Have you tried a proper USB isolator in your USB connection? These range in price from a few dollars for a hobby grade device to around $100 for a proper industrial spec isolator such as the Advantech brand. I can’t recommend what to buy because my setup uses an Ethernet connection to my motion controller so is galvanically isolated by the transformers in the Ethernet ports.
I must add that the fundamental physics behind shielding is not well understood and some of that misunderstanding is on display on this thread. Simplistically there are two sources of electromagnetic interference, electrostatic due to voltage and magnetic due to current flow. Electrostatic interference is typically mitigated by earthed screens or shields and in the first instance magnetic interference is mitigated by tightly twisted paired wiring. Cable shielding can also provide protection against magnetic interference but only if it is properly earthed at both ends. At this point arguments generally erupt about the folly of creating an earth loop. Creating a loop is deliberate and provides the mechanism for the magnetic shielding to work. The undesirable magnetic field causes current to flow around the loop which in turn radiates its own magnetic field. The two magnetic fields are in opposite directions and add together giving a resultant tending towards zero.? Rather than attempting to explain this further I would refer group members to a world renowned expert: Henry W Ott and his highly regarded textbook “Noise Reduction Techniques in Electronic Systems” and further, to carefully read installation instructions for VFDs from reputable manufacturers such as ABB, Siemens, Rockwell etc. Good quality VFDs usually have a substantial clamping arrangement for earthing the shield of the motor cable close to the VFD terminals.? The cable shield should also be well earthed to the motor terminal box using a good quality EMC grounding gland. Good quality shielded VFD cable and EMC glands are available from Lapp Kabel and other specialist manufacturers.
