While Sola's article is accurate, it has to do with distribution transformers, not wiring in your home, or a random project that hasn't been tested over and over again.

Why not just use copper wire for the choke you're making? It's easy to work with and won't burst into flames. Copper wire is thinner for any given current rating, so you'll have more room to work with, or be able to add more windings.

As for taking apart a MOT core. if you don't get a perfect fit between the laminations like they get at the factory, you will lose inductance. The welds on transformer cores are done with some sort of plasma device and don't warp the laminations.

Peter T. Keillor III wrote:
On Fri, 21 Jan 2005 04:12:38 GMT, "carl mciver"
wrote:

"Tim Williams" wrote in message
...
| For the welder rectifier I'm assembling, I need some filtering (so I
hear).
| Since I have some #0 AWG aluminum wire on hand I'm going to wind it around
| some MOT cores. I already made one (barely managing to squeeze ten turns
on
| it, there's space for two more but the wire is too tough!), with about
1/16"
| cardboard shim and welding the core back together (MOTs are normally
welded,
| I don't see anything wrong here).
|
| My question is, what's the best way to hook up the wire to the rest of the
| system? The welder's power transformer is wound with aluminum wire and
just
| clamped on the output terminals, so I'm guessing a washer and bolt will
do.
| But this is 7 strand, not solid... I might be able to do it, or it might
| squish apart. I suppose I could drill a hole in a block and use a clamp
| screw to hold it in place.
|
| Tim

Tim, the aluminum wiring has an issue that it is most critical that you
understand. When aluminum gets hot, it expands more than copper. So much
so that after a connection has been made, used to warm, then cooled, it will
get loose because the aluminum flows out of the connection. This poor
connection now creates heat due to the resistance, which starts it all over
again. Eventually a fire happens. The NEC is very concerned about the
history of this item, and has specific rules that you can't get away with
skipping.
Use connections that have "AL-CU" stamped on it and no others for that
connection. It is wise to buy a bit of antioxidizing gel, like NoAlox or
similar, and fill the connection before you assemble it with this gel. It
keeps the dissimilar corrosion away, which makes the fire issue crop up
again. Hey, it isn't inside the walls of your house, but nonetheless I
don't think you like any unnecessary action going on, even more so in a high
current connection like a welder.

Below is some interesting stuff off the FAQ on Sola's site:

Pete Keillor

--------------------------------------------------------------------------------

Q. Are copper windings better than aluminum windings?

A. As with most questions of this type pertaining to transformers, a
lot depends upon the application and the individual preferences of the
person specifying the transformer. Quite often the reason cited for
specifying copper windings is copper's high electrical conductivity.

During World War II, copper became scarce and was used primarily for
the war effort. Several industries turned to aluminum as alternative
to copper because it was in good supply, was very stable price-wise
and was less expensive than copper. In the 1940's, high-power
transmission power lines were converted from copper to aluminum and
secondary power distribution networks began utilizing aluminum in the
1950's. Today, virtually all standard transformer lines from the major
manufacturers are wound with aluminum. Although copper wound
transformers tend to be smaller than comparable aluminum wound
transformers offer some distinct advantages over copper wound units:

Both copper and aluminum oxidize over time. Aluminum conductors
oxidize until all exposed aluminum surfaces are covered with and oxide
layer.At that point oxidation stops unless the aluminum oxide barrier
is somehow broken and the aluminum conductor is re-exposed to the air.
Aluminum oxide inhibits chemical reaction of the metal with the wire
insulation. Aluminum oxide is also a good electrical insulator. Copper
on the other hand oxidizes completely over time. Copper also acts as a
mild catalyst, hastening the decay of the wire insulation. All of
these factors combine to give aluminum wound transformers a longer
life than comparable copper wound units, typically about five years.

The heat storage capacity of aluminum is approximately 2.33 times that
of copper (specific heat of aluminum is 0.214 cal/gram/?C, specific
heat of copper is 0.092 cal/gram/?C). With aluminum wound transformers
having a superior thermal storage capacity than copper wound units,
they can withstand more surge and overload currents than copper units
(normal exhibited when a motor starts.)

Although the conductivity of copper is better than that of aluminum,
on a per pound basis aluminum is over twice as good a conductor as
copper.

Aluminum wire has received a negative connotation over the years
primarily because of the care that must be taken in making
connections. Copper proponents are quick to refer to hotel and mobile
home fires that occurred where aluminum wire was present. Upon close
examination it was found that the root causes of these problems is
related to incorrect wiring devices being used. Copper and aluminum
expand at different rates when heated. If aluminum wire is used with
wiring devices solely rated for use with copper wire, the connection
could loosen as the connection heats up causing the resistance of the
connection to increase and the temperature to continue to escalate.
Most transformer manufacturers address this problem by making a
transition between the aluminum windings, either to a copper lead wire
(or bus bar) or by terminating to an AI/Cu lug (or connector).

Q. So why are copper wound transformers still specified?

A. Copper wound units may be specified because of space limitations.
Copper wound units can also be specified due to the environment in
which the transformer will be exposed. If the environment would be
corrosive to aluminum, copper wound transformers would make sense. Of
course, some people may just like copper wound transformers for their
own reasons. Sola/Hevi-Duty manufactures aluminum wound transformers
but can manufacture copper wound transformers upon special order.
Contact your Sola/Hevi-Duty representative for pricing and
manufacturing lead times.