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

Hi Travis,

First, Glad that you seem to have fixed your problem. Your alternator
must have had a bum diode, and my prediction was wrong.

Second, I want to point something out to you. Disconnecting the battery
while the alternator is spun-up (and maintaining field exitation) is
EXTREMELY DANGEROUS. Doing this could cause the bus voltage to spike
beyond 100+V, and destroy every piece of avionics in your panel. Rather
than trying to explain why here, do a Goggle search on the phrase
"ALTERNATOR LOAD DUMP".

Every aircraft I have seen is wired such that turning off the battery
master simultaneously breaks the path to the alternator field, thereby
shutting down the alternator. Some aircraft accomplish this with a
mechanical interlock on the switch (Cessna split master/ALT switch), or
by the way the ALT switch is wired. The fact that your Lake is NOT
wired this way seems to be a major screw up.

Disconnecting the battery in order to diagnose an alternator is NOT an
accepted diagnostic tool.

Third, to show the expected bus voltage under different conditions, I
did four different Spice simulations and put the results he
http://tinyurl.com/lkzt4

Normal.jpg = 14V with Battery, loaded to 25A, showing typical
resistances. (50mv of ripple)

NoBattery.jpg = Same as above, but with no battery to act as a filter.
(2V of ripple)

BadDiode.jpg = same as Normal, except one diode is open. (400mv of
ripple)

BadDiodeNoBattery.jpg = same as above, but no battery to act as a
filter. (15V! of ripple)

Look at all four, and you can see the difference in the waveshape and
amount of ripple.

I also posted four more files based on the four simulations above,
which show the Spectra (FFT) of each case. Note that when a diode is
missing, the fundamental frequency is related to the rate of rotation,
while with six intact diodes, the fundamental frequency of the ripple
is six times the rotation rate.

MikeM

"mikem" writes:


First, Glad that you seem to have fixed your problem. Your alternator
must have had a bum diode, and my prediction was wrong.

Second, I want to point something out to you. Disconnecting the battery
while the alternator is spun-up (and maintaining field exitation) is
EXTREMELY DANGEROUS. Doing this could cause the bus voltage to spike
beyond 100+V, and destroy every piece of avionics in your panel. Rather
than trying to explain why here, do a Goggle search on the phrase
"ALTERNATOR LOAD DUMP".

What he said (WHS). Although the figure I recall wasa mere 65 volts;
that from a discussion of such in a GE-MOV applications book many
years ago. Further, the resulting spikes is not just big (V) but
fat (long persistence) and thus high total energy.


Every aircraft I have seen is wired such that turning off the battery
master simultaneously breaks the path to the alternator field, thereby
shutting down the alternator. Some aircraft accomplish this with a
mechanical interlock on the switch (Cessna split master/ALT switch), or
by the way the ALT switch is wired. The fact that your Lake is NOT
wired this way seems to be a major screw up.

WHS!!!!

Disconnecting the battery in order to diagnose an alternator is NOT an
accepted diagnostic tool.

The ONLY good news is you'll blow up only the avionics; the magnetos
will not care and even an electric fuel pump may likely survive.

Do that stunt on modern car and you shall regret it...


Third, to show the expected bus voltage under different conditions, I
did four different Spice simulations and put the results he
http://tinyurl.com/lkzt4

Hmm, zip seen here, alas....
Is it working?


--
A host is a host from coast to
& no one will talk to a host that's close........[v].(301) 56-LINUX
Unless the host (that isn't close).........................pob 1433
is busy, hung or dead....................................20915-1433

Try this:
http://home.utah.edu/~mgm17160/Alternator/

"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.)




--
A host is a host from coast to
& no one will talk to a host that's close........[v].(301) 56-LINUX
Unless the host (that isn't close).........................pob 1433
is busy, hung or dead....................................20915-1433

Mike,

Thanks for publishing the alternator simulations. I found them very
interesting.

You said, "... with six intact diodes, the fundamental frequency of the
ripple is six times the rotation rate."

I read the fundamental ripple frequency as 3 kHz. If that's six times the
rotation rate, then the rotation rate must be 500 Hz. That's 30,000 rpm.
That doesn't seem reasonable. What am I missing?

Regards,

Jon

In all the years of chasing alternator whine in aircraft audio systems,
it sounded to me that it is at a couple of Khz.

Jon,

If the engine is turning 2600 RPM, that is 43 revs per sec. The pulleys
on the alternator have a ratio of about 4:1, so that would make the
alternator spin at about 173 revs per sec, or 173Hz. That would make
the alternator whine (assuming no bad diodes) of 1038Hz.

I should alter the sinsoidal current sources to have a freq of 173Hz,
instead of the 500Hz I used.

btw- for the electrical geeks out there, you are familiar with LTSpice,
arent you? If not, go look at www.linear.com and get a download. Best
Spice implemenation out there, and it is FREE!

Mike,

Thanks for the clarification and the link to LTSpice!

Jon

"mikem" wrote in message
oups.com...
In all the years of chasing alternator whine in aircraft audio systems,
it sounded to me that it is at a couple of Khz.

Jon,

If the engine is turning 2600 RPM, that is 43 revs per sec. The pulleys
on the alternator have a ratio of about 4:1, so that would make the
alternator spin at about 173 revs per sec, or 173Hz. That would make
the alternator whine (assuming no bad diodes) of 1038Hz.

I should alter the sinsoidal current sources to have a freq of 173Hz,
instead of the 500Hz I used.

btw- for the electrical geeks out there, you are familiar with LTSpice,
arent you? If not, go look at www.linear.com and get a download. Best
Spice implemenation out there, and it is FREE!

mikem wrote:
: Try this:
: http://home.utah.edu/~mgm17160/Alternator/

I see you didn't try to model the load dump transient... Trouble with that
one is that it's dependent on all sorts of other loads on the bus.

For those that didn't look up the potential damage of load dump transients,
the trouble is the tremendous amount of inductance in automotive claw-pole
alternators. Yes they're 3-phase synchronous machines, with six-pack diode rectifiers
to get "DC.".... very crappy 3-phase machines. When charging a battery that's sucking
down tens of amps, the backEMF necessary to overcome the AC voltage drop across the
(large) stator inductance can often measure 60-90V. If that battery load suddenly
goes away, all that magnetic energy stored in the inductor tries to keep the same
current flowing by increasing the output voltage. Then all your avionics go *poof*.

Now, if you've been flying for awhile and your battery is all charged up, it
will only be taking an amp or two. All other loads remaining constant, you could
disconnect the battery and run on just the alternator without hurting anything.
Trouble is, any other transient load shedding (gear motor, pitot heat, etc) will not
have the battery there to absorb it.

It seems like it might be useful as an emergency limp-home mode if the battery
somehow fails. I'm having a hard time envision a scenerio where it would be
safe/useful to do (other than a fried master relay).

-Cory

--

************************************************** ***********************
* Cory Papenfuss, Ph.D., PPSEL-IA *
* Electrical Engineering *
* Virginia Polytechnic Institute and State University *
************************************************** ***********************

I can't get at the link but your textual description fits what I was seeing
and hearing. I'm a believer! I'm a believer! I will never shut the battery
off again!!

At the next annual, I will have my mechanic replace the master as well.

Without seeing the waveforms, it sounds like it would be very hard to
differentiate a bad connection from a bad diode problem. I apparently lucked
out in my ill-informed diagnosis.

Thanks for all you input.

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


"mikem" wrote in message
ups.com...
Hi Travis,

First, Glad that you seem to have fixed your problem. Your alternator
must have had a bum diode, and my prediction was wrong.

Second, I want to point something out to you. Disconnecting the battery
while the alternator is spun-up (and maintaining field exitation) is
EXTREMELY DANGEROUS. Doing this could cause the bus voltage to spike
beyond 100+V, and destroy every piece of avionics in your panel. Rather
than trying to explain why here, do a Goggle search on the phrase
"ALTERNATOR LOAD DUMP".

Every aircraft I have seen is wired such that turning off the battery
master simultaneously breaks the path to the alternator field, thereby
shutting down the alternator. Some aircraft accomplish this with a
mechanical interlock on the switch (Cessna split master/ALT switch), or
by the way the ALT switch is wired. The fact that your Lake is NOT
wired this way seems to be a major screw up.

Disconnecting the battery in order to diagnose an alternator is NOT an
accepted diagnostic tool.

Third, to show the expected bus voltage under different conditions, I
did four different Spice simulations and put the results he
http://tinyurl.com/lkzt4

Normal.jpg = 14V with Battery, loaded to 25A, showing typical
resistances. (50mv of ripple)

NoBattery.jpg = Same as above, but with no battery to act as a filter.
(2V of ripple)

BadDiode.jpg = same as Normal, except one diode is open. (400mv of
ripple)

BadDiodeNoBattery.jpg = same as above, but no battery to act as a
filter. (15V! of ripple)

Look at all four, and you can see the difference in the waveshape and
amount of ripple.

I also posted four more files based on the four simulations above,
which show the Spectra (FFT) of each case. Note that when a diode is
missing, the fundamental frequency is related to the rate of rotation,
while with six intact diodes, the fundamental frequency of the ripple
is six times the rotation rate.

MikeM