I was reading an earlier post in this thread. It was wondering if you needed a VFD (variable frequency drive) on a mill. The question was answered but just in general terms. To be a little more specific,this is just a way to control the speed of the spindle motor (or any motor). Depending on which type of motor you have (AC or DC) will determine the method of control. AC will use a VFD where you could run a 3 phase AC motor on single phase for example. A DC motor runs via an inverter.