Uninterruptible power supply
The on-battery run-times of most UPSs are relatively short (only a few minutes) but sufficient to "buy time" for initiating a standby power source or properly shutting down the protected equipment.
The UPS will be designed to power certain equipment, such as a personal computer, without any objectionable dip or brownout to that device.
This type of UPS is able to tolerate continuous undervoltage brownouts and overvoltage surges without consuming the limited reserve battery power.
Depending on the design, changing the autotransformer tap can cause a very brief output power disruption,[5] which may cause UPSs equipped with a power-loss alarm to "chirp" for a moment.
Autotransformers can be engineered to cover a wide range of varying input voltages, but this requires more taps and increases complexity, as well as the expense of the UPS.
The main advantage of an online UPS is its ability to provide an "electrical firewall" between the incoming utility power and sensitive electronic equipment.
The online UPS is ideal for environments where electrical isolation is necessary or for equipment that is very sensitive to power fluctuations.
[6] Although it was at one time reserved for very large installations of 10 kW or more, advances in technology have now permitted it to be available as a common consumer device, supplying 500 W or less.
However, it typically costs much more, due to it having a much greater current AC-to-DC battery-charger/rectifier, and with the rectifier and inverter designed to run continuously with improved cooling systems.
It is called a double-conversion UPS due to the rectifier directly driving the inverter, even when powered from normal AC current.
[8] This style of UPS is targeted towards high-efficiency applications while still maintaining the features and protection level offered by double conversion.
[8] 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.
Many systems used in telecommunications use an extra-low voltage "common battery" 48 V DC power, because it has less restrictive safety regulations, such as being installed in conduit and junction boxes.
There has been much experimentation with 48 V DC power for computer servers, in the hope of reducing the likelihood of failure and the cost of equipment.
[10] It is traditionally used in conjunction with standby generators, providing backup power only for the brief period of time the engine needs to start running and stabilize its output.
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.
Newer rotary units use technologies such as magnetic bearings and air-evacuated enclosures to increase standby efficiency and reduce maintenance to very low levels.
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.
[12] Tower models stand upright on the ground or on a desk or shelf, and are typically used in network workstations or desktop computer applications.
[14][15] When a UPS system is placed outdoors, it should have some specific features that guarantee that it can tolerate weather without any effects on performance.
Factors such as temperature, humidity, rain, and snow among others should be considered by the manufacturer when designing an outdoor UPS system.
Solar inverters have special functions adapted for use with photovoltaic arrays, including maximum power point tracking and anti-islanding protection.
These often utilize simple switching AC power supplies and the output resembles a square wave rich in harmonics.
The voltage distortion then can cause problems in all electrical equipment connected to that power source, including the UPS itself.
To reduce voltage distortion, this requires heavier mains wiring or generators more than twice as large as the UPS.
This completely eliminates the need to oversize the generator (and transformers), without additional filters, investment cost, losses, or space.
To avoid the dependency on Ethernet infrastructure, the UPSs can be connected directly to the main control server by using a GSM/GPRS channel also.
The SMS or GPRS data packets sent from UPSs trigger software to shut down the PCs to reduce the load.
The run-time for a battery-operated UPS depends on the type and size of batteries and rate of discharge, and the efficiency of the inverter.
These issues also apply to other situations where series-parallel strings are used, not just in UPS systems but also in electric vehicle applications.
