You are right. I read a little bit on motor management on Wikipedia because I don't have too much experience with motors. I didn't know that there were so many solutions to limit starting current on AC motors and how a speed variator is built.
With motors, can you run with your UPS on by-pass, or with a load transfer switch ? Just switching on batteries when network is in trouble ? In that case, you don't have so much the starting issue and you avoid too many peak loads on the inverter. There could even be a security check in order not to start a complete industrial process when there is a breakdown of the network (at least to wait until the generator has started), I guess that in most cases, it is the process that needs to be protected, not the starting of the process. Well, here we enter in specific configuration and it is not so usefull to make therory without real example behind.
In our cooling system, we have different power source for regulation (UPS protected) and for power systems (heat pumps... are diesel generator protected). When network breaks down, we first wait 30 seconds to see if it comes back, than the generator needs 30 seconds to take the load. We have just one pump that has sometimes starting problems after a power cut, so a small UPS with a load transfer switch would be a perfect solution, but I don't know if this could work, the supplier didn't suggest it. It's a redundand system and it is not each time problematic, so we need at least 2 or 3 power cuts to get in trouble, the shorter cuts are the worst. Problem was solved with specific training, but in case of major trouble, it's not sure what problem will be handled first by the technical team.