The Power Protection Blog

When we’re talking about UPS systems we are talking here about Uninterruptible Power Supplies that generate AC power. There are basically two types, rotary and static.

Rotary Systems
Traditionally rotary systems are so called as they basically are a motor-generator. In a nutshell, the incoming mains powers a motor that spins a flywheel that then powers the generator. When the mains goes, the flywheel keeps the generator part going. This may be supplemented with Lead Acid batteries to extend the run time. Lately I’ve heard UPS called rotary systems because they use flywheel technology as the energy storage mechanism, but this strictly isn’t correct. Rotary systems are typically used for large installations, and are particularly suited where you have mechanical loads that demand high inrush currents.

Static UPS Systems
Are so called as they don’t have any moving parts and generate AC by what is known as an inverter. With static systems these are divided into basically three technologies, offline or standby, line interactive and online or double conversion systems.

Offline UPS:
The basic UPS system. What goes in goes out except when the mains supply goes out of tolerance, that is lower than about 207VAC or higher than about 253VAC. When this happens, AC is produced by an inverter. However this AC waveform is 99% of the time non-sinewave. Sometimes called square wave or pseudo-sine wave, it’s basically non-sine. These systems are cheap and cheerful and used for low grade protection of non-essential equipment, such as the odd PC where loss of data is more of an inconvenience than anything else. According to the new classification of UPS EN64020, these systems are VFD. These are available upto about 3KVA, but in fairness I’d stick to a max of about 1KVA for a PC or so.

Line Interactive:
There’s a lot of confusion as to what line interactive is. I’ve heard that a line interactive unit has to have a sinewave inverter, or that it contains a common charger/inverter so that the inverter is “alive” all the time and so reduces transfer time, or that the UPS will interact with the (power) line to keep the voltage within tighter limits. Well, the standard EN64020 has kind of defined the latter as line interactive or VI (Voltage Only Independent). So basically, any system that provides a degree of voltgae regulation is therefore line interactive. However this really only adds to the confusion as there is now a huge gulf in technology of line interactive units. Take the MiniGuard on this UPSMart shop. It’s line interactive as it provides voltage regulation and has a square wave output. The manufacturer (Kehua) market it as an Offline product (which is correct in my view). APC define their line interactive units as those with a common charger/rectifer and so distinguish the technology and hence price between their Backups and Smartups series. Both may now be defined as line interactive. Just check the price difference – again on this site- between the Opti – PS series – a high end line interactive product and the Mini Guard. Luckily the EN64020 standard does have some other means of defining using a series of letters and numbers after the initial classification, but who really will understand this? There’s no easy answer really, but take advice and read the specs for any line interactive unit.

Line Interactive systems are generally available upto around 3KVA, although 5, 7 and even 10KVA units are available. There advantage is that they are highly efficient, as the only power conversion is done when the unit is on battery. They are a good mid choice of technology. Disadvantage is that at higher power rating they tend to become very heavy due to the need to include a transformer for the voltage regulation, and a better bet is to opt for online technology.

Online UPS
Considered by many to be the only “true” form of a UPS. The input power is rectified (or converted) to DC which creates a DC buss that feeds the inverter. The inverter then converts the DC back into AC. As you can see, there are 2 conversions, hence the term “double conversion”. The obvious draw back here is efficiency, but the advantages are a very wide input voltage window (some systems will even except 100V without reverting to battery), a stable, clean output power waveform, and protection against practically all power problems. Online UPS also contain a bypass line, which means, in the event of UPS failure or overload, the system will divert power from the inverter back to the mains, and back again if the problem is removed, thus ensuring higher availability.

Online technology is the mainstay of all UPS systems above 10KVA, and is available from around 700VA. It is used for the protection of systems that are critical in nature. Traditionally expensive, the prices are now becoming competitive and overtaking line interactive as the preferred choice of mid level protection. I’ve seen here at UPSMart, a 1KVA online cheaper than any high grade line interactive unit. So unless you want higher efficiency or are in a very quiet environment than I’d opt for online units everytime. (note – online systems are “online” all the time and use forced cooling to keep them from overheating. This of course generates fan noise).

The new standard determines online units as VFI, where both the Voltage and Frequency are Independent.

I guess there’s a lot of text here, but I hope it makes sense. Just add a comment if you want clarifiation or disagree with anything said here.