Written pole motors have been around for some time. Manitoba Hydro used to promote them. In the smaller horsepower ratings they are not at all cost competitive with a VFD and three phase motor.
As to the original question, I would not even consider a soft-start on a single phase motor. A VFD and three phase motor is proven technology that works. I have done experiments with a VFD starting a 15 hp. motor on a 30 second ramp time to see what the inrush current would be...essentialy no inrush. Motor just started turning and slowly ramped up to speed. Current likewise slowly ramped up to operating current...no spike at all.
Soft starts on a single phase
