Transformer Wiring

[srnoth]

Hi there,

I recently posted on making a charger from old UPS transformers and a car alternator. Somehow it got removed from the front page after a couple hours, anyway, here it is:

Hey there,

I recently blew the MOSFETS on two identical UPSs by running them for too long (even with cooling fans, they over heated and gave me some Magic Blue Smoke). They were very cheap, very poor UPSs, and I am almost glad to get rid of them, since they obviously could not handle extended use connected to my battery bank.

After reading the article on the battery charger built from an old inverter, I started thinking. Would it be possible to wire the two transformers in parallel to the 120VAC power, which would then convert to ~12VAC. Then put this 12VAC into the AC side of the rectifier (i think that is what it is) of a car alternator, which would convert the 12VAC to 12VDC, which I could use to charge some batteries?

These are the figures:

   2. x 250W (500VA) UPS transformers, which if wired in parallel, should be capable of converting 500W @ 120VAC to 500W @ 12VAC. This should mean it would output 500/12 = about 40 amps.

   1. x 65amp Alternator Rectifier, which converts 12VAC to 12VDC, at about 40 amps.

Thus charging my batteries with 40 amps of power.

The reason for this is that I want to be able to run 2 x 400W inverters from AC power, in effect creating an inline UPS:

120VAC          ---->          12VAC          ---->          12VDC          ---->          120VAC

(wall)    (UPS tranformers)                 (rectifier)                       (inverter)

I currently see several possible problems with this design:

*Old UPS transformers may overheat with constant use, since only built to run for about 5 mins a couple times a year.

*Can the two tranformers be wired in parallel, ie with all the phase stuff in AC, or will this cause trouble (wrong voltage or something). Will these transformers even work 'backwards'?

*Will the rectifier decrease or increase the voltage? It needs to be atleast 13VDC to charge the battery bank.

*Will this whole system be able to run continuously, powering about 200W continuos AC from the inverters, which means the charger needs to be able to put out 400W @ 12V, and a bit more to charge the batteries, or keep them on float?

*All this calculations are without allowing for efficiency. Will this cause a problem?

And lastly, the transformers have 3 wires coming out at each end. I know that the middle one on the 12V side conects to the 12v +ve terminal. How am I supposed to connect these transfomer to the 120VAC and the alternator rectifier?

Thanks for all your help,

Stephen.

P.S. I know that there is an article on this under 'experiments', but it does not go into detail about these aspects.

[srnoth]

K3CZ Wrote:

The idea is sound and practical, especially if the two UPS's are indeed identical.  Only the transformers themselves are necessary, plus the alternator rectifiers(still mounted on the alternator heatsink).  The 3 leads on the low voltage side of the transformers are a center-tapped secondary, probably 16-0-16 volts for full wave rectification.  The AC (and the resulting DC voltage after rectification)voltages are already optimized for 6-cell charging.  Of course, there will be no automatic charge rate control with this simplified setup.  If the transformers are the same, they may be connected in parallel on both primary and secondary sides, and use one set of rectifiers. (observe AC polarity when paralleling secondarys to prevent fireworks.)  Otherwise, connect the units together only after the rectifiers (DC).

This setup should work FB, but watch the transformer temperatures; as you have noted, these units were not meant for continuous service.

                                             K3CZ,  PE

[srnoth]

Finsawyer wrote:

Any imbalance in the transformers will cause a problem as the one putting out a higher voltage will treat the other one as essentially a short for the voltage difference resulting in it putting out more (unusable) power.  I think you're better off rectifying each individually and dealing with any voltage difference by using two series resistors to make sure each provides exactly its share of the power.

GeoM

[srnoth]

Thanks K3CZ. I'm glad that it sounds like it might work. But can someone please tell me exactly what leads to attach to what? I'm not used to transformers with 3 leads. There are three on the low voltage side and three on the high voltage side. Exactly where to these go? Also, I want to test to make sure that the transformers are working before I hook them up to the alternator's rectifier. How do I do this?

Stephen.

[Opera House]

A simple test you can perform when you are done is to wire a 100W 120V lamp in series with the 120V supply to the transformer with no load. If everything is ok then it won't glow.  

[justanotherguy]

"Somehow it got removed from the front page after a couple hours"

sometimes i think the dans are just to dam nice of guys to run a board. they let you guys ride rough shod all over them. any other board i hang out at if you reposted a thread that had been moved you would get a strong public warning the first time and the second time you would be banned, and that would be that because the guy who pays the bandwidth bill said so.

[srnoth]

Hey another guy,

Thank you for your most greatly appreciated input, but I am afraid it is a bit off the topic. However, I will tell you that my post was not removed, but I had posted it in Homebrewed Electricity, and it got moved to Control. Somehow this meant that it was no longer on the frontpage. I'm sure anyone can tell you that a topic gets most of its comments while on the frontpage, and when I post, it is so that I get intelligent answers from the intelligent people that are on this forum.

If there is anything wrong with my post, I will be glad to recieve some constructive criticism from you guys.

Stephen.

P.S. That was a great idea about the lightbulb in series, Opera House, thanks a lot.

[K3CZ]

I, too wondered what happened to your/my posting, so I did a bit of searching - your problem and my comments ended up over in "controls", but I see the Dans have brought it back to the main board for the reception of additional comments.

Anyhow, you said the two xfrmrs were identical, eliminating load share problems.  Also, my diagnosis for the 3 wire secondary being CT (32/36volts) seems OK; but the line side 3 wires is not quite so obvious.  Most likely, there is a common and two possible input hiside wires, 110 volts and 120 volts, to cover Japan, etc.  The technique mentioned above, to use a 40/60 watt bulb as a current indicator is a good one: use such a bulb in series with your test clips and the AC line to check out the various combinations of the  3 input wires.  The pair of leads that does not lite the bulb is the proper input to the transformer.

                              Good luck!     K3CZ