Most forms of uninterruptible power supply (UPS) can be either powered by battery or flywheel energy. These are ready for immediate use at the instant that the mains electricity fails, but the small amount of stored energy they contain makes them suitable for a few seconds or minutes of use only. To get uninterruptible and continuous power supply, a diesel-generator back-up system is needed.
Diesel rotary uninterruptible power supply devices (DRUPS) combine the functionality of a battery-powered or flywheel-powered UPS and a diesel generator. When mains electricity supply is within specification, an electrical generator with a mass functions as motor to store kinetic energy in an electro-mechanical flywheel. In combination with a reactor or choke coil, the electrical generator also works as active filter for all sorts of power quality problems, like harmonics, RFI, and frequency variations. When mains electricity supply fails, stored energy in the flywheel is released to drive the electrical generator, which continues to supply power without interruption. At the same time (or with some delay, for example 2 to 11 seconds, to prevent the diesel engine from starting at every incident), the diesel engine takes over from the flywheel to drive the electrical generator to make the electricity required. The electro-magnetic flywheel can continue to support the diesel generator in order to keep a stable output frequency. Typically a DRUPS will have enough fuel to power the load for days or even weeks in the event of failure of the mains electricity supply.
The main advantages of DRUPS equipment compared to battery-powered UPS combined with a diesel-generator are the higher overall system energy efficiency, smaller footprint, use of fewer components, longer technical lifetime (no use of power electronics) and the fact it does not result in chemical waste (no use of batteries).
The main disadvantages of DRUPS equipment are a more frequent maintenance regimen due to the number of moving parts. DRUPS are also typically installed in external buildings due to noise concerns from the generators.
A DRUPS can provide a ride-through time of 15-40 seconds. A flywheel UPS can be installed ahead of typical UPS battery systems to reduce the effects of lightning & switching transients and to increase battery life.
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