On Mon, 10 Sep 2007 15:44:11 -0500, Ignoramus2883
wrote:

Bruce, forgive my ignorance, but let's say that a compressor has an
unloader valve (even better, electrically controlled). Then the motor
can be slowly ramped up to speed using a electronic drive, with the
unloader engaging at the proper moment, right?

If it has a centrifugal unloader that will not start pumping till
the compressor comes up to full speed and the oil pressure comes up,
like the higher end units from I-R and Quincy, yes the motor is
essentially starting unloaded - well, except for getting the rotating
mass of the motor and compressor moving.

You will still see a current spike at start, but not as big and it
will ramp down faster because the motor isn't trying to produce work
before getting up to speed. And for that, a start controller or VFD
would be even easier on the motor and your power bill.

My compressor is single phase, but my Bridgeport is 3 phase, on a VFD,
set to accelerate over 1 second when turned on. I am sure that it does
not draw anywhere close to the LRA when accelerating that slowly.

What am I missing?

That your Bridgeport is manually started when you are standing right
there, and if something goes seriously wrong you can kill the power.

Most compressors are designed to a price point (keep the mfg. costs
as cheap as practical while meeting the specs for capacity and
durability) so they won't spend for a soft-start. Unloaders cost
more, so they are only built into the higher end units. And the
compressor is expected to run unattended and start itself reliably
every time air is called for, for decades, without anyone needing to
run up and slap the power switch off if it stalls.

So the K.I.S.S. Principle says leave the electronics out if you
don't really need them.

A VFD will work if you just set it at 60.0 Hz, but the whole idea
behind it is Variable Frequency. The only reason I'd install one on a
compressor is if you want to play around with Logic Controllers and
adjusting the motor speed to the air consumption.

You can install one if you have one laying around and you want to
(Play) experiment. But I still stand by "If it ain't broke..."

-- Bruce --