HI Al, yes, I have been considering that.  My field is external like yours.  In that supply I have a 10amp fuse and the switch itself, along with the necessary crimps on the spade connectors.  I suppose I could tryed a temporary short across those connections to see if that that fixed the problem, but it did not.  Also voltage measured at the field is the same as on my engine monitor.
I wonder if we are wearing out the brushes more quickly because of our high continuous RPM.
-- David Leonard

Turbo Rotary RV-6 N4VY
http://N4VY.RotaryRoster.net
http://RotaryRoster.net


On Sun, Dec 20, 2009 at 7:55 AM, Al Gietzen <ALVentures@cox.net <mailto:ALVentures@cox.net>> wrote:

    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
    <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.


    --     David Leonard

    Turbo Rotary RV-6 N4VY
    http://N4VY.RotaryRoster.net <http://n4vy.rotaryroster.net/>
    http://RotaryRoster.net <http://rotaryroster.net/>