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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.

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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