Editing Uninterruptible power supply (section)

== Other designs ==

===Hybrid topology/double conversion on demand===

These hybrid rotary UPS<ref name="Rotary UPS">{{cite web|url=http://www.pscpower.com/site/wp-content/uploads/2013/06/Hybrid-Rotary-UPS.pdf|title=Hybrid Rotary UPS|archive-url=https://web.archive.org/web/20141204152131/http://www.pscpower.com/site/wp-content/uploads/2013/06/Hybrid-Rotary-UPS.pdf|archive-date=December 4, 2014|url-status=dead}}</ref> designs do not have official designations, although one name used by UTL is "double conversion on demand".<ref name="HPman">{{cite web|url=http://h20000.www2.hp.com/bc/docs/support/SupportManual/c01173322/c01173322.pdf <!-- originally http://h20000.www2.hp.com/bc/docs/support/SupportManual/c01173322/c01173322.pdf--> |archive-url=https://ghostarchive.org/archive/20221009/http://h20000.www2.hp.com/bc/docs/support/SupportManual/c01173322/c01173322.pdf |archive-date=2022-10-09 |url-status=live |title=Increasing energy efficiency with modular HP three-phase power distribution|publisher=[[HP Inc.|HP]]}}</ref> This style of UPS is targeted towards high-efficiency applications while still maintaining the features and protection level offered by double conversion.

A hybrid (double conversion on demand) UPS operates as an off-line/standby UPS when power conditions are within a certain preset window. This allows the UPS to achieve very high efficiency ratings. When the power conditions fluctuate outside of the predefined windows, the UPS switches to online/double-conversion operation.<ref name="HPman" /> In double-conversion mode the UPS can adjust for voltage variations without having to use battery power, can filter out line noise and control frequency.

===Ferroresonant===

Ferroresonant units operate in the same way as a standby UPS unit; however, they are online with the exception that a [[Voltage regulator#Constant-voltage transformer|ferroresonant transformer]], is used to filter the output. This transformer is designed to hold energy long enough to cover the time between switching from line power to battery power and effectively eliminates the transfer time. Many ferroresonant UPSs are 82–88% efficient (AC/DC-AC) and offer excellent isolation.

The transformer has three windings, one for ordinary mains power, the second for rectified battery power, and the third for output AC power to the load.

This once was the dominant type of UPS and is limited to around the {{nowrap|150 kVA}} range. These units are still mainly used in some industrial settings (oil and gas, petrochemical, chemical, utility, and heavy industry markets) due to the robust nature of the UPS. Many ferroresonant UPSs utilizing controlled ferro technology may interact with power-factor-correcting equipment. This will result in fluctuating output voltage of the UPS, but may be corrected by reducing the load levels or adding other linear type loads.{{Elucidate|date=May 2012}}

===DC power===
A UPS designed for powering DC equipment is very similar to an online UPS, except that it does not need an output inverter. Also, if the UPS's battery voltage is matched with the voltage the device needs, the device's [[power supply]] will not be needed either. Since one or more power conversion steps are eliminated, this increases efficiency and run time.

Many systems used in telecommunications use an [[extra-low voltage]] "[[common battery]]" 48&nbsp;V DC power, because it has less restrictive safety regulations, such as being installed in conduit and junction boxes. DC has typically been the dominant power source for telecommunications, and AC has typically been the dominant source for computers and servers.

There has been much experimentation with 48&nbsp;V DC power for computer servers, in the hope of reducing the likelihood of failure and the cost of equipment. However, to supply the same amount of power, the current would be higher than an equivalent 115&nbsp;V or 230&nbsp;V circuit; greater current requires larger conductors or more energy lost as heat.

High voltage DC (380&nbsp;V) is finding use in some data center applications and allows for small power conductors, but is subject to the more complex electrical code rules for safe containment of high voltages.<ref>{{cite web|first1=My|last1=Ton|first2=Brian|last2=Fortenbery|first3=William|last3=Tschudi|url=http://hightech.lbl.gov/documents/DATA_CENTERS/DCDemoFinalReport.pdf|archive-url=https://web.archive.org/web/20101008075806/http://hightech.lbl.gov/documents/DATA_CENTERS/DCDemoFinalReport.pdf|url-status=dead|archive-date=2010-10-08|title=DC Power for Improved Data Center Efficiency|publisher=Lawrence Berkeley National Laboratory|date=January 2007}}</ref>

For lower power devices that run on 5&nbsp;V, some [[Battery charger#Power bank|portable battery banks]] can work as a UPS.

=== Rotary ===

A rotary UPS uses the inertia of a high-mass spinning [[flywheel]] ([[flywheel energy storage]]) to provide short-term [[ride-through]] in the event of power loss. The flywheel also acts as a buffer against power spikes and sags, since such short-term power events are not able to appreciably affect the rotational speed of the high-mass flywheel. It is also one of the oldest designs, predating vacuum tubes and integrated circuits.

It can be considered to be ''on line'' since it spins continuously under normal conditions. However, unlike a battery-based UPS, flywheel-based UPS systems typically provide 10 to 20 seconds of protection before the flywheel has slowed and power output stops.<ref>{{cite web|publisher=Active Power|url=http://powertechniquesinc.com/wp-content/uploads/2015/08/Active-Power-WP-107-15-Seconds-vs-15-Minutes.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://powertechniquesinc.com/wp-content/uploads/2015/08/Active-Power-WP-107-15-Seconds-vs-15-Minutes.pdf |archive-date=2022-10-09 |url-status=live|title=15 Seconds versus 15 Minutes: White Paper 107 Designing for High Availability|date=2007}}</ref> It is traditionally used in conjunction with standby <!-- not necessarily :diesel--> generators, providing backup power only for the brief period of time the engine needs to start running and stabilize its output.

The rotary UPS is generally reserved for applications needing more than 10,000&nbsp;W of protection, to justify the expense and benefit from the advantages rotary UPS systems bring. A larger flywheel or multiple flywheels operating in parallel will increase the reserve running time or capacity.

Because the flywheels are a mechanical power source, it is not necessary to use an electric motor or generator as an intermediary between it and a diesel engine designed to provide emergency power. By using a transmission gearbox, the rotational inertia of the flywheel can be used to directly start up a diesel engine,<ref>{{Cite book |last=Dorf |first=Richard C. |url=https://books.google.com/books?id=n2C1DwAAQBAJ&dq=rotary+ups&pg=SA9-PA81 |title=The Electrical Engineering Handbook - Six Volume Set |date=2018-12-14 |publisher=CRC Press |isbn=978-1-4200-4975-6 |language=en}}</ref> and once running, the diesel engine can be used to directly spin the flywheel. Multiple flywheels can likewise be connected in parallel through mechanical [[countershaft]]s, without the need for separate motors and generators for each flywheel.

They are normally designed to provide very high current output compared to a purely electronic UPS, and are better able to provide inrush current for inductive loads such as motor startup or compressor loads, as well as medical MRI and [[cath lab]] equipment. It is also able to tolerate short-circuit conditions up to 17 times larger than an electronic UPS, permitting one device to blow a fuse and fail while other devices still continue to be powered from the rotary UPS.

Its life cycle is usually far greater than a purely electronic UPS, up to 30 years or more. But they do require periodic downtime for mechanical maintenance, such as [[ball bearing]] replacement. In larger systems, redundancy of the system ensures the availability of processes during this maintenance. Battery-based designs do not require downtime if the batteries can be [[Hot swapping|hot-swapped]], which is usually the case for larger units. Newer rotary units use technologies such as [[magnetic bearing]]s and air-evacuated enclosures to increase standby efficiency and reduce maintenance to very low levels.

Typically, the high-mass flywheel is used in conjunction with a [[motor-generator]] system. These units can be configured as:
# A motor driving a mechanically connected generator,<ref name="Rotary UPS" />
# A combined [[synchronous motor]] and generator wound in alternating slots of a single rotor and stator,
# A hybrid rotary UPS, designed similar to an online UPS, except that it uses the flywheel in place of batteries. The rectifier drives a motor to spin the flywheel, while a generator uses the flywheel to power the inverter.

In case No.&nbsp;3, the motor generator can be synchronous/synchronous or induction/synchronous. The motor side of the unit in case Nos.&nbsp;2 and 3 can be driven directly by an AC power source (typically when in inverter bypass), a 6-step double-conversion motor drive, or a 6-pulse inverter. Case No.&nbsp;1 uses an integrated flywheel as a short-term energy source instead of batteries to allow time for external, electrically coupled gensets to start and be brought online. Case Nos.&nbsp;2 and 3 can use batteries or a free-standing electrically coupled flywheel as the short-term energy source.