I'm plagiarizing myself from another forum, but I'm lazy. The question there had to do with the power utility requiring that they install load shedding contactors ahead of everything, including VFDs. Same concept applies. The statement at the end had to do with his wondering if he should install a circuit to turn the VFD output off before allowing it to be powered down, not germane to this thread.
The risk is from when the VFD uses what's called a "pre-charge resistor" to deal with the capacitor charging current inrush. Capacitors charge virtually instantly, and do so by pulling the available fault current. So to avoid having that damage the VFD rectifier and the caps themselves, when power is first applied, it passes through some sore of "pre-charge circuit". ALL drives must have this, but there are several ways to do it. Small drives (up to maybe 15A) use what's called an NTC thermistor, a resistor that DEcreases its resistance drastically as it heats up. So when the circuit is dead, the resistor is cold and the resistance is high, so it acts as a current limiter when power is applied, then the resistance drops to near zero. But it's never actually zero, so it always has some heat hence the size limit. Next is to use a resistor, but have a contact that shunts power around it a second later. That means a relay and controls for the relay and a resistor and its heat. So above about 100A, that too becomes problematic. So after that, many drive designs use SCRs for the rectifier instead of diodes and when power is applied they ramp the voltage into the caps, just like a soft starter. That means having a firing control circuit, but that's now boiled down into a chip set. Then above another size again, maybe 400A or so, many mfrs go back to a pre-charge resistor because of economics.
So why do I mention all this? IF the drive in question uses an NTC thermistor or a pre-charge resistor circuit, energizing and de-energizing the drive every time it is not needed will stress that resistor and it will fail early. Not right away, but sooner than it was designed to. Adding a line reactor can help a little (and is a good idea anyway), but in general most VFD mfrs will tell you not to cycle power if it can be avoided, mostly because it increases THEIR risk if having to replace the drive under warranty (indicating how risky it can be). But if your drive is large enough to use an SCR front-end, but not so large as to go back to a pre-charge resistor, then the concept of removing power every time you are not using it becomes basically a non-issue.
How can you tell? You will have to find someone in the vendor's organization that knows, but sometimes you can figure it out by looking at the spare parts list. If it shows a DC bus resistor, bingo. As to whether or not it's worth it to trench a power-down circuit, no. Telling the drive to power down is necessary for opening a switch on the OUTPUT side, but has zero effect on the INPUT side. So if you have no control over the PoCo issue, I would just inform the owner of the increased likelihood of premature failure so that he knows what to expect and doesn't blame you, use a 5% line reactor and say your prayers.
