Help, any gurus with alternator experience or knowledge?

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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 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(?)

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.

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.

I can do some empirical testing with a sacrificial coil when the vinyl
ester resin gets here.  

Anyone with experience/ideas in rewinding rotors and do you think this
should work?  Smaller gauge wire - same DCR, but lower ampere turns
and consequently more power used to cause more heat and excite the
field..

The potting resin is supposed to be good for ~240 F so I might be
pushing the limit there.


The whole story:

Five years ago my alternator failed.  The rotor had shorted -
resistance of the coil would vary from point five to five ohms - five
ohms is supposed to be typical.  Replacement rotor $350 . . . with no
guarantee that it wouldn't fail like the original in two years . . .

The regulator is the common type usually used with excited field
alternators - a two transistor circuit that pumps voltage to the coil
when the battery drops below the set point.  The voltage that goes to
the rotor (rotating field) is derived from an extra three diodes on
the positive of the six diode, three phase rectifier - so it is
isolated from the battery.

One effect of using extra diodes is that when the output of the
alternator drops (when the field is shorted, for instance) the
excitation current is also lower - doesn't do much to charge the
battery, but it doesn't kill the battery in an effort to excite a
shorted field, and doesn't kill the regulator pass transistor.  A good
design . . .

The original coil failed because the enamel on the magnet wire and or
varnish holding it together failed (probably because of heat or
vibration - at least that's what the wire looked like).  It was a self
supporting coil - made in a mold and had no bobbin.

I didn't have the stuff to make a self supporting coil so I made a
bobbin out of very thin two sided epoxy pcb material, and insulated
the inside with pieces of thin Mylar plastic.  Wound a layer - painted
it with epoxy and built up the entire coil that way.  It lasted 5
years and then failed because the lead wires to the coil opened - The
wires were in a sleeve of Teflon spaghetti and probably opened due to
metal fatigue - that's what the ends looked like - when you bend a
wire back and forth until it breaks.  I repaired one open to the
finish end and it worked for a few weeks and then the start end also
opened.  The coil is pretty much a goner now - the clear epoxy shows
the wire to be in excellent shape - no charring like the OEM part.

I want to wind another coil but would like to avoid using a bobbin
since that took me over a day to construct with hand tools, and the
bobbin didn't survive pulling apart the pole pieces.

So I found some wire wrap wire with "Tefzel" insulation and wound a
coil with that on the mandrel that supported my original bobbin.  I
secured it into a toroid shape using nylon lacing cord.  Fits the pole
pieces and looks like it will work. Plan B was to serve leads to it
made of fine braid - to avoid metal fatigue and sleeve it in cambric
spaghetti then dip it in vinyl ester resin, epoxy that assembly into
the pole pieces and reassemble the rotor.

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.

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Re: Help, any gurus with alternator experience or knowledge?

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Could you temporarily stick it in place with hot glue, just for testing
purposes? Hot glue should just be able to just peel right off between
tests and so on.

Good Luck!
Rich


Re: Help, any gurus with alternator experience or knowledge?

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Probably long enough to start the engine and measure the current.  I
was sort of hoping to get it to work without a test except perhaps a
coil alone - with a fixed dissipation.  

Even the hot melt adhesive isn't a pleasant idea - run long enough to
get real data and I'd probably have the adhesive slinging off into the
stator or space between the rotor and stator - too risky to chance
that - adhesive being ground up would take out the brushes - unlike
auto alternators, this one has axial slip rings - any bit of epoxy etc
that comes off eats the brushes.


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Re: Help, any gurus with alternator experience or knowledge?

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Lacquer


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Re: Help, any gurus with alternator experience or knowledge?
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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.

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

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

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

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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

Re: Help, any gurus with alternator experience or knowledge?
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Again no guru, but from what I remember of winding my own transformers,
more coil turns equals greater voltage, so this alternator MAY produce a
greater voltage for a given rotational speed.

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But as the Alternator produces an Alternating current, don't they also
work as a Bridge Rectifier?

Like I said NO GURU, but its my pennys worth ;-)

Best of luck with the build

Vaughn

Re: Help, any gurus with alternator experience or knowledge?
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  Automotive alternators are usually three phase with six diodes.  this
helps reduce the ripple current in the charging system, and reduces
filtering requirements for the electronics.


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Re: Help, any gurus with alternator experience or knowledge?
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Anytime you rewind a motor or alternator, you want to use the same gauge
wire.  Your alternator was designed with 22 AWG, for the number of
turns, the dc resistance, the current through it, the power dissipation,
and the space available.  If it would work just as well with 24 AWG,
they would have used 24 AWG originally.  If you're going to all the
trouble of winding these coils, placing them, and wiring them up, you
might as well use the right size wire to start with!

Re: Help, any gurus with alternator experience or knowledge?


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I wind transformers too.  If one winds the stator (non movable outside
part on my alternator) with more turns, the voltage will increase for
the same rotational speed.

We are talking about a DC electromagnet here - the rotating field --
magnetizing a hunk of cast iron (pole pieces). I ran some calculations
on wire size and cross sectional area of the coil.  All things being
equal - same magnet wire insulation - the resistance goes up (as wire
gets thinner) the current goes down (greater resistance) but the
magnetic field strength stays the same if the cross section (filled
with wire) stays the same.

I'm contemplating changing the size of the insulation with a (possibly
silver plated) copper wire and increasing insulation thickness and
decreasing wire diameter two gauges  - that's what gives me pause.

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Three phase delta connected output that goes to six diodes for primary
rectification and three additional (smaller) diodes to provide field
excitation independent of the battery.
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Thanks  - I enjoy tinkering with it - but don't want to get stuck
hundreds of miles from home with no way to repair it.

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Re: Help, any gurus with alternator experience or knowledge?
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only if you can increase the voltage to compensate for the resistance
which increases twice - due to greater length and reduced cross-section
of the conductor. the extra turn compensate for only one of those increases.

annealed, and oxygen free, copper wire will have slightly loer resistance
but I don't think it'll be enough.

Bye.
   Jasen

Re: Help, any gurus with alternator experience or knowledge?

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Well I calculated for three sizes of wire, in the same physical space.
The coil resistance in going to larger gauges went from 5 ohm to 1
ohm,  Power dissipation went from 28 watts to 144 watts.   Magnetic
force went from 1,415 Gilbert's to 1,417 Gilbert's.

Always the chance I made a mistake somewhere.

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Re: Help, any gurus with alternator experience or knowledge?
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I think so...  if you show your working I might be able to spot it.

Bye.
   Jasen

Re: Help, any gurus with alternator experience or knowledge?

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I don't have my data - it was from the original winding five years
ago.  The formula to get from ampere turns to Gilbert's is simple
(current *number of turns*1.257).  

If my memory serves, I used something like the number of turns that
could fit in a cross sectional area times the mean length per turn to
get wire length and resistance, from a 1914 book on solenoid
construction . . . .

But logically I see your point and it seems likely I went wrong
somewhere in there . . .  For the sake of argument:

If each turn of wire has one ohm resistance, and I have a four turn
coil and 8 volts to drive it I have a current of 2 amps and 8 ampere
turns dissipating 16 watts total.

If I halve the diameter of the wire the cross section drops by a
factor of four so the resistance should increase by a factor of four.
So now I have 16 turns fitting where 4 where, and the resistance is 64
ohms.  Four times the turns, with four times the resistance per turn.
With the same 8 volt supply that's 0.125 amps for 2 ampere turns
dissipating 1 watt.

Conclusion:  I was full of shit to state that wire size didn't matter.

Efficiency: To produce the same 8 ampere turns with a 1/2 size wire
will take only 4 watts - so it becomes four times more efficient to
decrease the wire size by one half (keeping the volume the same), or I
could produce 32 amp/turns of field strength for the same 16 watts
that produced 8 A/T with larger wire.  (if the volume were to
increase)

Practically speaking there's something like a theoretical increase of
7% or so when wires lay in the interstices created by the layer below.
Increase the turns by a factor of four and that 7% becomes significant
too.

See any flaws in the logic?

I'm glad we had this chat.

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Re: Help, any gurus with alternator experience or knowledge?

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Huh?  It comes out exactly the same either way:

8V,   4 ohm,  4T = 16W, 8A-T
32V, 64 ohm, 16T = 16W, 8A-T

The only issues as far as efficiency goes are the insulation thickness
as a percentage of the winding, and the wasted space due to the imperfect
packing between the wires.  You can write equations for each, and calculate
the best wire size for an 8A-T coil, if you want.

-Chuck

Re: Help, any gurus with alternator experience or knowledge?
On Tue, 11 Jul 2006 15:36:47 -0400, Chuck Harris

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If a single coil will give me two ampere turns with 1 watt of
dissipation won't four parallel coils give me the equivalent of 8
ampere turns at 4 watts dissipation?  All running at 8 volts in
parallel.

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Re: Help, any gurus with alternator experience or knowledge?
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Of course, but you will have twice as much wire volume as the original
coil.

TANFL!

-Chuck

Re: Help, any gurus with alternator experience or knowledge?

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Quit bloody wating your time(and likely effort) and get one at a salvage
yard...

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Re: Help, any gurus with alternator experience or knowledge?

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Would that it was that simple.  These things are scarcer than hen's
teeth, and all have a reputation for failing as originally made.  

There is some emotional satisfaction to be derived from solving
problems.

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Re: Help, any gurus with alternator experience or knowledge?

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when we send out motors for repair we also request them to use teflon
instead of the various ranges of enamel's they have out there.
   in one location we use AC motors as tension units, they don't last
long when running at the max with out proper ventilation and cool air
going across the unit. so we started to have these units repaired with
teflon and they have been great ever since with the exceptional bearing
blow out now and then which we can fix our self's.



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Re: Help, any gurus with alternator experience or knowledge?
I have to point out (since I am a mechanic in an electronic newsgroup) that
you are taking the electronic approach to solving your problem. The
mechanics approach would be to adapt a longer lived, less expensive, more
popular alternator form another brand of car onto your engine.

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