Uninterruptible Power Supplies(UPS)


1) Offline / Standby


     Offline UPS works the simpliest among different types of UPS. Its functions are mainly surge protection and battery backup. When the electricity supply is stable, offline UPS runs bypass mode and the device is directly connected to the city power. In case there is a sudden black out, the UPS mechanically switches from city power to DC-AC inverter output(from battery) in less than 25 milliseconds, in order to avoid equipment shutdown. The backup time varies depending on the size of the battery as well as degree of loading.


2) Line-Interactive

                    Line-Interactive UPS Diagram.png

       Line-interactive UPS works almost the same as offline UPS. The only difference is the addition of a multi-tap variable-voltage autotransformer. It uses the rationale behind the electromagnetic induction to control small undervoltage or overvoltage conditions. In other words, This circuitry provide stable voltage from the relatively unstable city power(city power may have voltage fluctuation from times to times) to connected equipment, also preventing equipment from long-term wear and tear.

     In case, the equipment suffer from a large over/undervoltage or even power lost. This line-interactive UPS will act as offline UPS to convert DC from the battery to AC for the equipment.


3) Online UPS


     The online UPS is ideal for environments where electrical isolation is necessary or for equipment that is very sensitive to power fluctuations. When city power sags, outages and other anomalies are frequent, online UPS provides advantages to sensitive IT equipment, backup generator, etc.

     The basic technology of the online UPS is the same as offline or line-interactive UPS. But it costs more due to the integration of rectifier and inverter into the UPS. Basically in an online UPS, the batteries are always connected to the inverter, so no power transfer switches are necessary. The rectifier also drives the inverter. When there is a blackout, the rectifier simply drops out of the circuit and the batteries keep the power steady and unchanged. When power restores, the rectifier resumes carrying most of the load and begins to charge the batteries.

     The main advantage of the online UPS is its ability to provide an electrical firewall between the incoming city power and sensitive electronic equipment. While the standby and line-interactive UPS merely filter the city power. The online UPS also provides a layer of insulation from power quality problems. It allows control of output voltage and frequency of input voltage and frequency.


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