Help, any gurus with alternator experience or knowledge?

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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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

Reply to
Rich Grise

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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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.

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

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

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

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

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   Jasen
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Reply to
jasen

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.

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

Reply to
Vaughn

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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Reply to
Michael A. Terrell

"Michael A. Terrell" wrote in news: snipped-for-privacy@earthlink.net:

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!

Reply to
Jim Land

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.

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

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

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Apricale, Italy
Reply to
Ban

jasen wrote in news: snipped-for-privacy@clunker.homenet:

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

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me

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 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.

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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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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You are forgetting the extra diodes in the regulator - they provide the current to the field rotor - not the battery. The warning light on automobiles serves two functions - it provides the "tickle" of a few hundred milliamps get things started as well as indicate that the alternator is working. Some autos won't charge the battery at all when the warning light burns out. My system has no warning light just a small resistor to start things off.

Fed pulsating DC albeit three phase rectified DC, inductance may be a factor. I don't know. Right now it is an unknown variable - when I switch on a load the voltage drops for ~100+ milliseconds and overshoots again when I switch it off - if the regulation were better I'd say inductance didn't matter.

I disagree there -

No fan, no pulley, driven - bolted directly to the crankshaft. No venting what so ever - no way for rain to get in. Conduction and radiation and convection around the cast aluminum cover.

That's a good idea. screw terminals of brass tabs imbedded in the epoxy to solder to

Here if I send a mini CD through the mail in a regular envelope - it comes back with the shiny side in the bottom of the envelope - Have to see how the post office does that - no scouring necessary, perfectly transparent.

Actually I have a crude coil winder. I have a large tape recorder, reel drive, DC, permanent magnet, motor. It has a taper on the end of its 2" long shaft. To the end of the motor I can bang on a block of wood and turn it down with a chisel and rasp to any diameter from

1/2" to 5" I already built a three part mandrel to wind the coil out of wood - the tricky part is figuring out how to get some plastic on there that will act as a mold release and mold a self supporting coil.

The coil winder was made for winding long Tesla coils and I did use it for a large induction coil. Works very well with an adjustable DC supply to power it. Crank up enough voltage to pull wire from a supply reel and guide it on by hand. Lower the voltage to keep tension on it to paint with varnish, epoxy, or to get a beer. Paint a Tesla coil with epoxy and set it to rotate slowly while it cures and it is possible to get a really smooth finish.

The winding lathe has a tail stock made from a carriage bolt and is threaded through a long nut mounted in blocks of wood - for Tesla coils - short coils go on the mandrels mounted to the motor shaft and turned to diameter.

I have some of those mini CDR discs and they are the ideal diameter for side plates (have to bore out the center to about 1-5/8") I don't know that I'd trust the poly carbonate plastic at those temperatures for a permanent bobbin - a tin can lid would make better sense there.

I'm toying with the idea of using a couple of polyethylene recyclable lids like one sees on re closable food cans to make temporary side plates. Coat them will silicone wax and epoxy the coil together and then remove it from the mold. The spindle (hub/center) could be wrapped with Teflon pipe joint tape and making the side plates undersize and sliding it over the tape may give a liquid tight seal to make a self-supporting coil. Then that coil could be further insulated with tape or dipping in resin.

Thanks for the reply - more ideas to think about

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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.

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.

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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I have no experience wiring motors but, it sounds like you think your wire will work and you don't want to hear that it won't... It probably will work.. All you can do is try it and see.. That is the only way to get a definative answer.. I had a similar problem, well similar with everone telling me that it wouldnt work. I was going to try to run ethernet over

700ft of cat5 wire. Everyone said it wouldn't work, but I tried anyway and it worked just fine, with the exception of lightning..

Anyway my point is just do it and see if it works, it probably will...

- Mike

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Michael Kennedy

"Michael Kennedy" wrote in news:K86dnXE_epcv7SzZnZ2dnUVZ snipped-for-privacy@comcast.com:

LOL! We should all be so lucky in our projects.

Reply to
Jim Land

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Reply to
Jamie

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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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Reply to
Jamie

Hmm, 700Ft?,. bet you r only getting 10MB ?

and btw, you can get FEP motor/mag wire with thin applied coatings on it. tefzel, well we had experience with that, it most likely will work how ever, if the motor burns up , toxic gas is released.. we use that at work for products that need to be smokeless. well it mite be Smokeless but it isn't toxless. .. if this stuff gets a little over heated you don't want to be directly in the path of its venting area. FEP isn't much better either actually, that is smokeless but removes the O2 of of the air and if your a smoker! watch out.

Reply to
Jamie

All that the inductance can do is smooth out the lumps in what's already pretty smooth.

unless there's extra turns on the output windings we're talking 1 volt maximum above what the main output is producing.

say the thing is going all out and the main rectifier doides are dropping

1.4v (each) the extra diodes for the field aur under less extreme load and maybe dropping 0.6V so you're only 0.8V above the voltage on the output terminal.

RTV silicone may be useful to reduce movement in the wires too, that blue stuff used for sealing tappet covers sounds suitable (Permatex ultra blue - nuetral cure, heat, and oil resistant)

Hmm, I wonder how they do that...

maybe their mail processing hardware bends the letters somewhere along the path... or maybe there's someone who sticvks them in a microwave oven :^)

Bye. Jasen

Reply to
jasen

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

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jasen

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