DC Voltage Step-Up


Old 09-20-15, 11:27 AM
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Question DC Voltage Step-Up

Hello Brothers!

Looking for some advice on a project I've got going here.

I'm building a "Stick Arch Welder" From a 18hp twin engine and a modified car alternator (modified by bypassing the voltage limiter).

I have the whole thing assembled now but its producing lower volts than needed (about 19v max) need 30+, Im maxing the engines rpm's out so I ether need to Increase the V-belt pulley ratio or step it up electrically.

My first thought just make a step up transformer for the output, problem is the alternator puts out DC (as far as I know you cant use transformers on DC), but as it turn's out alternators make 3 phase AC then rectify it to DC.

So what I'm thinking is if I take the 3 phase AC voltage and put it through a step up transformer and then rectify it.

So I guess my big question is can I take 3 separate phases and combine them into one phase then put it through the transformer? or do I need to make a transformer and rectifier for each one, then combine the DC for max AMP's.

Any Help is much appreciated!
Old 09-20-15, 12:35 PM
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Interesting project. You can't just combine the three phases. You can buy 3 phase transformers or you can use three separate transformers.

Roadblocks you will face are that the transformer(s) need to be able to handle the full current capacity of the alternator and those are going to be big and expensive. Most readily available transformers are going to be designed for 60 HZ ac, and the output from the alternator will be higher frequency, depending on RPM. Higher frequency is not usually a problem for transformers, but you will be paying for a lot of iron you don't need.

Perhaps you can find an alternator designed for 24 volt systems like I think are used on some tractor trailers or construction vehicles. May be a whole lot easier and cheaper.

Can the alternator really supply enough current for the welding you want to do?
Old 09-20-15, 04:22 PM
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My experience as a DIYer and as a tradesman sometimes seeing the results of DIY work:

Be humble about the projects you undertake. Repairing existing equipment to the specifications they were designed for is a relatively safe thing to do.

Redesigning equipment for other purposes can cause a variety of complications you are probably not going to recognize.

You might wind up electrocuting yourself or someone else, or starting a fire your insurance company might refuse to cover.

I'd look for a simpler way to do what you want to do....
Old 09-20-15, 04:27 PM
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That alternator is not going to like having it's output shorted by arc welding.

In my opinion.... a regular car alternator is no where near robust enough to handle arc welding.

To answer your question directly..... you'd need a transformer with a three phase primary winding and a single secondary winding.

You'd need something like... 20v 3 phase wye to 50v single phase transformer.... if you could find it.
Old 09-20-15, 04:49 PM
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Mother Earth News many years ago ran a DIY article on a DC welder but use used a deep discharge battery and the alternator to keep it charged while welding. The battery asks as a buffer so reduces the strain on the alternator. I know you said you need 30 volts but a 24 volt marine alternator and 24 volt marine battery might do the job for you. Or to get fanciful three 12v alternators and three 12 volt batteries, each series wired.
Old 09-20-15, 08:25 PM
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Hey thanks for the input guys, This is a hobby project and practice for an event where you need an emergency welder.

I didn't really give you guys a background on these welders but there nothing new check out youtube "alternator welder". I'm using a 140 Amp Delco alternator

I didn't know they made 3 phase transformers. I could probably rewind this monster old center tap transformer I got. I have previously made a stick welder powered on 110v AC by rewinding two old microwave transformers.

Dose any one know the wrap ratio for a 3 phase to single phase transformer? Is it the same as a regular transformer? ie 1:1 or is it 3:1 ect?

So if I had 12v AC in on 3 sets of 10 wraps on the primary what would the secondary coil pick up? (also 10 wraps)
Also what would the HZ be? 120 like the 3 input coils or multiplied by three?
I might try it un-rectified as an AC welder, probably has a really smooth arch at 120hz-360hz

Note: I'm trying a setup with a larger pulley ratio tomorrow hopefully this fixes the voltage issue. It seemed to be the easiest route right now.
But I'm still interested in learning more about 3 phase transformers.
Old 09-20-15, 08:58 PM
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Here's a decent link to info on 3 phase transformers: Three Phase Transformer Connections and Basics

I think you may want to stick to 3 phase in and 3 phase out. When you convert to single phase using a transformer, the input phases are unevenly loaded so you don't get full output power. And if you do rectify to DC, you get "nicer" dc coming from a 3 phase bridge than single phase. Takes two more diodes though...
Old 09-21-15, 06:13 AM
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I'm not too optimistic on the 3 phase transformer. Depending on shaft speed, the alternator frequency will approach 400Hz. So, a "common" mains transformer will overheat with eddy currents. You would need a surplus aircraft transformer. As far as a 3 phase aircraft transformer, that will take some searching at surplus sites.

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