fusible link questions

[Raymond C Drouillard]

eilds 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.

That's the point of the whold discussion.  I would run the alternater at
a higher voltage and use an efficient switching regulater to step it
down.  Much like a transformer, a switching regulater will step up the
current when it steps down the voltage.

A switching regulater does not regulate by dissapating the extra power.


>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 discussed this very thing with one of my electronics professors.  Yes,
silicon diodes drop 0.6 or 0.7 volts normally.  The diodes in an
automotive alternater, however, will drop more like 2V under heavy load.

An automotive alternater is wired in delta, and uses six diodes to
rectify the output.  The current from any coil will go through two diodes
in its path through the load.  That gives us a total voltage drop of 4V. 
Since we're supplying around 14V, the diodes dissapate about 28% of the
total output.  This means that if the alternater is providing 1000 watts
to the load, it is dissapating an additional 280 watts in the diodes. 
The stator windings are dissapating some power, too.

Ray Drouillard


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