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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