[Prev][Next][Index][Thread]

Re: Skin effect and aluminum question (fwd)





---------- Forwarded message ----------
Date: Sat, 14 Feb 1998 08:43:11 -0500
From: Thomas McGahee <tom_mcgahee-at-sigmais-dot-com>
To: Tesla List <tesla-at-pupman-dot-com>
Subject: Re: Skin effect and aluminum question (fwd)



----------
> From: Tesla List <tesla-at-pupman-dot-com>
> To: tesla-at-pupman-dot-com
> Subject: Skin effect and aluminum question (fwd)
> Date: Friday, February 13, 1998 11:15 PM
> 
> 
> 
> ---------- Forwarded message ----------
> Date: Fri, 13 Feb 1998 08:40:26 -0700 (MST)
> From: Chip Atkinson <chip-at-sophie.bolix-dot-com>
> To: tesla-at-pupman-dot-com
> Subject: Skin effect and aluminum question
> 
> Greetings,
> 
> 
> I have seen it mentioned on the list that aluminum is a poor choice of 
> conductor because the oxide layer that forms on the surface is an 
> insulator, and that if, for example, your oxide layer is 0.001" thick and 
> your frequency is such that you only have current going through the top 
> 0.006" of the conductor, you will then lose 1/6 of your available conductor.
> 
> However, I fail to see the difference between Al2O3 on the surface acting 
> as an insulator and a coat of varnish on the surface.  Both the Al2O3 
> coating and the varnish coating don't conduct, and are unaffected by 
> magnetic fields.
> 
> Can anyone convince me that I'm wrong?  I am aware that the aluminum 
> conductor may provide more resistance at connections because the 
> electricity has to penetrate the oxide layer though.
> 
> 
> Thanks!
> 
> Chip

Chip,
Sometimes I think that certain things are repeated so many times that
they eventually take on the ring of authority without, however, having
a solid basis in fact. At other times a material or technique may be
rightly labelled as below par, but for the wrong reasons.

As far as the skin depth problem with aluminum goes, the same could
be said for almost any metal that develops an oxide coating. I
don't know of a single oxide that conducts better than the elemental
metal. I believe that aluminum does, however, develop a thicker
oxide coating than, say, copper. The implication here is that the
effective size of the conducting surface is, indeed, reduced. The
percentage of reduction is a function of oxide thickness, as the 
growing oxide layer results in a shrinking aluminum 'surface'. Thus
the effect would be greater for a thinner aluminum wire or length
of tubing, as the oxide thickness would be fairly constant.

You are right that the oxide coating is no more detrimental than
a coating of varnish *as far as its surface conduction effects are
concerned*. Just remember that a coating of varnish is added on
top of the metal, without affecting its surface area, whereas an
oxide layer grows at the expense of the underlying metal. In other
words, the effective surface area shrinks as the oxide layer grows.

The oxide layer that builds up on aluminum is a much better insulator
than, say, copper oxide. You can't readily solder to aluminum
tubing, it is generally harder to find, has a lower conductivity
than copper, and the oxide layer really *is* a source of problems
when it comes to connectivity. Even if you clean the surface and
clamp down really well, air will infiltrate and eventually create
the oxide layer. There is also the problem of what do you use to
clamp onto the aluminum? Copper to copper connections aren't that
bad, but with aluminum you will most likely end up with an
aluminum and *something else* metal connection. Since the two metals
will be dissimilar, corrosion at the joints is almost guaranteed.


I see the major problems with aluminum being the LONG TERM 
reliability problems. Will aluminum work as a material for a
Tesla coil primary? Most assuredly. But it will be inferior to
copper in almost every category except weight, and possibly cost.

I have used aluminum in the past, and I can tell you that the ozone
attacks it and causes it to become blotched and speckled. I would
only use it if I had no copper tubing available.

Hope this helps.
Fr. Tom McGahee