On Jun 19, 10:09 pm, "RST Engineering"
wrote:

Forget I said compressor except for the calculation of wattage/amperage
required in a partially full compressor tank. All I need to do is to
rationally or experimentally match a generator with a motor that HAPPENS to
be connected to a compressor.

To hell with inflated horsepower claims, airplane engines, and the like.
Simple question. One horse compressor motor. How much peak
amperage/wattage to start it zero PSI in the tank, how much to keep it
running to max PSI of (say) 100 psi, and how much to REstart it when the
pressure drops to 80 psi.

A belt-driven compressor relies on its flywheel to drive
it through a compression stroke. The electric motor produces an amount
of torque that probably won't be enough to run a compressor without a
flywheel, or a compressor that has been stopped and has to start
against pressure. For that reason, most compressors have an unloading
system that exhaust the discharge line to let the compressor spool up
before it has to work against pressure.
I've tried to measure peak amps on a starting motor, and it
isn't easy. The mass it is driving affects it a lot, and even with no
load a squirrel-cage motor draws a bunch, maybe double its running
current, but it's so brief that the circuit breaker puts up with it.
Running off a generator means either a limited current supply that
causes massive voltage drop on start, so that the motor won't come up
to speed. And if you are starting against 80 psi with a non-unloaded
compressor you will need both a BIG motor and a BIG generator.
Sorry I can't give you numbers. Most of us won't be able to. There
are too many variables and I don't have even the basic info. Even the
motor type affects it: capacitor or non-cap start, compressor size and
mass, on and on. What is the motor rated to start? A low load like a
jet pump or table saw, or a heavy load like a big fan? I built my own
compressor years ago using an old truck compressor, and drove it with
a 1-horse 3450 RPM jet-pump motor. This motor has next to no starting
torque, so the compressor is set up to run continuously and the
original unloaders in it are engaged when tank pressure is reached. I
have to manually unload it to start it, and shut it off when I'm done.
Truck compressors have a device that hold the intake valves open when
the tank is full so that it can't compress anymore. The crankshaft
keeps turning.

Dan