Nathan Young wrote in message . ..
On Thu, 08 Jul 2004 03:53:23 GMT, "John Clonts"
wrote:

I am now able to reproduce my charging problem.

Engine off, Bat and Alt on:
Bus voltage: 24.8
Supply to Voltage Regulator: 22.6
Voltage Regulator Output to Alt Field: 21.6

I'm thinking maybe I should rig a jumper from Bus to the Voltage regulator
to see if the problem has to do with that 2.2v drop through the "Alt/Reg" 5A
circuit breaker, the Alt side of the master switch, and the Overvoltage
relay.

John, I didn't see your original charging problem, so cannot comment
on that, but will make a few general comments on this post.

21.6 Volts seems like a high field voltage to drive the alternator.
What kind of load was on the system during this measurement? I'm not
certain, but believe wih minimal load, the field voltage on my 14V
system was around 6-7V.

The 2.2V drop between the master bus and VR is not good and can lead
to several problems in the electrical system. I would fix this before
doing anything else.

#1. The bus voltage will be high. The VR will crank up the field
voltage until its input 'sees' 24V. Given a constant drop of 2.2V
between bus and VR, the bus will be at 26.2.

#2. If the resistance causing the 2.2V drop is caused by a bad CB,
switch, or loose/corroded connection before the OVR, the resistance
will vary depending on temperature and turbulence knocking the system
around. This will cause OVR cutouts as the OVR will occasionally see
the full bus voltage which is 24V + the voltage drop from a second ago
(26.2V or higher).

#3. Resistance between the master bus and the VR input sets up an RC
circuit which (when extreme) can cause charging system oscillation.

I like your idea of trying to eliminate the drop between the bus and
the VR input by direct connecting it to the bus. (Make sure to have
your avionics master OFF when doing this kind of troubleshooting!).

Another approach would be to measure the voltage after each of the
field CB, field switch, OVR, and VR input. This will probably tell
you where the drop is, and you can do this without the plane running.

BE EXTRA CAREFUL when probing around under the panel, it is easy to
short things with voltmeter probes.

-Nathan


Nathan,

As posted elsewhere I did finally solve the problem. The main problem
was the filter capacitor inside the alternator had come loose and was
bouncing around in there. Explained my intermittent charging, and
then eventually it wiped out the alternator completely.

After replacing the alternator the bus voltage was higher than
desired, but we were expecting this. We jumpered from +bat to the VR
input as described above, and the voltage came down-- confirming the
resistance problem. We cleaned the connections on the master switch
and on a connector to the VR itself. That solved the high bus voltage
problem.

It also eliminated my "bouncing ammeter/flickering panel lights",
which had been that way ever since we got the plane so I didn't even
think it WAS a problem!

Thanks for everyone's input-- it's been a great learning experience!

Cheers,
John Clonts
Temple, Texas
N7NZ