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ALT field switch... why, really?

But given your initial narrative, the driver smelled smoke under his dash. I just took a quick stroll through some of the car's wiring diagrams, and there are 'hot at all times' notes all over them at DC supply points feeding stuff in the dash. It's really difficult to see how disconnecting the field of the (internally regulated) alternator would make any difference whatsoever, with so many different 'always hot' supply points from the battery itself. In any case, assuming the driver turned off the ignition switch, the alternator wouldn't be a source of power, and the feeds *to* the alternator are protected by fuse links. So...If I were an NTSB investigator, I'd be leaning more toward not having a master contactor to disconnect the battery.

Feel free to point out errors in my logic.
 
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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.

If you crank with the alternator field on you're taking away 4-5 amps of cranking power plus the added mechanical resistance of the alternator during cranking - neither of which you really want.

For those reasons - and probably others - autos turn off the alternator field while the engine is cranking. And unless you want to pull a fuse or circuit breaker every time you start your plane you won't be able to do that without a switch.

Other reasons have been given, but those by themselves are good reasons for an alternator switch.
 
How would you shut down a stuck starter solenoid? :eek:

Bevan

Uh, I suspect you weren't following the conversation. My posts were responding to the post about a burning dash in a Chevy, & whether it was at all relevant to the presence/absence of an alternator field switch in an airplane. The NTSB comment was comparing the likelyhood of a missing alternator field switch vs a missing master solenoid as likely culprits in the Chevy fire; not an a/c fire.
 
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Based on this conversation, I asked B&C Tech Support about starting the engine with the alternator switch on or off. I have a 40 amp B&C alternator on an O-540 plus the SD-20. This is how they responded:

"I think there is a perception out there that having the alternator on during the start cycle creates a higher torque hurdle for the start to overcome. However, this is not true, because at low rpm the torque draw of the alternator will be close to negligible. Thus, whichever way works best for you is just fine."

Jerry
 
Care to speculate???

On mine I have a no name externally regulated alternator with OV and separate Master and Alternator field switches. Prior to reading this thread, I never really thought about it but always turned on the alternator after start. This was reinforced a few times when for whatever reason (not paying attention), I started with the alternator field on and the 5 Amp alternator field breaker was always popped. I simply reset it and all is well. I see that everyone else on this thread seems to be able to start with the alternator on with no side effects like mine. Any ideas why mine might be different before I take a deep dive? This has otherwise worked flawlessly for 15 years +.

Cheers, Sean
 
Maybe check the installation document for your OV module. See if it's schematic hints on a momentary over 5A power draw that could happen in your described situation.
 
Hmmm...

Maybe check the installation document for your OV module. See if it's schematic hints on a momentary over 5A power draw that could happen in your described situation.

Thanks Ralph. The OV module is covered in Heat Shrink so i'll have to pull it apart to get a part number. That certainly bears further investigation.

Cheers, Sean
 
Field switch and field breaker tripping in flight

On a recent return flight home to Chicago from New Jersey, to my great surprise my alternator field breaker tripped shortly after lifting off from Teterboro. Given what you see when you look down just off of Teterboro, that can really get your attention! Luckily, my B&C backup alternator jumped right in and allowed us to get home. Repeated attempts to push the field breaker back in resulted in either an immediate trip or normal operation for about ten seconds, and then tripping.

Once home, a check of the field resistance and all field wiring revealed no cause, and furthermore the problem was gone on subsequent flights. Talking to the fine folks at B&C, it was suggested that the cause was high resistance in the field switch contacts. My sense was that the trips were due, not to actual field over current, but rather a sensed over voltage by the regulator resulting in crowbar tripping. How that could be triggered by excess switch resistance evades me at this point, but the reason I mention this in the context of folks discussing the need, or not, of a field switch, the B&C folks pointed out that the classical Cessna split master switches that many of us dutifully installed, are known to chronically develop high resistance and need replacement, which is something useful to know, especially if you have the B&C regulator and alternator, as apparently it can cause the filed breaker to trip.

So, I measured the voltage drop across mine in flight, and sure enough, with from 1 to 3 amps of filed current, there was anywhere from .5 to 1 V drop across the switch, so indeed it had resistance. What it was in flight that one time I don't know, but I wish I had thought to re-cycle the switch, not just the breaker....I bet the field tripping might have gone away!

In the 1500 hours on my plane, I've had 3 instances of alternator outages. If you don't already have a backup on your vacuum pad, I highly recommend it!!
 
On a recent return flight home to Chicago from New Jersey, to my great surprise my alternator field breaker tripped shortly after lifting off from Teterboro. Given what you see when you look down just off of Teterboro, that can really get your attention! Luckily, my B&C backup alternator jumped right in and allowed us to get home. Repeated attempts to push the field breaker back in resulted in either an immediate trip or normal operation for about ten seconds, and then tripping.

Once home, a check of the field resistance and all field wiring revealed no cause, and furthermore the problem was gone on subsequent flights. Talking to the fine folks at B&C, it was suggested that the cause was high resistance in the field switch contacts. My sense was that the trips were due, not to actual field over current, but rather a sensed over voltage by the regulator resulting in crowbar tripping. How that could be triggered by excess switch resistance evades me at this point

I believe excess resistance in the field switch can cause it by this.. if there is excess resistance, then the field voltage will drop (across the resistance), causing the regulator to demand higher voltage, the Crowbar senses the higher voltage and grounds the field, tripping the breaker and preventing the over voltage from damaging anything.
 
EarthX

After reading this post years ago, I have been keeping my field switch on at all times. Now I just installed an EarthX battery with way more cranking amps than my old PC680.

Question: Could the new battery send too much current to harm the alternator during start? I don't really how this all works.

Thanks in advance for your advice.

Matt
 
Troubleshooting location

I would recommend not resetting a circuit breaker in flight for any system. If the circuit has a fault (causing the initial breaker trip) the power has been removed rendering the system inoperative. Pushing the breaker back in applies power to a possibly faulty system. A possible electrical fire is my concern here. My policy is to do ALL electrical troubleshooting regarding fuses or circuit breakers on the ground. Submitted for your consideration.
 
Pull-able breakers are not designed to be used as switches. Every time a breaker trips, it gets just a little weaker. So it will have a limited life when used like a switch.
 
Question: Could the new battery send too much current to harm the alternator during start?
No, The load determines the current, not the battery. The voltage regulator will
regulate the current. Of course an EarthX battery is capable of sending more
current because it has slightly higher voltage and less internal resistance. But
regardless, the load takes what it wants.
 
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