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Re: [TCML] primary tubing - now Aluminum wiring failures
Paul -
I think the heavy gage aluminum wire used in your MOT primaries is just
coated with a standard high-temperature magnet-wire insulation that happens
to be a transparent golden-brown color.
MOTs are built to the OEM specifications of the oven manufacturers, so there
is probably no attempt "trick" anyone into thinking the primary wire is
copper.
Regards,
Herr Zapp
----- Original Message -----
From: "Paul Brodie" <pbbrodie@xxxxxxxxxxxxx>
To: "Tesla Coil Mailing List" <tesla@xxxxxxxxxx>
Sent: Thursday, November 12, 2009 10:54 AM
Subject: Re: [TCML] primary tubing - now Aluminum wiring failures
I recently discovered that the primaries in several of my MOT's are aluminum
wire with a copper colored insulation. Is this typical or unusual? I have
noticed that all of the ones I have found with aluminum primary wire are
newer ones. I find it rather humorous that they went to the trouble of using
copper colored insulation, evidently in an attempt to disguise the fact that
the primary wire is aluminum. The secondaries are all copper.
The way I found this out was when I disassembled a couple of MOT's and found
the primary windings were extremely light. I scraped a little bit of
insulation off and low and behold, there's aluminum. I very carefully take
the MOT's apart without damaging the windings. These windings come in handy
for all sorts of experiments and tests. Of course, I also save the
secondaries unharmed as well.
A really neat little trick is to take 3 secondary coils wired in series and
loop several turns of 12AWG magnet wire through them. I then run this
primary with one of my ignition coil driver circuits and it makes some nice
arcs. I am considering disassembling a few more MOT's to see just how far I
can go with wiring them in series this way with a common primary. I suppose
it might help if I add some sort of core???
Anyone got any other ideas about ways to play around with these secondary
and even the aluminum primary coils or any other suggestions?
Paul
Think Positive
----- Original Message -----
From: jimlux
To: Tesla Coil Mailing List
Sent: Thursday, November 12, 2009 09:41
Subject: Re: [TCML] primary tubing - now Aluminum wiring failures
Quarkster wrote:
> Jim -
>
> The terminations of the aluminum wire become oxidized, develop some
> resistance across the termination, and eventually overheat.
>
> There are several contributing factors:
> 1. Aluminum exposed to air immediately develops a high-resistance oxide
> film;
> 2. Aluminum wire is quite soft and "cold-flows" under pressure;
> 3. Aluminum's high rate of thermal expansion can cause a
> "self-degenerating" thermal-cycling problem where a small amount of
> resistance creates localized heating, which causes the wire (and any
> clamped connections) to expand. When the electrical load on that circuit
> is removed, everything cools off, and if the aluminum has cold-flowed,
> there is now less clamping pressure, which allows further oxidation in
> the connection, which creates more resistance, which gets hotter the
> next time the circuit is loaded, etc, etc.
>
> I've personally seen wire-nuts on aluminum wiring that got so hot that
> the plastic wire-nut body completely melted away, leaving only the
> conical steel spring from inside the wire-nut holding the wires
together.
>
> Aluminum house wiring was a cost-saving measure that was implemented
> without adequate testing, and turned out to be a complete disaster, a
> great example of shoddy Engineering.
>
> Regards,
> Herr Zapp
It's a bit more complex than that, even. Aluminum comes in dozens of
different alloys, some more suitable than others for wiring. Another
aspect implicated in the house wiring issues is that about the same time
that they started using aluminum wire as a cost savings measure, they
also changed from brass to steel screw terminals on receptacles and
switches. That aggravated the differential thermal expansion problem.
There are modern aluminum wires that are perfectly safe, used with the
correct terminations.
As others have pointed out, most distribution transformers these days
use aluminum wire. My old Piper Cherokee 140 used aluminum wire (for
weight saving). There are miles of coaxial cable out there with
aluminum shield.
From a TC standpoint, I don't think aluminum buys you a whole lot. On
the primary, the difficulty in making a good adjustable tap point is
probably a deal breaker, even if copper tubing costs as much as it does.
On the secondary, it might be an OK trade, if you go up in size enough
to account for the increased resistivity. That will mean you get fewer
turns per inch, so, for a given size coil you'll get fewer turns, so the
inductance will be less, which will affect the tuning and the Lp/Ls ratio.
I know several folks have made experiments of one kind or another, but I
don't think they've spanned the necessary range of conditions to fully
account for all the various factors;e.g. how do you disentangle the
effect of changing the inductance in the seconday.. you'd need to wind
one with aluminum wire, then wind another with (smaller) copper wire so
the AC resistance is the same, but making sure the turns per inch makes
the inductance come out right. And for close wound coils (like a
secondary), the AC resistance calculation is non trivial, because the
fields from turns i-1 and i+1 affect the current distribution in turn i.
(aka proximity effect). That would change for the spacewound copper
wire coil.
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