120vDC to 120v AC invertor

What may be worth looking at is a computer UPS. I have one which has 10 x 12 volt sealed lead acid battieries inside and terminals* to connect an external 120vdc battery bank. It generates 3kVA.

Only a few UPS use 120v dc internally - some use 48v, some 72v and some

• You cannot use ac-rated kit such as contactors and switches with dc unless the manufacturer says so - and then it will be massively de-rated. I got hold of some high current dc stuff from a Government surplus place - I think they may have come from a submarine..I also have some lovely ebony-handled, spring loaded open frame knife switches - which are rated for 120 v dc. I only use them on 24v...to comply with IEE regs.

Sue, Yes I was thinking about a UPS. I have an older one lying around. Almost tempted to just plug it in and see how it likes seeing only DC coming in. Umfortunly it has two 24dc batteries, giving 48vdc to the controller. Step down transformer comes to mind. I have a few, but the closest I have is

What to do, Chris

You cannot use a transformer on dc. Sorry about that. And no, you wouldn't connect your dc supply to the ac input of the UPS - it wouldn't like that at all. You would connect your dc supply in place of the 120 v battery bank. Now, if you had a 48 v dc supply, you would be all set with your existing ups. Try shopping around for a different UPS - I paid

You are probably aware that dc is scary stuff. It can really set your house on fire. IIUC dc shocks are particularly nasty as they tend to leave you gripping the wires, rather than throwing you away. I have been very, very careful to avoid getting one.

Sue,

You caught me on the transformer. I am still thinking that it is AC coming out of the welder. Still cannot beleive that they would bump that much DC at 15 amps out to a regular AC plug (yes it was designed that way, way back then). Worse yet, no breaker or fuse.

Chris

Frankly, with what you've said in this post I think the best course of action would either be to drop the idea completely, or at least hire a professional to do the work.

Transformers d> Sue,

I would say be prepared to give up the idea. A PC UPS will not work for you, unless you intend to use low power motors. Your compressor probably needs FAR more power than a typical PC ups can provide.

You might be better served by buying a generator sized to your needs.

Ed

Already canned the idea for the sake of safety.and cost plus I still have the welder. :) Actually the UPS that I have is good to 2250watts, more than enough to drive a 3/4 hp compressor.

Thanks again all, C

Can it handle the starting inrush current?

| You caught me on the transformer. I am still thinking that it is AC coming | out of the welder. Still cannot beleive that they would bump that much DC | at 15 amps out to a regular AC plug (yes it was designed that way, way back | then). Worse yet, no breaker or fuse.

It does sound like a dangerous design. But DC tends to be useful for welding. I remember that someone did explain why it is useful that way, but I don't recall any details.

| * You cannot use ac-rated kit such as contactors and switches with dc | unless the manufacturer says so - and then it will be massively | de-rated. I got hold of some high current dc stuff from a Government | surplus place - I think they may have come from a submarine..I also have | some lovely ebony-handled, spring loaded open frame knife switches - | which are rated for 120 v dc. I only use them on 24v...to comply with | IEE regs.

What if the DC is pulsed and goes fully to zero 100 to 120 times per second? I do know that steady DC makes it much harder to snuff out arcs. But what about pulsed DC?

What mark space ratio? With the same peak voltage or the same RMS voltage?

All other things being equal, whatever that means, I would expect that a n arc with a pulsed unipolar supply would be harder to snuff out than an ac one, but considerably easier than a steady dc one. A voltage across the arc would be needed to sustain it, and the periods without such a voltage would be longer with ac than with a unipolar square wave pulse train. Thus the ac arc should snuff first, then the pulse arc and finally the steady dc arc..

But that is just an opinion. I can't quote anything to substantiate it.

Doubtful. Do you know what the compressor will draw when starting?

Ed