On 2006-07-08, default wrote:
The quandary

I'm rewinding an automobile style alternator rotor. I want to use
some "Tefzel" insulated wire I have on hand, instead of magnet wire, I
have to special order. I wound a test coil and it fits and looks like
it will work.

My concern is that I have no idea how much current it takes to excite
the field and if the potting compound will survive the heat. The
Tefzel coil has the same DCR (5 ohms) as the original magnet wire, but
it is two gauges thinner (went from 22 AWG to 24 AWG). So I would
assume that it will dissipate more power to achieve the same ampere
turns in the field.

I'm no guru but:

if the wire is thinner, but the coil the same dimension then you
theoretically have more turns - this means for the same current you have
more magnetic field.

if the coil is the same resistance as the old one then you will have the
same max current, and the same worst-case resistive heating as the old coil.

if the conductor is thinner but the insulated wire is thicker you have fewer
turns and threfore a less-effective altenator.

I can't test it very well without potting the coil and epoxying it
into the pole pieces, mounting it to the engine, etc. - and if it
turns out to be bad, it is a real bitch to pull apart and do over with
different wire.

I figure the excitation power probably drops with speed of rotation -
alternator voltage output tracks speed so it should need less
excitation as speed increases - and the frequency goes up so inductive
reactance also increases(?)

the coil is fed DC, inductance doesn't enter into it.

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.

those diodes don't give a greatly elevated voltage, they're mainly to
provide a way to power the generator warning light.

I tried powering the coil with a dc supply and pushed 2.5 amps through
it for 3 hours - no idea how hot the coil was, but the area between
the coil and pole pieces was 70 degrees F over ambient - around 150 F.
On the engine, it is driven directly off the crankshaft and probably
has an ambient of closer to 170 F - enclosed with no ventilation just
conduction and radiation cooling, and whatever air the rotor itself
stirs up.

your altenator doesn't vent slots or a fan behind the pulley?

The whole story:


Plan A is to laboriously construct a new bobbin (1+ day of effort) and
order the right gauge wire and do the wind - epoxy routine, then serve
leads made of braid instead of wiring directly to the slip rings. A
lot of work.

maybe you can epoxy in screw terminals this time incase the leads fail.

maybe you could use old CD-Roms (the shiny can be removed using a metal pot
scourer) for the ends of the bobbin and a dowel (or threaded rod?) wrapped
in paper and cling-wrap for the centre?

hmm, if I could fix a 3-jaw chuck to the back of a sewing machine that'd
make a good tool start for winding magnet coils

--

Bye.
Jasen