If I were designing that circuit, I'd put a resister in front of each
diode pair,and one behind the output. If you only had two phases you
would get the big voltage drop that Iggy sees. Adding a cap won't help.

Jon Elson wrote:
Ignoramus19266 wrote:

After a lot of thinking, I am beginning to suspect that the VFD does
more than just monitor bus voltage. I will try to ascertain facts a
little better before going on the assumption that only bus voltage is
the issue.
Ugh! If it uses an algorithm to detect the ripple frequency in the DC
bus, you have a big problem. If it sees 360 Hz ripple, all is OK, if it
sees 120 Hz ripple, give phase fault indication.

That would be bad news, and if it is in the microcontroller's firmware,
really hard to get around.

But, you ALSO see a large dip in DC voltage, which I find really hard to
understand, especially at no load. Given the thing is probably drawing
just a few watts to drive the electronics, (and I think most of that was
supposed to come from the control supply, powered separately) there
should be extremely little ripple and the DC voltage should go all the
way up to 1.414 * RMS line voltage. That has to be the key, and until
you know why it is well below this, the rest is speculation.

Jon