Headset Noise

Brought my plane home last night after having the alternator rebuilt.

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.

Prior to having that alternator rebuilt, I had a whine that increased in
frequency with RPM and got louder with load. If I shut the battery off, the
noise was intolerable. Clearly, the alternator was creating an A/C component
that the battery could just not dampen.

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.

By the way, before this, I had no idea that you could shut the battery off
and leave the alternator on. In 10 years of flying airplanes with split
master switches, I have never, ever, not once had the alternator on without
the battery. Trouble shooting scenarios always assume that if there is a
problem, it will be with the alternator and discuss either shutting the
alternator off or both. Some people start their plane with the alternator
off to reduce starting load and startup power spikes. I actually thought
that the switches were mechanically coupled so that it was either both or
just the battery. The first time I did shut the alternator off, my first
thought, "That's curious, I wonder if my switch is broken!"

Anyway, I picked up my plane last night. Started it without the alternator.
Cautiously turned the alternator on. No whine. Added load. No whine. Reved
it up a bit. No whine. Shut the battery off. No Whine.

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 continue to believe that the alternator should be creating a steady
voltage that should be usable without the dampening effects of the battery.
I understand that bad grounding connections, in particular, could impede a
radios ability to shed the A/C component but it should only have to shed
something like 50mv of ripple - not the .5 to 1.5v that I was seeing.

The bill was for 2.5 hours of labor and $273 for the rebuild. $450 total.
This was at Kenosha, WI.

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.

-------------------------------
Travis
Lake N3094P
PWK


"mikem" wrote in message
oups.com...

Travis Marlatte wrote:
. . .

I had a whine show up recently. It's was only bothersome with ANR
headsets.
If you concentrated, you could also hear it with regular headsets. I
checked
grounds and switches. Nothing made the least bit of difference.

Finally, used an oscilloscope to look at the alternator output. Nice
consistent ripple. No spikes. But, it was minimally .5 volt p-p growing
to
1.5v p-p with load. Typical alternator output ripple should be more like
20mv to 50mv p-p.

Travis, if you connected the alternator Field directly to a 2 to 5V DC
supply, connected a heavy load (like a couple of landing lights across
the alternator output), then you spun the alternator in a drill press
or lathe, while viewing the voltage across the load with a scope, you
would see about 4V p-p of ripple. Is that a reason to re-build the
alternator?

No!! It is actually the aircraft battery which "filters" the output of
the alternator, and removes the ripple. If you see ripple, it is
because there is some resistance in the path between the alternator
output and the battery posts, or a bum battery. It could be either in
the positive path or in the ground return. I predict that you will
still have your ripple when you get your freshly o/h alternator back...

Several hundred mV of ripple measured at the B terminal of an
alternator is perfectly normal...

MikeM