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Up Topic Welding Industry / Technical Discussions / magnetic pinch effect
- - By Plasma-Brain (**) Date 11-16-2007 14:40
Greetings all,
Pinch effect is something ive been wondering about for a while now, so I figured id throw it up here.
All I know about it is that it results from the magnetic forces of the current and manifests at the tip of the electrode, where it squeezes the wire and detaches the drop. Thats what the text book told me, so what im wondering is: would it be of any benifit to find a way to make this effect stronger?
ex: Would a higher pinch effect lower the transition current for globular to spray transfer over identical settings with less pinch?

Thanks once again, Clif
(Ps: if im poking at classified/porprietary information, please let me know and ill leave it alone)
Parent - - By js55 (*****) Date 11-16-2007 15:51
The pinch effect is essentially a physical manifestation of the magnetic field caused by current flow through the electrode (Maxwell and all that stuff). There is actually potential for pinch effect throughout the electrode wherever current flow is manifest. It actualizes at the wire tip because of the increasingly molten state of the electrode.
Pinch effect is directly related to the current density of the electrode. Therefore there is two ways to lower transition current through pinch effect. Increase current and reduce electrode cross section.
Also, changing weld gases can alter the electrical and ionization potential relationships of the welding process which can alter transition current (as CO2 users are well aware)but I don't believe it acts through the pinch effect directly.
Stephen is the guy to chime in on this to fill in the blanks I've clearly left in theory here.
And I may even have tromped on theoretical precision a bit. Stephen may help to clarify that as well.
But this in a nutshell is my understanding.
Parent - - By 803056 (*****) Date 11-16-2007 22:45
Some machines have a control that permits varying the "pinch effect" by varying the inductance.

Best regards - Al
Parent - By js55 (*****) Date 11-19-2007 15:51
thanks Al. No matter how hard we try and elevate ourselves and our companies and clients out of the technological dark ages, there are particulars that we will forget.
Parent - - By Lawrence (*****) Date 11-19-2007 16:04
Hey Al,

Can you say more about inductance and pinch effect and their relationship with spray transfer GMAW ?

My total experience with inductance has been with short circuit GMAW. In short it goes something like this:
   1. Inductance controls the rate of rise up to peak current during short circuit/dip transfer.
   2. A low inductance setting will give higher short circuiting frequency and a relatively cold weld.
   3.High inductance will give lower short circuiting frequency and a relatively hot weld due to longer arcing periods between short circuits.
   4. Inductance controls are primarily used for extra control on open root welds and short circuiting of stainless

I have worked with CV power supplies with flat and steep slope lugs to facililtate short circuit and spray. But I thought that was somehow different than inductance.

I would like to understand more

Thanks
Parent - By 803056 (*****) Date 11-19-2007 16:23 Edited 11-19-2007 16:37
You pretty well hit the nail on the head. The pinch effect, "controlled" by inductance, is primarily used when using the short circuiting mode of metal transfer.

I tell welders that inductance is like the shock absorbers on the front of the car. If the shocks are worn (little inductance) the car bounces uncontrollably (erratic unstable arc). If the shocks are too stiff, the ride is hard (too much inductance). In the case of the welding arc, the rate of change in the amperage relative to the arc voltage determines how the metal droplet detaches from the end of the electrode. If the rate of change is too rapid, the droplet detaches violently and produces excessive spatter. It the rate of change is too slow the metal droplet doesn't detach cleanly and the arc is unstable. I would have to do some serious digging through my old text books to go much beyond my brief description.

To the best of my recollection, inductance doesn't play a major roll in spray transfer.

Because welders are not taught electrical theory and don't understand the influence of inductance on a system, they had difficulty understanding how to set the inductance. Most manufacturers simply removed the variable inductor in favor of a fixed inductance that is best suited for welding carbon steels.

Slope is not quite the same in that slope is the relationship between the (static) voltage and (static) amperage of the power supply. Slope tells you the amount of change in amperage as you vary the arc voltage. Steep slope will produce a small change in amperage with a change in arc voltage. Flat slope results in a large change in amperage with a change in arc voltage. Slope doesn't indicate the rate of change. The primary difference between a CC and a CV power supply is the slope of the machine. The CC machine has a steep (drooper) slope and the CV has a flat (shallow) slope that results in an arc that maintains the proper arc length by varying the amperage (melt-off rate).

Most CC power supplies favored by pipewelders has a slope that is "shallow" to provide the ability to vary the amperage (thus penetration) by varying the arc length slightly. A steeper slope is favored by shop welders because slight fluctuations in arc length results in only a slight variations in amperage, i.e., it isn't as responsive and doesn't provide the "digging arc" many pipewelders favor.

Best regards - Al
Parent - - By CWI555 (*****) Date 11-16-2007 23:11
Some further info on the subject. A similiar effect is used to shrink coins and other things. It's amazing what it can do.

http://205.243.100.155/frames/gallery/coins4.jpg
http://en.wikipedia.org/wiki/Pinch_%28plasma_physics%29
http://www.americanmagnetics.com/tutorial/magnetp.html
Parent - By Plasma-Brain (**) Date 11-18-2007 21:46
Thank you all for the input, I have to read up more on phsyics to find out what I want to know so im going to go work on that.
- Clif
Parent - By 803056 (*****) Date 11-19-2007 16:05
Cool web sites.

I wish I had heard of the Tesla turbine before I located my steam turbine for fun and games.

Best regards - Al
Up Topic Welding Industry / Technical Discussions / magnetic pinch effect

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