AC to DC 1

[vreijb]

I have to change single phase AC 220V welding machine into DC welding Machine. I have used full wave diode bridge rectifier but the output voltage and Current are not smooth. Is it possible to add inductor in series and capacitor in parallel to get better results, and how to calculate the parameters.

 

[Sparkey5774]

The size of the capacitor (and/or inductor) that you would use will largly be dependant upon the current draw. What kind of current does the machine draw?

 

[vreijb]

Well the output current of the welding machine is normallly ranged from 100-350A

 

[bogeyman]

Using a capacitor across the welding supply will not help much, the cap will discharge in next to no time and you will still have some ripple on the welding current. The capacitor current will be huge and probably shorten the life of the cap(s) considerably. A ball park value could be 100,000uF or larger with a 1000A ripple rating (if such a beast is manufactured).
An inductor in series is essential to smooth out the welding current but with a single phase rectified power source you have little to no chance of getting a continuous output current from your welding set.

Big welding sets were once built using a motor generator set that gave good smoothish DC for the welding supply, it is immaterial what the prime source of power (single phase motor, 3 phase or IC engine) is in these sets since the output current comes from the torque applied to the generator shaft.

A three phase supply can be rectified and produce an EMF that only falls to 86% of the peak AC voltage at 6 times the input frequency (50/60Hz in ,300/360Hz out)this in association with an inductor (choke) will provide a suitable source of welding current for DC welding.

If the same rectifier/filter is used with a single phase input the source EMF will fall to zero twice per input cycle and result in a discontiuous weld current.
By installing an inductor you will improve the smoothness but will not eliminate the high ripple current, an increse in rectifier input voltage will be required to accomodate the voltage drop over the inductor.

If you can use a 3 phase input for the power source if you require a very smooth welding current. Its a wee bit more complex but it will work.

 

[CarlPugh]

Buy a DC welding machine. A new machine will cost less and work better than trying to rework an AC welder.

The DC welding machine's AC characteristics are critical in how well the welding machine welds.
When Glenn Pacific was building welders, the procedure was to design the welder and then make changes to improve the welding characteristics. A change would be made and if it improved the characteristics good, if welding characteristics were worse, the an opposite change would be made and tested. And so on.

 

[Skogsgurra]

Agree completely. I was involved in the improvement (read making it work at all) of a spot welding machine sold to auto repair shops and we started doing all the math we thought necessary. We got a thing that worked. But it was only when we started the testing/changing/testing/changing loop that we finally got it real good. Well - as good as you can have it under the 32 A mains fuse limitation we had.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[Sparkey5774]

If you were to use a 200mF cap you would experience around a 10V ripple at 300A load current. 100mF cap will provide approximatly a 20V ripple. You will never get "perfect" DC. The larger the capacitance you use, the less ripple effects you will get, but in the end it all depends on how much ripple you can tolerate. Also keep in mind that you can put any number of capacitors in parallel to acheive a higher total capacitance. Although, as the others mentioned, this can get very expensive very quickly.

 

[vreijb]

Thanks for all your advices. Actually the company I work for we have 50 nos AC single phase welding machines. The new application requires DC welding and we If there is a way to utilize the Welding units we have we will try to do it before we move to a next step to purchase DC machines

 

[itsmoked]

50 huh.. I see your motivation!

What brand and model,(mostly),are they? Maybe we can figure out a solution...

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com