Subject: More on welding with power supplies

From: [email-masked] (Frank Evan Perdicaro)
Date: 27 Jun 1996 09:57:22 -0700

OK, it is clear that my Lamda power supplies are not what is needed for
MIG, TIG, arc, etc.

But, in addition to these power supplies, I do have about 1 ton of
isolation transformers.  These are very nice ones, with about 12 tap
points, and can do a 4:1 voltage reduction.  They weigh about 150 lbs
each.  And they are free.

Plus, I have a huge, very nice auto transformer, also for free.

Say I feed 220 into the autotrans, then step down 4:1.  Do I have an
adjustable-power AC arc welder?

I understand some of the safety concerns.  This is on my mind. But
this stuff is just sitting around......

-- 
Frank Evan Perdicaro 			Dainippon Screen Engineering of America
 Legalize guns, drugs and cash...today.	   3700 Segerstrom Ave
  inhouse: frank@server, x258		      Santa Ana CA
   outhouse: [email-masked], 714-546-9491x258	 92704       DoD:1097

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From: [email-masked] (Marty Jones)
Date: 28 Jun 1996 17:04:53 GMT

>this stuff is just sitting around......

Man, I wish I had a supply of free stuff like that!!  I haven't done what you're asking,
but it seems that your transformers are at least as hefty as the ones in most
of the 200-amp "crackerbox" welders I have seen.

I'd also be tempted to take it a step further - buy some of the 300-amp rectifiers from
Surplus Sales in Lincoln, Nebraska, and parallel them with enough series impedance
(use equal lengths of wire as ballast resistors) to get a rectifier bridge that will handle 
the surge current.  Voila - DC welder as well.  Since they're free, you could also use
one of the big transformers as a super arc stabilizer - just wire the secondary in series
with the DC like a big inductor.  

Marty Jones

[email-masked]

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From: [email-masked] (Robert Nichols)
Date: Fri, 28 Jun 1996 20:20:06 GMT

In article <[email-masked]>,
Frank Evan Perdicaro <[email-masked]> wrote:
:
:OK, it is clear that my Lamda power supplies are not what is needed for
:MIG, TIG, arc, etc.
:
:But, in addition to these power supplies, I do have about 1 ton of
:isolation transformers.  These are very nice ones, with about 12 tap
:points, and can do a 4:1 voltage reduction.  They weigh about 150 lbs
:each.  And they are free.
:
:Plus, I have a huge, very nice auto transformer, also for free.
:
:Say I feed 220 into the autotrans, then step down 4:1.  Do I have an
:adjustable-power AC arc welder?

No, you won't.  That setup lacks any sort of series impedance for
controlling the current.  The transformer in a common AC arc welder is
constructed with an adjustable element that allows the magnetic flux
produced by the primary winding to bypass the secondary.  That variable
flux bypass shows up in the equivalent circuit as a series inductance
that is the primary element for controlling the current flow.  Without
some sort of series element in the circuit, the only thing
you can control is the voltage, and that's an extremely poor way to
control current in a welder.

Yes, you can do the job with a conventional transformer and an external
inductor, but the inductor needs to be almost as large as the
transformer, so there is a big advantage to building that function into
the transformer itself.

-- 
Bob Nichols         [email-masked]

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From: [email-masked] (Alan Shinn)
Date: Fri, 28 Jun 1996 18:22:31 -0800

In article <[email-masked]>, [email-masked]

> this stuff is just sitting around......

4/1 with 220 gives 55V - about right - then think about useing the
autotransformer as a variable inductor in series with the transformer and
the load to set the current.

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From: [email-masked] (Jonathan M. Elson)
Date: 5 Jul 1996 04:46:55 GMT

Frank Evan Perdicaro ([email-masked]) wrote:

: But, in addition to these power supplies, I do have about 1 ton of
: isolation transformers.  These are very nice ones, with about 12 tap
: points, and can do a 4:1 voltage reduction.  They weigh about 150 lbs
: each.  And they are free.

: Say I feed 220 into the autotrans, then step down 4:1.  Do I have an
: adjustable-power AC arc welder?

Not quite there yet!  If you built this, the first time you touched the
stick to the work, it would spot weld, and the circuit breaker in your
fuse panel would trip.  You need a way to limit the current, and a way to
stabilize the arc.  Most 'buzz-box' welders do both with the same
mechanism, adjustable poor coupling between the primary and secondary.
They accomplish this with a movable piece of iron core material in the
transformer.  You don't have an easy way to do this, all your transformers
have good coupling between the primary and secondary.  You need to insert
an inductance (modest in Henries, but LOTS of Amps) in series with either
the primary or secondary.  You could use just a couple of turns through
one of those cores with welding cable, or put the tapped autotransformer
in series with the primary - but NOT using it as an autotransformer.
it will take a bit of tinkering to find the right settings for all this.
You want to have a good open circuit voltage (55 Volts is just about
right).  But you also need good current regulation, and a simple series
inductor won't quite do that.  The magnetic saturation of the buzz-box
core sections are tuned by design to have a linearizing effect on current,
always adapting closed-circuit voltage to keep arc current close to the
setting on the box.  Still, what you want to do may work fine.  You won't
know without trying it.

But DEFINITELY DON'T try to build it the way you suggested.  Without
current limiting, you are likely to demonstrate exploding wire technology!
if the current rises without limit (except leakage inductance in your
transformers and wiring resistance) then the highest resistance in the
system may be the welding rod, and it may literally explode when you try
your first arc.  That could be a hazardous experience.

					jon