Repurpose UPS transformer as a power supply (3Kva 24v) - Page 1
Forum section: electronics / projects

author: guipoletto
author_rank: Contributor
timestamp: February 18, 2014, 12:56:05 am
content: Hi guys! first post here! i think it is the correct area, anyway, tell me i'm nuts, but i want to repurpose a 3kva UPS transformer, as a power supply to a remote site that is otherwise wind-powered.I've been looking for designs online, but it seems to me that the classic 2n3055 design is impractical, especially due to the low gain of the transistors, that is, in order to put out 60amps i would need another 30 just to drive the 3055's, also meaning that the thing would be more of a toaster than a power supply.I wonder if i could use the original IRFP-2907's (8 total) from the UPS as the linear regulators, but i couldn't find any mosfet-based design, so my guess is that there would be problems balancing the current.I'm open to suggentions.Thanks in advance!

author: dannyf
author_rank: Super Contributor
timestamp: February 18, 2014, 01:38:51 am
content: One approach would be to use a typical 3-terminal regulator as the error amplifier and use the mosfets to expand the current capability. You get the best of both worlds: simplicity of the 3-terminal regulator + current capability of a discrete regulator.

author: johansen
author_rank: Super Contributor
timestamp: February 18, 2014, 04:06:34 am
content: there are dozens of buck control IC's that you can use with or without the existing H bridge driver that was used to drive the fets you have.you'll want to add some dozens of uf of film caps to the existing electrolytic caps, though much closer to the fets., and you'll need to source an inductor or two. (i would find a two phase control chip and build a two phase buck converter)the output capacitor can be some crappy electrolytic caps, the esr actually stabilizes the system.

author: guipoletto
author_rank: Contributor
timestamp: February 19, 2014, 11:34:23 pm
content: I opted for the linear design for the robustness, and the lack of harmonics in the output.I plan to use this power supply to feed a radio retransmitter (UHF), so i am trying to avoid switching regulators. If i were to go for buck regulators, i would probably go for one of those industrial modules from artesyn (emerson power).But i could really use some clean regulation here.I've been working on a design, as soon as i have some (real) time i'll scan it and post it.

author: Magicmushroom666
author_rank: Contributor
timestamp: February 20, 2014, 08:25:53 am
content: Sending 60A at 24V is going to be expensive cabling wise, would be better to send the power over at higher voltage if possible.

author: nctnico
author_rank: Super Contributor
timestamp: February 20, 2014, 09:23:01 am
content: The biggest problem of this project is that the transformer has not been designed to operate for more than an hour or so. It will get too hot at full load over a longer period of time.

author: johansen
author_rank: Super Contributor
timestamp: February 20, 2014, 03:04:13 pm
content: I have two transformers from 1500VA pure sine wave UPS and they have relatively thin laminations, and they are wound with a foil secondary, not wire.Yes they will overheat at *rated* load, but that is because they are designed to run for 30 minutes maximum.These pure sine wave machines don't run saturated either, unlike the MSW inverters, so no load losses should be relatively low.Given that copper loss follows I^2, you should be able to run a 3KVA ups transformer at 2KW continuously.Keep in mind however that capacitor input filters consume many more VAR's than you think...If the capacitors are too big, you could overheat the transformer at only 1KW *load*

author: woodchips
author_rank: Frequent Contributor
timestamp: February 20, 2014, 06:26:38 pm
content: Must confess that I am still not sure whether you are trying to use a 24V battery to generate mains, 230V or so, or 230V to generate 24V DC?Assuming it is 24V DC to 230V AC, why start with such a low voltage? If you had 240V DC battery bank then you could simply use a variable frequency motor inverter to get the 230V AC, and it would also be 3 phase. Low voltages mean lots of current, 3kVA is about 150A on full load? You will need copper bus bars for that.

author: peter.mitchell
author_rank: Super Contributor
timestamp: February 21, 2014, 08:06:43 am
content: Quote from: woodchips on February 20, 2014, 06:26:38 pmThe majority of VFDs I've use just PWM the output and rely on winding inductance to average the current. The output looks nothing like a 3 phase sinewave, who knows what this would do to electronics.

author: guipoletto
author_rank: Contributor
timestamp: February 21, 2014, 11:47:27 pm
content: Quote from: woodchips on February 20, 2014, 06:26:38 pmI am planning on using the transformer to generate a filtered 24v high amps outout from the mains, pretty much like a 2n3055 classical linear power supply.The guys up there seem to want the opposite, to get mains voltage from solar cells (not my case).For them, i would advise avoiding those UPS transformers like the plague. they are designed to get a high surge load, therefore the primary (when inverting), is most times designed to take the 11v(9 to 11,5) (from sealed batteries under load) and boost to 115.If you continuously feed those transformers with more than those 11 volts, they can prove really inneficient.

author: dannyf
author_rank: Super Contributor
timestamp: February 21, 2014, 11:55:52 pm
content: QuoteOf course, the answer is yes.The simplest and a very good approach is to take your typical 3-terminal regulator and use the mosfet to expand its output current. The typical datasheet would have a schematic for such, using a pnp transistor on top of a 3-terminal regulator. You just need to put a mosfet on top of that pnp (with an extra resistor) and you are done.However, I doubt you will the kind of amperage you want, at least without some *****serious******* heatsinking. Maybe putting the mosfets in a tank of ice water.

author: guipoletto
author_rank: Contributor
timestamp: February 22, 2014, 12:57:51 am
content: The original heatsink from the UPS weighs about a kilo, and there is a mains-powered 25cm fan too.