> This is where specmanship comes in. Some (most? all?) manufacturers of
> compressors for home use will stick an over sized motor so they can make
> horsepower claims in their advertising. So you have a 5 horse
> motor but it
> is only loaded to 1.5 so it doesn't pull more amps than a 15 amp circuit
> will allow.
That's possible certainly, but I've never seen one that had a too-large
motor on it. On the contrary, all of the units I've looked at have motor
nameplates that call the "hp" rating a lie.
For example, my Sanborn is a "2.5 hp" compressor, but the motor nameplate
says 15 amps @ 115 volts, which is about 1725 volt-amps. Induction motors
have a lousy power factor, call it 80, so that's about 1380 watts.
Efficiency is likewise not wonderful for induction motors, maybe 90% if it's
really good, so that's about 1242 watts power output or 1.6 horsepower.
A simpler rule of thumb is that it takes 1000 volt-amps (1 kva) to make 1
hp, and you can't reliably get more than about 1800 volt-amps out of an
ordinary house outlet.
BTW, it's also rated for 8.1 scfm @ 40 psi, and I believe it met all these
specs pretty close when it was new. Now that it's old and worn, it doesn't
make as much air as it used to, and no longer blows the breaker if there's
anything else active on the circuit.
> The only spec that matters is SCFM at the pressure of concern
Except for my original point of where you can plug it in, I agree.
Randall
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