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Yet another alternator problem



 
 
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  #1  
Old July 9th 04, 11:03 PM
Chris Kennedy
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Default Yet another alternator problem

My turn in the barrel with alternator issues.

I have a PA-24 250 with an InterAv 50A alternator conversion installed
that has a chronic but transient issue that appears to be associated
with regulation. Observed symptoms include:

- Overvoltage relay kicking the alternator offline when the gear motor
shuts down when running the gear down (but not when running the gear up).

- Overvoltage relay kicking the alternator offline with demand spikes
(like hitting all the landing lights at the same time)

- Transient discharge indications on the EI volt/ammeter when running
the strobes.

I've replaced the spike guard cap, rebuilt the wiring harness, made sure
that the grounds are clean and that there are no ground loops. Measured
resistance between the regulator output and the field terminal on the
alternator is less than one ohm. The battery is new and the ship has
copper cables.

This issue isn't new; it's been observed with the aircraft since we
purchased it and we're still trying to track it down. The issue with
running the gear down is both the most common and the most disturbing,
since the last thing I want to do is take all the avionics offline in
order to recycle the alternator while swimming through the marine layer.
I'd considered putting MOVs on the gear motor, but that doesn't
address the real problem, which is that the regulator seems slow to
respond to changing load, causing overshoots when heavy loads drop off
and droops when loads come on.

So, two questions for the group:

- Anyone have a clue regarding what in the world is going on here and
- Does anyone know of an alternative to the InterAv conversion for the
O-540? Even my hangar mate with the '68 Cherokee has 60A alternator...

Thanks,
Chris.
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  #2  
Old July 10th 04, 02:14 PM
Nathan Young
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Default

On Fri, 09 Jul 2004 22:03:23 GMT, Chris Kennedy
wrote:

My turn in the barrel with alternator issues.

I have a PA-24 250 with an InterAv 50A alternator conversion installed
that has a chronic but transient issue that appears to be associated
with regulation. Observed symptoms include:

- Overvoltage relay kicking the alternator offline when the gear motor
shuts down when running the gear down (but not when running the gear up).

- Overvoltage relay kicking the alternator offline with demand spikes
(like hitting all the landing lights at the same time)

- Transient discharge indications on the EI volt/ammeter when running
the strobes.

I've replaced the spike guard cap, rebuilt the wiring harness, made sure
that the grounds are clean and that there are no ground loops. Measured
resistance between the regulator output and the field terminal on the
alternator is less than one ohm. The battery is new and the ship has
copper cables.

This issue isn't new; it's been observed with the aircraft since we
purchased it and we're still trying to track it down. The issue with
running the gear down is both the most common and the most disturbing,
since the last thing I want to do is take all the avionics offline in
order to recycle the alternator while swimming through the marine layer.
I'd considered putting MOVs on the gear motor, but that doesn't
address the real problem, which is that the regulator seems slow to
respond to changing load, causing overshoots when heavy loads drop off
and droops when loads come on.

So, two questions for the group:

- Anyone have a clue regarding what in the world is going on here and
- Does anyone know of an alternative to the InterAv conversion for the
O-540? Even my hangar mate with the '68 Cherokee has 60A alternator...

Thanks,
Chris.


Have you measured the bus voltage? Sounds like it might be set too
high, and possibly on the threshold of the OVR cutout. If so, any
inductive spike would be able to push the voltage higher forcing an
OVR cutout.

Also, what is the voltage at the input to the OVR and the input to the
VR?

-Nathan


  #3  
Old July 10th 04, 11:53 PM
Chris Kennedy
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Posts: n/a
Default

Nathan Young wrote:

Have you measured the bus voltage? Sounds like it might be set too
high, and possibly on the threshold of the OVR cutout. If so, any
inductive spike would be able to push the voltage higher forcing an
OVR cutout.


14v and change. Even if the set point was too high it doesn't explain
the tendency to intermittently show discharge with spiking loads, nor
the occasionally-observed behavior of a largely resistive load (turning
on the landing lights) tripping the thing off (the working theory is
that the halogens have a high cold inrush current but quickly settle
down to a lower operating current -- quick enough that the regulator
overshoots).

Also, what is the voltage at the input to the OVR and the input to the
VR?


Sorry. I forgot to include that all of the voltages in the system are
within InterAv's specs for troubleshooting "common" failures. The
conditions for these measurements are engine not running, master and
field on, 12.5v at the alternator output terminal. Under those
conditions the alternator regulator terminal has 0.9 volts on it, the
regulator's regulator terminal has 0.9 volts, the regulator's field
terminal has 1.2 volts and the alternator's field terminal 1.2 volts.
The OVR is in series between the regulator's field output and the
alternator's field terminal; its sense inputs are tied to the bus and
the regulator ground.

I'm wondering if brush chatter could be contributing. I can't find any
log entries suggesting that they've been changed, and the thing has
about 400 hours on it. I also haven't stuck a scope on it to see if a
diode has given up the ghost, but I'm not seeing or hearing anything in
the avionic stack that would suggest I've got a noisy bus.

What I really want is a 100A conversion, but at this rate I'm going to
end up ripping this alternator and its attendant bits out of the
aircraft and installing Yet Another Copy of the InterAv kit :P

Chris.
  #4  
Old July 11th 04, 03:35 AM
Aaron Coolidge
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Posts: n/a
Default

Chris Kennedy wrote:

: 14v and change. Even if the set point was too high it doesn't explain
: the tendency to intermittently show discharge with spiking loads, nor
: the occasionally-observed behavior of a largely resistive load (turning
: on the landing lights) tripping the thing off (the working theory is
: that the halogens have a high cold inrush current but quickly settle
: down to a lower operating current -- quick enough that the regulator
: overshoots).

You are right about the high inrush for halogen (or, in fact, any
incandescent) lamp. Don't forget that the full name is "tungsten-halogen",
and the cold resistance of these 12V units is probably like 0.2 ohms.
Of course, the resistance rises as the filament temp goes up. Handling
tungsten inrush is a very tough job for switches, both mechanical and
solid-state.

Instead of a MOV across the gear motor, I would try zener diodes in inverse
series, say about 14V 5W units.

It could also be bad connections to your battery - the battery is supposed to
soak up the junk on the bus, as well as handle transient inrush currents.
Did you check the voltage drops across the battery ground braid, and the
master contactor? If you've got an oscilloscope current probe, check the
battery current when you get these OV trips. If the OV is reaching the
battery terminals its current should spike too. A 400 Hz current probe,
such as those sold by Sperry or Amprobe probably won't have the bandwidth
needed to see these transients. Where are you located?

--
Aaron Coolidge
  #5  
Old July 11th 04, 07:42 AM
John_F
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Default

What you have not said is what the voltage at the battery is when the
aircraft is running and the alternator is charging. This voltage
should be 13.9 to 14.2 volts at the battery at about 70F degrees. The
trip point on most over voltage limiters is set at 16.0 volts.
Both of these points needs to be measured with a simple bench setup
using a small light bulb for the alternator field load and a regulated
variable voltage power supply, Increase the voltage to the regulator
until the field light goes out. This is the voltage the regulator is
set to regulate at. Continue to increase the voltage until the over
voltage relay opens. This is the voltage the OVR is set to. Do not
exceed 17 volts.
Until you know these two voltages you are guessing and trying to fix
the problem with the hope and poke method.

A bad diode in the alternator can cause the over voltage to trip due
to the excessive voltage ripple. A bad diode in the alternator will
also limit your maximum output current to less than 2/3 of rated
current and may be as low as 1/3 if it is shorted.

Turn on all of your lights and other loads on and measure the current
from the alternator. Measure both the AC and DC currents. If the AC
current is more than 15% of the DC current when you are generating at
least 50% of rated current you have a bad alternator diode or stator
winding.

Make damn sure you do NOT have any stainless steel washers between the
battery post and the battery cables.

Do not use stainless parts in electrical circuits because they are
high resistance. The film that forms on stainless parts to keep them
from rusting can be very high resistance.

John

frerichsatrodotcom

On Fri, 09 Jul 2004 22:03:23 GMT, Chris Kennedy
wrote:

My turn in the barrel with alternator issues.

I have a PA-24 250 with an InterAv 50A alternator conversion installed
that has a chronic but transient issue that appears to be associated
with regulation. Observed symptoms include:

- Overvoltage relay kicking the alternator offline when the gear motor
shuts down when running the gear down (but not when running the gear up).

- Overvoltage relay kicking the alternator offline with demand spikes
(like hitting all the landing lights at the same time)

- Transient discharge indications on the EI volt/ammeter when running
the strobes.

I've replaced the spike guard cap, rebuilt the wiring harness, made sure
that the grounds are clean and that there are no ground loops. Measured
resistance between the regulator output and the field terminal on the
alternator is less than one ohm. The battery is new and the ship has
copper cables.

This issue isn't new; it's been observed with the aircraft since we
purchased it and we're still trying to track it down. The issue with
running the gear down is both the most common and the most disturbing,
since the last thing I want to do is take all the avionics offline in
order to recycle the alternator while swimming through the marine layer.
I'd considered putting MOVs on the gear motor, but that doesn't
address the real problem, which is that the regulator seems slow to
respond to changing load, causing overshoots when heavy loads drop off
and droops when loads come on.

So, two questions for the group:

- Anyone have a clue regarding what in the world is going on here and
- Does anyone know of an alternative to the InterAv conversion for the
O-540? Even my hangar mate with the '68 Cherokee has 60A alternator...

Thanks,
Chris.


  #6  
Old July 11th 04, 06:54 PM
Martin Kosina
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Posts: n/a
Default

I'm wondering if brush chatter could be contributing. I can't find any
log entries suggesting that they've been changed, and the thing has
about 400 hours on it.


Chris,

For what its worth, I had the same symptoms after replacing a bad
voltage regulator with a Zefftronics unit (I did this after having
chronically low voltage in the system, turned out the old VR was an
automotive one, a very bad thing with a split master switch, but
that's another story). The new regulator was keeping up nicely, but
when a large load was suddenly placed on the system (like flaps, or
landing light), the voltage would momentarily shoot up to 16V, the
built-in OVR then (correctly) tripped. Anyway, the problem was in the
tired brushes, I had to get it going ASAP so I just swapped in a
rebuilt alternator, but I am sure brush replacement would have fixed
it also.

Martin
  #7  
Old July 12th 04, 10:56 PM
external usenet poster
 
Posts: n/a
Default

We had a similar problem in our 172M. The over voltage detector was
firing at too low a voltage, and we would get multiple dropouts.

To simplify diagnosics we used a handheld electric motor and Vee belt
to drive the alternator and then an adjustable power supply or fat
resistor load while looking at the 14 V lead with an oscilloscope. It
is a lot better than working around a spinning propeller. Found the
problem right away.

I noticed that the original overvoltage wiring "blob" had a small
tweekpot adjustment screw showing on one end. I suppose we could have
adjusted the trip voltage a little higher with that, but we already
had purchased a new one. Pipers may not be the same.

BTW - when measuring voltage drops, be sure to measure across as many
faying (joint)surfaces as possible. Your resistances sounded kinda
high.
  #8  
Old July 16th 04, 04:28 PM
John P
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Posts: n/a
Default

Chris,
I think what John_F said is great!
The clues a
It's happened since new
It's a PA24-250
All grounds cleaned and "checked"
Any big load trips the OVP ( except gear up which is less)

I still thinks it's a ground loop. I believe it's the battery cable to
the frame(?) I forget how this
attaches on the PA24. Measure the voltage drop on the battery from minus
terminal of battery to the frame
and with a long "wire" to the bolt on the starter ground while someone
"cranks". How much voltage?
I thinks it's a ground resistance problem! Et all, does this make sense?
A ground problem would raise
the voltage seen to the OVP artificially, trip it.... The alternator seems
to keep the battery up, lower current
will not trip OVP, only higher current...
I know on our Cherokees I always took the battery cables off the belly
and cleaned em.....often..
and every low resistance high current connection also.
Did I miss a clue here that blows this out of the water? I looked at
this that the alternator is working fine!

John N3DR

wrote in message
om...
We had a similar problem in our 172M. The over voltage detector was
firing at too low a voltage, and we would get multiple dropouts.

To simplify diagnosics we used a handheld electric motor and Vee belt
to drive the alternator and then an adjustable power supply or fat
resistor load while looking at the 14 V lead with an oscilloscope. It
is a lot better than working around a spinning propeller. Found the
problem right away.

I noticed that the original overvoltage wiring "blob" had a small
tweekpot adjustment screw showing on one end. I suppose we could have
adjusted the trip voltage a little higher with that, but we already
had purchased a new one. Pipers may not be the same.

BTW - when measuring voltage drops, be sure to measure across as many
faying (joint)surfaces as possible. Your resistances sounded kinda
high.



 




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