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July 18, 2012 10:15 AM   Subscribe

Why are my UPSs not pulling power from the generator?

We have a 1500VA battery backup powering 2 servers and a 1250VA powering our network and phone system. The 1500 will power the servers for 5 minutes (before initiating shutdown) and the 1250 will run the network and phones for 35 minutes, both reporting almost no load. We have a 15kw generator with a 60 amp breaker connected to the server circuit that kicks on when the power goes out, taking about 45 seconds to start. Our problem is that the UPSs don't go back onto line power when the generator comes on.

We've tested the voltage - from the wall it's a steady 120v, when the generator kicks on it fluctuates from 120-130v. We've used several UPSs (including a brand new one purchased for testing) but none will use the power of the generator, they just run down to nothing. The light in the server closet (generator powered) works with no problem, as well as several other things connected to it. Our electrician has tested all, says everything should be running fine and is baffled as well.

So, hive mind, I'm wondering - what the hell is going on? The electrician theorizes that the UPS is more sensitive to fluctuations, doesn't see the generator power as a good source of power and won't use it. Does anyone have any other insight?
posted by dozo to Computers & Internet (7 answers total)
You will need a line conditioner between the dirty generator power and the UPSs. They are going to look at the sloppy voltage and frequency coming off that beast and say "hell no I am not passing that through to your very expensive electronic equipment."
posted by seanmpuckett at 10:19 AM on July 18, 2012 [1 favorite]

Seconding the line conditioner. I have had this problem with some fancy battery chargers in the past where they work fine on the mains but not on generator power. More so than the voltage fluctuations the actual waveform might not be clean enough to work properly.
posted by JayNolan at 10:28 AM on July 18, 2012

Awesome, thanks for the suggestions. Ordered a line conditioner and will update after we've switched them over.
posted by dozo at 11:17 AM on July 18, 2012

Once you have the conditioner in place, be sure to check the output voltage that the UPS units are drawing from. I recently had a problem where I couldn't figure out why a UPS was always triggering a low voltage alarm. Turns out the conditioner was taking in 120 V but only outputting 104 V. If you have a similar situation, you may need to adjust the UPS input voltage range that triggers the alarm. The UPS I have, an Eaton PowerWare 9130, can handle that via the control panel (but NOT the web interface).
posted by AMSBoethius at 11:55 AM on July 18, 2012

Sounds like the fluctuating voltage is your problem, which should be corrected with the conditioner. If that doesn't fix your problem, also check the frequency, which the conditioner may also correct. The UPS will monitor both voltage and frequency and try to predict if the input power will be stable enough to run on. Even though both may stay within acceptable limits, the fluctuation make the predictions seem like they are always about to become unacceptable.

Another problem could be the grounding, but if an electrician has looked at it, it should be grounded correctly. We have had to use a special plug in one of our generator outlets that tie the ground to one of the legs (neutral I think) in order for the UPS to accept their power.
posted by Yorrick at 8:55 PM on July 18, 2012

UPSs generally monitor the line conditions and will refuse to run off that outlet (generator, what have you) if the line is too far out of spec. Voltage too low or too high; frequency off, whatever. Generators are often fine for running lights and simple appliances, but are usually terrible at powering precision electronics. I service and sell copiers and printers, and there are a lot of people out in the San Juan Islands running off generators at least part time.

I wind up repairing a lot of power supplies. (ka-ching!)

Here are some of the issues I've run into with generator power:
-The generator is producing square-wave, instead of sine-wave, output. Some generators fake it with a "stepped square" that approximates a sine curve, some do not. This is a problem because many devices monitor zero-cross as a reference point, which you rarely get with a generator's rough square wave output.

-The generator is producing the correct voltage, but at the incorrect or an unstable frequency.

-The generator is producing the correct voltage, but the power and current waves are out of phase, which can stress a power supply (or be refused by a UPS).

-The generator produces fluctuating voltage, or worse, a severe surge at start-up.

-There may be significant Neutral-Ground leakage, which also may cause the input to be rejected by the UPS (any halfway decent UPS should reject anything over 0.5V).
posted by xedrik at 11:24 PM on July 18, 2012

Just an update: We ordered the AVR but true to what AMSBoethius said, it was putting out 107v instead on 110. The UPS still wouldn't take the generator power and started sending underpower warnings so we had to return it.

We've ordered another and I'll update when it arrives.
posted by dozo at 6:49 AM on August 8, 2012

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