On Sun, 9 Jul 2006 09:59:37 +0200, "Ban" wrote:

default wrote:
The quandary


When your wire is thinner but has the same resistance, it will be
shorter(40%) and hence the field will be weaker, probably less than half. It
also means it will heat up much more for 2 reasons: The diameter is smaller
and the insulation is much thicker, so the heat cannot be transferred to the
metal or air so easily, builds up in the center and can blow up the sleeve
of epoxy. Another remark: how well do you think your insulation is sticking
to *any* glue? An additional property of PTFE is its tendency to "flow"
aggravated by vibration, temperature and pressure, all of which are present
in your application.

Thanks, you've given me some ideas.

I kinda lied about the resistance - my repair to the original
measures 4.5 ohms and the Tefzel coil measures 3.8 . Turns out my DVM
isn't great in the low ohms range - two different days and two
different readings.

The Tefzel wire does have what appears to be a silver plating if that
is enough to enter into it. What I can do is measure the thickness of
the ETFE and compare that to an enamel wire table and see how many
turns should fit in the same cross sectional area and pin down the
turns difference with some accuracy. My repair winding was relatively
sloppy and wasn't "perfect lay" by any means.

PTFE FEP PFA are in the same class when it comes to physical
properties ETFE is Tefzel and is in a different class - twice the
tensile strength, 20% harder, better compressive strength, similar
flexural strength. Lower temperature rating for the physical
properties than FEP (260 C) types but good to 155 degrees C.



Back EMF increases linearly with speed, but since you have less windings,
the current will be higher.

Back EMF in the excited field rotor? I didn't know there was any.

I'm not convinced current will be higher - the excitation current is
directly derived from the alternator output - it isn't fixed - it
doesn't come from the battery. A shorted or nearly shorted rotor is
less effective for output at the same rotational speed so it has less
voltage to send to the rotor - that's the reason for an extra three
diodes on the regulator - not for the warning light as one poster
suggested (it has no warning light). There is a fixed dropping
resistor that provides a tickle of current from the battery to get
things started.

So, I'm thinking worst case is probably close to idle speed. To
further complicate that idea, excitation also has to track speed to
some extent, since it is derived from an extra set of diodes from the
rectifier - lower speed means less current/voltage to work with.

forget about the tefzel for this app and get the right magnet wire suitable
for high temp.

Price of magnet wire in small quantities has quadrupled in the last
year and doubled in large quantities - place I bought one pound from
for $12 now wants $35 for a half pound (8 ounces) and I need about 12
ounces. At that rate it would probably be cheaper to buy a 6 pound
reel of the stuff.

Yeah - you're right to point out ETFE won't stick to epoxy, to work it
has to penetrate between the wires - I'm using vinyl ester resin,
similar to polyester resin (water thin compared to epoxy and good
wetting properties) higher temperature rating than epoxy. I'll use
epoxy to hold the coil in place or thickened vinyl ester resin for
that.

Blow it apart due to thermal expansion? That should be no problem to
discover with empirical testing - vinyl ester resin sets up hard -
but it isn't glass hard - just hard compared to unfilled epoxy. The
coil will always have some air trapped in the center of the windings
unless I can figure out how to turn my mandrel into a mold (I'm
working on that now)

Buying wire is always an option - but I'll kick this around some more
before I give up. I have two weeks to get it working.

Thanks for the reply.

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