oldjag wrote:

Well I am using a rotary phase converter. I've never checked the
output voltage/current balance at light load, ie the grinder at 3/4
hp. It's balanced to less than 5 volts difference on each phase when I
run my 10hp motor on my lathe. I'll try running the grinder with the
lathe motor on to see if there is a difference.

I really really doubt it's your phase converter. I suggest you balance your
wheel, dress it, break the corners with a dressing stone, and balance it again,
make sure the ways are lubricated correctly, check the belts, and try your test
again. If your wheel adapter isn't set up for balancing you can get one that is
from Chevalier's office in Los Angeles. I know because I ordered 2 of 'em.

GWE