Why do alternator rotors fail?
15-E
rotorsfail.htm
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I have thought, many times, about why rotors fail. I have
some basic ideas. Back in the early days, rotor windings were held in place with
some sort of lacquer or shellac substance. After enough heating and
cooling cycles, it might begin to flake off, letting windings move about
from centripetal force. I don't
think the stuff did nearly as much good as modern epoxies applied
in a vacuum chamber, but it was pretty much standard treatment for motor/generator/alternator windings back then.
I have personally seen rebuilt rotors fail from not being
assembled correctly...one was a brand-new rebuild, right out of the box. I have seen rotors short circuit; but most have OPENED, and
sometimes that is intermittent, and the internal connection can close at some
rpm....or open. It's a mixed bag. If the rotor is properly assembled, and then vacuum
impregnated with epoxy, it should hold up darn near forever.
Rotors run fairly hot in the core, particularly when a high
percentage of alternator maximum output is used. Rotors on the faired RT
models (particularly the early ones with NO louvers in the front
fairing piece) run hotter. I think that the wider the heat-cold range the rotor
is exposed to; and, the more often, the less the reliability.
I believe that higher output usage is part of the problem. With rotors, voltage regulators, and diode boards....the higher the output, the more heat is generated in these parts, and cycling heat-cold tends to bring about failures in things electrical and electronic. A combination of a RT fairing, and high output, likely is a contributing factor.
Another possible factor is that the air that passes over
the stator, rotor, and diode board, is not likely the same quantity,
nor velocity,
between some models. The air flow is not the same, for instance, between the
clamshell air cleaner models and the rectangular air cleaner models. Take a look at
the air flow differences....look at the outer timing chest metal cover
which is NOT the same design on all models. Look at how air
gets inside, passes around and over the parts, and exits....and you will see why I
think this is a factor.
I have NOT run any sort of survey to prove this
one way or the other....by belief that a goodly percentage of rotors fail due to how
the the rider shifts gears, and how often, over the many miles. Those that shift abruptly downward might be particularly prone to
the sudden acceleration forces on the rotor windings. I have not made
up my mind as to whether or not JUST high rpm is a factor, but I
suspect that it IS involved.
The Omega alternator rotor runs very hot in its core. That
alone might detract some from its reliability. But if
the rotor is vacuum chamber impregnated with epoxy, it may be fine. It
remains to be seen how many fail over the coming years, as miles on them
increases and the number sold increases. The EnDuraLast should be immune to rotor failures, but whether
or not the system is more or less reliable than stock...or the
Omega.....it will likely be some years before we know.
I think we just do not have enough data to know if these two
aftermarket alternators will be more or less reliable than the stock
alternators, as the miles are accumulated.
It appears that the last of the airheads rotors are more
reliable than the earlier ones, but it is difficult to make an informed
statistically good statement....after all, they are also YOUNGER, and have had,
in general, fewer heat-cold cycles and miles.
I suspect that the only way to determine what is going on
would be to do a rather detailed survey of miles, number of heat-cool cycles, rpm and
shifting types
(smooth, match rpm?...or tends to shift a bit jerkily....)....
average trips being very short, short, or long....RT? type of front outer
cover? Clamshell model?....etc.
Many things could contribute...making it difficult to say what
is going on....and making it somewhat meaningless (not totally,
perhaps) on how long rotors last, or do not, for any one particular rider.
I have had ONE rotor fail on one of MY
Airheads; it was an improperly made aftermarket rotor. I've also had ONE diode
board fail (standard non-bent leads on as delivered Wehrle board, 1983
R100RT). Since "Airheads" begins with the /5, I suspect that means half a
million miles. I have a LOT of miles on /5 bikes...with NO alternator
problems. I am not so sure ANYthing can be concluded from THAT. Can we conclude anything much from my statement about MY
rotor experiences on RT's? MAYBE...maybe NOT. I have always done FEW short
jaunts; never overtaxed my alternators; don't shift down with snap rpm
rises. On the other hand, much of the mileage has been on RT's with rectangular
airboxes...which are definitely harder on the alternators, due to heat effects
(granted one of my three RT's came with the NON-louvered front
fairing and I DID soon after purchase, modify it).
I think it would take quite an in-depth survey to determine
what was causing the various failures. Even if we could determine trends, etc......what would you DO
about it? Increase cooling to the parts?...HOW??...yes, there ARE a few
changes that help...some I am not sure about.
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© Copyright, 2010, R. Fleischer