Re: MIG Welding, CC Inductor, and bypass thereof

On Mon, 14 May 2007 11:46:11 -0500, Ignoramus20083
<ignoramus20083@xxxxxxxxxxxxxxxxxxxx> wrote:

This is in regards to an old Hobart CyberTIG welder that I modified by
putting in a new microcontroller that I programmed in BASIC.

http://igor.chudov.com/projects/Welding/11-New-Rectifier/

Basically because I have a separate SCR controller, and separate
module for controlling it, I can program it to do anything I want, to
supply any voltage and any current, within my power limits, of course.

So it is basicallly an arbitrary DC power supply, with a few extra
relays that control high frequency arc starting and gas valve.

This thing works just fine in constant current mode, letting me stick
weld as well as TIG weld.

Note that it has a big inductor that helps with constant current
welding.

Now that I am done with CC mode welding, I started exploring MIG
welding, which requires constant voltage. Found a LN-25 wirefeed unit
on Craigslist.

The issue at hand is that the output of my welder goes through a large
inductor that is helpful in CC mode, but counterproductive for wire
feed welding. This inductor weighs perhaps 50 lbs. (?)

So, for nice wire feed welding, I would need to bypass it. I see two
ways of doing it:

1) Make a simple bypass circuit using a big contactor and big cable
(to be turned on when MIG welding is being done).

2) Instead of bypassing the inductor, install a capacitor that would
"cancel out" the effect of the inductor.

Can that be done at all, is this an insane idea or not? My hope is
that it would be done with a smaller "work envelope", utilizing
smaller wires and a smallish capacitor, instead of a bulky contactor
and big wire, etc.

Any thoughts on bypassing the inductor?

thanks

i

First, the LN25 can be used on a CC machine. It'll work better with
a CV machine, but it can be used with a CC machine. See page 2 of
http://content.lincolnelectric.com/pdfs/products/literature/e8100.pdf

A wirefeed welder is nominally a CV machine, but it still must limit
current. There is typically some inductance in the secondary circuit.
Some MIG machines (e.g., Millermatic) also have substantial
capacitors across the output. The result is that the cap maintains
voltage relatively constant on a short-term basis (droplet transfer,
etc) but avg current is limited so it doesn't melt the wire inside the
feed tube or blow breakers in event of a "stick" or short.

The Ready Welder is another example of a wirefeed that can be used
with a CC (stick) machine.
http://www.readywelder.com/

Average current in CV GMAW is a function of wire speed. (Ref:
Miller's book on GMAW, recommended reading and homework assignment if
you want to do good wirefeed welding.) A wirefeed can be made to
approximate CV operation from a CC machine by sensing arc voltage
and adjusting wire speed accordingly. The average current is
essentially constant from the CC machine, wire speed is
automatically controlled so the arc voltage is "right" for that level
of current and wire size. These units may contain capacitors, don't
know.

RTFM!
.

Loading