Don / Frank,
I do not know enough about soft start motors, but I have never heard about a problem using the chinese speed controllers with motors with soft start. These chinese speed controllers work very much as a VFD, the main difference with VFDs, is that they do not rectify the current before creating the new frequency. This creates a very obnoxious wave form. That is why usually these speed controllers do not reduce the speed to less than 50%. This answers Frank’s question. Yes, if you reduce the resistor or pot value you could go lower, but I would not do it if I where you.
I know a lot of people have bad things to say about these speed controllers, but they do work for most users.
Thanks,
Arturo Duncan
http://cnc4pc.com