Soft start for servo motor

[Edsbear]

I thought VFD's were to control 3 phase motors if only single phase was available 

[dikman]

Yep, that is their primary function, but the fact you can vary the speed is what makes them so useful. It is very difficult to make a single phase AC motor variable speed - grinders, lathes, drill presses etc - but by replacing the single phase motor with a 3-phase and adding a VFD you can adjust the speed on-the-fly with a simple potentiometer.  My belt grinder, lathe and drill press have all been so modified, much better than changing belts to adjust the speed.

I haven't, however, bothered with using such a setup on any of my sewing machines as the ubiquitous servo is perfectly adequate (and a lot simpler to set up).