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> Yet, I think it should be possible to use limiting on the
> secundary side too. However, becouse of the high voltages
> it will probably lead into so much trouble it might not be
> worth the trouble.
No, I would not think it would be worth the trouble when something
simple and "off-the-shelf" like an arc welder is fully rated and
works so well in the 240 volt AC supply main.
Richard Quick
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Quoting Richard Quick:
> Wiring one in the circuit is easy. Simply place the primary winding
> on the arc welder in series with the variacs. Short the secondary
> on the arc welder. By adjusting the arc power level on the welder
> the current through the variacs is varied.
How is the arc welder actually done? I'm not familiar with those
devices. If I know what it is made of I'd figure the principle
out pretty fast. The ready-made unit seems nice!
> electric oven elements make the coil much more manageable.
Yes, I knew about that method. That is surely one possibility.
> No, I would not think it would be worth the trouble when something
> simple and "off-the-shelf" like an arc welder is fully rated and
> works so well in the 240 volt AC supply main.
Agreed. In principle a small amount of salt in water and the thing
in proper plastic tube would make suitable limitter for primary or
secundary side. Managing the electrodes or cross section (current
path) in one way or another is still a must if it is wanted
variable. Much more work than with arc welders though.
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Quoting gcerny-at-ix-dot-netcom-dot-com (Glenn Cerny ):
GC> I guess I still don't get it. Sorry, I am a little slow at
GC> times. Does a coiler connect to the secondary of the welder
GC> or the primary with his feeder leads? I assume that the
GC> welder winding that is not used is simply "stored".
Current limiting (ballast) is usually placed in series with the
240 volt 50/60 cycle mains somewhere between the breaker box and
the primary winding on the step up transformer (xfmr). Current
limiting can be resistive, inductive, or a combination of both.
When using an arc welder as a current limiter, the primary
winding on the welder is placed in series with one leg of the 240
volt 50/60 cycle feed to the step up xfmr. Now we want current to
flow through this circuit, we simply want to be able to control
the amount of current. In order for current to flow at all, the
secondary winding on the arc welder, that is the welder cables
themselves, must be shorted out. The welding cables are equipped
with clamps, so I just clamp the ends of the cable together.
Now current will flow through both windings on the welder. The
low voltage secondary winding is shorted, the 240 volt primary
winding is in series with the step up xfmr.
The welder will be equipped with some means of increasing and
decreasing the power that is supplied to the welding arc. By
power what we really mean is current. A dial, slide bar, tap
holes, or the like is used to vary the current that is supplied
to the welding arc. With the secondary winding directly shorted
there is virually no resistance or impedance to current flow
here. If it were not for the inductive current limiting designed
into the welder core, every time you struck a welding arc the
breakers would trip and/or things would get pretty dark. The
welder is designed to operate with a low impedance (even a
dead short) across the secondary winding for limited periods
of time, described as the "duty cycle" which will be noted
somewhere in the manual or on the welder case.
With the 240 volt primary winding of the welder wired into the
coil control circuitry as outlined above, the current settings on
the welder are in effect limiting the number of amps at ~240
volts (there is some voltage drop) that are allowed to pass
through the primary winding on the step up xfmr. When you
increase or decrease the power setting on the welder cabinet, you
are increasing or decreasing the current available to the step up
transformer.
This is the simple way to wire things. When operating Tesla coils
I like to have control over the voltage as well as the current.
Variacs are available off the shelf that can be ganged together
to handle all of the current that can be passed through the arc
welder primary. In actuality I run the arc welder primary in
series with the variacs in my control cabinet. The outputs from
the variacs are sent to the step up xfmr, and it is this output,
or "conditioned power", that I monitor with the meters in the
control cabinet face.
Richard Quick
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Quoting mrbarton-at-ix-dot-netcom-dot-com (Mark Barton)
> I have had great success by winding large gauge stranded wire
> around a 4 inch dia. phenolic or PVC tube about 3 feet long.
> Close winding a few layers is usually enough. A straight core
> is raised and lowered in and out of the tube by pulley means to
> regulate the inductance and hence the power draw. It is almost
> impossible to saturate a straight core.
> I know I said that the usual practice is to use a Variac. Like
> I said, I've seen it done all over, but I have never personally
> used one in that role. Sorry. Zap, Mark
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>Quoting mrbarton-at-ix-dot-netcom-dot-com (Mark Barton)
>> I have had great success by winding large gauge stranded wire
>> around a 4 inch dia. phenolic or PVC tube about 3 feet long.
>> Close winding a few layers is usually enough. A straight core
>> is raised and lowered in and out of the tube by pulley means
>> to regulate the inductance and hence the power draw. It is almost
>> impossible to saturate a straight core.
Richard Quick said:
Well Mark, Ed Sonderman and I were wondering if anybody else had tried
this. It seems that three of us in the other group had happened upon
the same idea, and in fact I posted the exact same dimensions as you
just gave.
For a core I had suggested buying a spool of bailing wire, then
cutting the wire into 3 foot lengths. The sections would be lightly
coated with secondary sealer, tightly bound in a few spots with some
friction tape, inserted into a 4" diameter ABS plastic tube, and potted
with epoxy to prevent vibration. You can find four inch ABS and PVC pipe
that will telescope fairly neatly, so the winding can be wound on the
larger tube and can be coated with sufficient thickness of sealer to pot
the winding to prevent loosening. Since we already had some resitive
ballast in the circuit, and the arc welders pretty much opened up all the
way, I figured one layer of winding would provide enough inductive ballast
to do the job we needed.
Questions:
1) How much does your core weigh? so we have an idea of how many
pounds of wire to purchase.
Mark Barton replied:
>>
I have made 4 of these things over the years and the cores all weighed
in at about 20-30 pounds. Don't forget, all my coils are 10KVA
minimum. I used cannibalized transformer laminations from large
transformers cut and lined up straight. One time I used 2in by 3ft
sheets of galvanized steel each individually wrapped in waxed paper.
Another time I used a clump of welding rods each rod dipped in varnish.
Huh, huh, the varnish melted and ran.
<<
2) How many actual turns of winding are on the coil, and how many
layers?
>>
This varies with the amount of L you need. The less power you want to
run, the more L you need. On the biggest limiter we built, we wound 4
layers of #2 wire on a 4in by 3ft phenolic tube. This was sufficient
to run a 15KVA unit from full on with the core out, to full off with
the core in. Sorry, no data on actual number of turns. Wind more than
you need and then take it off if you can't draw the power.
<<
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I forgot who popped this question:
>I am not clear on this one yet. Are we talking arc welding rods?
>The arc welding rods I am familiar with are about 18 inches long
>and are coated with flux. I think they are mild steel.
To which DWP replied:
Welding rods come in a variety of steels, to match the material
being welded. They are specced for WELDING, so getting
electrical (magnetic...) data is likely to be tricky. A welding
supplier will have a variety of rod materials to choose from. In
the practical world, its probably better to try something than
worry about the details. "Welding rods" can be used either with
gas welding or arc welding. If a sufficient supply of old
coat hangars, or iron wire or ... was handy, try it. If using
"gas" rods (less likely to have flux on them) I would consider
insulating them (justa quick paint dip, nothing fancy, the
voltages are low....) as this will reduce eddy current losses.
In doing a pro design, at these power levels, a mag amp sort of
approach would be my guess, BUT they are real tricky, even for experts
(which i am not). A movable core, of salvaged laminations, or
welding rods seems a neat approach & is used in some conventional
work at power frequencies.
The inductance is set by the amount of core AND by the air gap. A
small air gap will drop the inductance tremendously, then, assuming
a movable core, the inductance will drop further as the core is
withdrawn. Design details, for a math approach, will be in any good
Electrical Engineers Handbook, tho they will ASSume that the mag
characteristics of the core materials are known.
regards dwp
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