Sorry, but this comment shows that you don't understand the physics that would be involved in something like this.
That means it can spin at any frequency and it doesn't really matter what state the grid is in.
It's a synchronous rotor which is spun by the mains. When the mains frequency tries to change, the momentum of the flywheel supplies massive leading or lagging current to prevent the change.
It behaves exactly as an un-powered generator idling on the mains.
There is no DC motor.
In the industry they are known as a "Synchronous Condenser" or Syncom. They have been in use for a long time.
https://www.pv-magazine-australia.com/2019/04/06/long-read-b...