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Alternator Exciter Wire Light

Discussion in 'Automotive Electronics' started by MCE Mike, Jun 17, 2016.

  1. MCE Mike

    MCE Mike New Member

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    Hello,

    I just finished wiring a boat with a carbed Chevy big block for a friend, and wired an alternator for the first time. It's a very simple boat with only enough electronics to fire the ignition, bilge pump, and horn. I wired the alternator according to the manufacturer's spec (POS, NEG, EXC to key on power source). I wired the ignition light (supplied with a switch panel that was purchased for this boat) into the alternator EXC wire in this fashion: Key-on power to the + side of the light, and the - side of the light to the EXC post on the alternator. When the key is ON, the ign. light illuminates. However, once the engine starts, the light remains on, and the battery shows a discharging state. =The alternator is not charging.

    If I disconnect the EXC wire at the alternator, the ign light goes out on the switch panel. So this is telling me that no wires were crossed, etc. However, here is the weird part: With the EXC wire disconnected at the alternator, I can put a test light on the end of the wire and to the battery negative. This completes the circuit again and the dash light comes back on, but the test light does not light up. I verified the test light is working.

    Any ideas? I am wondering if the ign light is an LED. However, I can't find any info on it. I am thinking this is probably something small and stupid that I just can't see.

    Thanks,

    Mike
     
  2. MikeMl

    MikeMl Well-Known Member Most Helpful Member

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    Many different alternators, many different way to wire the idiot lamp. Most modern alternators do not support an external idiot lamp...
     
  3. MCE Mike

    MCE Mike New Member

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  4. dave

    Dave New Member

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  5. shortbus=

    shortbus= Well-Known Member

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    Some alternators also need a resistor in parallel with the idiot light. Any good reason that you didn't use an AC Delco alternator seeing as it is a Chevy/GM engine? They are about the easiest to wire. Is there a ground connection back to the battery from the dash board? Since this is probably a fiberglass boat.
     
  6. spec

    spec Well-Known Member Most Helpful Member

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    Hy MCE,

    Welcome to ETO.

    You have come to the right place- there are plenty of petrol heads here.

    Just in case it helps, here is a schematic of a typical automobile charging circuit showing the function of the ignition light.

    spec

    2016_06_19_SCHEMATIC UPDATED IN LINE WITH RODALCO'S OBSERVATIONS IN POST #14
    2016_06_17_CAR_ALTERNATO_CIRCUIT DIAGRAM.png

    NOTES
    (1) Before starting, with the ignition switch set to auxiliary, the ignition light is illuminated because its right side is at 12V and its left side has a path to earth via the regulator and rotor coil.
    (2) When the engine starts and the alternator is spinning the auxiliary field coils produce 12V so that the ignition light has 12V on its left side too. As its right side is also at 12V no current flows through the ignition light so it extinguishes. In fact there always is a small current through the ignition light because the right side is 12V DC and the left side is 12V with a ripple voltage imposed on it due to rectification of the auxiliary diodes.
    (3) A voltage regulator is effectively just a switch. If the battery voltage is below 14V (or so) it closes and puts current through the alternator rotor which causes the field coil voltage to increase. If the battery voltage is above 14V the switch opens and thus no current flows through the rotor and the main field coil voltage drops. It is known as 'bang-bang control'.
    (4) In case you are wondering why the ignition does not come on when the rotor current is zero, the answer is that the rotor current is never reduced completely to zero and also there is normally reminance in the rotor magnetic core to keep the voltage on the left of the ignition light at near 12V. The regulator switching is also quite fast.

    This is just a thought, but it may be necessary to flash the alternator rotor to give it some remnant magnetism so that it starts generating current- 5 amps or so, I think, should do the job.
     
    Last edited: Jun 18, 2016
  7. MCE Mike

    MCE Mike New Member

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    Thanks for the replies guys. I've got it wired up like spec's schematic, I wonder if it needs the resistor in parallel with the light? I was thinking only LEDs needed the resistor.

    shortbus - The only reason for that alternator is because it was already installed and used before. But the old wiring was a rat's nest.

    spec - can you tell me how to flash the alternator? I've never done that before.


    This is the exact switch panel and light I am using. http://www.jegs.com/i/JEGS-Performance-Products/555/11010K/10002/-1?parentProductId=763174
     
  8. debe

    debe Active Member

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    Check the idiot light, if its not atleast a 2 Watt globe then theres not enough current to excite the rotor. The first clue was when you put the test light on the excite wire & it lit up the idiot light & not the test light. Almost suggests the idiot light is an LED & wont work.
     
  9. MikeMl

    MikeMl Well-Known Member Most Helpful Member

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    Debe has it. The current flow through the idiot lamp has to be high enough to provide sufficient current to bootstrap the field excitation. The resistance of the field (rotor) in such alternators is quite low (~7Ω), so it takes a 12V lamp with a cold resistance less than ~25Ω to supply enough current to get the alternator going...

    If the indicator in the panel you linked is a LED (which passes only ~0.02A), it will have to be replaced with a real incandescent 12V 3W lamp that passes >0.3A. In this type of system, if the lamp burns out, the alternator will not bootstrap during start.
     
  10. spec

    spec Well-Known Member Most Helpful Member

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    I have had this conversation before and the general consensus was that there is normally enough (magnetic) reminance in the alternator rotor core to excite the alternator- like you, I didn't think so. But, in any case, if the alternator is brand new and has never had any current flowing through it there will be no reminance in the rotor, hence the flashing advice.

    I can't swear to this, but I think I have seen alternators still charging with a blown ignition light. It probably depends on the alternator type and possibly the automobile wiring.

    spec
     
  11. spec

    spec Well-Known Member Most Helpful Member

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    The other members have effectively covered this with a higher current ignition light.

    But to flash the rotor you need a 1 to 2 Ohm 20W or more resistor which you connect from the battery positive to the rotor. But if you don't have a suitable resistor you can use a traditional tungsten headlamp bulb. The higher the wattage the better. Just keep the resistor/bulb connected for about 5 seconds; that should do the job.

    Of course, you can simply wire in a headlight bulb in place of/in parallel with the existing ignition light to see if the alternator will then charge.

    Lastly, there is the possibility that your alternator is simply faulty.

    UPDATE- I now see that the alternator is not new and presumably was working so the flashing theory is not valid. That leaves the lower resistance ignition light.

    spec
     
    Last edited: Jun 17, 2016
  12. shortbus=

    shortbus= Well-Known Member

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    May be that things have changed over the years that I'm not aware of, but when I started out in mechanic life, generators were flashed but alternators were not. The idiot light is what is used to "flash" the alternator to start it charging. I'm not sure but think "flashing" a modern alternator with a built in regulator would not be wise thing to do.
     
  13. spec

    spec Well-Known Member Most Helpful Member

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    I don't know of any change to the design of alternators and as you are a professional with obvious experience I would take your good advice. Some time ago though I got into a similar discussion and held your point of view. But I was told that an alternator would self excite due to rotor remanance.

    I am not arguing here but isn't the ignition light current quite low- would that be enough to magnetize the rotor? I only ask out of interest.

    Flashing, even in the traditional sense ( momentarily connecting the rotor to the positive of the battery) should not harm the regulator. After all it has the full output from the auxiliary field coils, via the auxiliary field diodes, across it when the alternator is working normally.

    spec
     
  14. JMW

    JMW Member

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    Very interesting, I haven't been to this site for awhile and coincidentally I've been stumped on a boat with the same alternator. Very common 65 amps. There are several regulators available. Which do you have, 1 screw terminal or 3 on the regulator. The schematic for the alternator shows an idiot lite.
    http://www.pbase.com/mainecruising/alternator_conversion
    I'll dig out some more tomorrow or Monday
     
  15. RODALCO

    RODALCO Well-Known Member

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    Post Nr 5

    I think the lower set of diodes are the wrong way around in there.

    Normally a 12 Volt 3 Watt bulb is enough for excitation.
     
  16. spec

    spec Well-Known Member Most Helpful Member

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    Hell RODALCO, that is a big one. Well spotted :cool:

    That diagram came from some training handout, so many people must have viewed it. It is worrying that such a gross error should have gone unnoticed. My excuse is that I didn't look at that part of the circuit- just assumed it was OK :banghead:

    Plagiarism always gets you into trouble in the end.

    spec

    (post #5 corrected 2016_06_19)
     
    Last edited: Jun 18, 2016
  17. shortbus=

    shortbus= Well-Known Member

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    In my understanding of how the alternator works, the idiot light only supplys the rotor current to get the alternator to start charging at a lower RPM. Then the output of the stator thru the regulator then takes over from there and this is what turns off the idiot light, ~13V on each side of the light. Other than the idiot light there isn't a direct connection to the rotors coil.

    The flashing of a generator wasn't so much to magnetize it as to provide the correct polarity to it. It only needed to be done when installing a new regulator or generator to the vehicle.
     
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  18. RODALCO

    RODALCO Well-Known Member

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    Great, you have corrected the diagram. It looked weird as soon I saw it. have worked a lot with alternators, so I am familiar with the circuit.
     
  19. RODALCO

    RODALCO Well-Known Member

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    Yes, the light only provides the initial excitation current.
    As soon there is enough magnetic flux in the rotor, the rotor will take over and the voltage regulator adjusts the voltage to the correct level.
    Sometimes you can see the light dimly glow between 14.4 volts from alternator output and the ± 0.8 Volts voltage drop in the wiring loom which may be sitting at 13.6 volts when loaded with dash blowers, headlamps etc.

    The flashing of an alternator is usually done when the exciter bulb is burnt out, or a temporary alternator is fitted and not connected up properly, best is to use a 12 volts 21 or 55 watt bulb in series just in case you short something out
     
  20. spec

    spec Well-Known Member Most Helpful Member

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    Just to get things straight for me. Is it actually confirmed that you need to have current flowing through the rotor to get any voltage output from an alternator or will rotor reminance do it. Or is it a halfway-house where some alternators will and others won't?

    Also, does anyone know the typical current flowing through the rotor when the alternator is producing 10A and maximum current, say 80A.

    spec
     
    Last edited: Jun 20, 2016
  21. shortbus=

    shortbus= Well-Known Member

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    Things have changed over the years with alternators. The early ones, with a separate regulator, would not to my knowledge self excite. Think this was due to the heavy inductance in the regulator coils/solenoids, and length of the wiring between reg and alt.

    Can't say that I've ever seen a value put on that.
     
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