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.
On Sun, Dec 20, 2009 at 7:55 AM, Al Gietzen <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] 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.
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