1
\$\begingroup\$

First, let me premise this question with the fact that I'm not well versed in electricity.

Here's the problem: I am trying to power a UPS with an inverter and two deep-cycle batteries run in parallel. Connected to the UPS are a laptop, router, RFID reader and printer. When everything is turned on, the UPS repeatedly (every two seconds exactly) kicks on and off. We've tried different inverters and the same thing continues to happen. The UPS' red "building fault" light illuminates as well. Can someone help me understand what we're doing wrong?

My best illustration of the configuration: http://i.stack.imgur.com/fqi0J.jpg

\$\endgroup\$
  • \$\begingroup\$ Laser printers consume a lot of power at roughly 2 second intervals. Are you using a laser printer or ink jet? \$\endgroup\$ – user3624 Jun 21 '13 at 19:59
  • \$\begingroup\$ +1 @David A consumer UPS will not be able to hold up a laser printer. Peak demand is almost 2,000 Watts (my measurement) for a home-office sized machine. \$\endgroup\$ – DrFriedParts Jun 21 '13 at 20:19
  • 3
    \$\begingroup\$ The building fault light is on because you have no connection to Earth, which would normally be provided by the ground pin, and also less directly, the neutral line of the AC supply coming into your building. \$\endgroup\$ – Phil Frost Jun 21 '13 at 20:31
  • \$\begingroup\$ I've run a laser printer off of a 1.5KVA UPS (Eaton Powerware online UPS), but I found the UPS to be very sensitive to input power -- it kept tripping over to battery power when running off a cheap "contractor grade" 7.5KW generator, but when we switched it over to a nice Honda 5000W inverter generator it was fine. I suspect that in this case, the inverter is not putting out a nice enough waveform for the UPS. \$\endgroup\$ – Johnny Jun 21 '13 at 21:30
  • 4
    \$\begingroup\$ Another possibility: Unless you paid extra for a true sine wave inverter, your inverter is producing "Modified Sine Wave", or a stepped waveform that is more square than sine. Possibly your UPS does not like operating from this modified sine wave power. \$\endgroup\$ – jwygralak67 Jun 23 '13 at 17:51
8
\$\begingroup\$

Too small

Your inverter and your UPS are too "small". You have too much electrical load attached for the power ratings of those devices.

Why is there a UPS in there? A UPS (to the first-order) is just a battery + an inverter... which you, obviously, already have in your system.

Too inefficient

Also this is NUTS inefficient...

Approximately:

Battery (DC) --> Inverter (AC) --> UPS (DC) --> Devices (AC) --> Device Internals (DC)

You are converting DC and AC at least 3 times. Each time you lose about 20% of the power. So you are wasting half of the battery energy (e.g. it is running half as long as it could).

Follow-up

The reason we have a UPS is in case the battery bank runs dry we have time to hook a generator.

Monitor the voltage of the batteries and "hook a generator" when the voltage drops below 80% of nominal.

You can use a $20 multimeter for this and just check it every half-hour or so. There are also off-the-shelf devices for this.

That will be much cheaper, simpler, and more efficient. It will also improve battery runtime, produce much cleaner power, and continue to operate during the transition.

The tiny battery in the UPS will likely not be able to provide the instantaneous power to keep things running since it is much smaller than the larger batteries you use for primary power.

Can you explain how we can eliminate the building fault with this current configuration? Is there a way to create a ground?

uh... ground it! ;-) Connect the center pin of the UPS's power cord to a copper pipe at least a meter long and bury the copper pipe in the ground (vertically if possible -- e.g. so it's at least a meter deep).

This will only work if you also have a ground-post (connection point marked "ground" or with the Earth Ground symbol) on your inverter that you connect to the same pipe.

If your inverter doesn't, I agree with @Johnny when he says:

I don't think there's a safe way to eliminate the warning light while plugged into a portable inverter, and there's probably not much need to do so since I haven't run across a UPS that refuses to operate when that light is on.

You can buy copper electrical conduit pipe from a home improvement store for a few dollars.

I should also point out, that the goal here is simply to get the ground-fault light to turn off (by grounding the network). My suggestion should in no-way be considered a lightning protection system. Nor does it make use of anything that would give you ground-fault protection. Further, it probably doesn't meet the US National Electrical Code (NEC), which calls for a maximum 25 Ohms (DC) to ground. Typically, more than twice my recommended length is used to insure compliance.

\$\endgroup\$
  • \$\begingroup\$ DrFriedParts, Thanks for the detailed answer. The reason we have a UPS is in case the battery bank runs dry we have time to hook a generator. We're trying to keep things quiet. I understand the inefficiencies now, and we can investigate a work-around. Q: Can you explain how we can eliminate the building fault with this current configuration? Is there a way to create a ground? \$\endgroup\$ – clayperez Jun 21 '13 at 21:27
  • \$\begingroup\$ Revised as requested. \$\endgroup\$ – DrFriedParts Jun 21 '13 at 21:46
  • \$\begingroup\$ Simply grounding the inverter probably won't affect the site wiring fault indicator, since it is probably looking to make sure that neutral and ground are at the same potential. And I definitely would not bond neural+ground together to eliminate the fault light without consulting an electrician or being instructed to in the inverter docs. I'm not even sure that grounding the inverter is a safe thing to do if it doesn't not have a grounding post designed for this. \$\endgroup\$ – Johnny Jun 21 '13 at 21:56
  • \$\begingroup\$ @Johnny -- the building fault is on the UPS, not the inverter. I made a typo. Apologies. Completely agree with you, it won't work and shouldn't be attempted if the inverter doesn't have a grounding post. \$\endgroup\$ – DrFriedParts Jun 21 '13 at 22:04
  • \$\begingroup\$ Yeah, sorry I wasn't clear, I was talking about the wiring fault indicator on the UPS too. I don't think there's a safe way to eliminate the warning light while plugged into a portable inverter, and there's probably not much need to do so since I haven't run across a UPS that refuses to operate when that light is on. Maybe if he upgraded to a household inverter made for powering a house from a battery bank he'd have a ground terminal and the neutral bonding that the UPS expects (but I'm not sure about that). \$\endgroup\$ – Johnny Jun 21 '13 at 22:08
2
\$\begingroup\$

Rule of thumb: Never hook up a laser printer to the battery side of a UPS. They draw a ton of current for split second when they charge up. This will cause many(most?) UPSs to fault.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.