Starting with Alternator ON/OFF

Janekom · #1 02-09-2009, 12:02 AM

Standard RV7A with 60A alternator and wiring more or less as per Van's diagram.
We are debating which way is better/safer. Most Cessna POH's say start with the alternator on. I personally like to start the 7A first, then switch on the alternator and then when everything has settled the Avionics master switch. Any comments would be appreciated.

Ironflight · #2 02-09-2009, 05:48 AM

I always liked turning alternators on AFTER engine start as well Jan - gave me a good feeling that it was actually working. But while that was OK with older, externally regulated alternators, there is significant empirical evidence that it will kill the internally regulated automotive alternators. GMCJETPILOT has posted extensively on this in the past - search for some of his posts on alternators, and you can get the theory. I have gotten to where I just leave the alternator switch on all the time now, and can always turn it off if I have an electrical problem.

Paul

Jamie · #3 02-09-2009, 05:53 AM

Ditto Paul's comments.

Yeah, and there really isn't a practical reason to turn it on after startup.

I have an "idiot light" on my panel for my alternator. It turns on when I switch on the master switch and turns off when I start up, so if the light goes off I know the alternator is working.

Also, I always check my main buss voltage before take-off and during cursory ops checks in flight. If it's ~12.2 volts, the alternator is not working, if it's 14.1, it's working. Simple.

pierre smith · #4 02-09-2009, 06:24 AM

....a good friend of mine is a for real engine electronic type expert. He said that after the engine is running, it puts quite a "shock" on the armature from the instant load when you turn it on, whereas if it's already on while you're cranking, the load increases slowly as the revs build up.

I now do as he says....no alternator problems in 3 1/2 years 440 hours,

Regards,

jthocker · #5 02-09-2009, 06:56 AM

Like Paul said, I used to turn on the alt. after starting. After toasting an alt. on my 6A after 100 hrs., and heeding GMC George's advice, I now "gang" the alt. field switch with my master switch. Of course on the 3 planes since the 6, only 1 has over 100 hrs., so we'll see.

frankh · #6 02-09-2009, 02:37 PM

Every car in the free world starts this way and there a lot more of them than there are of us....Hard to argue with sucess..

Frank

N941WR · #7 02-09-2009, 02:47 PM

In addition to what the others have said, do not "test" the alternator by pulling the excite field breaker as that will also cause failures.

If you want to verify your alternator is working, add and remove load, such as turning on your fuel pump and landing lights and observe your engine monitor.

L.Adamson · #8 02-09-2009, 05:51 PM

Quote:

Originally Posted by N941WR

In addition to what the others have said, do not "test" the alternator by pulling the excite field breaker as that will also cause failures.

Some how, some way, on one of my cross country flights during phase one testing; I "accidentally" turned off my alt switch. I don't know how long it was off, but the voltage had dropped to 11+ volts. I noticed this as I shut down, and wondered how I missed the blinking red low voltage light. Not only that, but I still had the B-lead relay from the "discontinued Bob K." over voltage setup, which cut off my B-lead from the alt. to the battery,too. I had already disconnected the over voltage module that is apparently prone to nuisance tripping.

I'm a hundred miles from home, and figured my Van's 60 amp internally regulated alternator would now be toast. This is the alternator they were selling before "Plane Power" models.

Happily, the engine cranked over, and my amp meter went to about 20+ amps, and slowly went back to zero. At least I learned, that perhaps shutting off the alt-field actually WILL cut the alternator power, although I've read that it might not on some internally regulated units. And that these alternators are not always toast if we chop the alt field. However, I have no plans to re-test!

L.Adamson -- RV6A

erich weaver · #9 02-09-2009, 09:02 PM

I monitor Bob Knuckolls posts on his aero-electric list and this subject came up recently. According to Bob, when using B&C components (alternator and regulator) and wiring per his schematics, switching the field switch off while under power does not present any risk of failure. I believe him.

erich

L.Adamson · #10 02-09-2009, 10:14 PM

Quote:

Originally Posted by erich weaver

I monitor Bob Knuckolls posts on his aero-electric list and this subject came up recently. According to Bob, when using B&C components (alternator and regulator) and wiring per his schematics, switching the field switch off while under power does not present any risk of failure. I believe him.

If Bob is still going with the external regulator; then that is correct.

L.Adamson