Electrolytic capacitors have a finite lifespan, when under power at maximum operating conditions something like 2000-5000 hours, but can vary quite a bit. VFDs have internal timers which keep track of the component operating life hours. Electrolytic capacitors are also prone to failure when powering them up after many months/years of not being used. If a VFD sitting up the shelf for more than a year, they should slowly be brought up to full operating voltage. Each manufacturer has a specific sequence they recommend, but suffice it to say, VFDs in storage should have voltage applied to them at least annually. Also rapid on/off cycling of a VFD can also cause damage to the precharge circuit which inserts a resistor or PTC into the capacitor charging circuit on power up. My general recommendation is to turn on the power of the VFD equipment when you plan on using it, shut off the power at the end of the day or when you are done using your machines. If you turn off your VFD and decide to turn it back on, wait at least 5 minutes before doing so. Also from a safety issue, I have a power disconnect at each of my machines. If I am working on them, I unplug them from the wall (or turn off the breaker) and wait for the VFD capacitors to fully discharge (about 5 minutes).
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