It's actually a fan. It keeps constant pressure (vacuum, underpressure) in a big filtering silo. It works better now, PID parameters were too aggressive.
It didn't occur to me that this type of behavior could create mechanical problems, I was only worrying about VFD.
Thanks for replies fellas.
There are no forbidden frequencies, it is just an unstable PID. I was worried about VFD, if frequent run/brake/run/brake... cycle could harm it.
Dear all,
I have one installation with VFD running a PID loop and keeping a constant pressure. At some lower set point values it starts to oscillate, not much, at maximum output fluctuates +/-1Hz per second. It doesn't affect the process, but I have to leave it that way for about a week before...
