Subject: Re: Hi-Freq module for mobile TIG

From: [email-masked] (Jim Craighead)
Date: 22 Mar 1995 13:21:03 GMT

In article <[email-masked]>, [email-masked] (Dan Wold) says:
>
>Greetings,
>
>I'd like to put together a hi-frequency attachment for my portable welder.
>The welder is a 2-cylinder gas-engine driven capable of 225 amps AC output.
>DC output is 200 amps max. I believe the open circuit voltage is around 50v
>and the welding voltage around 25v. There is an aux 125v @30 amp AC outlet.

 I am also interested in any info on high frequency exciters. If someone 
could post the frequency used for this purpose, I would be happy to post a
schematic for a high power oscillator. Using brute force, it should be 
pretty simple to make a transformer to couple the HF to the welder's main 
output.
 I have found texts that explain how to weld, but I have not found any
that explain the theory behind the technology.

Jim Craighead
Development Electronics
Mayo Clinic

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From: [email-masked] (Henry Kroker)
Date: 22 Mar 1995 02:38:54 GMT

>High frequency current is essential for TIG welding aluminum as the high 
>frequency current breaks up the oxide coating on the surface of the 
>aluminum.  Without the HF, you will have to break up the oxide 
>mechanically, which does not produce as sound a weld.

>Craig Hodne

Don't know much about TIG welding, but based on the repairs I'm doing to a sorched plasma, the 700 or 800 dollars Lincoln wanted for their attacment sounds like a good deal. The power board for this torch (a 40 amp unit ...SMALL) is $1200US. The sixteen power mosfets are 7.00 each US. This still leaves control, rectifiers, filtering, heatsink etc. I have to confess that I have considered this possibility myself, but the ecconomics seem a bit doubtfull.

The plasma torch i'm repairing has "fair" schematics with it. For a TIG attachment these could be simplified, but they give you some idea of what's involved with switching high power, at a high frequency.

I'm trying the repair, but the welding shop wrote it off.

I'd be interested in knowing of referance texts that dealt with both of these subjects (plasma high freq TIG) in depth, from a technical perspective. ie. physics and electronics. Any help would be appreciated.
Thanks
henry

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From: Michael Kline <[email-masked]>
Date: Mon, 20 Mar 1995 18:14:51 GMT

> mechanically, which does not produce as sound a weld.
  I don't think this is exactly correct. In the TIG welding course
that I took it was explained that the high freq attachment kept
the argon ionized when the main AC power crossed through zero.
tests of aluminum welding with just an old Miller Buzz-box and
an air cooled TIG torch showed that without the high Freq attachment
you could still weld aluminum but the amperage had to be cranked up
so high that you burned thru anything less than 1/4" thick.

	I too would be interested in any schematics or documentation
on building a high freq attachment. I think I could build something
now that would generate the high voltage but I'm a little worried
about what it would do to me when I Zap myself as I know I will.
A high enough frequency should handle this aspect but I too am 
unclear as to how to couple the high freq in.

	Any knowledgable welder/electrical engineers out There?

					Mike
					[email-masked]

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From: [email-masked] (Dan Wold)
Date: 20 Mar 1995 21:16:59 -0500

Michael Kline ([email-masked]) wrote:
: [email-masked] (Craig Hodne) wrote:
: > 
: > High frequency current is essential for TIG welding aluminum as the high 
: > frequency current breaks up the oxide coating on the surface of the 
: > aluminum.  Without the HF, you will have to break up the oxide 
: > mechanically, which does not produce as sound a weld.
: > 
:   I don't think this is exactly correct. In the TIG welding course
: that I took it was explained that the high freq attachment kept
: the argon ionized when the main AC power crossed through zero.
: tests of aluminum welding with just an old Miller Buzz-box and
: an air cooled TIG torch showed that without the high Freq attachment
: you could still weld aluminum but the amperage had to be cranked up
: so high that you burned thru anything less than 1/4" thick.

I go along with Mike's analysis above. The effect of the main AC weld
current crossing through zero can be experienced by setting up to weld 1/8"
alum plate (for example) , run a bead with hi-freq on, then run a bead with
it shut off- it doesn't work very well as the arc stutters or shuts down
completely.

: 	I too would be interested in any schematics or documentation
: on building a high freq attachment. I think I could build something
: now that would generate the high voltage but I'm a little worried
: about what it would do to me when I Zap myself as I know I will.
: A high enough frequency should handle this aspect but I too am 
: unclear as to how to couple the high freq in.

I believe the hi-freq is coupled in with air-core coil transformer. About 5
or 6 years ago I had the cover off my old AIRCO AC-only "Heli-Arc" and
remember seeing an air-core coil about 4" diam and about 8" long. This coil
must be energized by another coil which produces the main boost to
hi-voltage. Maybe an automotive type spark coil would work?

In my shop I have a 375 amp Lincoln TIG. Without grounding my body to the
welding table I can start the hi-freq arc on my fingertip and pull it out to
about 1/2" before it breaks. After 3-4 seconds of arcing my finger will feel
warm but I don't get any sensation of electrocuting myself. This is a
very nice feature! And it never fails to impress visitors when I strike a
buzzing little arc on my finger. Another aspect of hi-freq is EMI or
radio-noise. My shop Lincoln seems to supress EMI very well. I play a radio
constantly and the lady in the antique store above me plays her TV all day
too, both without interference.  

: 	Any knowledgable welder/electrical engineers out There?

I've lost the owners manual to my TIG. :-(  Maybe someone has one they can
look at for schematics. 

-Dan
-- 
[email-masked]
Daniel Wold                               239 City Island Ave,Bronx,NY,10464
finger [email-masked]
Sysop: WORLD CITIZEN BBS  718-885-2346  14.4  24 hrs  FREE  Fidonet BahaiNet

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From: [email-masked] (Gale Carlisle)
Date: Mon, 10 Apr 1995 02:43:36 GMT

In a previous article, [email-masked] (BobHarbour) says:

>: I am also interested in any info on high frequency exciters. If someone 
>:could post the frequency used for this purpose, I would be happy to post
>a
>:schematic for a high power oscillator. Using brute force, it should be 
>:pretty simple to make a transformer to couple the HF to the welder's main
>
>:output
>
>I have a Miller AC/DC hf machine and it uses a very simple setup for the
>hf source. It has a 
>transformer driven off the ac line generating about 3000 VAC (sez the
>manual) with a spark gap
>and a coupling trasformer/capacitive resonant circuit. I have not had the
>cover off it in a long time
>so I don't know if it's an air core transformer or what. There is also an
>RC bypass across the rest of the power supply to keep the RF out. The
>manual says it oscillates at about 2Mhz. 
>The oscillator circuit has the secondary of it's power transformer in
>parallel with the spark gap and in parallel with the series combination of
>the coupling transformer and capacitor.
>It is very reliable. I bought the machine used about 8 years ago and use
>it as a hobby machine and
>other than setting the gap on the spark gap when I got it, I havent had to
>mess with it at all.
>
>Bob Harbour
>[email-masked]
>[email-masked]
>

	Bob Harbour

		Specifications on 1994 model Thermal-Dynamics AC-DC
	Inverter are as follows :

	32 KHZ IGBT based inverter design
	True square wave DC
	250 Amps
	28 volts

	options=  GTAW  Pulser
	          GTAW  Sloper

	Iam realy interested in a fairly cheap conversion for a AC-DC
	welder. Let me know if come up with any more info

						Thanks  Steve Carlisle