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

and then times three for starting
under some volume of air left in the compressor reservoir or ...

That SHOULD not be an issue. A properly designed compressor will
start "unloaded" so that only the motor inrush, assumed at 4-8 times
running for an industrial motor, less for the capacitor start single
phase units, is an issue.

Before I tell elebendy bazillion Kitplanes readers that the Harbor Freight
Subaru 2200 watt generator will drive the Sears 1 horse compressor (and even
worse, buy the Sears compressor only to not have it work), will somebody
please do a reality check on me for horsepower/watts for this lashup.

(Note ... convert watts to amperes by dividing watts by 120 volts).
860w=7a 1725w=14a 2600w=21a

Sears and Harbor Freight are damned near clueless about this sort of stuff.
If anybody has a source I can reference for running/starting/starting under
load for air compressors it would be well received.

I'd LIKE to buy a 3/4 horse compressor but they go from the kiddie's 1/4
horse toy straight to one horse with darned little in between.

Based on the units with bigger motors, the HP is a marketing game, not
real. Industry assumes ... ASSUMES ... about 4 cfm (to 100 psi) per
horsepower. Small units will be less efficient ...maybe 3 cfm. Large
(50 hp+) units will be a little better, perhaps 4.4 or so.

It is often missed that capacity is INLET air.

From what I've read and seen, trying to do calculations, most CFM
ratings are 30% to 100% unrealistic. Most horsepower ratings are 50%
to 300% unrealistic.

Look at it this way ... 2 HP is 1500 watts ... throw in single phase
efficientcy of maybe 65%, you are at 2200 watts in for 2HP out ...
about 20 amps on 120V. It would be my OPINION that the 2200 watt
generator would start and run a 1HP motor with little trouble.

There is a guy (a lawyer, darnit) who has done some research in this;
I understand that he was involved in a complaint against the wild
numbers used in advertising ... take alook at
http://users.goldengate.net/~kbrady/motors.pdf for some interesting
info.