Ignoramus3971 wrote:
It is not able to fully spin up when unloaders kick in. That's the
issue. It starts spinning, accelerates slowly, then unloaders kick in,
and it slows down, then it blows the overload.

Are unloaders adjustable?

Well, it depends on how the things work. In most systems, air
pressure operates the unloader. Apply air pressure, and a pin
drops down into the intake port and tips the valve disc to hold
the intake valve open. On many of the larger Quincy's, the
unloader is operated by a valve that is controlled by oil pressure.
That valve has to cut off air supply and then bleed the pressure
off the unloaders to make the compressor start pumping. If you
can find where the air escapes when the valve releases the air,
and put an orifice there, you should be able to delay the
loading of the compressor. The Quincy hydraulic unloader
control valve # 110827 (may not be similar to the one on your
compressor) has a 1/4" NPT oil port, and a 1/8" NPT air supply
port opposite it. It has a 1/8" NPT outlet to the unloaders on
the side. Opposite that is a slanted passage marked "vent" on
the drawing. You might be able to epoxy a thin brass tube to
the vent, and then attach a needle valve to the brass tube to
get the delay you need. If your phase converter takes too long
to spin it up, my guess is the VFD won't be able to go a whole
lot faster. On the other hand, it could manage a nice
acceleration with perfect phase balance, so that the overloads
don't trip. (You wouldn't have the overloads conneced, anyway,
as the VFD can sense an overload.)

Jon