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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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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

--

Bye. 
   Jasen
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.

--
Service to my country? Been there, Done that, and I've got my DD214 to 
prove it. 
Member of DAV #85. 

Michael A. Terrell 
Central Florida
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

Oh, I don't know . . . where's your sense of adventure man?

All engineering is a matter of compromise. You assume the factory got it right

- the original lasted 2 years my first repair lasted 5. It would still be working if I'd thought to serve some stranded wire to the start and finish ends and not used Teflon tubing to support the wire.

There are some alternators that last until the brushes die from wear or the diodes rust through - this alternator has a very high, well documented, rotor failure rate. I'm assuming the factory did not get it right.

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.

The power dissipation goes down as the wire gets thinner, lower current producing the same ampere turns in the same available space assuming the voltage stays constant - higher efficiency.

I'm committing two sins here - going down in wire diameter and increasing insulation thickness - fewer turns of thinner wire. Conductivity may be better because it looks like silver plating on the wire. I wish I'd measured the 1,000 foot reel before I started using it . . . but I bought it for Tesla coils not alternator rotors.

The downside of using very thin wire is it is more likely to break under stress, physical or thermal cycling. At some point wire insulation may take up more room than wire itself with very thin wire gauges - but within the practical range of several gauges it shouldn't matter all that much. Theoretically.

I would expect the output to go up slightly with thinner wire due to an increase in efficiency but don't expect it to be enough to be worth changing to a different gauge. I'm just trying to save a buck on wire cost.

I have Tefzel insulated wire I already own . . . . Copper prices have doubled in the last year or two. One pound was costing me $12, one half pound now costs $35 from the same source - but in larger quantities it has (only) doubled. I need about 12 ounces.

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Yeah was hoping to find a person who's done a rewind himself. The repair group is too busy, figured I'd drop off the screen in a day or two.

My latest plan is to try a self-supporting coil. I found some 22 gauge wire I could afford. Rather than make a bobbin, I put some polyethylene facings to my mandrel and plan to spray it with silicone as a mold release agent. I'm serving some small braided wire to the ends of the magnet wire and bringing those up through fabric sleeves and anchoring the ends in wraps of cotton twine.

I'm using the same epoxy that worked last time for this winding. The stuff I ordered didn't get here and won't until Friday. I may still do a second coil with vinyl ester resin since I have more than enough wire now.

There's no doubt the wire and sleeves I used were anchored well enough the last time - but I think it was a mistake to use Teflon sleeves - the wire had about .5 millimeter of play in the sleeve and it was 1.5 centimeters long embedded in epoxy - but epoxy can't penetrate the sleeve.

Fabric sleeve (called spaghetti in the repair trade here) has been supplanted with Teflon and heat-shrink tubing. I have some Military type, small diameter (2.5mm), silver plated coaxial cable with a fabric braid. Probably used for airframe applications from sensors to cockpit. I'm using the braid and fabric and discarding the Teflon center conductor.

Thanks for the ideas

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