What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

Alternator OFF until after start a good idea?

A

ao.frog

Well Known Member
Hi.

I'm in the process of writing the POH to my -7 and was wondering if it would be a good idea to wait until after engine start before turning on the alternator?
In other words: turn on only the battery switch before engine start.

Then the entire elec system would be protected for "spikes" during start up?

And vice verca on shutdown? (Turn off the alternator before mix is pulled to cutoff)

Or is it something I've missed or don't understand?
 
I wouldn't

I don't honestly think it makes any difference but my preference is to leave the alternator switched on during start up and shutdown.

Reason being is that if ever I have seen an alternator fail its when it has been disconnected from a running engine.

Frank
 
Nope, turn it all on.... this lets it come online slowly. Turning your alternator on and off while running is not recommended and should be limited to testing only.
 
I agree with Frank.

Do you ever turn off the alternator in your car?
 
Alf - I have a slightly different view.

I leave it off 'til the engine is running, then turn it on.

What I think is much more important is dont turn it off (if you can) until after the engine has stopped turning. That is where the problems happen.

Have fun, Steve.
 
Mel said:
Do you ever turn off the alternator in your car?
Click to expand...

Exactly. At my gliding club, I fly the tow plane along with gliders ... unfortunatly with shared aircraft come shared habbits. There was one particular person who had a 'thing' for turning the alternator switch off. The kicker is that it is under a switch cover so that nobody touches it!

I think it is particularly a saftey issue having it off when it comes to numerous pilots as everyone carries out their own checklist. Realistically if you are the only one flying it, and you have it in your checklist ... by all means, do what ever the heck you want .... but think to yourself what the purpose is for it first. That's the difference between a pilot and a good pilot. Pilots do things ... good pilots understand why they are doing the things they do. If you get some electrical engineer to tell you that it will prevent spiking and give longer lasting alternator life. By all means ... and let us know too!

I'm not a fan.
 
Last edited:
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.
 
Last edited:
I think turning it off for startup and off for shutdown provides an excellent opportunity to forget to do something...your alternator will be fine going through the startup/shutdown...the vast majority of them live their whole lives this way.
 
I used a Cessna style split master switch and just like all the other planes I have flown, both sides are turned on, the engine is started. It stays on until after I shut down.

One builder in our chapter used to be a co-owner in a 172. He has told me on more than one occasion that one of their co-owners would always turn the master on and off to check the alternator load. He went on to say they replaced three alternators in the two years this guy was flying the plane. After he sold his share they never had another alternator failure.

Who knows if it is cause and effect but since I've never read a POH that suggests testing the alternator by any other method than apply a load (landing lights, fuel pump, etc.) to it and watching the ammeter guage.
 
Standby Alternator?

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.
 
It's ON then...

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...??
 
Avionics master needs left off prior to start up .... could be a costly mistake with spikes on that one.
 
ao.frog said:
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.
Click to expand...

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

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.
 
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."
 
Leave it off

rmcgann said:
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.
Click to expand...


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
 
sjhurlbut said:
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
Click to expand...


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.
 
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)
 
Different industry but . . .

In heavy truck electronics, it is recognized that the load shed/load dump cases tend to occur in real life as an alternator goes offline the hard way. Second best way to fry an armature (or lay some heavy carbon tracks on the brush comm) is to bring it up while rotating an under a heavy load(low voltage - an instantanious loading, which we simulate in validation. You may think an engine going from stopped to idle does so quickly, but in elecrical terms, it's s l o w.

Having understood that, I was toying(90% sold) with the idea of eliminating an 'alt fld' switch from my panel. Sure, I'll have a pullable breaker, but it will be on the master. This thread seems to confirm that as a reasonable approach. Bob may not like it . . . but that's ok. I'm still very cautious using what I know from work on the airplane.

Rick 90432
 
Here is another data point for what it is worth. Been working with ND internally regulated alternators since they came out about 30 years ago (auto repair business) and I've switched field power on and off in aircraft and race cars almost as long (many hundreds of times) . Never seen one cave yet by doing this. Brushes might wear out after a very long time and a Lycoming might eventually vibrate some parts loose on one but these are generally very reliable electrical devices in my experience.

While theory may be great, I tend to rely more on actual experience. I don't believe that switching the field power on or off with the engine at idle will have any negative effects on a Denso IR alternator.
 
Last edited:
Captain Avgas said:
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.
Click to expand...

I called Plain Power about your question of resetting the "FIELD " circut breaker. You can reset the breaker once to see if the problem is corrected. But if it pops again, then leave it off line till it is repaired.

Also keep in mind that any time that you have the master and alt switch on and the motor is not running, the field reads the 12+ volts of the battery and wants to bring it up to 14.2. This can heat up the alternator if it is not spinning.
 
Another Data Point

I routinely start my Rocket with the Master switch turned to battery only, and the Magneto on but the Electronic Ignition off. Hit start and once the engine idles, turn Master to On and EI On. THe EI module contributes to the kickback problem. With the Master on Battey only I have maximum battery power delivered to the starter without any other electrical drain.

I do use B&C alternators with B&C alternator controllers.
 
Turn alt off & on?

As Mel said "Do you turn the alternator on & off in your car?". The alternator has been designed to not having to do this. As said its just some thing else to remember. Delco makes all thier 10si alternators with an integral regulator. Lighter and less parts - Hm. If you want to see what a brand new one costs go to your chevy or buick dealer and ask! Airplanes use AMP meters because they always use to! A volt meter can tell you a lot more and is LIGHTER. By the way airplanes always use to have generators!
 
IMO, there's no point in having the alternator field on prior to engine start. The alternator is not spinning so it won't generate any power. Since the voltage regulator is trying to bring the voltage up to 14 volts (from 12 volts), it is drawing current (typically 3-4 amps) and not producing any benefit with the engine stopped.

When the field is on, the alternator also generates a load on the engine proportional to the amount of electrical load needed. I haven't done any formal testing, but consider the electrical load during start.

After you start the engine, the Vertical Power VP-200 then automatically turns on the alternator field, then the avionics. Each device is turned on with a short delay (1/10 second) before the next device is turned on. This gives the voltage regulator plenty of time to adjust to each new load being turned on. :D
 
Mel said:
Do you ever turn off the alternator in your car?
Click to expand...

Actually you do but might not know it. When you move the key to the ACC position, typically the alt field is turned off. When you switch to IGN the field wire is powered, and then when you move to START it mechanically turns off uneeded electrical loads like fans, electric windows, alt field, etc.

This may not apply to all cars, especially the newer ones with digital electronics and solid-state switches. The computer handles that function rather than a mechanical switch.
 
MCA said:
IMO, there's no point in having the alternator field on prior to engine start. The alternator is not spinning so it won't generate any power. Since the voltage regulator is trying to bring the voltage up to 14 volts (from 12 volts), it is drawing current (typically 3-4 amps) and not producing any benefit with the engine stopped.

When the field is on, the alternator also generates a load on the engine proportional to the amount of electrical load needed. I haven't done any formal testing, but consider the electrical load during start.

After you start the engine, the Vertical Power VP-200 then automatically turns on the alternator field, then the avionics. Each device is turned on with a short delay (1/10 second) before the next device is turned on. This gives the voltage regulator plenty of time to adjust to each new load being turned on. :D
Click to expand...

Marc, does the VP-X Pro do the same thing as the VP-200 in your last paragraph?
 
The VP-X only turns the alternator on and off based on the position of the alternator switch. There is no automation like on the VP-200.
 
A data point to add to the discussion: (and yes, I realize it's four years old...)

When I was doing my private training in the DA-20, the checklist I used called for starting on battery alone and then switching the alt on after start. I double-checked my memory using Google, and the first checklist I found corroborated my memory. But then I got curious and dug the POH I have out of my closet, and the start checklist in there does not specify starting on battery alone.

Which I find very interesting...

In any case, when I first started flying Archers, I retained that habit of starting on battery alone, until one day when I missed a step. I think I'd been in the air a few minutes when I noticed I hadn't switched the alternator on. Oops!

Since then, I've just followed the real checklist instead of inventing my own additional complexities.
 
Us old dogs had to learn that modern avionics have new rules. Dynon Skyview recommends turning it on BEFORE hitting the starter! I checked with Garmin on their radio, and they say the same thing. This definitely makes aircraft more "carlike" and eliminates the extra steps and switches.
 
As a point of reference......

I used to start with the alt fld off. After going through a few alternators, I began the practice of leaving the field ON for start.
I have not had to replace an alternator since.
 
One thing that you don't want to do is turn on the master to sit and play with your panel and still have the alt field on. If you turn on the master and are not going to start the motor, keep the field off.
 
Have been starting my plane with the ALT field off since the beginning. I use a PP alternator and it has performed flawlessly.

May die on the next flight who knows but it has worked fine so far this way.
 
One thing that you don't want to do is turn on the master to sit and play with your panel and still have the alt field on. If you turn on the master and are not going to start the motor, keep the field off.
Click to expand...

How will that ruin your alternator?
I can see the additional draw on the battery but other than that?

I have always started the engine with the alternator field on and turned off after the engine is shut down, so far so good.
 
The real story.......

In days of old, the "spike" loads during startup of generators and early alternators were harmful to older avionics. Modern avionics can tolerate the spikes without harm, therefore, it is the same as "why do we pull the prop through by hand on flat engines?" Old school vs modern. On many alternator systems, once the engine is running with the alternator online, turning off the field has no influence on the "exiting" depending on the type and how it is wired. Either technique works, neither is harmful.

Regards,

Gary Brown
ATP A&P IA CFI DAR
 
N427EF said:
How will that ruin your alternator?
I can see the additional draw on the battery but other than that?

I have always started the engine with the alternator field on and turned off after the engine is shut down, so far so good.
Click to expand...

I can't tell you the "why" of this, but we have lots of anecdotal evidence that the internally-regulated automotive alternators do NOT like being cycled when they are already turning - they weren't designed for it. Externally-regulated units seem less inclined to be damaged by this....and most of the alternators we all learned with on certified airplanes were of this type. So turning on the alternator after start is a habit that many of us have "unlearned" to keep them alive longer.

Paul
 
Mike Busch on alternators

Mike Busch has an EAA webinar on alternators at:http://www.eaavideo.org/video.aspx?v=1549006054001

About 9 minutes from the end of the webinar, during the questions, he talks briefly about startup and shutdown - his recommendation (in line with many posts in this thread) is to start the engine with the alternator field already on unless the engine is being started while connected to a ground power unit (in that case, he recommends turning the alternator on after they've disconnected the cart) or if the battery is quite depleted and you think the additional power draw for the alternator would leave you without sufficient energy to start.
 
Yet another data point

rv6ejguy said:
Here is another data point for what it is worth. Been working with ND internally regulated alternators since they came out about 30 years ago (auto repair business) and I've switched field power on and off in aircraft and race cars almost as long (many hundreds of times) . Never seen one cave yet by doing this. Brushes might wear out after a very long time and a Lycoming might eventually vibrate some parts loose on one but these are generally very reliable electrical devices in my experience.

While theory may be great, I tend to rely more on actual experience. I don't believe that switching the field power on or off with the engine at idle will have any negative effects on a Denso IR alternator.
Click to expand...

I purchased a rebuilt Denso IR alternator on eBay for my 7A for $50.
It's the one for the 87 Suzuki Samurai. It was still in the box from an autoparts store.

I have always switched the alternator on AFTER starting the engine.
After 4 years and 770 hours it's still working great! (knock on wood).

Mark
 
When starting your car do you start the engine first then turn the alternator on...I don't think so so why would you do any different in your plane?
 
Norman CYYJ said:
When starting your car do you start the engine first then turn the alternator on...I don't think so so why would you do any different in your plane?
Click to expand...

No, the cars computer or ignition switch does that for me.

With the good luck I've had, I don't intend to change my ways now!

Mark
 
This needs to join the never ending debate sub forum....

Evidence suggest that both ways work so pick your poison and run with it...
 
MauiLvrs said:
Don't think so ...
See Mel's note below.
Click to expand...

Seems for every nay sayer there is someone that says no problem. The proof would be in an unbiased scientific test in controlled conditions.....never gonna happen.....so looks like we are stuck with opinions, personal beliefs and myths.

Personally I don't think it matters one way or the other.
 
I've decided to "change".

Since I purchased my RV-8, I have used the startup technique I was taught in flight school.

I am now in the process of creating a "modern simple" panel. As part of that redesign, the separate switch for the alternator field circuit is going away (the CB remains).

Will it matter? I don't know.

Addendum: I have an internally regulated alternator. Most likely an automotive type unit.
 
Last edited:
I have an electron dependent engine...ignition, injection, fue pumps.... And as such installed two alternators. I have always been afraid to cycle the field due to all the stories about short life. I have idiot lights on both alternators and rely on those to let me know if there is a problem, but have never felt great about it. I wish there was a more definitive way to test both alternators without cycling the field.
 
here is some data

When we bought our plane the alternator field was wired to the master and was always on. I wanted a way to turn it off so I could test the backup system on the ground. I changed it so I could switch the field on and off. About 2 months later the alternator failed.

Still have it wired that way but I always have it on thru startup and shutdown now. If I am going to test the backup system, I just leave the main alternator off thru the startup-test-shutdown sequence, then restart normally.
 
Turning on the field switch while the engine is already running is almost like the electrical analogy to having a car with a manual transmission and jamming the shifter into gear while the engine is running and not using the clutch.

Some of the RVs at my airport have had repeated failures of the internal VR of their Nippon Denso alternators until they started leaving the field switch on all the time, then the failures stopped happening.

I've got the little 35A ND alternator in my RV-6 and when I bought it last May, it was equipped with the small,square 2-wire Mopar-style external regulator under the panel. The previous owner/builder followed the practice of startup with the field off and turning it on after the engine was running. When I bought the plane, he'd just put in a brand new battery and I noticed the charging system wasn't charging worth squat... maybe putting out 12.1-12.2v tops at 1500-1800 RPM. The regulator was toast and I changed it out for a Ford-style unit and now have a solid 14.3 volts. I leave the field switch on all the time and consider it there in case I ever need to shut off the system for emergency only.

My old Cherokee had no field switch, only a pullable breaker for the field circuit, designed this way from the factory so maybe Piper knew something about keeping the old Chrysler alternators with external regulators happy. Or maybe they were just too cheap to use an expensive split master switch like the Cessnas :p
 
humptybump said:
I am now in the process of creating a "modern simple" panel. As part of that redesign, the separate switch for the alternator field circuit is going away (the CB remains).

Will it matter? I don't know.
Click to expand...

That's the way we wired.
If we need to kill the field ... pull the breaker ...

Works like a champ ...
 
Brantel said:
This needs to join the never ending debate sub forum....

Evidence suggest that both ways work so pick your poison and run with it...
Click to expand...


I Agree.

I have my alternator (B&C L-60) switched OFF for all my starts the past 15-years and over 2,600 hours. No alternator issues and yes I replaced the alternator at 2,200 hours and it was still working. Have had other issues with the charging circuit and each time it has come back to a bad connector at the alternator field "Fast-on" contact.

IF I were building new, I would leave the Alternator Field switch out and just have an alternator field "pull type" Circuit Breaker. Since I have the switch, I use it.

I am one data point with 15-years and 2,600+ hours that turning the alternator field switch on AFTER starting caused no issues.

IMHO, I think that field switch came about back when a Generator was used before alternators and batteries did not have the cranking power that we have today. Also, the starters did not crank as fast as they do now. To get a good start, as much load needed to be off-loaded from the battery to turn the starter.
 
You must log in or register to reply here.
Back
Top