Hello Hamid,
The ability to model the generator/motor exciter mass and corresponding internal subsynchronous resonance is currently not available for the Synchronous Machine blocks.
To view how this block is modeled, you can right-click and navigate to 'Mask > Look Under Mask'. If you would like to then implement the exciter mass and model the corresponding resonance yourself, you can copy the internal components, create a custom block, and modify the mechanical and electrical subsystems inside.
Joe
