In article ,
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.

One classic maneuver is to slam the switch from forward to reverse in
one fluid motion, thus putting a spinning motor across the power line
backwards. The applied power opposes the rotation of the motor, forcing
it to deaccelerate to zero and then accelerate in the opposite
direction, all very quickly. The motor has no problem with this, but
the current surge is substantial.

I'm sure that there are also substantial voltage spikes involved.

I've heard of people getting away with this, if the VFD was sufficiently
oversize compared to the motor, but I bet that the VFD manufacturer is
agast.

I suppose that another approach is to use the VFD only for generation of
bulk 3 phase power, giving up on the variable part. If one has a large
3 phase motor always running, that motor will supply the startup surges,
so long as the large motor is large enough to dominate the lathe motor.

Joe Gwinn