Alternator OFF until after start a good idea?

[ao.frog]

Hi guys.

Thanks for all the feedback and thanks for preventing me for doing a exepensive mistake!

I'll turn on and off the alternator together with the batt switch then and write my checklist accordingly.

What I think I'll also will do, is to wait to turn on the avionics (GPS, VHF, TX etc) until after engine start, so the avoinics doesn't go through the startup sequence.
For engine monitoring during startup, I'm thinking to let my AFS-3500 run on it's internal battery.

That way, the avionics are put on line after start and after engine instruments are checked. (Volt and charging for example)

How does this sound...? Or maybe I'm overthinking again...??

[Adam Oke]

Avionics master needs left off prior to start up .... could be a costly mistake with spikes on that one.

[sjjonesnz]

That's exactly what I'm planning on doing....can you please let us know how you get on??

[jetjok]

Originally Posted by ao.frog

What I think I'll also will do, is to wait to turn on the avionics (GPS, VHF, TX etc) until after engine start, so the avoinics doesn't go through the startup sequence.
For engine monitoring during startup, I'm thinking to let my AFS-3500 run on it's internal battery.

Oh Yeah Baby!! No avionics on when you are starting!! It would really ruin your day to watch them all go "blink..blink..off!"

[Bob Brown]

It's not the upspikes that are so bad...it's the down spikes. Read all about it at the Aeroelectic Connection...

This would logically lead us into a discussion about the wisdom of avionics master switches, which all of us know follow the discussions of primers, nosewheels, fuel injection, electronic ignition and Sikaflex. While I have not had a canopy come loose from Sikaflex, I HAVE had an avionics master switch fail...at night. All of that stuff is searchable...

Alf, do you have RPM now?

[Captain Avgas]

Originally Posted by gasman

Planepower has an internal regulator like toyota and many other auto type alternators. They say if you turn the field off with the motor running, DO NOT TURN IT BACK ON with the motor running.
http://www.plane-power.com/Tech_Bits_value.htm
BTW.... Van's has the best price on PLane POwer.

My understanding is that the Plane Power alternator has an inbuilt overvoltage device. Presumably this can trip (as all overvoltage devices tend to do from time to time). My guess is that if this happened in flight you'd need to turn the alternator off and then on again to reset the device. That entails turning on the alternator with the engine running. So I'm not certain what Plane Power's advice means in that context. Can you enlighten.

[mahlon_r]

It has been my experience that turning the alternator on and off with load on the system, causes spikes that can damage or disable one or more of the diodes in the alternator. Thus, limiting it's capacity or even disabling all output. I have always told my customer?s the ?once on, leave it on? story but that if they just had to turn it off, to do so with a charged battery and everything else turned off. Have had many flight schools, that had alternator replacement issues because they simulated electrical malfunctions by turning off the alternator or pulling the field breaker and then seeing how long it took the student to notice the gauge was on discharge. Then after the student noticed, several minutes later, they would flick the switch back on or recycle the breaker/fuse, with everything else on, and then have a real electrical malfunction.
Good Luck,
Mahlon
?The opinions and information provided in this and all of my posts are hopefully helpful to you. Please use the information provided responsibly and at your own risk."

[sjhurlbut]

Originally Posted by rmcgann

Looking forward to a final answer on this question.

So what are people doing with a backup alternator? Leave powered all the time, or only operate if the primary fails? With a 2 alternator, single battery setup, I am very interested.

cheers
-10
almost done.

I have B&C 60Amp and SD8 6 amp backup. I leave the backup alt OFF all the time unless main alt fails. If the main alt fails and the electrical load is small (never the case for me) you may not even notice that the main alt has failed since voltage will remain >13V. Instead leave the backup off, get low voltage warning from main alt failure, turn backup alt on and load shed if required.

Steve
Rv7A
Flying

[airguy]

Originally Posted by sjhurlbut

I have B&C 60Amp and SD8 6 amp backup. I leave the backup alt OFF all the time unless main alt fails. If the main alt fails and the electrical load is small (never the case for me) you may not even notice that the main alt has failed since voltage will remain >13V. Instead leave the backup off, get low voltage warning from main alt failure, turn backup alt on and load shed if required.

Steve
Rv7A
Flying

That sounds like a reasonable course of action to me. It's a little hard on the alternator to "splash" the field coil to full power by turning it on at full load and RPM, but realistically with the very low number of lifetime cycles that the backup alternator will experience, I call that acceptable. If you have exceptionally bad luck over the life of your plane you MIGHT power up that alternator under those conditions 10 times over its life with maybe that many hours of runtime. I would certainly ops-check it on the ground treating it just like the main alternator after any in-flight use, though.

With the backup being only 6-amps output, I wouldn't think you would need to worry too much about a positive voltage spike hurting the avionics - the battery would easily be able to absorb any momentary overvoltage at that low a current.

[Ironflight]

My original Ops scheme in my plane was to turn the alternator on after startup (to see the voltage rise), then during pre-take-off I'd take it off, watch the volts drop, turn on the SD-8 (backup alternator), watch the volts rise, then take the SD-8 off, and bring the main back up. Very complicated, makes you feel like you're really working!!

At about 250 hours or so, I had some voltage wandering, and before ti got worse, I replaced the alternator, and read some really good posts by George (haven't heard from George in awhile....where has he been?) about how the ND alternators work, and decided that it wasn't good for them to be powered on/off while rotating, so I haven't done it to the second one. 500 hours now, and no trouble. You might call this superstition (a sample of ONE is hardly science), but I'm sticking with it.

Now I start and end with the main alternator "ON", and test the back-up only during maintenance - the thing is a permanent magnet dynamo - it really can't not work, unless the coupling has sheared (I check that on pre-flight when I check oil). It's about as complex as operating a car, a lot less "fun", and much more reasonable (to me).

Best,

Paul (EFIS on aux battery for start, Avionics of until after start)