Opening a set of contacts on the output of a VFD is a setup for blowing transistors. As contacts open, they strike an arc until the dielectric of the air in the separation of the contacts is enough to extinguish it. Because technically the output of a VFD is DC pulses, that arc lasts longer than it otherwise would, and the voltage across the contacts increases as the separation starts to act like a capacitor. This sets up a condition in which the voltage on the output of the transistors becomes very high, usually high enough to cause the silicon layers making up the switch portion of the transistor to get microscopic burn through holes. That damage is not usually immediately noticeable but it is incremental; it keeps getting worse every time you do it until the transistor shorts and ruptures. It's all over then and the damage is often spread out across the drive to where it isn't repairable.
But other than that, no problemo...
As mentioned, you should never open contacts down stream of a VFD that is running a motor. If your operators are too lazy to follow simple rules, you must add an aux. contact in the disconnect switch wired back to one of the drive inputs programmed to disable the transistors (base block) so that by the time the main contacts of the disconnect open, the transistors are already turned off and not conducting.
