I tried connecting the VFD to the compressor. The VFD is a 18.5 kW (25
HP) rated Siemens SED2 VFD. Its target market is HVAC. The compressor
is a 10 HP motor.

It was kind of unsuccessful. I made quite a few observations.

1) The motor, when VFD turns on, only spins at maybe 60 to 90 RPM. And
struggles to turn the compressor.

2) The VFD has no troubles spinning up an unloaded 1 HP motor (just
for test).

3) I measured DC bus voltage when the VFD was powering up the 10 HP
motor, it was about 220v.

4) I used a straight piece of 10 ga wire instead of a DC choke to
connect the DC bus.

5) If I measure voltage on the output, with my nice multimeter
(knowing that I should not trust the result too much due to square
wave), I get something like 70-90 volts instead of 220. That's what
the read on panel reads if I set it to display voltage.

Logically, a 25 HP VFD should spin up the 10 HP motor.

I looked into the manual, available at

http://igor.chudov.com/manuals/

and found reference to parameter 1300, V/F curve. It talks about
different selections, such as linear V/F, parabolic etc. It says that
linear V/F is better for constant torque and "positive displacement
pumps".

Due to family stuff, I had no time to actually try changing this, does
it sound as though this could solve it? The wikipedia Pump article
seems to imply that my piston compressor is a positive displacement
pump.

Anyway, it was a somewhat frustrating day in this respect. I will
followup to this once I try it in an hour or so.

i