Author Topic: Homemade double conversion UPS from parts  (Read 1730 times)

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dnix71

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Homemade double conversion UPS from parts
« on: April 11, 2017, 03:56:13 PM »
Does anyone see anything overtly dangerous in this?

The back story: Where I live the highly trained  ::) ??? contractors for the local power company have been working on the system live during the day adding cross-tied feeders so we are supposed to be less likely to suffer the loss of the grid again like we did during hurricane Wilma in 2005. You can imagine the damage they have been doing to connected equipment by working live. I have personally seen fire alarm panels damaged (false alarms/dead batteries), point of sale computers wrecked (dead hard drive/scrambled modems) and business down-time because 3-phase equipment just hangs and quits when phases get dropped during switching.

The POS computer required over $500 to fix, and a complete replacement will be $3k.

Regular uninterruptible power supplies don't work when something like this is going on. The transients and brown-outs go right though. Big data centers don't use the home grade stuff like an APC 350, they do double conversion. Make dc from the line and feed that to a high quality sine wave inverter with battery backup.

I put this together from stuff I had unused in my apartment. The dc supply is 19 amps at 13.8v, the inverter is a true sine 600 watt AIMS, and the battery came from a automobile jump starter that I scrapped.




george65

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Re: Homemade double conversion UPS from parts
« Reply #1 on: April 11, 2017, 04:37:08 PM »

Seems to me the only real protection you have there is the battery to clamp any spikes.  May or may not be effective.

You can get UPS units that do this double conversion. I have a couple of APC units that designed this way.  It's easy to tell the Double conversion types, you can Cold boot them, IE, boot them on the batteries without and AC power.
The line interactive Types won't do that. They need to be connected to the AC and powered on from there. Once they have started you can unplug the AC and they will run on battery power but they won't START on battery power alone like the line active / double conversion types.

Bought them off fleabay for under $100 ea. Just have to keep your eye out and make offers when they come up.
I have one of these on my play Solar System. I removed the battery pack and have a couple of large car batteries connected to the UPS.
 Control the Solar input with a simple charge controller and the UPS does the rest.

joestue

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Re: Homemade double conversion UPS from parts
« Reply #2 on: April 11, 2017, 05:38:03 PM »
Imo, overkill. If your equipment can be damaged by a brownout, get that taken care of. Modern PFC computer power supplies can operate between 85 and 265 vac if they are run at about half load. Some of them may continue to operate at just 60 vac if run at much less than half load.


Spikes are a problem but the load dump transients should not make it past the surge protection in your terminal strip. (Lightning is a different kind) Some of them use the circuit board as a fuse. On some of them if the green led doesn't light up then them mov's did their job.


Traditional ups take at least one cycle of 16ms to transfer because they use relays. While a spike can get through in that time, it's only clamped if it's more than 330volts (typically, I have seen 150vac rated mov's in 120v equipment) but the spike is not very long.

Simple solution is increasing the impedance of your grid supply. A 5% line reactor might be better than replacing mov's all the time. If the spike doesn't last long enough to cause enough current to flow through the diode rectifier (causing it to fail or over volt the downstream components) in your computer powersupply, then nothing will break


Anything run from 3 phase that will break if a phase drops out= your problem imo.
« Last Edit: April 11, 2017, 05:55:04 PM by joestue »

dnix71

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Re: Homemade double conversion UPS from parts
« Reply #3 on: April 11, 2017, 06:24:09 PM »
We are having rapid short phase drops as they switch each line. The a/c overhead made banging noises and then a loud buzz while the lights dimmed many times. The aircompressor stopped, the conveyor stopped as they are all 3 phase. It's just a matter of time before something fails. A electronic ballast in the hall with 3 tubes appeared to fail one day. We put in a maintenance request, but 2 days later in the middle of the day it suddenly started working again.

Computer hard drives can't handle a bunch of sudden hard stops, and the modem scrambles is scary, too. This is an ATT business class modem. If the modem gets scrambled, no connectivity, which means the POS stops working after 4 hours and a hard reboot is needed to restore offline sales capability. People don't care much cash anymore, and the credit card readers will only allow 4 hours off-line before they refuse to work.

Back in the 1980's I would have just gone out and put an Isobar filter in front of the UPS. Those had capacitors, choke coils and resistors, instead of throw away MOVs.

OperaHouse

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Re: Homemade double conversion UPS from parts
« Reply #4 on: April 11, 2017, 10:04:27 PM »
I just question what the power supply is set at.  I would prefer the inverter connected to the battery through a diode and once the power supply drops out the battery takes over.  That would allow the battery to be on a separate smart charger. Not sure how the battery would fare if just connected fixed voltage of say 13.5V through many discharge cycles and long term float. There are commercial UPS that do this exact thing, constantly running on inverter.

george65

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Re: Homemade double conversion UPS from parts
« Reply #5 on: April 12, 2017, 04:53:09 AM »
We are having rapid short phase drops as they switch each line. The a/c overhead made banging noises and then a loud buzz while the lights dimmed many times. The aircompressor stopped, the conveyor stopped as they are all 3 phase. It's just a matter of time before something fails. A electronic ballast in the hall with 3 tubes appeared to fail one day.

This is an ATT business class modem. If the modem gets scrambled, no connectivity, which means the POS stops working after 4 hours and a hard reboot is needed to restore offline sales capability. People don't care much cash anymore, and the credit card readers will only allow 4 hours off-line before they refuse to work.

So what are you going to put this setup that you have made on?  Doesn't seem practical for anything you have mentioned.
The battery appears to be a 17-20 Ah unit. If you pull 600 or even 300W from it, it isn't going to give you a very long run time if the power goes out.
Even with the charger running, you are heading downhill at anything over about 250W which would be less than most desktop computers and an LED monitor would pull let alone any add on's.

If things were as bad as you describe, I'd be getting hold of a proper used UPS, removing the battery pack and hooking it up to some used car batteries or whatever was better you could find and then running my equipment off that.  Would probably last all day and you wouldn't have to worry about lost sales, damaged equipment or anything else.  When the need had passed and the power was stable, you could sell it off and recoup most of your money.  As a system you might get more for it.

Whatever the cost, which I imagine should not be more than $2-300 if you bought a used UPS and batteries, it would be cheap insurance against the potential losses to the business and you could probably write the cost off anyway. Once you sold it off afterwards, you may even be in front.

joestue

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Re: Homemade double conversion UPS from parts
« Reply #6 on: April 12, 2017, 10:03:25 AM »

Seems to me the only real protection you have there is the battery to clamp any spikes.  May or may not be effective.

Spikes can't get through a dc power supply unless lightning strikes it. Usually when the input is over volted the semiconductors fail shorted, or they blow up and fail open. the output drops to zero as fast as if you turned it off, the only voltage surge you'll see at the output is the increased current flowing through the transformer for the few tens of pwm cycles that it takes for the control loop to react and reduce the current, and this voltage surge will be similar in magnitude to the voltage surge you get when you simply disconnect half of the load. maybe 100millivolts on a 5 volt supply.

It would not surprise me if most computer power supplies could handle a transient that has just enough energy to push the 400 volt capacitors up to 600 volts. The question is, does the capacitor explode in the time it takes the power supply to discharge the capacitor from 600 to 400. my guess is no, if its less than 5 seconds. last story I heard was on a cnc forum someone plugged his 240v vfd into 400 vac and it blew up in 15 seconds. http://www.cnczone.com/forums/spindles-vfd/332720-repair-vfd-1-5kw-huanyang-never-connect.html the drive itself was fine.

unregulated transformer, rectifier, these stop spikes according to the nonlinear impedance/saturation of the transformer and the dc filter capacitors. spikes won't get through but sustained over voltage will.
« Last Edit: April 12, 2017, 10:10:34 AM by joestue »

dnix71

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Re: Homemade double conversion UPS from parts
« Reply #7 on: April 12, 2017, 05:46:20 PM »
OperaHouse The inverter runs off the power supply now. I had thought of adding a diode for better isolation. I have a 40 amp adjustable 13.8v power supply on the way. That will give me 500 watts of usable output. I'm not worried about the short battery run time. If they power fails for more than a few minutes, you can't serve food anyway, as the speed oven will have cooled down, and it doesn't work properly anyway without electricity to force air around inside. The power is only failing for a second or two at a time most times.

There was one outage that lasted more than 4 hours. We just shut down and closed up. People who live in "3rd world" countries are used to this kind of thing. They have adapted with generators. The newest generators are inverter-based now. No nasty harmonics and phase shifts and a solid, stable frequency.

One of our buildings has a 400kw Volvo based genset. That one only takes a flip of a switch to bring up, but it won't run everything in that building.

Transients are not as harsh on the equipment as the dropping of one phase. That causes nasty brownouts and hard stops anything 3 phase.