I can't say I've dealt directly with this, but it could be an interlock system between the VFD and a disconnect near the equipment. VFDs don't like having their load shut off while running. If you have a piece of equipment and a required disconnect near it, then the VFD powering the equipment somewhere else, these contacts would signal the VFD to shut down prior to the actual equipment being powered off.
Edit to add: I would guess the voltage would be that of the VFD inputs. The contacts would somehow need to send their signal before power to the equipment is actually removed.
Correct. In a NEMA type
* disconnect switch, the Aux Contacts of the switch are always operated by the handle, not the switch mechanism, so the Aux will change state PRIOR to the main contacts. So you wire up the Aux contact of the disconnect to an "Enable" contact in the VFD, so that if the VFD is in a Run state when someone pulls the handle on the local disconnect, the Aux contact opens to disable the output of the VFD, BEFORE the main contacts of the disconnect switch open.
If you don't do this and someone opens the disconnect switch while the VFD is in a Run state, the PWM pulses from the VFD, being DC, cause a very high voltage transient in the power circuits, which can damage the transistors in the VFD, i.e. "blow it up". While most newer VFDs have SOME protection built in against this now, you never know for sure, so it is just prudent to do this as a standard practice. In addition even if a VFD has the internal protection built-in, it still stresses the components every time it happens, so you can think of it as "incremental damage" that will EVENTUALLY cause the transistors to fail.
* Some of the inexpensive small IEC disconnects that people sell now will use a 4th pole of the main contacts
AS the "Aux Contact", but that will not work for this application because they will change state at the same time, so too late to protect the transistors. You have to check to make sure.
