors
List Cognoscente,
Well, you've got me looking at these tool mail order houses now, and I
return to my often visited dream of near limitless compressed air in the
garage. I think prices are lower now than when I last investigated.
It's hard to compare performance because the power figures used are
misleading. If Ohm's law still reigns, P=EI and 746 Watts still make 1 Hp,
and
you're not going to get more than about 1 3/4 hp. out of a 120 volt, 15 amp
circuit, given some inefficiency. My question is some of these compressor
motors have the ability to be wired @240 volts as well. Can one assume if
the
motor is so wired it will produce twice the power, or will the current
simply
be halved to produce a more efficient wimpy result? Is looking at amperage a
good way to compare these things, or is cubic feet at pressure best? I want
a
US made, two stage, belt driven, cast iron w/crankcase twin, right?
Thanks, Bob Jepson in Beantown
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