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Ralph Mowery

In article ,
says...

Especially since
the VFD manual says to not connect anything across the output.
Switches and capacitors are specifically mentioned.

We have the VFD powering only the spindle motor, which requires 3
phase power from a single phase source. We have no filtering on the
VFD output. Everything else I previously mentioned is powered by
220VAC single phase, which is full of AC line filters. If you suspect
that the spindle power is somehow getting into the computah, perhaps
some shielding conduit might be an easy fix?

http://eecoonline.com/vfd-load-filters/
See comments under "VFD Load Filters".

Where I worked we had hundreds of VFDs. They ranged from around 1/2 hp
to over 200 hp. Nothing but the motor goes on the output. On the
supply lines which were mostly 480 v 3 phase or some 120 V single phase
was a reactor. This is to keep the electricl noise generated by the VFD
out of the supply lines.

As the plant had many microprocessors/PLC and other computer equipment
the AC lines needed to be somewhat clean. Very difficult with all the
various equipmnet in the plant. Some times strange thing would seem to
hapen due to a spike of noise. It may not be on the AC line powering
the equipment,but on some of the sensor wiring.