What is a VDF anyway?
VFD – variable frequency drive. You know how regular AC is 60hz (60 times per second cycles the AC current wave) – a motor connected to mains, runs at some multiple of “60x/sec” natively. Getting different speeds, depend on cutting power to it for some duration of each cycle (thus you can control the speed, but you have to sacrifice output (horsepower) in the process.
A VFD takes the incoming mains, makes it high voltage DC. Then uses an inverter to make it back into AC but a variable frequency (usually 100hz -> 400hz for cnc spindles) – so the motor can then be fine tuned to any RPM without limiting its output power.
Why is this a thing in the CNC world? Well if you do really advanced machining, you want the exact rpm of the bit spinning to calculate just how big a chip of material its cutting per rotation. Then you move the bit along at a exact optimal speed to take just the perfect size of chip to a) remove as much material as possible and b) ensure optimal tool life.
However, thats on big steel framed industrial machines. Our hobbygrade machine, flex a little so you can never dial that recipe in that close anyway as you need to accomodate the flex and loose belt drives and cheap leadscrews. So its pointless adding a heavy, complex, VFD spindle on our machines. It like putting a V8 with twin turbos and the worlds best engine management computer, into a golf cart