? 4 Electrical engineere or other expert

I am trying to follow the electrons and what is softer on the charging system. I read that some turn on the charging system after start and some don't, by either the field contact or other means. I would think that connecting the field or energizing the regulator after the engine is running, there would be a load shock to the battery and on the alternator diode pack. What say Ye, charging system powered up during start and shut down, or no?

bret said:

I am trying to follow the electrons and what is softer on the charging system. I read that some turn on the charging system after start and some don't, by either the field contact or other means. I would think that connecting the field or energizing the regulator after the engine is running, there would be a load shock to the battery and on the alternator diode pack. What say Ye, charging system powered up during start and shut down, or no?

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Until you get a real expert, I agree it seems like a shock. I do know that some vintage of alternators are sensitive to activating the field once it is at speed. Apparently, many of our are not prone to their failure. I will be engaging my field before start, just like my cars for more than a million miles.

Maybe, someone that has worked with the alternator manufacturers can tell us why/why-not to your question.

If the engine is already running when the alternator is turned on, the regulator will send maximum field current to the alternator to increase the battery voltage. It might take a second or two for the voltage to stabilize. I do not think that will hurt anything. But what is the advantage? And what if you forget to turn the alternator on? Cars do not require shutting off the alternator while starting the engine. Most aircraft electrical loads are a small fraction of the starter current which can be hundreds of amps. A few extra amps will not hurt the starter performance. I routinely turn on everything including avionics prior to engine start.

Just do what all cars do. The alternator field (or for the internally regulated alternators the source) is on anytime the master is on. The old school Cessna split master/alternator switch approach is not required or desired.

WRT any "shock", I offer that this is not an issue. One a month or so I turn off the primary alternator source breaker to make sure the backup alternator picks up the load. Then I turn the primary back on.

Carl

I use an internally regulated alternator, & don't make any attempt to take it off line during normal operations. For normal startup sequence, I never worry about it, and wouldn't do it if I had an externally regulated alternator, either.

With an externally regulated alternator, it's good to have control of the field (pullable breaker or switch) for those times that you need to sit in the a/c & perform some task that requires electrical power, but without a need to crank the engine. Reason is that many external regulators will bring the field winding on line any time the bus is hot, in an attempt to get alternator output up to the set point (not happening when the engine's not running). So you'll have a needless additional 2A-5A of drain on the battery if you don't shut off the field.

Charlie



It's worth mentioning that some older designs (which are still in common use in homebuilts) can have their regulator circuits destroyed if they're shut down while under load. When the load was(is) dumped, those early designs can't catch the field voltage fast enough to prevent a large internal voltage spike, which exceeds the voltage rating of some of the semiconductors in the regulator. Guys were checking their low voltage detector circuits by shutting down the alternator in flight, and killing the alternator.

Unless you trace all the wires out, I don't think it's fair to say a car has the alternator on when cranking. Are you 100% sure that when the key is in the START position on your car, the alternator field is active? Heck, modern cars cycle the alternator on and off all the time for fuel economy (off going up a hill, on going down).

Alternators don't come on "hard". They have impedance that prevents a quick ramp up. Suddenly disconnecting them is hard on everything, but cycling them by turning the field on and off is pretty much a non-event. It partially matters what alternator you have and what control you have over it.

But this has all been discussed before:

http://www.vansairforce.com/community/showthread.php?t=31739&page=5

rv7charlie said:

--snip--



It's worth mentioning that some older designs (which are still in common use in homebuilts) can have their regulator circuits destroyed if they're shut down while under load. When the load was(is) dumped, those early designs can't catch the field voltage fast enough to prevent a large internal voltage spike, which exceeds the voltage rating of some of the semiconductors in the regulator. Guys were checking their low voltage detector circuits by shutting down the alternator in flight, and killing the alternator.

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There's a big difference between turning off the field vs. disconnecting the B lead (output) when the alternator is producing current. The former is what happens when a field switch is used, and it does not produce a voltage spike on any alternator design I am aware of. The second, however, can produce a damaging voltage spike to any load connected to the alternator. And some of the aircraft wiring diagrams I have seen - which when the master is turned off disconnect the B-lead from the battery at the same time as turning off the field - also can cause a voltage spike. This is because it takes a bit of time for the magnetic field to collapse after the field is turned off.

You really want to turn off the field first before taking any action that disconnects the B-lead from the battery.

Mark,

That's correct; it was/is internally regulated versions. I suppose I could have been a bit more clear than just saying, "can have their regulator circuits destroyed if they're shut down while under load. When the load was(is) dumped...etc".

Charlie

molson309 said:

There's a big difference between turning off the field vs. disconnecting the B lead (output) when the alternator is producing current. The former is what happens when a field switch is used, and it does not produce a voltage spike on any alternator design I am aware of. The second, however, can produce a damaging voltage spike to any load connected to the alternator. And some of the aircraft wiring diagrams I have seen - which when the master is turned off disconnect the B-lead from the battery at the same time as turning off the field - also can cause a voltage spike. This is because it takes a bit of time for the magnetic field to collapse after the field is turned off.

You really want to turn off the field first before taking any action that disconnects the B-lead from the battery.

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Disconnecting the field might create a voltage spike, it is one big coil around the rotor making a magnet, then at disconnect the magnetic field collapses and Bang? just like our master and start relay......yes, no? sorry, I shouldn't say bang while talking airplane talk.........