Here is a few of the differences between VFD and Rotary Phase Converters.

VFD or Variable Frequency Drives are a type of adjustable speed drive.

They are used in electro - mechanical drive systems to control things like your AC motor speed and torque.

They do this by varying motor input frequency and voltage.

A Rotary Phase Converter  is made from a three phase electric motor and/or idler (generator) and normally requires a capacitor and some type of starting aid to help with phase balance and improved power factors.

These are equipped with two motor solutions, one is solely to produce the three phase power and the other motor driving the load runs on the power produced.