“Ok, so I used shielded VFD cable in my powerflex installation…where do I put the shield again?”
Electrical noise, or more specifically high frequency electrical noise (also called common mode), tends to be the gremlin of modern electrical panels. Noise issues can cause random faults and headaches, not to mention lost production. In fact, I have seen cases in the past where two “identical” panels were deployed, and the gremlin decides to plague only one! Inside any modern electrical cabinet, the major source of this type of noise are IGBT based inverters, or VFDs (Variable Frequency Drives).
Several documents talk about how to mitigate the issue of high frequency electrical noise. Rumsey recommends using a shielded VFD cable, galvanized Hoffman back panels, and worry about bonding, not just electrical grounds.
So now that you’ve done all that – what should you do with this braided shield? I’ve seen some terminate it on the backplane with a strap clamp, and this is good. Others twist the shield into a single strand and land it under a ground (GND) screw which is not as effective. Still others simply don’t connect it at all, use unshielded cord or even use heat shrinkable tubing to tidy it up, then land that under a strap. Not good! If there are no other components in the cabinet this may be acceptable. However, most machines have multiple drives and noise sensitive devices in the same cabinet and quite possibly less than ideal separation between the “clean” and “dirty” noise areas. Have you ever seen a PLC input or output blink oddly? This is a sure sign of trouble.
Rumsey recommends an EMC grounding plate from Rockwell Automation. EMC plates, available for all the 520 and 750 series drive, are marketed as a requirement for CE. An EMC plate provides a convenient location to land the power cable shields with clamps included and solves the bonding problem at the drive end. The shielded VFD cable is continued beyond the cabinet wall and helps with mitigating noise INSIDE the cabinet in addition to outside. Taking this extra step can also make your installation easier, provide a modicum or strain relief for the screw terminals, and is more robust to boot. Top it off with a shield terminating cable gland at the motor terminal box, and you’ve completed a solid solution to your electrical noise from VFDs.