fusible link questions

[Clarence L.Snyder]

Raymond C Drouillard wrote:
> 
> >What is the current in and voltage across the field windings necessary
> to
> >generate the power proposed? I'm willing to bet that it isn't that huge.
> >Certainly the full power output proposed by the original poster will not
> be
> >dissapated in the field windings.
> >
> >I've no idea what current is normally presant in the field windings, but
> I
> >wouldn't expect it to be large, and I would expect that it could be
> increased
> >significantly before you have problems with overheating.

Standard early Delcotron fiels was 1.8 to 2.3 amps on 45 amp, 2.8 to 3.2
on 60 amp units. The early Ford alternator field drew 3 amps +/-. This
is 36 watts nominal dissipation. 
This is a small amount of current - and power loss - but on many of the
compact alternators tody, running them at close to capacity for any
length of time (shorted battery, for instance) will fry the alternator -
feilds or stator, or both.
> 
> I hadn't even considered the power dissapated by the field coil.  It
> should be small compared to the power dissapated by the rest of the
> system.  If not, it can be replaced with a ceramic or rare earth
> permenent magnet.
> 
Which would produce a non-regulatable device. You would then require an
active regulator to dissipate the extra power produced externally.
> >
> >The other issue noone has mentioned is the voltage output, can the
> diodes and
> >other components survive the high voltage?
> 
Most good alternator diodes are rated at 300PIV. 
If the diodes have a .6 volt forward drop, at 100 amps they are
dissipating 60 watts each. In 3 Phase, they share the load, so likely
see closer to 35-40 amp peaks, and 33 amp average for 20 watts per
diode, or 200 watts? total diode dissipation at full load. This does not
change with voltage, but does with load. Even if the load is split
between the two diodes of each pair, that would be 100 watts total diode
dissipation - a good reason to have fins and a fan. This dissipation is
an order of magnitude higher than the field or stator losses.

> I didn't mention it, but my plan is actually to remove the diodes when I
> remove the regulater, and bring three or four wires (depending on if I go
> with Y or Delta) out and mounting the rectifiers and switching regulater
> in a friendlier location.  I would use rectifiers with the appropriate
> ratings.
> 
> Ray Drouillard
> 
> >
> >--
> >Tom Parker - tparker at nznet.gen.nz
> >           - http://www.geocities.com/MotorCity/Track/8381/
> >
> >
> 
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