For large power units, dynamic uninterruptible power supplies are
sometimes used. A synchronous motor/alternator is connected on the mains
via a choke. Energy is stored in a flywheel. When the mains power
fails, an Eddy-current regulation maintains the power on the load as
long as the flywheel's energy is not exhausted. DUPS are sometimes
combined or integrated with a diesel generator that is turned on after a
brief delay, forming a diesel rotary uninterruptible power
supply (DRUPS).
A fuel cell UPS has been developed in recent years using hydrogen and a
fuel cell as a power source, potentially providing long run times in a
small space.