Q: I’ve been getting quite a few questions from clients lately about UPS (uninterruptible power supply) systems.The problem is, most either want to run out to their local office supply store and buy what’s cheapest, which I’ve insisted probably isn’t the best idea, or they’re an audiophile who’s heard that on-line UPS systems are the way to go.
What can I tell people who don’t understand the value of the (many) other types of UPS systems, or don’t even know they exist?
A: No doubt about it, you’re certainly stuck between a rock and a hard place. As you mentioned, it’s widely believed that there are only two types of UPS systems, namely Standby and On-Line.
And, to make matters far worse, these two terms are often incorrectly applied to many UPS systems on today’s market.
As to what can be done I’ve always found that the best defense in your situation is a good offense, as many misunderstandings about UPS systems are cleared up when the different types of UPS topologies are properly identified.
Often time when clients are presented will ther entire picture (which often involves you educating them a bit), they realize which unit is best for their situation themselves.
To assist in your UPS client education, the major systems have been defined below (Note: All references to kVA sizes are general.). Hopefully this helps!
This is the most common UPS type used for personal computers. The transfer switch is set to choose the filtered AC input as the primary power source and switches to the battery/inverter as the backup source in case of a failure of the primary source (AC).
In the case of power failure, the transfer switch must operate to switch over to the battery/inverter backup power source. The inverter only starts when the power fails, hence the name “standby.” Benefits of this model include high efficiency, small size, and low cost.
Line Interactive UPS
This is the most common UPS used for small business, web, and departmental servers.In this design, the battery-to-AC power converter (inverter) is always connected to the output of the UPS. Battery charging is provided by operating the inverter in reverse during times when the input AC power is normal.
When the input power fails the transfer switch opens and the power flow is from battery to the UPS output. The fact that the inverter is always connected to the output provides additional filtering and yields reduced switching transients when compared with the Standby-type UPS.
The inverter also provides regulation, operating to correct brownout conditions, which would otherwise force the UPS to switch to battery operation. This allows the UPS to operate at sites with very poor power. The inverter can be designed such that its failure will still permit power flow from the AC input to the output, which eliminates the potential of single point failure and effectively provides for two independent power paths.
This topology is inherently very efficient, which leads to high reliability while at the same time providing superior power protection. High efficiency, low cost, high reliability coupled with the ability to correct low- or high-line voltage conditions make this the dominant type of UPS in the 0.5-5 kVA (volt-ampere) power range.