Re: Making a 300 Amp square wave AC inverter

Ignoramus965 wrote: 
On Fri, 09 Sep 2005 07:17:10 GMT, ehsjr <ehsjr@xxxxxxxxxxxxxxxx> wrote:

Ignoramus965 wrote:

On Thu, 08 Sep 2005 20:56:00 GMT, Rich Grise <rich@xxxxxxxxxxx> wrote:


On Thu, 08 Sep 2005 18:48:43 +0000, Ignoramus965 wrote:



I am considering a project to build a 300 amp, about 40V inverter that
would invert DC into square wave AC. That's for converting a DC
TIG welder into a square wave AC TIG welder. I do not yet have the
welder, but may get it in a few days.

I am coming here after some preliminary discussions on
rec.crafts.metalworking. 

I have some basic knowledge of physics and some knowledge of
electricity to start with, but I am not well familiar with solid state
electronics. I built a well functioning phase converter, and the only
solid state piece that I ever used was a crydom relay with zero
crossing.

Then bite the bullet, and buy the AC welder. You couldn't possibly
design and build anything like that for less than the difference in
cost of the two models - parts alone would do it, and building something like that is definitely not a project for a beginner. 


You are, quite possibly, right, but I would like to know why that is
so, what are the technical reasons.

i


Rich *is* right.  Out of courtesy to your request,
I'll mention a few technical issues below.  First,
let me say I think experimentation is great and I
encourage it.  But what you are talking about should
be undertaken only by those who would not need to post
as you did.  This is not a knock on you. Screwing around
with 300 amps is not a "give me the schematic and a few
pointers and I'll cobble it together" project.

Some technical issues:
1) You want a 40 volt, 300 amp inverter. Thats 12,000
watts. Twelve THOUSAND watts. Have you considered
this?


Well, I am aware that it is 12 kW, but, quite possibly, not aware of all implications of that.


How will you determine what to use to connect
your inverter to the DC? I'm assuming you already
have the DC source - do you?



Not yet, but if I acquire a CC DC welder on Monday, I will have a DC
source. If I do not acquire such a welder, I will not go ahead with
this project.


2) Assuming you do have the DC source, how do you
know it is capable of driving the inverter? How
do you intend to protect the source?



I suppose that a DC welder has internal protection.


3) Can you predict the possible failures within a
circuit and take steps to prevent them, and, more
importantly, do you understand what will or could happen
if one (or more) of those failures does occur?



Well, I would attempt to do that.

i



Ok.  When you can advise someone how to correctly handle
the issues raised in those questions, you're ready to
start the project.  Bring deep pockets (for replacement
parts), an in depth understanding of mosfets and gate
drive circuits and excellent construction techniques
to bear. You'll need a dedicated 60 amp 240 volt AC mains
circuit to power the DC welder.  A flameproof working
environment might restrict the failures to the equipment
if you stand far enough away.

Ed
.

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