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:
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
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?
PWM to control 12-16v output: http://www.bakatronics.com/shop/item.aspx?itemid=581
another diy tig: http://myweb.cableone.net/rschell/TIG.htm