UPS tolerance question - Under/over V & Hz

[domwild]

Hi,

My son claims the "Boost" icon on the panel of the UPS (a sine wave with increasing amplitude and the word "Boost" below) means the UPS will use the battery plus the mains to maintain the 110 or 220/240 AC during brown-outs.

We all know the UPS will handle a complete mains failure for about 20 min. or so, at least with the existing battery pack. And if my son is correct, then it will also handle lower AC feeds with the help of the battery.

1. Now does this mean a UPS could, at least theoretically, be connected directly to one phase of a mill, hydro or wind? Would need three identical UPSs/batteries for three phases.
   
2. Does a UPS disconnect during overvoltage condition?
   
3. As an F&P with its 42 coils generates a high frequency AC current, how tolerant are UPSs with their transformers anyway before they saturate?? To put it in another way, as higher voltage means higher frequency -> saturation, will it harm the UPS?
   
4. Reminds me of another (silly?) question for power engineers: Can a 3-phase current of varying frequency be easily changed to a single phase current by phase shifting the other two phases 120 or 240 degrees?
   

[commanda]

This all depends slightly on exactly which UPS you have ! I don't claim to be an expert on every ups ever made; however I would expect the following to apply to most.

Most ups's are a 2 stage device. Switch-mode down-converter, batteries, switch-mode up-converter (inverter).

1: The input stage is a switchmode down-converter. You can't design it with a wide enough input voltage range (to connect directly to F&P). Generally limited to about 3:1 (80-240 volts AC).

Nando & I have had long discussions about this via email. Look in my diary at the recent posting about the mppt. I believe there it explains my basic approach. First stage has to be a boost converter.

2. : I wouldn't expect it to. Unless you count popping a fuse or letting all the magic smoke escape.
   
3. : See 1. The frequency from the F&P is not that high. Might consider changing the input diode bridge for fast-recovery types. If it comes to that, just fit a 3 phase bridge, and use one UPS. Unless the control electronics requires that you really have AC at the input.
   
4. : No. I know someone will chime in here, and point out that under certain conditions it can be done. Just take my word for it & don't. Usually either direct couple 3 phase motor to single phase alternator; or rectify the 3 phase to dc and drive an inverter.
   
   

Amanda

[Nando]

Yes there are UPS that have a mode of complement, which means that if the sine wave of the GRID SAGS the UPS balances the output to maintain the sine wave complete, also if the GRID power goes OFF the UPS will continue supplying the power without any sine wave glitch transition.

These type of UPS with a very light modification can be used to supply energy to the GRID.

The UPS is synchronized to the GRID frequency but it is not producing any voltage or power per se, the circuitry is following the GRID voltage and Current and if a discrepancy is present the UPS generates enough voltage and power to keep the GRID power clean and correct from a few volts to total power if the GRID goes away.

It is a more complicated design = like a GRID tied Inverter.

Nando

[Nando]

.1) NO, THE frequency changes and differences are too great for this type of UPS.

.2) Some UPS do, specialized like the one you seem to have , the others may go ON in a few milliseconds and stay ON for certain time to insure that the GRID voltage is stable.

.3) PURE sine wave UPS are NOT tolerant to low frequency since the circuit internally may be at very high frequency > 17 KHZ

.4) 3 PHASE can can NOT be changed by phase switching of the other phases , MAY I SUGGEST THAT you search basic electrical books.

Nando

[robl]

I have a 1KW APC sinewave SmartUps,and according to my investigations, "boost" happens prior to the inverter/charger section. Essentially, different taps on a autotransformer are connected to the grid to keep the output voltage steady without requiring the services of the inverter/battery element.

Rob

[domwild]

Thanks for all the interesting answers.

Back to the drawing board!

Regards,

[marvin]

UPS Tech here with comments:

1. Most small UPS (under 5kva) and some larger ones are off line and only turn on inverter during periods of utility failure loss or +- 15% deviation.
   
2. Boost as mentioned above appears to be an automatic tap changer on the input transformer. A way of regulation that attempts to keep output of transformer within +-5% without an inverter. Some UPSs and PDUs use this technology, from my experience they are problematic.
   
3. I worked for a Co that designed/mfgd a small on-line UPS back in the mid 80s that used buck regulation for battery charging/DC bus. Input was FW rectified then reduced to charge 18v, 36v or 72v battery string. It didn't care about frequency or voltage  (up to the rating of buck regulator device). I know it would operate from DC up to 400hz as one was used on the "Vomit Comet".