ROTARY UPS  

A flywheel driven rotary UPS is used for applications requiring ride-through of short duration power system outages, voltage dips, etc. The flywheel driven rotary UPS typically does not include batteries, and support times are usually on the order of a few seconds to a few minutes. The use of a generator driven rotary UPS can provide extended power for an indefinite power outage and also supply air-conditioning and lighting loads. Data processing equipment rooms will typically overheat within a 15- to 30-minute period if the ventilation system is not working, making the generator set a near necessity for outages in excess of this time. Analysis of battery cost will often justify a generator set at lower cost than choosing a long battery support time with an accompanying restriction of eventually implementing an orderly critical load shutdown. A battery support time of as little as a few minutes may be specified with generator set backup; however, longer support times in the range of 15 minutes are more typical.
Basic Principle Diagram	1) Diesel engine 3) Electromagnetic clutch 6) Synchronous motor /alternator 4) Flywheel
Some of rotary UPS systems consists of an AC Motor Generator (M-G) set with a flywheel, as well as a rectifier, storage batteries, inverter, static switch and solid state circuitry. Both the motor generator set and the rectifier/battery/inverter combination are supplied by the incoming utility service. They represent parallel supply paths and either path is capable of supplying the load. A static switch selects the path to be utilized to supply the load. During normal operation, the M-G set powers the computer loads, while the off-line static section is on "stand-by" and charges the system batteries. Upon loss of the utility feed, the control circuitry will disconnect the M-G set from the utility by opening the static switch and closing the inverter-output circuit breakers, allowing the system batteries to power the M-G set through the inverter. Mechanical energy "stored" in the flywheel allows the M-G set to continue to deliver its full-rated output for (generally) a minimum of 200 milliseconds. This provides sufficient time for the control system to sense a loss of utility supply and to transfer to the battery/inverter combination for supply of loads.
Basic schematic diagram

Sources: - US Army Corp of Engineers - AKF Engineers - The figures are from Hitzinger GmbH