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Re: Secondary wire

Hi Steve,
               I have some comments:

> Original Poster: "Steve Cook" <Steve-at-g8cyerichmond.freeserve.co.uk> 
> 
> Looking back at some old notes, I found something I had written in '95,
> unfortunately I have no idea where the idea came from. The gist was this,
that
> modern wire with self-fluxing insulation is not as good as the older types of
> enamelled wire. Two queries were attached in my notes, the first being
whether
> the modern insulation might be lossier, the second being that it had a lower
> puncture voltage. In the UK, at least, only the more modern type is readily
> available from the normal stockists. Personally I have used both, but has
> anyone any commments?
> Steve

I said nasty things about self-fluxing wire back in '95 in my 
magazine article. A more detailed explanation is warranted.

       Firstly, the insulation will not withstand the same voltages that 
polythermalize (eze?) wire will. However, unless you are running 
kV's per turn in a closewound secondary (nobody does to my 
knowledge), this is not an issue. The real problem is that the self 
fluxing insulation is nowhere near as robust as the glazed finish on 
the polythermalize wire and is far more susceptible to mechanical 
damage. For this reason, it is taboo to wind electric motors using it 
as magnetostriction occurs in the windings. Obviously, a dropped 
secondary is less likely to retain its electrical integrity with this type 
of wire. For the record, I have tested 0.8mm polythermalize wire with 
over 8kV (10kHz AC and DC) applied between two pieces placed 
together - it never gave despite corona in the AC tests.

        I also discovered that the self-fluxing insulation is prone to melt 
when some solvents are applied to it, particularly at moderately 
elevated temperatures. I discovered this to my cost when curing a 
varnished transformer at about 80 degrees C. I have never used 
such wire since. This was with a high grade electrical varnish. 
Subsequently, I learnt that the company I have been buying wire 
from for the last five years won't stock self-fluxing on principle.

         Although it is rare these days as I design and build my own 
transformers, on the odd occasion I do order a commercial 
transformer I always specify polythermalize wire.

         I have my doubts about formvar coated wire also. Unless I am 
mistaken (somebody correct me if I'm wrong), this type of insulation 
is a very dark brown colour and it has a tendency to age and 
eventually crack and peel off the wire leaving pristine copper in its 
wake. I've struck this in many old transformers from which I had 
hoped to recover the wire.

Regards,
Malcolm