What if I take a single phase 240V AC motor and attach it to a balanced 100 lb iron disc?
Once it is up to speed it shouldn't be pulling much power to stay running since there is no load.
If there is a 1 second brownout the power from the motor will flow into my subpanel and keep the power on.
Just like a kinetic battery backup but homemade.
Would this work?
Am I correct in assuming the load would be small on the already spinning disc?
I don't know how great that would work for single or split phase. That technique is already in use for 3-phase systems, it was what was done before efficient UPSs based on batteries came along. Some datacenters could run for up to 15 seconds with a continuous 2MW load on such a device when it was sized correctly.