What's new
Van's Air Force

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

ALT field switch... why, really?

DaleB

Well Known Member
Assume we have an electrical system with an alternator, external voltage regulator, field supply via either a fuse or breaker, and an overvoltage crowbar.

In what situation would I use an alternator field switch, if I had one? In other words, if the master switch is on, why would I want the alternator field off? What failure mode, in flight or on the ground, could be helped by turning the alternator field off?

I see an ALT switch in some form on every panel I can remember. Old or new, Piper or Cessna or RV. I understand what it does, but I've never once done anything other than turn it on and off with the master switch, nor have I seen anything similar on anything other than an airplane. Just wondering why.
 
ALT switch

I'll be interested in the responses to this thread.

FWIW--

I was trained that when there is a split or separate alt switch, it's there for a reason, so use it. That was never followed by an actual reason. Odd that I just accepted that since it's rather out of character for me.

When so installed, such as my RV, I leave the the ALT off while starting, then turn it on, and then when shutting down it's the last switch off BEFORE pulling the mixture, and when the prop stops I shut off both ignitions and finally the master.
 
Though I now forget the details, I do remember reading, and being convinced, that starting the engine was best done with ALT *off* ... I could try to find the exact technical explanation ...
 
Right, to prevent a voltage spike during start, that's what it was ... think I was convinced by a few stories of people actually suffering from this at one point of another ... I guess it's just extra safety in case the crowbar doesn't do its job? Or maybe the aircraft in question didn't have overvoltage protection?
 
Or if the voltage regulator fails and the voltage increases above acceptable levels, you can kill the switch or the breaker and turn the alternator off.

I have recently been somewhat convinced that the main alternator breaker is not as necessary as we always thought. Plane Power told a customer that the only reason we ever still use them is because we always have.
 
Best justification is to be able to sit off the alternator in case of a failure mode (regulator, etc) that would allow the voltage to run away. If you have a "crowbar" style over voltage protection circuit then that use case disappears in case of a voltage regulator failure and voltage spike the crowbar causes the field breaker to blow and does the job.

I can't think of a reason to have it if you have a crowbar protector. In fact, if damage from over voltage is your concern I would posit that by the time you determined you had such a failure and flipped the switch it would be too late.

A lot of things we do are rooted in historical practice that probable have no practical need in modern electronic environment. Why do I say that? Because I've yet to see an alternator switch in a modern automobile and they have more electronics in them that even the most tricked out panel I've ever seen documented here.

For me the Crowbar circuit is a worthwhile insurance policy of sorts. The alternator switch - probably a waste IMHO.
 
Think of this scenario for a minute... You're flying along and your alternator belt breaks. You now have field current flowing through a useless alternator, draining the already-limited amount of electrons available in the aircraft's battery. The alt field switch allows that flow of electrons to be easily controlled. If it were instead an alt field fuse tucked up under the panel, it would be far more difficult to control the field current.

The same story goes for when you're doing some ground testing... do you really want juice to be flowing through your alternator? For that kind of activity, keeping only the "BATT" side of the switch on makes sense. Again, it's a matter of ease of being able to isolate the power generation from the power storage functions of the electrical system.

As for turning the alternator on after engine start, there are a lot of opinions on this. Some alternators do not fare well with their field voltage being turned on and off while they are being turned by the engine. This has led to premature failure of some alternators. We often see recommendations to turn on both sides of the master switch prior to engine start, and there was a good thread on here discussing why we would want the extra drag of the alternator during engine start. The bottom line remains that powering the alternator field should be done in accordance with manufacturer's recommendations.

You might also want to think about how this is done in your car (the source of many alternators installed in RV's). When you turn the ignition switch on in your car, the alternator field is powered and its output gets to ramp up as the engine comes up to speed. Consider it a "soft start" feature for your electrical system.
 
My last panel had no "ALT" switch setting. It was "ON" or "OFF". I had to chamgevthect artery one and forgot to pull the field breaker. The alternator was not happy. When I re-did the panel, I talked thru the decision and was convinced to use a three position switch so I now have OFF-BATTERY-ALT. it's not a significant benefit given. Still have a CB for the alternator field. It's just a matter of having the correct procedures documented in the POH.
 
You're flying along and your alternator belt breaks. You now have field current flowing through a useless alternator, draining the already-limited amount of electrons available in the aircraft's battery.
The same story goes for when you're doing some ground testing... do you really want juice to be flowing through your alternator?

I did NOT include the field control switch in my setup. The above quote is my only regrets.

A pull type breaker is all you need for the times when you need to disconnect the alt.
 
My Alt field draws 3.5 amps

You turn on the master to do EFIS work, or other electrical work, testing, etc, and you're draining 3.5 amps for no reason. I have VPX and use the EFIS VPX option to turn off the alternator field while working on my plane. I chose to not install a panel switch for the alternator field.
 
The master contactor only disconnects the battery from the electrical system, not the alternator. Once up and running, some alternators will keep producing power after the battery is disconnected from the system. If there is ever smoke in the cockpit, it might be desired to shut off all electrical power including the battery and alternator, thus a need for an alternator switch. If the electrical system in not equipped with automatic over voltage protection, an alternator switch is needed to shut off the alternator in case of high voltage.
For an instant when de-energized, contactors and solenoids can produce a high voltage spike unless an arc-suppression diode is connected across the coil. Cranking the engine causes a voltage sag, not a voltage spike. In the early days of transistors, some avionics could be damaged by a voltage sag. That was the beginning of the rule to shut off avionics while starting the engine. That rule has been passed on from generation to generation. However, modern avionics are designed to withstand any aircraft electrical system anomalies. I routinely start my engine with all avionics turned on. Here is a quote from the Dynon D-180 manual, "It is acceptable to have the FlightDEK-D180 turned on during engine start." Other Dynon products have similar statements.
 
One thing to remember is if you are using a automotive style alternator, flipping the alt switch 'off' once it has been excited you aren't turning anything 'off'. To turn it 'off' in flight you have to kill the master and alt and then only turn the master back on.

Turning the alt field on after the engine is running seems to cause a lot of automotive alternator failures so I just have a pullable 5amp breaker for the alt field.
 
Think of this scenario for a minute... You're flying along and your alternator belt breaks. You now have field current flowing through a useless alternator, draining the already-limited amount of electrons available in the aircraft's battery.

You turn on the master to do EFIS work, or other electrical work, testing, etc, and you're draining 3.5 amps for no reason.

That's what I thought too... but I'm getting conflicting information from my search for how much current is consumed by a non-running alternator. Some sources say a few amps, some indicate that is only the case if you have a failed regulator and that under normal circumstances, a non-spinning alternator should only draw a few mA. I don't know which is correct, and I don't know enough about alternator and automotive regulator design to know for sure. I suspect it depends on the regulator.

I started thinking about this because we're (generally) running automotive or automotive-derived alternators, and I have never in my life seen a car, motorcycle, bus, boat or any other vehicle with an alternator switch. I am aware of the ancient history behind the avionics master switch, which I also plan to omit.

I had already planned on an overvoltage crowbar, so maybe just using a pullable breaker in an accessible location rather than a fuse tucked under the panel is the way to go.
 
For an instant when de-energized, contactors and solenoids can produce a high voltage spike unless an arc-suppression diode is connected across the coil. Cranking the engine causes a voltage sag, not a voltage spike. In the early days of transistors, some avionics could be damaged by a voltage sag. That was the beginning of the rule to shut off avionics while starting the engine. That rule has been passed on from generation to generation. However, modern avionics are designed to withstand any aircraft electrical system anomalies. I routinely start my engine with all avionics turned on. Here is a quote from the Dynon D-180 manual, "It is acceptable to have the FlightDEK-D180 turned on during engine start." Other Dynon products have similar statements.

I think we have a winner - this is what I was told by the Continental engineers when I first started to work there. I remember them saying there was a voltage spike at shutdown. But I would like to see some data either way. Vern -do you have an o-scope trace?

The field wire seems to cause more failures than it prevents, but that is just an anecdotal conclusion.
 
Last edited:
That's what I thought too... but I'm getting conflicting information from my search for how much current is consumed by a non-running alternator. Some sources say a few amps, some indicate that is only the case if you have a failed regulator and that under normal circumstances, a non-spinning alternator should only draw a few mA. .....

The VPX measures amperage on every circuit. It shows me the alternator field being ''on'' with a non -running engine is drawing 3.5 amps. When I use the VPX to turn the alternator field off, I see the total drain reduced by 3.5 amps. I am using a PlanePower alternator.
 
Last edited:
OMG

I guess next I'll stop waiting 1 hour after I eat before going swimming. :D

Very interesting thread!
 
With internally-regulated, automotive alternators like many of us use, experience and empirical evidence seems to indicate that turning the alternator on once the engine and alternator are turing is a great way to shorten the life of the regulator. Because we have a reason to believe this is potentially bad, and no one can come up with a reason that turning it on BEFORE engine start is BAD, I now turn alternator switches (if the airplane has them "ON" at all times - unless (as many have said), I need to shed load in the air or on the ground while doing other things such as EFIS maintenance.

I always use a pilot-accessible, pullable breaker for the field, so in the rare instances when I want the alternator OFF with the master ON, I just pull that breaker - and therefore, I no longer install a separate switch. Fewer switches is a good thing - less to forget or mess up, lighter, simpler, fewer things to break....
 
The VPX measures amperage on every circuit. It shows me the alternator field being ''on'' with a non -running engine is drawing 3.5 amps. When I use the VPX to turn the alternator field off, I see the total drain reduced by 3.5 amps. I am using a PlanePower alternator.

Measurement trumps all. Give that man a prize.

Is that draw for a 60 amp or 70 amp unit?

FWIW, I just use a pull-able breaker for the field. Alternator field is always on with master on, unless I pull the breaker. Without a switch or breaker, a Plane Power will remain live with the master switch off. Put another way, with the battery disconnected there would still be power going to the main bus, not a good thing if you have smoke under the panel. The field switch or breaker shuts down that electron source.
 
Sure. But...

A lot of things we do are rooted in historical practice that probable have no practical need in modern electronic environment. Why do I say that? Because I've yet to see an alternator switch in a modern automobile and they have more electronics in them that even the most tricked out panel I've ever seen documented here.

Sure, but when the smoke comes out of their panels they just stop the car and run...

Sayin'

Jerry
 
Sure, but when the smoke comes out of their panels they just stop the car and run...
Very true. But I think that's theoretical. I can't recall anyone I know who has had an electrical fire (or even substantial smoke) in a car built in the last 20 years or so, and some of my kids and their friends have driven some pretty sketchy cars. Isn't that what fuses and CBs are for? Wouldn't smoke coming from the panel be a sign that you designed something wrong?
 
If you have a setup like mine where ground maintenance functions need to be performed using electrical power after each engine shutdown, being able to have the battery on without the alternator field excited makes quite a difference, especially as I have only a small battery (engine cranking for me is not electrical). Makes quite an audible difference when using my oil scavenge pump.

To simplify the switchology though, I use only a single 3-position Master switch (off/batt/batt+alt).
 
Alt Field Switch

Whether you like the explanation or not the book by Bob Nuckolls, The AeroElectric Connection, in chapter 3 (Engine Driven Power Sources) gives a concise explanation regarding this switch.

Cheers,
JMA
 
Whether you like the explanation or not the book by Bob Nuckolls, The AeroElectric Connection, in chapter 3 (Engine Driven Power Sources) gives a concise explanation regarding this switch.
Yes, it does. I'm just asking when one would ever actually use it. If we accept the Nuckolls book as aeronautic electrical design Gospel (and we may or may not), I found Verse - I mean, Appendix Z, Note 22 interesting.

I think a 3-position toggle will do nicely, simply to accommodate ground operations with the master on and engine not running.
 
From the front page:
Q: ALT field switch... why, really?

A: voltage spike during start
A debunked myth, as far as I can tell.

A: voltage regulator fails and the voltage increases above acceptable levels
By the time you notice and flip a switch your avionics have already headed west, if they're going to at all. This is a job for an OV crowbar, not a switch.

A: alternator belt breaks
A very rare occurrence, but it does make a good case for a pilot-accessible breaker.

A: If there is ever smoke in the cockpit, it might be desired to shut off all electrical power including the battery and alternator, thus a need for an alternator switch.
Another good case for a pilot-accessible breaker.

A: when de-energized, contactors and solenoids can produce a high voltage spike
This is a true statement, but is unrelated to the ALT field switch.
Not mentioned is the best reason I have seen so far... running avionics, etc. on the ground with the engine off. It's something likely to be done regularly, so I would not want to use a pullable breaker for it. In an automotive setup this is handled by the ignition switch ACC position. In an RV, I think a DP3T OFF/BAT/ALT switch would do the job perfectly well. And, with that setup I can use a fuse for the field supply.

The need for a breaker and an over-voltage crowbar protection circuit was pretty obvious from the start. I was trying to figure out why you'd need or want an ALT switch separate from those two items. I'd seen reports that an alternator at rest would only draw a few mA, and that may be the case with some of the internally regulated automotive setups.

See, this is why I like this joint. Smart people here.
 
Nice finishing statement.... I agree

See, this is why I like this joint. Smart people here.[/QUOTE]
 
I have recently been somewhat convinced that the main alternator breaker is not as necessary as we always thought. Plane Power told a customer that the only reason we ever still use them is because we always have.

Amazing how true that is for lots of things, especially in light airplanes.
 
Data point

One more option. I have a 60 amp Plane Power main alternator and the 30 amp FS-1 backup alternator. I have their fields controlled through a single switch that can be Main-Off-Backup. So, if the main alt fails, I get a low volt/low amp warning from my VP-X and switch to backup. Or I can turn them both off if need be for the rare reasons that have been listed. I leave the main alt on at all times - startup, shutdown, everything - unless doing avionics work in hangar or testing the backup alternator in flight or during runup. I like the setup.
 
I have recently been somewhat convinced that the main alternator breaker is not as necessary as we always thought. Plane Power told a customer that the only reason we ever still use them is because we always have.
A fuse or circuit breaker is needed to protect the battery and wires from a short circuit within the alternator or its main "B" lead. The purpose of the fuse or circuit breaker is not to protect the alternator because the alternator is self current limiting. An alternator can not put out much more than its rated capacity no matter how much load is connected to it.
Since the purpose of the fuse or circuit breaker is to protect the battery and to protect the "B" lead from excessive battery current, it makes sense to locate the protection close to the battery. To prevent nuisance trips, the protective device should be rated well above the highest expected alternator output current. Since the "B" lead protection should only trip or blow in the event of a short circuit, my personal preference is to use an ANL or similar fuse.
 
Avoid shearing ALT gear?

I have the Plane Power alternator that bolts to the back of the engine and runs off the vacuum pad gear. The interface is a nylon gear, and I've sheared it once doing a hot start. So although I realize this goes against conventional wisdom, I turn on the field AFTER starting the engine, reasoning that with no field to the alternator it is freer to spin and I am thus less likely to shear the nylon gear. I am no electrical engineer, so please correct me if my reasoning is mistaken!
 
Alternator field switch

The alternator field is the only circuit breaker in my electrical system. Every other circuit uses a fuse per the AeroElectric Connection. 800 hours and no problems.
 
.... Is that draw for a 60 amp or 70 amp unit?
FWIW, I just use a pull-able breaker for the field. Alternator field is always on with master on, unless I pull the breaker.... .

That would be a 60 amp PlanePower alternator. VPX is my ''pullable breaker''. By design, I did not install an Alternator Field switch. Its always on if the Master is on unless I ''pull'' the breaker by bringing up the VPX on my EFIS screen.
 
Switch and Breaker

<snip>In an RV, I think a DP3T OFF/BAT/ALT switch would do the job perfectly well. And, with that setup I can use a fuse for the field supply.</snip>

I use a OFF/BAT/ALT toggle switch as you describe AND a pull-able circuit breaker for the field (they are literally right next to each other on the panel). The BAT position for the aforementioned ground ops reasons; the breaker so I have a chance to reset in flight from a spurious trip.

BTW - I have seen close to 5 amps for the field draw with the engine off (amp difference on the Skyview between the BAT and ALT positions) using a Planepower, 60 amp, internal regulator alt.
 
Last edited:
Switch Part Number

Hi All,
I like the sound of this off/batt/alt+batt switch. Does anyone have a B&C or Stein part number for this switch that they can point me to, as I can't seem to find it on their websites?
Thanks.
Tom.
 
Stein SKU: LT-003 is the DPDT locking toggle variant like I have for off/batt/batt+alt

It's a bit on the pricey side (pretty high quality switch) but just my personal preference for a master was to have one which needs a very deliberate action to switch on, because my aircraft lives in a public access warbirds display hangar. Not essential to go that upmarket though. The others mentioned here would be fine too.
 
My regulator has built-in overvoltage protection that latches the field off if triggered. In the event of a nuisance trip, the VR power has to be cycled to get the alternator back online so a field switch or pullable breaker is in order.
 
Very true. But I think that's theoretical. I can't recall anyone I know who has had an electrical fire (or even substantial smoke) in a car built in the last 20 years or so, and some of my kids and their friends have driven some pretty sketchy cars.
I know i'm replying to a two year old comment, but as the thread was revived I thought this would be of note... As luck would have it, two nights ago this happened in front of my neighbour's house. A guy was driving along, smelled something funny followed by smoke coming from under his dash... He pulled over, and just had time to grab his valuables before the flames started. 2005-ish Chevy Cavalier.

It took four city and district fire trucks, and a brief visit from the airport fire truck (which used up a good supply of foam on it) to put the fire out... An hour and a half later.

Video hosted on my Google Photos site, the link should work for anyone who has it... About 1min long, large fireball.
https://photos.app.goo.gl/KQRwadcDsIYwacIg2
 
Dramatic video, but is there any evidence whatsoever that the fire is related to not having a separate alternator field switch???????
 
I know i'm replying to a two year old comment, but as the thread was revived I thought this would be of note... As luck would have it, two nights ago this happened in front of my neighbour's house. A guy was driving along, smelled something funny followed by smoke coming from under his dash... He pulled over, and just had time to grab his valuables before the flames started. 2005-ish Chevy Cavalier.

It took four city and district fire trucks, and a brief visit from the airport fire truck (which used up a good supply of foam on it) to put the fire out... An hour and a half later.

Video hosted on my Google Photos site, the link should work for anyone who has it... About 1min long, large fireball.
https://photos.app.goo.gl/KQRwadcDsIYwacIg2
I suspect that if my airplane was maintained to the standards of most of the 2005 Chevy Cavaliers that are still on the road, I'd worry about it more.

Still... it's not like an electrical malfunction that could be fixed by killing tht ALT field (that is what we were talking about, right?) cannot happen, it's just a very low probability event. A meteor strike can happen. A bird strike can happen; we hear about that s lot more often than we do smoke in the cockpit. And yet we don't all add layers of polycarbonate to out windshields, or fly with helmets and face shields.

So, two-plus years later, I'm flying a plane with no ALT field control -- mostly because it has not alternator field, it's a PM alternator. I've had a regulator failure, fortunately in a mode that only put about 16V on the bus -- well within the limits of all of my electronic items.
 
I suspect that if my airplane was maintained to the standards of most of the 2005 Chevy Cavaliers that are still on the road, I'd worry about it more.

Still... it's not like an electrical malfunction that could be fixed by killing tht ALT field (that is what we were talking about, right?) cannot happen, it's just a very low probability event. A meteor strike can happen. A bird strike can happen; we hear about that s lot more often than we do smoke in the cockpit. And yet we don't all add layers of polycarbonate to out windshields, or fly with helmets and face shields.

So, two-plus years later, I'm flying a plane with no ALT field control -- mostly because it has not alternator field, it's a PM alternator. I've had a regulator failure, fortunately in a mode that only put about 16V on the bus -- well within the limits of all of my electronic items.

Um, take a look at Snowflake's avatar. :D

Only way he could be _any_ cooler is to sport an eye patch aboard the Space Angel, like me.
 
And yet we don't all add layers of polycarbonate to out windshields, or fly with helmets and face shields.

Um, take a look at Snowflake's avatar. :D

:) I get it... and I agree. Some people do wear Nomex flight suits, gloves, socks, skivvies, Nomex and carbon fiber helmets, face shields, etc. I would certainly never criticize their choices, it's a matter of risk tolerance and what you're used to. Personally, I think I'd look incredibly silly suiting up like an F-18 pilot to climb into my RV-12 and go for lunch somewhere.

Like I said, anything is a possibility, and there are plenty of possible events that would leave a guy really wishing he had that Nomex or helmet or Fiberfrax and titanium firewall or parachute or whatever. I feel that the probability for those events is sufficiently low that I'm OK flying in shorts and a T-shirt, with the normal complement of fuses and a Halon fire extinguisher on the floor. I figure the odds are on my side, but anything can happen (says the guy who just left the hospital yesterday).
 
One consideration not mentioned - testing.

At some point you will want to turn off your alternator while flying to:
- Verify your panel still works (do this only VFR and keep a close eye on buss voltage).
- Verify your standby alternator, if you have one, comes on line and maintains buss voltage at the point you set it.

Remember, you want to run such tests under your conditions. Don?t wait for the plane to run the tests on you.

I use a switch breaker for the Plane Around 60a alternator (http://www.aircraftspruce.com/catalog/elpages/pbcircuitbrkr2.php?clickkey=5950 ) and a pull breaker for the B&C backup alternator regulator.

Carl
 
Voltage regulation

Put another way, with the battery disconnected there would still be power going to the main bus, not a good thing if you have smoke under the panel. The field switch or breaker shuts down that electron source.

Also not good for the avionics. The PP alt has wild voltage fluctuations when there is no battery to load it!
 
I too run two alternators but not all the time. In fact I run the 60A main all the time xcept to occasionally test my backup alternator. Then I switch off the main and switch on the B&C to make sure it is still with me! Can't think of any reason to run both of them all the time?
 
Dramatic video, but is there any evidence whatsoever that the fire is related to not having a separate alternator field switch???????
No, none at all... I was responding to an assertion that no modern car has electrical problems that might make you want to turn off the alternator field. This driver turned the key off and took it with him when he left, too, so if that removed the alternator field, it didn't help.

Bill Boyd said:
Um, take a look at Snowflake's avatar.
LMAO... And for what it's worth, I also have a nomex suit and gloves. I wear it all when flying formation or aerobatics. I go back and forth between my Lightspeed headset and my helmet when pleasure flying, there is occasionally a lot of bird activity in my area and it might be rather difficult to see if I ever took one in the canopy. Also, the tinted visor is handy as I wear glasses and it saves me finding prescription sunglasses.

[size=-2]That being said I don't dislike feeling like a fighter pilot...[/size]
 
Dramatic video, but is there any evidence whatsoever that the fire is related to not having a separate alternator field switch???????

I think the point is that if the car had a field switch, it might have made a difference to the extent of the fire -- it might have gone out if power was removed. Assumes that the battery would disconnect with key to off also. Not all circuits do, varies by car. Many keep horn or headlights or other items 'hot' with the key off.
 
No, none at all... I was responding to an assertion that no modern car has electrical problems that might make you want to turn off the alternator field.
Not to get too picky, but that was not my assertion at all. What I said was, "I have never in my life seen a car, motorcycle, bus, boat or any other vehicle with an alternator switch", and "I can't recall anyone I know who has had an electrical fire (or even substantial smoke) in a car built in the last 20 years or so, and some of my kids and their friends have driven some pretty sketchy cars." Not a claim that it never happens, just an observation that it's certainly not a common occurrence.

Obviously it would be silly to contend that none of the many millions of cars sold each year have electrical problems. Even a newer, well maintained vehicle (which I would wager an '05 Cavalier was not) could possibly have some problem that would lead to smoke or a fire. It could also have a family of spiders living in the fuel vent line. The incidence of either, though, is so rare as to be notable, even given the lack of an alternator switch in any motor vehicle I've ever seen.

It's not a claim of impossibility, it's an observation of the risk level. And my original question was about the reasoning behind an alternator field switch. The need for a fuse or breaker is pretty obvious, and if said fuse or breaker is accessible in flight it would seem to cover any of the reasonable cases made for such a switch.
 
Back
Top