Tig arc start voltage vs OCV for DIY tig welder

[jdchmiel]

Let me preface this post with the confession I have never touched a Tig welder.

I have read a bit of the miller product literature and see for example the Dynasty 200 output specifications of:
TIG:
200 A at 18 V, 20% Duty Cycle
150 A at 16 V, 60% Duty Cycle
140 A at 15.6 V, 40% Duty Cycle
100 A at 14 V, 100% Duty Cycle

STICK:
200 A at 28 V, 20% Duty Cycle
130 A at 25.2 V, 60% Duty Cycle
100 A at 24 V, 60% Duty Cycle
90 A at 23.6 V, 100% Duty Cycle

and a max open circuit voltage of 80 V or 5-10v with low OCV enabled ( lift start mode)

My question is if one were building a DIY tig welder like I am from car alternator(s), what is the minimum voltage to start an arc? IS the higher OCV useful when doing scratch start, and then as soon as the welder is loaded the voltage drops down below 20v to the welding voltage?
IF that is the case, how does high frequency start come into play? Can an inverter tig machine start the arc at 14v with high frequency? Or is the OCV with high frequency also bumped to the 80v range to provide enough "oomph" to bridge the gap to the work piece?


I am asking these questions because I am going about the typical alternator welder DIY build a bit differently by using 1 or 2 ( got two in excellent shape from ebay for $130 shipped) 200 amp police crown vic alternators. I can use them in parallel with a bypassed voltage regulator to get ~400 amps at ~80 volts, or I can use the built in ECM controls by supplying a PWM signal 5-100% duty cycle to vary the voltage output from 12 - 16 volts individually, or 24-36v in series, by turning a pot ( ala foot pedal)

I theorize that the low Tig voltage from Miller is for one of two reasons, it is somehow useful / preferable, or they can get more amperage from the same silicone output devices at a lower voltage. If it is preferable, then using a single alternator to supply 16v @ 200 amps would work just fine, assuming that I could ever start the arc.

I can add HF to the alternator output pretty easily, (HV power supply +spark gap ) but if an high OCV is needed WITH HF then it would not help the arc start at all.
The other option is possibly to add in a pseudo high OCV by using a capacitor + charging circuit to have a brief spark discharge to begin the arc, but for this I have not worked out how to protect the alternator regulators.

So, in recap from the long explanation for a few things I need to learn:
-What voltage is present to start a Tig arc ?
-Is a 14-18v Tig output voltage (inverter specs) preferable to a ~30v range (transformer unit specs) for any reason?

reference links:
alternator: http://www.p71interceptor.com/altern...bishi/upgrade/
PWM to control 12-16v output: http://www.bakatronics.com/shop/item.aspx?itemid=581
weldernator: http://diy-welder.com/weldernator.shtml
another diy tig: http://myweb.cableone.net/rschell/TIG.htm

[DSW]

I'm not quite sure why you are doing this, other than just to make an electronics project... Tig requires a Constant Curent power supply. You can do basic scratch start tig with any DC capable stick machine and an aircooled tig torch. Tozzi did a good post a while back on his basic scratch start tig setup. You don't need to worry about what the Voltage is. Set the stick machine to an amp setting thats apropriate for the material you need to weld and go to town. High frequency will help with arc starts, but it's not required for DC tig. It is required for AC tig, but that's another animal entierly.

I forget who here even setup a small amp tig using one of the cheap HF inverter stick machines and managed some decent results at like 60-70 amps by addding an aircooled tig torch.

[jdchmiel]

I understand that you can use pretty much any CC machine to Tig with.. the alternator setup works pretty good for Tig and Stick from what I have researched. This is for a dirt cheap "fun" or "can I do this" type project. I suspect I am going to doing this the traditional bypassed regulator way that has been proven to work fine, but I would like to understand why / how / what of the output voltages engineered in the inverter machine.

[dbotos]

From the Miller TIG Handbook, chapter 4:

"The ionization potential of argon is 15.7 volts. So this is the minimum voltage that must be maintained in the welding circuit to establish the arc or to weld with argon."

http://www.millerwelds.com/resources/TIGhandbook/

This is why you see the outputs rated at ~16 V.

And from chapter 8's section on lift arc:

"This type of arc starting method was developed to eliminate tungsten contamination associated with the scratch start method.With touch start the tungsten is brought into contact with the workpiece. When this occurs, the power source senses a short circuit and establishes a low voltage current in the weld circuit. This voltage and current are not great enough to establish an arc, but do contribute to heating the electrode. When the electrode is lifted from the workpiece, the power source senses the absence of the short circuit condition and automatically switches to the current set on the machine. The fact that the electrode has been pre-heated assists in arc initiation."

What kind of circuitry are you going to use for current regulation?

[jdchmiel]

IT depends on which method I use... If I use the variable field drive, that regulates the output, but I think it is voltage and current together.. If I use the PWM signal to the alternators in series, then I was going to adjust the speed I spin them...