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Types & Specification of UPS
The UPS systems available in market today can be either a static UPS or Rotating type UPS. In a static UPS system there is no moving part inside the complete circuit resulting into absolutely no need of maintenance and reliability. However the rotating type UPS systems were developed initially and they contain heavy fly wheel along with the charged battery. The battery gives enough energy to the alternator for 10 to 15 seconds to provide a backup and in the mean time the diesel engine is started to rotate the alternator. These systems need maintenance and that is why in present day circumstances, specially with computer systems the static UPS systems have become more popular. Rather one can say that they are used in almost all computer using organizations.
Static UPS systems can mainly be classified into two main categories :-
1. Classification depending on output power required.
2. Classification depending on the working of inverter circuit.
Classification Depending on output power required :-
UPS systems, until recently, were only available in expensive high or medium power models to be used with large scale electronic or computer systems. But to reduce the power consumption and cater to the need of individuals, portable plug-in low power UPS systems have been developed for use with microcomputer, minicomputers, intelligent terminals, electronic cash registers, data terminals and other small systems.
Classification depending on the working of inverter circuit :-
Under this category there are three main types of UPS :-
(a) Off Line UPS
(b) On Line UPS
(c) Line Interactive UPS.
(a) Off Line UPS :-
Off line UPS filters the mains and feeds it directly to the load for most of the time. When the mains is unhealthy, perhaps due to a slight dip in voltage, the load is switched by a fast relay, in typically less than half a cycle, to an inverter. The inverters derive its power from the battery. Inverter generates a square mer stepped waveform to emulate the mains and this waveform is satisfactory for most computers. The offline UPS is also known as standby mode of UPS, because inverter is on standby long as the commercial power source is available. The inverter comes to action only when there is some problem with the AC mains.
An offline UPS switches on the battery power via a DC to AC inverter when it detects a power loss or drop. And this process takes a certain small but finite time which in off line UPS is typically four milliseconds. That is why off line UPS is also known as Short-Break UPS.
(b) On Line UPS :-
On line UPS converts AC mains into DC before inverting again to AC to power the load with a synthetic sine wave. A battery connected across DC link acts as the back up power source. This gives a supply for the computer that totally isolates the input mains from the load and thus removing all mains noise and with no break when the ac mains fails. That is why an online UPS is also known as No Break UPS.
It consists of a rectifier unit/battery charger which converts AC to DC and float charges the battery. It also powers the DC to AC inverter circuit. As the inverter is continuously running/working in an On-line UPS, it is also known as 'CONTINUOUS MODE UPS'. This means that fluctuations in the commercial AC supply does not affect the AC output of the inverter. When the AC power supply fails, the battery continues to supply power to the inverter, ensuring uninterruptible power supply to the load. So, there isn't any switching time when a power loss or drop occurs because the inverter is continuously ON.
(c) Line Interactive UPS :-
In line interactive UPS, the inverter runs continuously parallel with the mains all the time. Continuous operation of inverter avoids the potential catastrophe of a start-up failure. Inverter produces a pure sine wave, so that the critical load is fed on the same diet whether commercial power is present or when the battery is supplying powers.
In line- interactive UPS, the inverter plays the dual role of charging the battery when mains supply is present as well as regulating the output voltage. In the absence of main supply, it functions as a normal inverter.
A relay is used to disconnect the inverter from mains when there is power back to the main supply. Line interactive UPS commonly has mains boost feature to avoid draining the battery unnecessarily in brownouts/low voltage conditions.
Line interactive UPS is gaining ground and is replacing online and off line for price/performances criteria.
Some Important Specification Of UPS :-
> overload Capacity :- It is the output a UPS can supply which in greater than is continuous rating for a short period of time. It is usually expressed as a voltage of the continuous rating for a specified period of time.
> In-rush Current :- In-rush current is the current demanded by the load, when the load is activated and this current is substantially higher than its steady-state value.
> Output Waveforms :- The output wave form of a UPS should be a sine wave. Any other type of wave form produces overheating of the computer and electronic equipment power supply.
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