Ignoramus10340 wrote:
I have found approximately 170 microfarads of extra capacitance, in
bits and pieces. Wired them in. Set the V/f curve to linear (setting
0). I set the max frequency to 20 Hz.

With that, the compressor spins, runs, pumps, and the motor goes to
33% of its proper RPM. DC Bus Voltage drops from 320 volts to
approximately 255 volts when the motor is in action.

That proves the problem is where we expected it to be. But, 255
V is still really low. You should be able to develop 90 V RMS
at that cap voltage (half of 255/1.414). Hmmm, (90/240) * 60 Hz
= 22.5 Hz, so that is right on the money, too.
So it is a little victory, sort of.

Trying to set frequency to 25 Hz results in the motor getting stuck
and stopped.

So, it seems that extra capacitance helps, but I need much more.

I have ordered extra capacitors on eBay (electrolytic type, rated at
500 VDC).

I hope that maybe 1,000 uF would be about right.
Well, you need a hold-up time of about 8 ms, and the line
current for 10 Hp at 340 VDC is about 22 A, so 1000 uf (0.001 F)
will sag at a rate of 22,000 V/sec. Ugh, if I did this right,
it will still lose 176 V between line peaks. This doesn't sound
right, but I don't see my error. Increasing the cap bank to
5000 uF (wow, that's a lot of caps at 500 V) would reduce the
sag to about 34 V, which is probably the kind of voltage ripple
you are aiming for (~10%).

Jon