Dave;
Regarding the varying, or higher
charging voltage, I’ve had similar behavior. I’ve determined that
a possible cause is the difference where you are measuring the voltage, and
what the regulator is seeing. I provide the field current from a separate
source, having undone the internal connection from the alternator output. You
may have a similar setup. That means that any resistance – voltage drops –
from the battery to the alternator field, will result in the regulator seeing a
lower voltage, and increasing the output.
I have two potential contact resistance
points in that circuit; the battery contactor, and a pullable breaker that I
use to turnoff the field current if I want. A couple of weeks ago when I
started up the voltage (at the EM2) was reading over 15V, and triggering the
over limit light. After a couple of resets of the breaker, and a couple
of shutdowns (turning off battery contactor), the voltage settled down at 14-14.5.
I’m not fully convinced the contacts
were the problem, but I think it likely. May be relevant to your
situation.
Early this year I had replaced the
voltage regulator after seeing high voltage. Next time I’ll replace
the pullable breaker or the battery contactor.
(I bet the skiing at mammoth was great,
and the weather beautiful.)
Al
-----Original Message-----
From: Rotary motors in aircraft
[mailto:flyrotary@lancaironline.net] On
Behalf Of David Leonard
Sent: Saturday, December 19, 2009
5:52 PM
To: Rotary motors in aircraft
Subject: [FlyRotary] Real Life No
Alternator test
I have been having an intermittent and slowly
worsening problem with high voltage. Over most of the last 30 hrs or so
it settled in around 14.6 or 14.7 volts. But on yesterday's flight home
from Mammoth I noticed it hovering around 15V and some of my avionics were
acting up, so I decided to do a little alternator out experiment.
I turned off the alternator and all unnecessary draw -
radio, strobes, Blue Mountain EFIS, electric AI. I kept on the engine
instrumentation, transponder, music (a most critical item given the beauty of
the sunset), audio panel, rocky mountain microencoder, and trio auto pilot
(also allowing me to more fully enjoy the flight). I was showing 8 amps to run
the engine bus (coils and injectors).
With my 33 amp hour Panasonic SLA battery I flew for
45 minutes and 200 miles over the Sierras, passing dozens of
airports, and into the LA basin before it got dark enough that I decided
to turn the lights on. The plan was to turn the alternator back on when
the voltage dropped below 11.5 volts but it never dropped below 11.9V. It
did charge at a brisk 60amps when I turned the alternator back on so there
couldn't have been too much time left. Still, that did quite a bit of
confidence building regarding the ability to carry on in the event of an
alternator failure.
I also set a personal record for the most beautiful
and clear sunset I have seen while flying. I could see San Nicholas
Island from over 150 miles away.
Time to install a spare alternator.