"Travis Marlatte" writes:


As you can read in my earlier post below, I scoped out the voltage and
concluded that the alternator had blown a phase. If a diode had gone, I
expected to see ripple with injected spikes that the blown diode would have
rectified but didn't. Since I saw a steady ripple with no spike and at a
higher peak to peak level than I expected, I concluded that all the diodes
were rectifying the same and that the excess A/C component was because a
phase of the alternator had gone. This was my conclusion sitting the plane
with a scope and reinforced later by some web site that talks about trouble
shooting alternator waveform outputs.

I do not recall; did you test this alternator at full load? That
will sometime expose bad diodes that look good at lower currents.
In days past, the gas station [remember THOSE?] had a volt/amp meter
with a carbon-pile load; tighten the pile until the belt is REALLY
singing and look at the scope.

Below, mikem suggests that the problem was more likely the battery or
connections. I believe that the battery does absorb some ripple but it
doesn't make sense to me that the alternator output would be so noisy that
you couldn't operate the plane with just alternator power.

See other post...

The ticket says - "Rebuilt and tested." My mechanic says that they found a
bad diode. I don't really know if that was THE problem or just part of the
problem. Bottom line, removing and reinstalling a rebuilt alternator solved
my whine.

I have to wonder if the rebuilding/reinstalling also solved a partial
ground issue somewhere....


I continue to believe that the alternator should be creating a steady
voltage that should be usable without the dampening effects of the battery.

Please stop believing that. Or believe it, but do not practice it.

mikem, are we really talking about the same kind of alternator? Are these
alternators typically 3-phase? With 3-phases and the typical diode pack, why
would you see 4v p-p? That seems excessive to me.

Every auto alternator I have seen is 3-phase, Y wound. (But I've
heard tell of some that were delta..)

There are 6 main diodes, and 3 aux diodes.

(Note my Honda is a partial exception. For reasons I have yet to
grok; the center-point of its Y has 2 more main diodes, making 8
main. I can't wrap my head around what this accomplishes.)




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