Alternator Question

[Richard J Lebens]

I came into this discussion late but, go to
http://www.john-joyce.co.uk/electron/altoreg/
for a typical picture.  Grounding what you call the field wire or the
wire to the left of the charge warning lamp in the referenced schematic
would do bad things to your field rectifier diodes if the car was
running.  Probably let the magic smoke out.  That wire provides current
through the warning lamp to the regulator to energize the field winding
from the battery before the alternator starts to output current.  No
field current -> no magnetic field -> no output.  The lamp also serves
as a resistor to limit the current.  note: if you disconect the wire
your alternator may still work for a while because of the residual
magnetism in the rotor.  The lamp goes out because when the alternator
is passing current through the diodes both sides of the lamp are at
battery potential and no current flows.

The field rectifier diodes serve two functions.  They rectify the
output of the alternator to provide a DC current to the field to create
a mechanically spinning magnetic field.  Also the diode voltage drop
across them is about equal to the rectifier diodes so the output of the
field rectifier presented to the regulator is pretty close to what the
battery voltage is.  As mentioned above, before the alternator is
spinning converting mechanical energy into electrical, the field
current has to be provided from the battery.

--- Ben Swann <bswann at worldnet.att.net> wrote:
> Huw,
>
> As it turns out, the new/rebuilt alternator was bad after all - Sean
> will
> be getting a new one from the local dealer for about the same cost.
>
> I am still interested in exactly how the exiter/feild current wire is
> supposed to operate.  My understanding is that when it is hooked to
> ground
> (-) it enables the alternator to pass current.  Seems it's there to
> prevent
> the alt from passing current backward through the alternator when the
> engine is not running, therefore acting as a switch.  The oil switch
> is


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