Ignoramus19502 wrote:
So, everyone is saying that it is bad to place a switch between a
motor and a VFD (if it is switched when running, of course).

I am sure that it is true, but I am not sure why. Also, is there
something that can be done to mitigate a switch between VFD and
motor being switched when running.

The question is not entirely idle in nature and pertains to my lathe.

I think the inductive spike that occurs on opening the switch would be
really hard on the output FETS of the VFD.

The current surge of trying to start a motor at nominal speed would be
large. I am not sure it is always the case, but I have my VFD set up to
ramp up to 60 Hz over a couple of second period to reduce the startup
surge. I am not using the variable speed aspect of the VFD, just as a
source of 3 phase 60Hz because my lathe has a good variable speed drive.

As Joe mentioned, "plug" reversing would give really large current
surges and I think would put double the output voltage across the FETs
for a short time.

About the only thing I can think of that might mitigate the problem is a
large idler motor, but that would get rid of my two favorite aspects of
the VFD - quiet and low additional power drain. If you really want to
switch the outputs, I would probably just build a rotary phase
converter. You are 80% there with the idler motor.

Good Luck,

BobH