A variable speed drive is a soft start arrangement, but there are a lot of standards out there, so what aspect are you wanting to look at. For instance, there is IEC TS 60034-31 Rotating electrical machines - Part 31: Selection of energy-efficient motors including variable speed applications - Application guide. However if you are looking for a standard for the converter then it will be a different standard. Also the standard my be dependent on what the motor is driving and where it is located. A VSD motor in an assembly plant will be under very different requirements from a VSD motor in a chemical processing plant.