DIY welder

[Welsh Chris]

After searching the forum I know DIY welders have been covered before but here is a twist I cant find anything on...
Having seen this
http://www.metalwebnews.com/information/alternator-welder.pdf
I thought that might be interesting to make but here is the twist, I have an old NewArc viper tig add on lurking around. It requires 24-90 ocv and up to 200 amps (though in truth I only really need 100amps). The main problem is as you've probably already guessed I need constant current.
Could you get a stable current from an alternator welder given that they are intended to give constant volts in a car?
Help appreciated.

 

[shenion]

That setup has very little control over the current. Putting a rheostat in series with the field gives a much better constant current output.

Check out my setup with a 200A alternator and a rheostat: http://diy-welder.com/weldernator.shtml

 

[Welsh Chris]

Hello Shenion
Thanks for the reply, ive had a good read through and really must say, that's an excellent article, very informative.
That looks like it could serve the purpose. A question though....
How does the alternator cope when running but not welding?

 

[shenion]

Mine with no load outputs about 75VDC. That really helps maintain the arc and also helps starting.

The big issue is the diodes. I tested mine on the bench to 180V and they worked fine. Most older alternators have 200V diodes. Some of the new ones will fail at 50Vor less. Many either replace the stock diodes or use an external rectifier.

There are also large voltage spikes to deal with. Hence the MOV and capacitor I added. The capacitor reduces the current/voltage spikes inside the alternator.

A good alternator will be better than most single-phase mains powered arc welders. As it is 3-phase, the rectified DC is very smooth. On mine, the arc is just a soft hiss. Also any ripple is at 500hz or more.

 

[Jim Davey]

Here's a tip, from someone who knows. Always make sure that the Viper unit is fed by a power source capable of supplying at least 200 amps constantly and set it to at least that, preferably more if you've got it, regardless of wether you think you need that much power (you can always adjust the amperage on the Viper unit itself) as the Viper unit is very easily burned out by being under supplied. National hire companies have burned out dozens if not more this way as people just don't know how to set them up. The unit itself works great, just set the welding power source to max regardless of load to avoid damage to the tig unit.

 

[Welsh Chris]

Hi all. Thanks for the great input. Shenion, you're welder does look like it will serve the purpose and your comments will certainly help to make sure I select the right alternator.
Thanks Jim for your pointers, I really wouldn't have assumed under powering it would cause damage, I would have tried an alternator around 140 amps so you saved me there.
Shenion, your plans mention linking two alternators to increase output, perhaps that's the answer.
I will start with one though and see if I get it right but not link it to the viper.

 

[Jim Davey]

Make sure your power source is DC, the Viper won't like AC at all.

 

[Welsh Chris]

Good point, I should make a point of making sure the rectifier diodes are up to the job.

 

[Jim Davey]

And your smoothing capacitors if it has any?

 

[Welsh Chris]

Hi all.
Ive had a look into the capacitors, Shenion your plans do give good details on the capacitor, I have looked on digikey for the MOV using the part number you listed but also for capacitors, whats your opinion on...?
http://www.digikey.co.uk/product-search/en?x=13&y=20&lang=en&site=uk&KeyWords=cgs862u100r4c
Regarding the new arc add on, I think I am just going to build the welder as is and make sure I am happy I have done it right. I will then perhaps revisit the add on at a later date perhaps once I have added a second alternator to guarantee sufficient supply.

 

[shenion]

There are two things about that cap: 1 - it is not in stock. 2 - minimum order 100. Otherwise, it will work fine.

Really, ebay is where to get large caps. I have gotten several NOS ones cheap.

I looked at the New Arc box. From what Jim says, it looks like it is an invertor and a bit under rated. Like most invertors it needs a good supply. Say you are welding at 140A, it will draw probably 90A on the input side as the input would be a higher voltage. Once the input drops, it has to draw more current to get the same power out. At some port it exceeds the input current capability. The same happens on invertors on long mains leads.

You probably can just tie the field of the alternator directly to +12V power when using the add-on. Then the output will not drop as much when welding.

 

[Welsh Chris]

That will teach me to read the page properly, I looked at the spec and unit cost but didn't read the stock. I will take your advice and head to ebay.

Thanks for the explanation of the newarc, it all makes sense. I am going to put that to one side for now though.

I'm only really doing this for the sake of a project and I thought it would be nice to have a welder I can run off the PTO on my tractor, not for any specific purpose.

I'm sure I'm not alone in enjoying making things just for the sake of making them .