Uninterruptable Power Supplies (UPS)


UPS’s can be divided into various different types i.e. off-line units, line interactive units and on-line units. There is a considerable difference in the way that these units work and the different types should not be confused. An on-line unit is considered to be the best protection that money can buy.


The off-line UPS takes in the normal mains, drives the load and keeps the battery fully charged. The input is monitored and as soon as the mains drops below a pre-set level or fails completely the unit switches to battery inverted power. Switching time is typically less than 4 ms, which is well within the tolerance of modern computer power supplies.


The Line interactive UPS’s are designed to interact with the mains to supply a steady flow of power to the computer equipment. A high powered filter is incorporated into the machine to protect against spikes and surges. In addition an automatic voltage regulator is also added, this will help to increase the battery life of the UPS as it will be able to switch to battery power at a much lower input voltage.


On-line UPS’s have a few sub categories as well.

The first one – which are a bit old now but probably the most reliable and robust units in the world. The ferroresonant UPS. The incoming mains goes straight into a constant voltage transformer (CVT). The CVT creates an AC storage chamber which isolates the input from the output. No matter what happens to the incoming mains the output will remain constant. Once the mains fails the input of the CVT is fed from a battery powered inverter. There is no break on the output during the transfer from mains supply to inverter supply or vice versa. The use of ferroresonant technology reduces the component count on the machines which will automatically increase reliability.

The second one is the high frequency transformerless type online UPS – these are farily cheap units, not very robust (i.e. cannot tolerate too much abuse), but if properly sized and maintained they are very reliable. They are limited to size and up to a 3KVA they are great.

The third is a transformer based UPS. Obviously a bit more expensive but much more robust and available in single phase and three phase. Size is not much of a problem (although at the lower end the transformerless units are far cheaper.)

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