Alternator problem

Wow! A very informed response. I appreciate it immensely.

Thank you!

"Nathan Young" wrote in message
...
On Fri, 23 Apr 2004 13:28:40 GMT, "Tom Jackson"
wrote:

The other day, I was flying and noticed that my ammeter guage was reading
0
(it had been reading fine for a long portion of the flight.) Also, I had
been running a lap-top from the cigar lighter plug for the first time
ever.
I turned-off virtually all electric devices and continued to my
destination.
I also shut-off the alternator switch. Later in the flight, I tried
recycling the alternator switch, and the guage would read full
deflection,
so I then would shut the alternator switch off again.

Later in the day, I fired-up the plane - could tell that the battery was
weak because it could hardly pull the prop through. The ammeter guage,
however seemed to look ok - appeared to be charging the battery. I
figured
that somehow it had recycled itself, so I took off and headed home
without
incident (I watched the ammeter guage very closely throughout the flight,
and it appeared normal throughout.)

The plane is a 1974 Piper Warrior - 151.

Any thoughts? Was it merely a failsafe due to too much current being
drawn
from the lap-top? Should I have it looked at?

No way it was the laptop. Laptops run on a few amps at 12V, much less
than many accessories (pitot heat or landing light).

If the alternator had been off (and the plane running on battery) for
a long time the battery might be down at 9-10 volts. When the
alternator is re-energized, the current will go to close to max
deflect for a minute or two until the battery gets back up to 12-13V,
then it will slowly ramp down to normal as the battery gets back to
14V. The behavior you saw was normal.

Now the real question is why did your alternator go offline? The
Cherokee electrical system has about 5-6 components and all can be
failure points. Most pilots/mechanics replace the alternator, but
that rarely solves the problem.

In the Cherokee, the alternator output is wired directly to the master
bus. The field circuit breaker taps off the master bus, and then
provides current to the field switch, the field switch is connected to
an overvoltage regulator, which is connected to the voltage regulator.
The output of the VR controls the field current on the alternator, and
hence controls the output of the alternator.

Common culprits to the Cherokee electrical system include:
1. Field current breaker develops corrosion (it is probably 30 years
old, so not surprising), and begins to intermittently introduce a
resistance into the circuit. This allows a voltage drop across the
breaker, so the VR turns up the bus voltage until it sees 14V. The
problem here is that because there is voltage drop across the breaker,
the master bus might be sitting at 15,16,17V. Eventually the plane
hits some turbulence and the breaker gets moved a slight amount, the
resistance goes back to zero, and the overvoltage regulator sees 16V,
so it goes open circuit, which cuts the voltage to the VR, which then
has no power to give to the altenator field, so the alternator goes
offline. Solution: Replace the field breaker.

2. The Cherokee field current switch gets worn with age and will
begin to exhibit strange behavior. This is commonly seen as 'pulsing'
of the electrical system as the contacts in the switch heat, expand,
lose contact, cool, and then make contact again. Solution: Replace
the switch.

3. The overvoltage regulator in most older Cherokees is an
electromagnetic monstrosity, and the magnet/relay can fail to hold the
circuit open. This causes intermittent random electrical failures.
Solution: Repalce the OVR.

4. The VR can go bad.

5. In addition to all these components that can fail, the wiring
between them (after 30 years) can develop loose contacts, corrosion,
or cracks in the wire that allow open circuits or shorts to ground.

All of these can be really difficult problems to track down,
especially when the problem can't be reproduced on the ground.

Be patient, and be careful. Flakey electrical systems don't matter
much to a VFR pilot, but it is no-go issue for an IFR plane.

Also, if you use a voltmeter to probe around under the panel, be
careful especially if you have the master engaged - it is super easy
to short the bus while probing around the master bus.

-Nathan

I think that you will find that the voltage spikes generated by the
trim motor and switch are getting into the voltage regulator's over
voltage sense and tripping it off line. You can suppress the
generation of the spikes with a 50 volt MOV (metal oxide varistor)
across the trim motor or add a capacitor across the regulators voltage
sense input or both. If the regulator already has a large external
cap replace it with a new cap since the electrolytic cap tends to dry
out and lose capacity over the years.
Do not add a cap across the motor or the switch may fail soon. To see
these spikes requires a digital storage scope which most A&P's do not
have.
John

On Fri, 23 Apr 2004 18:51:49 -0700, "Mike Noel"
wrote:

For what it is worth, my alternator started going off-line regularly after
months of perfect operation. With some investigation I realized the trouble
started when I 'rediscovered' the electric trim on my 74 Archer. Even
though the trim would move OK with no noticeable deflection of the ammeter,
after a while I would look down and see no output from the alternator.
Cycling the alternator field rocker would put it back on-line. Not sure
what the problem is with the trim circuit since the trim wheel doesn't seem
to have too much friction, but now I trim manually and will eventually
investigate the wiring and motor.

Alternator goes out... ... and then comes back

I believe that somehow you tripped the over voltage relay.
That whams the alternator off line positively
until something is cycled. Even a momentary over voltage
will do it.

So the symptom then is as you had, there is no "load."

The "something" that is cycled will either be the alternator
switch or master switch. Check you POH. On older 14v planes,
it's often the master switch that must be cycled.

The overvoltage protection is there to keep the alternator
from ripping the **** out of everything if there is a failure
in the regulator or wiring that would command full output
from the alternator. Generally, they are set around 16v.

The most common reason for ov tripping is water in the battery
too low. Since you charge like a bandit, I think that's not
your problem.

When it was at full scale, you could have momentarily throttled
back to reduce rpm... I'd have expected the load to drop a bit. A
low battery could peg it out for awhile as the other poster pointed
out.

Bill Hale BPPP instructor A&P

"Brian Sponcil" writes:

I have a '75 151 with similar problems. Basically my Ammeter reads normally
(10 amps) while on the ground and through run-up. However once I'm in the
air for a little while it drops to zero and no amount of cycling the alt
switch fixes it. I replaced the voltage regulator but the only effect I saw
from that was a "calming" of the Ammeter needle. I suppose that was worth
$100 ;-) Anyway, I suspect that the Chrysler alternator, a rebuilt unit
installed at the last annual, is the culprit. I'm having the A&P look at it
monday so I'll get back to you with the verdict.

Depending on the meter, this sounds quite normal.

As the other poster said, there are 2 different ammeters out there.
A zero-center one reads current into/out of the battery. A single-ended
one reads output from the alternator. Not the same thing at all.

The zero center flavor will start out a flight by showing a discharge,
then after you start the engine, it should start to charge. After
you make up for the battery's amp-hours used in starting, it should
drop to zero -- the battery is charged and needs no more.

The alternator output meter is a different story. It should start
out showing some output at idle/taxi [recharging], then lots at
climb/cruise, [still recharging but more RPM=more amps possible]
then NEAR zero once fully charged. How far from zero depends on what
you else are running. (If you have a PL-12 with 3 navcoms,
2 transponders, one ADF, XM radio, a LORAN and twin GPS, it will be
a bit more than Ron's Fly Baby.)

Note the "zero" is not exact on either gauge.

There's an easy test while in flight. Turn on some load - the landing
lights will do nicely. The zero-center battery gauge should stay
the same, provided you've not exceeded the alternator capacity
{unlikely}. The alternator-output gauge should show 10+ amps more
output & drop again when you turn off the lights.

The OP mentioned the alternator-output pegged when he brought it
back on line. That's not unexpected. The alternator was attempting
to recharge a depleted battery. Safe? I've never seen an automotive-style
alternator damaged by overload. I suppose already-weak diodes might
fail; but in general, the output is limited by the available magnetic
field. That's limited by the field current, set by the voltage
regulator -- by changing the field current, it changes the field
strength, and that changes the output voltage. The regulator can't
do more than put a full 13v on the field, and that's within spec.

And the bigger lesson is to learn how your a/c normally acts, and
notice when it changes.



--
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

David Lesher wrote:

There's an easy test while in flight. Turn on some load - the landing
lights will do nicely. The zero-center battery gauge should stay
the same, ...

Actually, there is a short deflection to the right, lasting less than a second, after
which the needle centers again. I use this test during runup to simultaneously check
the ammeter and the lights.

George Patterson
If you don't tell lies, you never have to remember what you said.

"G.R. Patterson III" writes:



There's an easy test while in flight. Turn on some load - the landing
lights will do nicely. The zero-center battery gauge should stay
the same, ...

Actually, there is a short deflection to the right, lasting less than a second, after
which the needle centers again. I use this test during runup to simultaneously check
the ammeter and the lights.

True, but it may not be visible in all cases.






--
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

Nathan Young writes:

Common culprits to the Cherokee electrical system include:
1. Field current breaker develops corrosion (it is probably 30 years
old, so not surprising), and begins to intermittently introduce a
resistance into the circuit. This allows a voltage drop across the
breaker, so the VR turns up the bus voltage until it sees 14V. The
problem here is that because there is voltage drop across the breaker,
the master bus might be sitting at 15,16,17V. Eventually the plane
hits some turbulence and the breaker gets moved a slight amount, the
resistance goes back to zero, and the overvoltage regulator sees 16V,
so it goes open circuit, which cuts the voltage to the VR, which then
has no power to give to the altenator field, so the alternator goes
offline. Solution: Replace the field breaker.

Does this alternator have auxiliary diodes? Automotive alternators
have two sets of diodes. The six main ones are in a three-phase
array that provides the output.

There are also 3 small {~5A} diodes to provide field excitation. At
startup, the battery will supply the needed field excitation, but
as soon as the alternator is working, they take over the job. (They
are integral to the idiot line scheme in cars, however, so may not
exist in aircraft units.)



--
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

David Lesher wrote:


There are also 3 small {~5A} diodes to provide field excitation. At
startup, the battery will supply the needed field excitation, but
as soon as the alternator is working, they take over the job. (They
are integral to the idiot line scheme in cars, however, so may not
exist in aircraft units.)

Negative. Neither the Prestolite (Ford style) alternators used on
Cessnas, nor the Chrysler alternators used in Pipers have the additonal
"diode trio". Only the aftermarket STCed INTERAV conversion uses
a Motorola alternator with the diode trio.

MikeM
Skylane '1MM

Mike Noel wrote:

For what it is worth, my alternator started going off-line regularly after
months of perfect operation. With some investigation I realized the trouble
started when I 'rediscovered' the electric trim on my 74 Archer. Even
though the trim would move OK with no noticeable deflection of the ammeter,
after a while I would look down and see no output from the alternator.
Cycling the alternator field rocker would put it back on-line. Not sure
what the problem is with the trim circuit since the trim wheel doesn't seem
to have too much friction, but now I trim manually and will eventually
investigate the wiring and motor.

The inductive voltage spike from the trim motor (at the instant you
release the trim motor switch) is being detected by the alternator
OverVoltage Protection (OVP) circuit built in to the Piper's Alternator
Control Unit (ACU).

The solution: supress the motor glitch (at the motor), or make the
OVP less susceptible to a momentary voltage excursion above 16V.

The topic has been extensively discussed on r.a.o before. Google groups
it...

MikeM
Skylane '1MM
Pacer '00Z