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Thread: Alternator Field

  1. #1

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    Alternator Field

    I have a 30 amp alternator. What would be the approximate current draw of the alternator field with engine stopped?

  2. #2
    wireweinie's Avatar
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    Yes.

    Web
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    wireweinie's Avatar
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    Lol. To many variables. What brand/model? 14 volt or 28 volt? Old or new? 'A' circuit or 'B' circuit? And not sure why you'd want to check current draw when you could just do a voltage check.

    The only way to check this accurately is to compare it to the same model alternator on another installation. But is is easier to disconnect the field terminals and measure the resistance through the field. Or check the voltage at the field terminal. Then compare these results to the manufacturer's specs.

    On most alternators, you can check the voltage at the field terminal with the engine stopped and then with the engine running. Stopped, it's usually just below battery voltage. Running, it should drop down to something like 3 volts or less.

    But different alternators have different specs.

    Web
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  4. #4

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    Trying to figure out if the current draw I'm seeing is just the alternator field draw or if part of it is due to a possible short. Would like to confirm before I take to the air again.

  5. #5
    wireweinie's Avatar
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    How are you observing the current draw? Panel mounted ammeter? If so is it in the bus feed or is it in the alternator output?

    Web
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  6. #6

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    Panel mounted ammeter in the bus.

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    wireweinie's Avatar
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    Turn on the master. Note the amp reading. Pull the field breaker and note the amp reading. The difference between the two readings is the total current going through the field. Any reading on the ammeter once the field breaker is pulled is going to another circuit.

    Web
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  8. #8

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    Yes - that makes sense. So simple I overlooked that strategy!

    The whole story: After I got my JPI EDM-700 back from repairs in California I re-installed it in the panel. Obviously, I overstressed some of the wiring in the process. When I turned on the main master, and then the avionics master there was a bit of smoke (!!!). It didn't help that I had a fully charged Earth-X ETX-900 battery connected.

    Anyway, I think I have found and fixed the short behind the panel, and have been using a little 12-volt batter with a 20-amp fuse to confirm. Even on the ground, "smoke in the cockpit" is a little disconcerting!

    It also doesn't help that I'm working in minus 30C (minus 22F) temperatures.

  9. #9
    wireweinie's Avatar
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    Quote Originally Posted by NunavutPA-12 View Post
    It also doesn't help that I'm working in minus 30C (minus 22F) temperatures.
    Short circuits get hot.

    Web
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  10. #10
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    Quote Originally Posted by NunavutPA-12 View Post
    ..... When I turned on the main master, and then the avionics master there was a bit of smoke (!!!). .....
    As we all know, modern avionics are smoke operated.
    But when you let the smoke out, they quit working.
    Cessna Skywagon-- accept no substitute!
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  11. #11

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    Got this PM yesterday:

    alternator field amps

    0 amps with the engine stopped. Field amps vary with load on the alternator. No electrical load with engine stopped so no field ampere draw. There would be battery voltage voltage on the field but 0 amps.


    True?
    Last edited by NunavutPA-12; 11-27-2020 at 11:13 AM.

  12. #12
    wireweinie's Avatar
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    Quote Originally Posted by NunavutPA-12 View Post
    Got this PM yesterday:

    alternator field amps

    0 amps with the engine stopped. Field amps vary with load on the alternator. No electrical load with engine stopped so no field ampere draw. There would be battery voltage voltage on the field but 0 amps.
    That is incorrect. With the system powered on but the engine not turning, there will be no output from the alternator but there will be current flowing through the field windings. The reason is that the regulator senses this '0 amps/12volt' output and meters current into the field windings to increase the output. Once the engine begins turning the alternator, the regulator will sense the increased output current and voltage and will adjust the field current to maintain roughly 14 volts and supply output current.

    The current in the field windings will vary with load but still, only to maintain voltage and output current.

    Web
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  13. #13

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    Thanks Web. I was confused (as always!) by the message and thought I was back to square one for awhile.

  14. #14
    wireweinie's Avatar
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    Nope.

    The reg is trying but the alternator can't work until it's turning.

    Web
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  15. #15

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    If the field switch is on when the battery is on my Denso will draw about 4 amps, doing what Web says trying to light some dynomite under the alternator to get it off its but, butt it cant because you forget to start the engine..
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  16. #16

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    4 amps. Okay! That's at least a reference point. Thanks.

  17. #17
    wireweinie's Avatar
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    Quote Originally Posted by NunavutPA-12 View Post
    4 amps. Okay! That's at least a reference point. Thanks.
    Only if you have a Denso alternator and of the same model and size as Eddie.

    Asking what the field current would be in 'a 30 amp alternator' is like asking how hard the wind should blow. What part of the world? What season? What temps? Day or night? etc.

    Back to the basics; what charging system do you have? Brand, model, size (max amps).

    Web

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    Last edited by wireweinie; 11-27-2020 at 05:16 PM.
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  18. #18

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    Sorry for the delay in responding, Web. I ran out of internet in the last few days.

    The alternator is a B&C L-40. It was installed years ago by a previous owner who neglected to make a log-book entry, so it took some time to confirm.

    The next time the 'plane is in for a rebuild (not in my lifetime) I will see that the panel is made easily removable for servicing. Currently, it is not, and it's a right PITA to get at the wiring. I used to be flexible enough to lie on my back while I inspected wiring, installed and zip tied everything, but those days are long gone!

  19. #19
    wireweinie's Avatar
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    Ok, if you need to compare measured readings, the ONLY alternator to compare to is another L-40. I'd suggest downloading a copy of B&C's troubleshooting guide and following it. I've attached a section that may give you an idea of whether your field windings are good or not. But read through the entire guide.

    Web
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