Air Compressor Horsepower/Wattage/Amperage

On Sun, 24 Jun 2007 07:20:40 -0400, Ron Rosenfeld
wrote:

On Mon, 18 Jun 2007 13:49:59 -0700, "RST Engineering"
wrote:

I thought I had this compressor power thing down to a pretty good science
until they started screwing around with "rated watts" and "peak power" and
all that crap that makes their compressor look really good until you go to
use it.

Back when we were using "real" horsepower I used a figure of 750 (to make it
easy to calculate, I believe 746 is the actual number) watts per horsepower
and an efficiency factor of 85% so that a one horse motor would take 860
watts to do the actual work.

But then you multiplied that times two for "starting" wattage for a couple
of seconds to give 1725 watts under start and then times three for starting
under some volume of air left in the compressor reservoir or about 2600
watts.

That presses my 2200 watt continuous duty (2800 watts peak) fairly close to
the load limit, but certainly gives a margin for error that seems
reasonable.

I happen to have a Sears 1HP compressor (1.5HP Peak). Mine has a nameplate
rating of 10.5A @ 120V. I could not see/locate the nameplate on the motor
itself, so I figured a 52.5A startup surge (5X). My inverter has a 78A
peak capacity (46A continuous) so I figured things would work -- and they
have.

Your 2800W peak generator translates to 23.3A at 120VAC. If you have the
same Sears compressor as I do, I'm not surprised that the generator will
have a problem starting it.

I just measured my little suitcase compressor. It's a dry pump with a
nameplate rating of 115V, 15A, 7.9cfm and 4cfm (presumably at 40 and
90psi). Running off an inverter - tank empty starting current 33.8A.
Running current 11.6A. Normal starting current (after tank drops to
about 80psi, 32.4A (reflects warmed up compressor I guess). Despite
the startup current, this compressor runs fine off my $300 cheapie
3500W rated generator, which in reality is only good for about 2000W
continuous. (inflated rating plus 4500' elevation)

Wayne