I'm trying to understand the application of VFDs in industry. I
understand the basics,ie; by changing the frequency of AC current, you
can change the rpm of an AC induction motor. This is a good thingG. I
know that things like fans, pumps and conveyors are great applications
for VFDs because you can change the speed of the motor to match the
load. I also think that VFDs will work with machine tools because the
VDF allows the spindles to be run at an infinite number of speeds,
negating the need for some belts, pulleys or transmissions.

Looking at VDFs available on Ebay, it appears that the input on some of
them are single phase and some are 3 phase, but all VDFs have a 3 phase
output.It appears to me that one could use (a properly rated) VDF
instead of a phase converter to convert 230v single phase to 3phase at
230v or 460v. Am I correct?

The VDFs listed on Ebay are for fractional horsepower all the way up to
50 hp and higher. Does the VDF rating have to match the HP rating of the
motor, or could one use a VDF rated at 3 hp to drive a smaller motor
like a 1 hp motor? I know that it would not be a good thing to use a VDF
rated at 3hp to run a 25 hp motor.

Greg