# Basic Arc Welding Question- Ac And Zero Crossing



## markba633csi (Jul 7, 2016)

I have read that for tig welding the arc extinguishes when the ac goes thru zero,  and this is why HF needs to be superimposed on it.  
Why is this not a problem for smaw (stick) ac welding?  
thanks
Mark S.


----------



## Tony Wells (Jul 7, 2016)

The zero crossing period is very short, practically non-existent in the real-time domain, and the residual magnetism in the core of the transformer lags behind in the cycle and in effect "smooths" out the sine wave and it never is allowed to get to exactly zero. Works like a choke, in a fashion.

So then, when does it cross, if it is indeed AC? I've never scoped one. Never was that curious. But I suspect that when it does, there is enough current flowing to even use the length of cable as well as the length of the winding to smooth it enough to make the time insignificant.


----------



## mike837go (Jul 7, 2016)

Also the cloud of ionized gas allows the arc to re-establish.


----------



## markba633csi (Jul 7, 2016)

I think I'm starting to understand it,  more of an issue with "old iron" type tig machines at 60 cycles.  Newer inverter units are square wave and have much shorter crossings.  The square wave machines must be using the HF for arc starting only.
MS


----------



## Tony Wells (Jul 8, 2016)

I think I should clarify something. I didn't mean it never was actually zero. Of course any current that changes polarity, as full wave current does, is at zero for some time. But there are so many factors that govern the actual arc performance that the negligible time spent _at_ zero has no practical effect.


----------

