Alternator problem

Discussion in 'OnBoard Electronics & Controls' started by topcatii, Jul 17, 2009.

  1. topcatii
    Joined: Jul 2009
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    topcatii topcatii

    I have a 1977 four cyclinder (140 HP) Mercruser inboard w/outdrive, and have had some electrical problems, like my alternator is charging at 50 volts dc, anyone know why.
     
  2. CDK
    Joined: Aug 2007
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    CDK retired engineer

    Alternators don't do that, not even 32 years ago. With an open circuit and a defective regulator there may be 50 VDC between V+ and GND, but that is not "charging".

    More than likely that is the heart of the problem. You think it is charging but there is an open circuit between the alternator output and the battery. And that killed the regulator!
     
  3. PAR
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    PAR Yacht Designer/Builder

    Okay, I'll bite, how do you know it's charging at 50 VDC?
     
  4. pistnbroke
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    pistnbroke I try

    you carnt run an alaternator without the battery connected .....Most are battery excited by the warning lamp current and the voltage is held to 14.4 -14.7 by the voltge regulator which is usually internal . If you have 50 v it sounds like the battery is disconnected from the output terminal and the voltage regulator is short circuit ) buggered )

    Check you have 12v at the B terminal and if battery volts rise when it running then its charging .....if battery goes over 15v the regulator is buggered .
     
  5. topcatii
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    topcatii topcatii

    Checked from bat terminal on back of alternator to ground, 50 volts dc using a fluke 12 multimeter.
     
  6. pistnbroke
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    pistnbroke I try

    Dont run it again the more you run it the more chance you have of blowing the main and field diodes ...whats the voltage at the B terminal .with the engine not running ..should be battery volts ......if its connected to the battery .....rule is test dont guess.....
     
  7. Carioca
    Joined: Aug 2005
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    Carioca Junior Member

    Yes, you can !

    I have, and it is possible in the following scenario:

    The battery + lead was (most probably) loose and on a 50 km trip, the said lead 'took off' from the battery + terminal........and as the alternator was already 'on the go', it made little difference if it was connected to the battery or not....the 'field flash' requirement had already been met earlier on, when the car was started with the said lead in contact with the battery + terminal ...

    On reaching my destination, I tried to re-start the car again...... the whole instrument panel was dead.

    I subsequently learnt that the voltage regulator had been fried, but the alternator´s diodes had survived the over-voltage ordeal.

    ??????

    With the battery disconnected (capacitor, load ballast...or a combination of these.... which is the better analogy ?) - the alternator, its accessories and specially, sensitive electronic gear on the output , are at a total loss.

    Correct. The 'field-flash' current (energy) in my comment above.


    With 50 V Dc output, your alternator may have survived, at least partially, if not fully.
     
  8. pistnbroke
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    pistnbroke I try

    Please do not use incorrect terms it only spreads techno crap on the internet

    Field is a dynamo term an alternator has a rotor and stator
    Field flashing is a term used to change the polarity of a dynamo and does not relate to alternators

    An alternator is excited by battery current via the warning lamp ..a dynamo uses residual magnetic field and is not dependent on its warning lamp if it has one for excitation.

    glad you solved your problem .
     
  9. Carioca
    Joined: Aug 2005
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    Carioca Junior Member

    I take it you are into terminology, pure and simple !

    A 'motor-car dynamo' is fundamentally an alternator, ie, produces a source of alternating voltage/current (AC), which is converted to direct voltage/current (DC) through the use of a mechanical rectifier, comprising the comutator and brushes.

    With the advent of affordable solid-state electronics in the 60´s, the design of 'motor-car dynamos' was radically changed.

    It now became feasible to manufacture an AC alternator - analogous to those used in electrical power companies to electrify cities and factories with AC 50/60 Hz - and rectify its output using solid-state diodes.

    This move was accompanied by a cost-reduction benefit, in that it was no longer necessary to manufacture (tediously) the comutator and armature windings, which as you well know are standard dynamo parts.

    Field flashing is a procedure whereby the electromagnetic field - without which it would not be physically possible to generate the desired voltage/current output - is given a gentle shove, thereby allowing the voltage/current-producing device to self-excite (part of the device´s output is fed back to the windings of the electromagnetic field, through a regulating circuit, viz, the regulator)

    A 'motor-car alternator' does also have some residual magnetic field, although this is insufficient to get the alternator going. Thus, the need to flash the field, to get it on the move ! Once it gets going, however, it takes complete charge of the requirements of its electromagnetic field !

    Indeed, there are dynamos (expensive ones) that use powerful magnets (rare-earth magnets?), possessing high residual magnetic fields, and these dispense with field flashing.

    However, as they start to produce useful electrical output, some means must be provided to regulate the same. Control windings, fed by the output circuit, are provided to augment/ reduce the powerful, resident magnetic field.

    May I ask you a question ?

    Why would a dynamo have a warning lamp, if not to signal that the residual field has done its job - that is, got the dynamo on the move - and that the dynamo´s output is now taking charge of the rest, through a shunt (or series) field winding fed by its own output, via a regulator (obviousl) ? ?
     
  10. pistnbroke
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    pistnbroke I try

    " Why would a dynamo have a warning lamp"

    the original purpose of the warning lamp pre starter motors was to enable the driver to be sure that the ignition was on before he got out to swing the starting handle ...many times the engine would not start because the ignition was not on ..hence the warning light .. french cars did not have a warning light because french people are so clever ..( which is why they crashed concorde / Killed Diana / and failed to maintain the latest crash plane )

    Please accept that you are talking technocrap if you use terms other than rotor and stator when refering to alternators and I dont need the function of any alternator or dynamo component explained ..I have every auto electrical qualification available from the basic to degree
     
  11. Carioca
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    Carioca Junior Member

    If the warning lamp was destined for the purpose that you affirm above, then it should have been wired from the ignition switch to ground

    It is amazing that you can get away with displaying such thoughtlessness on this forum, without a mediator calling your attention.

    To be so eager to ridicule alternative ways of analysing issues could well have activated the 'warning lamps' of other forumites as well.

    From now on, we all bow our heads in reverence to your umpteen qualifications and superior knowledge !
     
  12. pistnbroke
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    pistnbroke I try

    If the light was from ign to earth it would be on all the time .....with the connection to the charging system it goes off when the engine starts

    thanks for your complements ...
     
  13. Carioca
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    Carioca Junior Member

    Need only connect the 'warning lamp' to the terminal which is activated (with +12 V) when the ignition is switched on (with a key).

    Not worth a mention. After all, you do a magnificent job of displaying your talents on this forum.
     
    1 person likes this.
  14. pistnbroke
    Joined: Jan 2009
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    pistnbroke I try

    your logic gives me a headache...so the WL glows as you drive along when its illumination is no longer required ...????

    I like Brazil nuts .......
     

  15. CDK
    Joined: Aug 2007
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    CDK retired engineer

    I answered your question correctly in post #2, so why stretch it needlessly. If you had watched the forum a bit you cannot miss the fact that PistnBroke knows what he is talking about.
     
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