Originally Posted by Steve Bruzonsky
Q: Can an ordinary, off the shelf, computer UPS product do the same thing as an AC regenerator?
A: No. Computer grade UPS systems are designed to support the specific requirements of computer power supplies and while they do an excellent job of protecting from surges and provide continuous power during blackouts (power failures) they have either no or very insensitive voltage regulation, do not remove harmonic distortion, allow some noise and transient voltages through and provide less than ideal sine wave AC output.
Whoa, whoa, whoa, way too broad a statement. That's like saying your Bryston amps are no better than a boxed 5.1 system amp. An amplifier is just an amplifier right? There is a wide range of quality in UPS systems just as there is with any hardware.
Let me break UPS system down to four basic classes:
1) The cheapest is an "offline UPS" which is nothing more than a battery powered simple inverter connected through a DPDT relay to the outlet. The relay coil is connected to the incomming AC line. When the AC line drops, the inverter starts up, the relay relaxes and the inverter is switched to the outlet on the back. This all takes time - many hundreds of milliseconds. SMPS power supplies can ride this out of they are not heavily loaded and that's why these systems work well in certain applications. The output of this class is almost always a nasty square wave full of noise. Also things like ethernet switches and monitors can handle a glitch. But there is no regulation and in fact when not on the battery, they are just simply passing the AC cord right trhough to the outlet. They may throw in a Pi filter and MOV, but a cheap power strip can do that too. Easy to find at Best Buy and Wall Mart.
2) "Interactive UPS" These use solid state switching that can react within an AC cycle. Though not totally glitch proof, they are much better than the above. Also with these the inverter can come on line and assist the incomming AC line voltage. They can increase it if it is low but typically do nothing if it's too high. This is a form of regulation. As the inverter must now be synchronous with the AC line, it's design is far more complex. Also the output is more often than not a modified square wave or "stepped" square wave to better approximate a sine wave. Better computer and electronic stores carry these.
3) "On-Line UPS" In this class there is no relay or switch for changeover. That's because the inverter is always powering the load outlet. The incomming AC is converted to DC and the battery pack is summed to it with diodes. That means the two DC sources are combined to feed the inverter. (It's actually a bit more complex in that the line obtained DC is a bit higher to keep the batteries baised off until needed, but there is no significant switch time.) While these system can get by with a square wave or stepped sinewave, they are almost always true sine wave as the quality class demands it. As the inverter is the power source, it is re-generated AC and we don't need surge protectors or filters in it's output. As the AC power is re-generated, it is easy to tightly regulate. These are typically not found on store shelves and must be ordered from specialty suppliers.
4) "Facility Wide UPS" These are the same as #3 except built on a massive scale to protect an entire technical facility such as bank and insurance headquaters. Also at home in TV broadcast centers and medical facilities where power loss is simply not an option. They typically take in 3 phase 480v and output the same. Then distribution transformers drop this down to 120/208v and distribute to breaker panels and dedicated outlets. This is because in large buildings, 480v can use thinner wire with less voltgae drop over long runs. You place the transformers in local electrical closets. These systems are also often paired with backup generators. That's why most of the se large systems only have 15-20 minutes of reserve. That's plenty of time for the generator to start and stabalize. These systems are very low in THD and have internal redundancy on many levels. They regulate within 1% which is way beyond any connected device requirements. They also have to be servicable hot and have synchronous bypass switches so they can be shut down and brought up without a glitch to the load. These systems are custom build and can cost upwards of a millions.