I have a few answers for you as I've been using VFDs for a number of years now. A VFD can have a number of different features depending on what model you acquire. Mine converts single phase to three phase (not all do this), through settings can run from 0 hertz through 180, will brake as slow or fast as you want through settings, and of coarse has reverse. In the USA electricity is normally 110-120 60 hertz. 3 phase motors RPMs are set for this standard. Say 1740 rpm at 60 hertz. At 30 Hertz the motor will be going half that speed, at 10 hertz it will be going around 175 RPM. Now you can also set your limits to go all the way to 180 hertz which would triple the speed of your motor to 5220 RPM.
Since you are running 3 phase you cannot hurt your motor going from full speed in one direction and throwing it into reverse so that it can run at full speed in the other but be a ware that although it will not damage the motor it could possible destroy the machine. If your lathe spindle has a screw on plate this sudden reverse can unscrew the chuck and you may catch it a couple of blocks down the street. I have my lathes wired so that my normal on/off forward/reverse lever on the lathe controls those operations as they always have. I have the VFD mounted above the lathes and use the rheostat on it to control the speed or to jog the lathe slowly. My lathes came with a 1720 RPM single phase 220 motor and I changed them to 3440 RPM 3 phase motors for the extra speed. I keep my belt on a lower speed and use the VFD to attain whatever speed that I desire without going so quickly that the chuck may explode. Right now my lathes will run between 0 and about 2300 RPM on this lower belt setting. If I want to cut threads I sometimes lower the belt setting or throw in the back gear so that I can run as slow as 3 or 4 RPM and still have plenty of power so that the motor doesn't stall.
After getting my first set-up I don't know how I went for so many years without one. It really makes it so much easier and quicker.
Dick